TECHNICAL FIELD The invention relates to the field of agricul- ture. More specifically, the invention pertains to the fumigation of soils, enclosed spaces, and agricultural commodities.

BACKGROUND ART Among the more economically serious plant para¬ sites are nematodes, which are roundworms, comprising as many as 10,000 species, of which at least 150 are known to adversely, affect plant life. Plant parasitic nematodes have been known since about the year 1750. Most of the nematodes which cause crop damage do so by feeding on plant roots, and therefore are found primarily in the upper few inches of soil in the roots or in close proximity to the roots. Nematode feeding causes hypertrophy or gall forma¬ tion, and the evidence of heavy infestation is plant stunting, pale foliage, wilting, and even plant death in extreme cases.

Virtually all of the world's crops and ornamental plants can be attacked by parasitic nematodes. Important destructive nematode species include the root knot nema- todes which are hosted by tomatoes, alfalfa, cotton, corn, potatoes, citrus and many other crops, the golden nematode of potatoes, the sugar beet cyst nematode and the citrus nematode. These, and a few other species, are described in "The Soil Pest Complex", Agricultural and Food Chemistry, Vol. 3, pages 202-205 (1955) . Also described therein is a further complication resulting from nematode infestation, namely a lowered resistance to the effects of plant attack by bacteria and pathogenic soil fungi.

Except for small volumes of soil which _. can be sterilized, it has not been found possible to eliminate nematodes. Parasite populations can, however, be kept at levels which economically permit agricultural operations by

soil fumigation, crop rotation using non-hosting plant ^~ varieties, and (to a much lesser extent) the development of plants which are resistant to infestation. In many in¬ stances, control of nematodes is achieved only by σombina- tions of these techniques, and most control programs have proven quite costly.

Another serious problem in agriculture is the attack of plants by pathogenic microorganisms, particularly fungi. Such pathogens are normally controlled by fumiga- tion, prior to crop planting, using broad spectrum bio- cides, many of which are no longer regarded as environmen¬ tally safe. Certain narrow spectrum fungicides are avail¬ able, but .are extremely expensive and lose effectiveness against successive generations of fungi, due to genetic adaptability.

The process of soil fumigation requires the movement of gaseous chemicals through the soil which is treated, and the ^' readily apparent necessity for a suffi¬ cient concentration of gas at a given temperature and pressure condition to be lethal to the pest which would be controlled. Volatility of the chemical agent is critical to successful fumigation, since a very volatile substance will disperse too readily and not develop an effective concentration except for locations very close to the point of introduction to the soil. Substances having a very low volatility are also undesirable, since they will not disperse in the soil, and will be effective only at loca¬ tions near the point of introduction.

Since fumigants typically are effective against a pest only during specific phases in the life cycle of the pest, some measures must be taken to ensure that the fumigant is present during the proper phases. This re¬ quirement normally has been met by either applying highly persistent chemicals, applying large enough doses of the chemicals so that the normal decomposition, leaching, volatilization, and other processes will have a lesser effect upon pesticide concentration in the treated

environment, or, for highly volatile chemicals, enclosing the treated area (such as by covering soils) for sufficient time to achieve control of the pest. Unfortunately, most ' of the persistent chemicals are now environmentally unde- sirable and the noted application methods are sometimes prohibitively expensive.

Carbon disulfide is the first reported soil fumigant, used in Europe during the 1870's to control the sugar beet nematode. This agent is commercially impracti- cal, however, since very large quantities must be applied, due to its high volatility. Further, the material is quite flammable, reportedly being ignited even by static elec¬ tricity resulting from pouring the material out of drums. In addition, carbon disulfide possesses a very objection- able odor, and its vapors are toxic to humans. When sold for fumigant use, the carbon disulfide is normally mixed ^" with an inert fire retarding compound, such as carbon tetrachloride, and occasionally also with another fumigant. Typically, these compositions do not contain over about 20 percent by weight of carbon disulfide.

In addition to soil uses, carbon disulfide has been proven effective in the fumigation of commodities, as an insecticide, as a rodenticide, and for controlling certain weeds. The chemistry of thioσarbonic acids and salts has been studied in some detail, as indicated in the papers by O'Donoghue and ^" Kahan, Journal of the Chemical Society, Vol. 89 (II) , pages 1812-1818 (1906) ; Yeoman, Journal of the Chemical Society, Vol. 119, pages 38-54 (1921); and Mills and Robinson, Journal of the Chemical Society, Vol. 1928 (II) , pages 2326-2332 (1928) . According to O'Donoghue and Kahan, derivatives of thiocarbonic acid were prepared by Berzelius, who reacted aqueous solutions of hydrosulfides with carbon disulfide, the reactions occurring as in (1) :

2 KHS + CS. κ ₂ cs ₃ + H ₂ S (1)

f OMPI

giving unstable solutions which yielded unstable crystal¬ line salts.

Other thiocarbonates were prepared and further characterized by O'Donoghue and Kahan. Their paper, at page 1818, reports the formation of ammonium thiocarbonate by reacting liquid ammonia with cold alcoholic thiocarbonic acid, prepared by dropping a solution of "calcium thiocar¬ bonate" into concentrated hydrochloric acid. The "calcium thiocarbonate" utilized by the authors is described as a double salt, including the calcium cation in combination with both hydroxide and trithiocarbonate anions.

The noted paper by Yeoman reports the further study of thiocarbonates (called trithiocarbonates therein) and also reports the preparation and properties of perthio- carbonates (or tetrathiocarbonates) , derivatives of tetra- thioσarbonic acid, H _^ CS.. Yeoman prepared ammonium tri- thiocarbonate by saturating an alcoholic ammonia solution with hydrogen suifide, and then adding carbon disulfide; dry ether was added to precipitate the product salt. Ammonium perthiocarbonate was prepared in a similar manner, except that after reacting the ammonia and hydrogen suifide, elemental sulfur was added to form the disulfide, (NH.) ₂ S ₂ ; adding carbon disulfide immediately precipitated the product. Yeoman states that "solutions of both ammonium trithiocarbonate and perthiocarbonate are very unstable" due to both decomposition to form thiocyanate as a product, and to "complete dissociation into ammonia, hydrogen suifide, and carbon disulfide." Considerable explanation is provided concerning the stability of thiocarbonates, as exemplified by sodium trithiocarbonate and perthiocarbonate. Sodium trithiocar¬ bonate solutions in water are said to remain stable only if oxygen and carbon dioxide are "rigidly excluded"; the presence of oxygen causes decomposition to form carbon disulfide and thiosulfates, while carbon dioxide decomposes the solution to give a carbonate and carbon disulfide.

Similarly, solutions of sodium perthiocarbonate are report¬ ed to be stable for a considerable time in. the absence of oxygen, the presence of air causing decomposition into thiosulfate and carbon disulfide, while carbon dioxide decomposes the compound to form a carbonate, elemental sulfur, carbon disulfide, and hydrogen suifide. The potas¬ sium thiocarbonates behave similarly, according to Yeoman.

Yeoman also attempted to prepare and characterize the stability of thiocarbonate salts of four of the alka- line earth metals. Yeoman was unable to prepare a "pure" calcium tri- or tetrathiocarbonate, but observed that the double salt of calcium trithiocarbonate that he prepared was more stable (probably because it was less hygroscopic) than the sodium or potassium thiocarbonates. The barium tetrathiocarbonate could not be isolated, although Yeoman believed that it existed in solution. Barium trithiocar¬ bonate was found to be stable, although it was alleged to behave like sodium trithiocarbonate when dissolved in water. The preparation of aqueous solutions of the tri- and tetrathiocarbonate of magnesium and strontium was alleged, but the magnesium thiocarbonates were not charac¬ terized. However, the stability of none of the magnesium or strontium salts or solutions was determined.

The previously noted paper by Mills and Robinson shows the preparation of ammonium thiocarbonate by digest¬ ing ammonium pentasulfide (obtained by suspending sulfur in aqueous ammonia, then saturating with hydrogen suifide) with carbon disulfide. A crystalline residue from this digestion was found to be ammonium perthiocarbonate. These authors prepared a "better" ammonium perthiocarbonate product, however, by extracting the ammonium pentasulfide with carbon disulfide in a Soxhlet apparatus.

DISCLOSURE OF INVENTION

. The invention is directed to the fumigation of soils, enclosed spaces, agricultural products and other

commodities, etc., using compositions which decompose to form carbon disulfide and certain other biocidal materials. Such fumigation can be used to control bacteria, fungi, insects, nematodes, rodents, and weeds, all of which are included herein in the term "pests."

Fumigant compositions are described herein as "thiocarbonates," including, without limitation, salts of trithiocarbonic acid and tetrathiocarbonic acid, composi¬ tions having empirical formulae intermediate to these acid salts (such as MCS-, _, wherein M is a divalent metal ion) , and compositions containing substances in addition to thio¬ carbonates [such as ^' a stabilized ammonium tetrathiocarbon¬ ate whic .contains ammonium suifide, i.e., (NH.) ₂ CS ₄ *

(NH ₄ ) ₂ S]. ^' The compositions are generally water soluble and can be prepared, stored, and used in aqueous solutions. Thiocarbonate solutions of the invention are stable during prolonged periods of storage in a closed container, exhibit a low vapor pressure, and are not flammable. For soil fumigation, thiocarbonates can be mixed with fertilizers to provide a multi-functional application.

The term "stability", as used herein, can be regarded as a composite of two concepts: chemical stabil¬ ity and physical stability. Since the effectiveness of a composition depends, at least in part, upon its ability to release carbon disulfide during decomposition, chemical stability is expressed accordingly; this can be quantified by, for example, chemically decomposing the composition at some time and measuring the amount of carbon disulfide which evolves. Alternatively, an indication of the amount of available carbon disulfide can be obtained by spectro- photometrically deteirmining the presence of the thiocar¬ bonyl bond (^C="S) in a sample of the composition. The absorbance at wavelengths corresponding to those at which thiocarbonyl is known to absorb energy can be used for a quantitative analysis.

Symptomatic of chemical stability, but having an independent significance, is physical stability. This concept is important due to the nature of the products formed during decomposition of the composition, partiσu- larly the ammonia, hydrogen suifide, and carbon disulfide, which each have a high vapor pressure. It is readily apparent that a change in the physical form of the composi¬ tion from a solution of low vapor pressure into a mixture of compounds, each possessing a high vapor pressure, imposes some rather stringent requirements upon storage containers. Vapor pressure above the composition of the invention, therefore, will be used herein as an indicator of physical stability; a condition of maintained low vapor pressure is the desired property. Another index of phys- iσal instability is the formation of undesirable insoluble precipitates, which frequently comprise sulfur, or of an immiscible liquid phase, such as carbon disulfide. The more general description of physical stability, then, is the maintenance of only a single phase in the composition. Assessment of the stability of a particular composition must involve consideration of both the chemical stability and the physical stability over a period of time during which stability is desired. Certain formulations do not form precipitates and do not develop high vapor pres- sures during a reasonable storage period and, therefore, may be preferred over a formulation which has a greater chemical stability, but develops objectionable physical characteristics during storage. As a further example, a composition which is intended to be used as an additive to irrigation water is likely to be selected for its freedom from precipitate formation upon dilution; to obtain this property, a composition having a lower chemical stability could be necessary.

Ammonium thiocarbonate compositions of this invention are normally prepared by mixing the components (ammonia, hydrogen suifide, carbon disulfide, water, and, optionally, sulfur) in the ^' proper proportions, and under

__ GGMMr?lI__

conditions which facilitate removal of the heat generated during the preparation. Most of this heat.results from the mixing of ammonia and hydrogen suifide, and from the addition of carbon disulfide to the other components. No particular order of component addition is required, except that ammonia must either be present prior to hydrogen suifide addition or must be added concurrently with the hydrogen suifide. In a typical batch preparation, the required amount of water will be introduced into a con- tainer (which has cooling coils or other heat exchanging means) , followed by the sequential additions of gaseous or liquid ammonia and hydrogen suifide, sulfur (if required) , and carbon disulfide.

Many variations in the foregoing preparation are possible. For example, ammonia can be added as an aqueous ammonia solution, to satisfy all, or some part, of the ammonia requirement, reducing the amount of cooling needed. A further reduction in cooling can be obtained by using an ammonium suifide solution or solid to provide any desired amount of the ammonia and hydrogen suifide requirement. Sulfur, if required, can be added as the element or as a solution in carbon disulfide.

It is possible to replace a portion of the ammonia and hydrogen suifide with a soluble suifide materi- al such as alkali metal suifide, alkaline earth metal suifide, or any mixture thereof. The maximum replaced portion will usually be equivalent in suifide content to that amount of hydrogen suifide which would exceed the carbon disulfide molarity in a particular composition. These alternative compositions are especially useful for soil treatment, when it is desired to incorporate plant nutrients not otherwise present, e.g. , potassium and magnesium, for correcting a soil deficiency.

A typical continuous-flow production of the composition includes dissolving molten sulfur in carbon disulfide, using a mixing vessel which can be cooled, for example, by external recycle through a heat exchanger,

followed by combining the sulfur solution with water, liquid ammonia and liquid hydrogen suifide.in a cooled reactor vessel.

The reactor in either a batch or continuous process should be maintained at a somewhat elevated temper¬ ature, e.g., about 25° C. to about 70° C, to promote the rapid formation of a clear solution. Stirring or other mixing of the reactor contents also is useful in this regard. A holding time of about one hour is normally sufficient for obtaining the desired product solution.

A stabilized fumigant which is obtained by the above preparations comprises an aqueous solution of up to about fifty percent by weight solute, in which solute the molarity' of hydrogen suifide is greater than the molarity of carbon disulfide, and is about one-half the molarity of ammonia, and in which sulfur can also be present. Were it not for the requirement that the hydrogen suifide molarity exceeds that of the carbon disulfide, the range of solute compositions could include the stoichiometriσ equivalents of ammonium trithiocarbonate and ammonium tetrathiocarbon¬ ate. This requirement, in fact, is an important factor in obtaining the enhanced stability exhibited by the composi¬ tions of this invention.

One theoretical basis for explaining the enhance- ment in stability which is obtained by means of the inven¬ tion can be inferred from the following equations, although we do not intend to be bound by any one particular theory, since other possible explanations could be developed. In the equations, likely equilibrium conditions are indicated by the double arrows, while reactions which are considered to be primarily irreversible are denoted by a single arrow. Equilibration between ammonium tetrathiocarbonate and ammonium trithiocarbonate and its components is represented by (3) ; a possible decomposition route of ammonium trithio- carbonate into ammonium dithiocarbamate with ammonia and carbon disulfide in an acidic environment is shown by (5) ;

OMPI

the decomposition of ammonium dithiocarbamate into ammonium thiocyanate is represented by (6) .

(NH ₄ ) ₂ CS ₄ ^=Z___± (NH ₄ ) ₂ CS ₃ + S (2)

NH ₂ CS ₂ NH ₄ > NH ₄ SCN + H ₂ S (6)

From (2) , a prediction can be made that increased stability will result from an excess of elemental sulfur in the composition. This effect has been confirmed. Using the expression of (3) , it can be inferred that an excess of a component will shift the equilibrium to favor maintenance of ammonium trithiocarbonate. This has been disproved in the case of excess carbon disulfide, and also for excess ammonia. The effect of ammonia, however, appears to be expressible as a quadratic function, desta¬ bilizing solutions of ammonium trithiocarbonate as the excess ammonia increases, then reversing to provide increas¬ ing stability with continued increases in the ammonia level. For excess hydrogen suifide, however, a stabilizing effect has been found, expressible as a quadratic function to reflect the stabilization as hydrogen suifide concentra- tion is increased to a particular level, then a decrease in stability for higher levels of hydrogen suifide.

The reactions of (4) and (6) show a mechanism for the decomposition which results in forming ammonium thiocya¬ nate, thereby destroying the thiocarbonyl bond and prevent- ing the release of carbon disulfide by the composition.

According to (5) , however, acidic conditions can cause the intermediate product to release carbon disulfide.

Some general parameters which have been deter¬ mined to effect composition physical stability are as follows for a composition which is an aqueous solution of about 45 percent by weight of a solute comprising hydrogen

•11-

sulfide, ammonia (at twice the molarity of hydrogen sui¬ fide) , carbon disulfide, and sulfur:

(a) the composition is stable for several months without hydrogen suifide evolution if (1) sulfur molarity is greater than or equal to carbon disulfide molarity, and (2) hydrogen suifide molarity is less than 1.5 times the carbon disulfide molarity;

(b) for the case described above in (a) , carbon disulfide will separate into a separate phase if its molarity is greater than that of hydrogen suifide; and (σ) the composition is stable for several months without sulfur precipitation if (1) sulfur molarity is less than or equal to carbon disulfide molarity, and (2) hydrogen suifide molarity is equal to or greater than carbon disulfide molarity. The solubility limit of an ammonium thiocarbonate composition is approximately 50 to 55 percent by weight solute, showing some variability which is dependent upon relative amounts of the various components present. Release of carbon disulfide is rapidly accelerated upon dilution of the composition with water. Some of the possible compositions of the invention, however, are not suitable for uses which require dilution, because of the resulting sulfur precipitation. In general, sulfur precip¬ itation occurs within a few days if (1) hydrogen suifide molarity (present with approximately twice its molarity of ammonia) is less than about 1.5 times the molarity of carbon disulfide, and (2) sulfur molarity is greater than carbon disulfide molarity, and (3) carbon disulfide is less than about 2.5 percent by weight in the composition. As a practical matter, the least tolerable manifestation of physical instability is gas evolution, since this causes stresses on the storage container which

could result in releasing toxic and flammable or explosive vapors.

The ammonium thiocarbonate compositions are stabilized by excess sulfur against significant increases in vapor pressure, and against significant solid or immisci¬ ble liquid phase formation, during reasonable storage periods, and also maintain acceptable chemical stability during such periods.

Alkaline earth metal (i.e. , magnesium, calcium, strontium, and barium) thiocarbonates are somewhat more stable against loss of carbon disulfide than is an ammonium thiocarbonate. Moreover, neither alkaline earth metal nor alkali metal (lithium, sodium, potassium and cesium) thiocarbonate solutions form the phytotoxic thiocyanate species upon decomposition, so such solutions generally are more suitable for long-term storage.

Alkaline earth metal thiocarbonates can be prepared by reacting alkaline earth metal sulfides, either alone or mixed with elemental sulfur (when tetrathiocarbon- ate is to be prepared) , with carbon disulfide, preferably in aqueous media, to directly form aqueous fumigant compo¬ sitions. Alkaline earth metal sulfides can be generated in situ, by reaction of hydrogen suifide with an aqueous solution or dispersion of alkaline earth metal salts, oxides, hydroxides, and the like. This same procedure is applicable to preparation of alkali metal thiocarbonates. The preparation is conveniently carried out at temperatures about 15° C. to about 35° C, but may be conducted between about 0° C. and the boiling point of carbon disulfide, preferably under an inert or reducing gas atmosphere, to avoid oxidation of sulfur compounds to sulfur oxide moieties such as thiosulfates. Reactants are preferably provided in approximately stoichiometric amounts: one mole of alkaline earth metal suifide per mole of carbon disulfide, to form alkaline earth metal trithiocarbonate, and one additional mole of elemental sulfur added to form alkaline earth metal tetrathiocarbonate. Products have the

empirical formula M CS wherein n is 1 when M is alkaline earth metal, n is 2 when M is alkali metal,, and x is 3, 4 or values between 3 and 4.

The solubility limit for alkaline earth metal trithiocarbonates in water is approximately 55 percent by weight; the limit for corresponding tetrathiocarbonates is about 45 percent by weight. Solutions are normally diluted with water to concentrations less than about 33 percent by weight, to avoid precipitation at low temperatures. Salts may be recovered from the aqueous solutions by evaporation of the water and filtration of the resulting precipitate (under an inert or reducing atmosphere) if it is desirable to store the alkaline earth metal thiocarbon¬ ate for extremely long periods prior to use as a fumigant. However, the aqueous solution is substantially stable in and of itself; therefore, there is usually no need to recover the salt as a substantially anhydrous solid. Moreover, it is generally easier to handle the liquid solution than the solid alkaline earth metal thiocarbonate. While the above-described alkaline earth metal thiocarbonates are the active fumigants and therefore may be used in any form (e.g., as a powder admixed with inert solids, as solution or dispersion in an organic solvent, etc.), it is preferred to use the aqueous solutions direct- ly as fumigants. Therefore, the fumigation method of the invention may be carried out by the application of aqueous solutions of alkaline earth metal thiocarbonates.

The above aqueous reaction solutions may be diluted prior to application to provide a solution concen- tration of as low as 0.01 percent by weight of the alkaline earth metal thiocarbonate. The aqueous solution may incorporate surfactants to assist in application as a fumigant. Preferably, a strong base, e.g., an alkali metal hydroxide such as sodium hydroxide, is added to the aqueous solution of alkaline earth metal thiocarbonate to increase the stability thereof during application.

The alkaline earth metal thiocarbonates (like the ammonium and alkali metal analogues) decompose upon expo¬ sure to the atmosphere, at ambient temperatures and humid¬ ities, to yield carbon disulfide. Therefore, the aqueous solution will yield (upon evaporation of the water) a solvated alkaline earth metal thiocarbonate which decom¬ poses to carbon disulfide, in the presence of atmospheric gases at ambient temperatures.

The aqueous solutions of alkaline earth thiocar- bonates utilized in the method of this invention are stable against significant increases in vapor pressure, and significant solid phase formation, during storage periods. These solutions also maintain acceptable chemical stability during such periods, as measured by their ability to decompose to carbon disulfide upon application as a fumi¬ gant.

Soil application of a thiocarbonate composition can be accomplished either prior to planting or after plant growth is established. It should be noted, however, that different plant species exhibit differing tolerances to chemical agents. In addition, phytotoxicity to a particu¬ lar plant can be dependent upon its growth stage. Germin¬ ation is not inhibited for most plant seeds after soil treatment, and growth of established plants is not signifi- cantly altered. Some seedlings, though, show phytotoxicity symptoms. Postplant applications of the composition to such diverse crops as corn, cotton, tomatoes, potatoes and grapes have given no indications of phytotoxicity at effective nematocidal application rates, but cucumber plants have been shown to be somewhat sensitive to thiocar¬ bonate.

The compositions can be applied in undiluted form (to minimize the amount which is required per acre) by spraying onto the soil surface, preferably followed within several hours by water application to move the composition into the soil before a significant amount of free carbon disulfide is released. Injection into the soil, using a

shank or knife, is also a useful method for applying the compositions. This application can either.be "flat," wherein the injectors are closely spaced to treat essen¬ tially the entire field area, or can be "localized" by spacing the injectors such that only the plant growing bed is treated, in bands.

Alternatively, those forms of the compositions which are physically stable upon dilution can be mixed into irrigation water and applied by any customary manner, such as through sprinklers, in the water for furrow or flood irrigation, and in drip irrigation systems. The composi¬ tions will move into the soil with the water, and decompose to accomplish their fumigation functions.

Decomposition of the thiocarbonates in the diluted solutions, prior to movement into the soil, can be retarded by increasing the pH of the solutions. With waters having a high total hardness, however, even the inherent alkalinity of thiocarbonate salts can lead to the precipitation of insoluble carbonates, i.e., of calcium, which tend to plug drip irrigation emitters or sprinkler nozzles. Such precipitation can be retarded by the addi¬ tion of a hardness-complexing agent, such as sodium hexa- metaphosphate, to the water.

The thiocarbonates can be combined with other agricultural chemicals to provide a multifunctional prod¬ uct. For example, the stable salts may be combined with solid or liquid fertilizers such as urea, ammonia, ammonium nitrate, calcium nitrate, etc. and other sources of plant nutrients. The compositions also can be used in non-soil fumigation procedures, such as in the chamber fumigation of commodities which are introduced into commerce. In this type of procedure, dilution of a composition or the appli¬ cation of heat, or both, can be used to promote a rapid decomposition into the fumigant components. Upon termina¬ tion of the fumigation procedure, vapors in the chamber can be drawn through a scrubbing system, e.g., one containing

O PI

an alkaline aqueous solution, to remove the fumigant and prevent atmospheric pollution when the chamber is opened.

Another important use of the compositions is as a fumigant for stored grains and other agricultural products. If applied to products which are to be stored, a fumigant composition can be applied simply by spraying into the product as it is being transported to the storage enclosure with a conveyor, auger or other device. The composition can be applied to agricultural products which are already in storage, by spraying onto the exposed products and sealing the storage enclosure.

It is also possible to use the thiocarbonate compositions for fumigating rooms or storage enclosures; this is accomplished by spraying the floor and walls with the composition, and sealing the space until the desired fumigation is accomplished. As an alternative to spraying, a technique similar to chamber fumigation can be used, wherein heat decomposes the composition in an enclosed space. The fumigating ability of compositions described herein has been expressed primarily in terms of the avail¬ able carbon disulfide content. It should be noted, how¬ ever, that other components can contribute to efficacy as a fumigant. Ammonia, for example, is a fungicide for use on harvested grapefruit, lemons, oranges, and on grain for feed use. In addition, sulfur is very widely used as a fungicide-acaricide-insecticide, so any of the compositions of the invention which decompose to form sulfur will have similar properties in addition to the properties attribut- able to the carbon disulfide content.

Upon dilution, acidification, heating or intro¬ duction into the soil (which is a form of dilution) , the compositions of the invention break down into their compo¬ nents by a process which can be conceptualized as a phys- ical dissociation. In a soil environment, the inorganic cation, sulfur, and hydrogen suifide components are rapidly withdrawn into soil particles, and thereby rendered more or

less immobile, depending upon soil characteristics, mois¬ ture, ambient temperature and the like. Certain of these species will be used as plant nutrients. Carbon disulfide, however, is not tightly bound to the soil and readily migrates to perform the fumigation function.

MODES FOR CARRYING OUT THE INVENTION

EXAMPLE 1 Preparation of an ammonium thiocarbonate composi¬ tion is accomplished, using a 12 liter, three-neck, round- bottom flask, fitted with a sealed stirrer, gas delivery tube, and, a U-tube manometer. A 5461 gram charge of water is placed in the flask, and 1266 grams of anhydrous ammonia are added with cooling of the flask and stirring. With further cooling, 1266 grams of hydrogen suifide are added. To the resulting solution are added 595 grams of finely divided sulfur and, with resumed cooling, 1412 grams of carbon disulfide are also added. Stirring is continued while the mixture is maintained at a temperature between about 24° C. and about 38° C. for a period of about one hour. The resulting clear, deep yellow solution has a composition as follows:

Component Weight Percent Mol«e Percent

NH ₃ 12.66 16.46

H ₂ S 12.66 8.22

S 5.95 4.11 cs ₂ 14.12 4.11

H ₂ 0 54.61 67.1

This solution has a specific gravity at 21° C. of 1.130, and a crystallization temperature of about -10° C.

EXAMPLE 2

Solutions corresponding in stoichiometry to an ammoniated ammonium trithiocarbonate are prepared by the

procedure of Example 1. The chemical stability is deter¬ mined at 23° C. by measuring absorbance at .wavelengths corresponding to those of the thiocarbonyl group (11.0 microns) and the thiocyanate group (4.85 microns) at the time of preparation and at subsequent times, using Fourier- transform infrared spectrophotometry.

When the infrared data are expressed as the result of thiocarbonyl absorbance divided by the sum of thiocarbonyl absorbanσe plus thiocyanate absorbance (called "absorbance ratio" in this and subsequent examples) , a plot σan be made versus elapsed time since composition prepara¬ tion. The natural logarithm of the absorbance ratio is a linear function of elapsed time, so a linear regression by the method of least squares is used to calculate the equation of this line. By solving the equation for an absorbance ratio of one-half of its original value, the "half-life" of the composition is calculated.

Results are obtained as follows:

Composition, mole percent Absorbance Ratio Half-Life,

NH ₃ H ₂ S cs ₂ H ₂ 0 0, 2, 4.7 Months Months

9.93 4.14 4.13 81.80 1, 0.45, 0.18 2.0

11.57 4.13 4.13 80.16 1, 0.42, 0.16 1.9

13.23 4.13 4.13 78.51 1, 0.44, 0.19 2.2

EXAMPLE 3

The experiment of Example 2 is repeated with solutions containing sulfur and varying amounts of other components, yielding compositions as tabulated:

Formula Composition, Mo].e Percent

Number NH ₃ H ₂ S cs ₂ ■S H ₂ 0

1 9.38 4.69 4.70 4.70 76.53

2 13.06 6.53 4.76 4.77 70.88

3 13.32 6.66 4.86 7.42 67.74

4 14.52 7.26 4.79 4.79 68.64

5 16.47 8.23 4.11 4.11 67.07

6 16.80 8.40 4.18 6.73 63.89

It should be noted that Formula 1 corresponds stoiσhiometri- σally to a solution of ammonium tetrathioσarbonate.

Infrared absorption determinations are made using these σompositions giving the following σalσulated half- lives:

Absorbanσe Ratio

Formula 0 5.5 12 15 Half-life,

Number Months Months Months Months Months

1 0.95 0.63 0.62 0.37 11.9

2 0.96 0.74 0.66 0.53 17.7

3 0.96 0.80 0.72 . 0.62 25.8

4 0.96 0.78 0.67 0.37 13.1

5 0.96 0.67 0.58 0.48 14.2

6 0.95 0.70 0.60 0.48 14.8

These data show that increasing the content of soluble suifide enhances chemical stability, and that a further enhancement can be obtained by increasing the sulfur content.

EXAMPLE 4 The compositions of Example 3 are evaluated for physiσal stability by plaσing the prepared solutions in a σlosed σontainer and measuring absolute vapor pressure by flashing the liquid into an evaσuated chamber which is conneσted to an opentube manometer. The following measure¬ ments are obtained:

Formula Absolute Vapor Pressure, mm. Hg

Number 0 Months 6 Months

1 222 -

2 93 -

3 154 _

4 99 -

5 112 274

6 204 224

All of the formulae have an aσσeptable vapor pressure at the time of formulation, but the first four formulae eaσh beσome strongly effervesσent during storage, rendering the subsequent vapor pressure measurements unreliable. In addition, an unidentified solid is formed in the σontainer with Formula 1, prior to the six month measurement.

These data demonstrate the enhanσement in phys¬ ical stability which is attributable to an excess of soluble suifide in the composition.

EXAMPLE 5

Using the procedure of Example 2, chemical stability (in terms of solution half-life) is determined over a period of six months for various compositions prepared acσording to the method of Example 1. In addi¬ tion, absolute vapor pressure over the liquid in a σlosed container is measured at the time of preparing the σomposi- tion.

Results are as tabulated :

Composition, Mole Perσent Half-life, . Absolute Vapor

NH ₃ H ₂ S cs ₂ s H ₂ 0 Months Pressure, mm. Hg

9.74 4.87 4.64 4.64 76.11 13.0 254

11.66 4.87 4.64 4.64 74.20 9.1 102

13.60 4.86 4.63 4.63 72.28 7.6 81

15.52 4.86 4.62 4.62 70.38 6.6 80

10.70 5.34 4.65 4.65 74.65 11.9 209

12.81 5.34 4.65 4.65 72.56 10.9 83

14.94 5.34 4.65 4.65 70.44 7.6 80

17.05 5.34 4.65 4.65 68.35 7.2 87

10.77 5.38 4.68 5.62 73.54 17.2 323

12.91 5.38 4.68 5.62 71.41 11.8 92

15.04 5.38 4.68 5.62 69.31 7.8 73

17.19 5.38 4.68 5.62 67.17 7.0 90

10.85 5.43 4.72 6.61 72.34 17.7 —

13.00 5.43 4.72 6.61 70.27 11.7 107

15.16 5.43 4.72 6.61 68.12 8.1 79

17.30 5.43 4.72 6.61 66.01 7.0 77

9.92 4.96 3.97 3.96 77.19 15.2 158

11.89 4.96 3.97 3.96 75.22 10.9 83

13.87 4.96 3.97 3.96 73.26 7.9 77

15.81 4.96 3.97 3.96 71.33 7.4 80

9.98 4.99 3.99 4.79 76.24 18.0 203

11.97 4.99 3.99 4.79 74.27 11.3 81

13.96 4.99 3.99 4.79 72.29 7.9 71

15.92 4.99 3.99 4.79 70.36 7.4 81

10.05 5.03 4.02 5.63 75.28 15.3 226

12.04 5.03 4.02 5.63 73.30 10.5 78

14.04 5.03 4.02 5.63 71.34 7.7 70

16.02 5.03 4.02 5.63 69.38 7.4 80

Composition , Mole Percent Half -life . Absolute Vapor

NH ₃ H ₂ S cs ₂ S H ₂ 0 Months • Pressure,mm. Hg

14.32 7.16 4.72 4.72 69.08 19.4 118

18.56 7.14 4.70 4.70 64.89 12.8 106

22.79 7.13 4.69 4.70 60.69 10.8 140

14.54 7.27 4.79 6.70 66.70 20.7 129

18.84 7.25 4.77 6.68 62.46 13.3 101

23.13 7.23 4.76 6.67 58.20 10.9 135

14.64 7.32 4.82 7.71 65.51 20.7 129

18.99 7.31 4.81 7.70 61.19 13.3 96

23.29 7.28 4.80 7.67 56.95 10.80 133

19.20 9.60 4.80 4.80 61.59 14.6 152

24.89 9.57 4.79 4.79 55.96 12.8 168

19.47 9.73 4.87 6.82 59.11 14.6 145

25.24 9.70 4.85 6.79 53.41 12.80 166

19.63 9.82 4.91 7.86 57.79 16.9 150

25.44 9.78 4.89 7.83 52.04 13.9 168

Using a multiple linear regression technique, an equation is derived from the data of this example, which can be used to predict the chemiσal stability of a σomposi- tion. The equation (7) is as follows, wherein t is the solution half-life (in months) and X is the mole perσentage of its subsσripted σomponent:

-2.0 X _cs +0.65 X _s +0.21 X _H (7)

The data are found to fit this equation quite well, as indiσated by the regression σorrelation of 0.95.

A similar regression σalσulation is performed, using the vapor pressure data, to prediσt this physiσal property of a σomposition. In the following equation (8) ,

ln(VP) is the natural logarithm of the absolute vapor pressure (millimeters merσury) and X is again the mole percentage of the subsσripted σomponent.

In(VP)=1.907-0.447 X _NH +0.013 X^ _R +0.578 X _{H g} -0.027 x _g

J J -ά £•

+0.258 _cs + 0.0248 X _g + 0.040 X _{H Q} (8)

__ - _*

The fit of data is measured by the σorrelation of 0.86 whiσh is obtained.

EXAMPLE 6

The rate at whiσh σarbon disulfide is lost from diluted ammonium thioσarbonate σompositions is determined by bubbling nitrogen through the solutions, and measuring the σarbon disulfide σontent of the gas whiσh leaves the solution, using a mass speσtrometer.

In the determination, the solution, σorresponding to that of Example 1 (σontaining 14.1 perσent by weight σarbon disulfide) , is σompared to pure σarbon disulfide, and to serial dilutions of the Example 1 solution with water, whiσh prepared 10, 1 and 0.1 volume perσent solu¬ tions of the original σomposition.

Results are as tabulated, wherein k is the σalσulated first order rate σonstant for loss of σarbon disulfide, and t is the solution half-life.

Composition ^{Jt l} hour' t (hours) cs ₂ 2.0 -

Ex. 1, 100% 0.003 230

Ex. 1, 10% 0.14 5.0

Ex. 1, 1% 1.09 0.6

Ex. 1, 0.1% 1.35 0.5

It should be noted that the value of k for the 0.1 perσent solution is approximately 70 perσent of the value obtained for pure σarbon disulfide. Similar results

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are obtained when various dilutions of other thioσarbonate solutions are tested.

EXAMPLE 7 The utility as nematoσides for ammonium thioσar¬ bonate σompositions is demonstrated in a greenhouse experi¬ ment with tomato plants.

In the experiment, eighty σontainers are used, eaσh σontaining about 500 grams of sterilized sandy loam soil. Eaσh σontainer is given four 5-milliliter injections of extract from nematode-infested pepper roots, one inch below the soil surface, producing an initial population of 2000 rootτknot nematode larvae per container.

Twenty treatments are replicated four times, each treatment consisting of solution injection into the soil at a two inch depth. The treatments inσlude eaσh of the six σompositions from Example 3 at three levels, plus one level of the known nema ocide l,2-dibromo-3-chloropropane (DBCP) , and a control with only water injected. After injection, each container is enclosed in a plastic bag and placed in the shade for three days. Upon removing the bags, the soils are aerated by stirring, and allowed to stand undis¬ turbed for eight additional days. Following an additional aeration, a tomato seedling is planted in each pot. Eaσh σontainer reσeives 25 milligrams nitrogen

(as σalσium nitrate) immediately after planting, followed by 2 grams of a slow release σomplete fertilizer. The plants are harvested after 37 days of growth, and soil is removed from the roots by a gentle washing with water. By use of a magnifying glass, the number of root galls is σounted on eaσh plant. Roots and tops are then separated by σutting, oven dried at 80° C. and weighed.

Results are as shown in the table, in whiσh the "Application" represents milligrams of treatment per kilogram of soil, calσulated as σontained σarbon disulfide for the Example 3 solutions. Gall σounts and weights are mean values from the four repliσates.

Treatment Application, Gall Dry Weight, Grams

Solution ppm Count Total Roots

None - 24.3 1.338 0.335

DBCP 50 0* 1.238 0.273

1 22 1.3* 0.933 0.175

1 43 3.8 1.058 0.178

1 65 1.3* 0.750 0.155

2 22 8.3 1.323 0.298

2 43 5.3 1.393 0.325

2 65 5.0 1.350 0.292

3 22 6.5 1.135 0.253

3 43 2.0* 1.505 0.325

3 65 4.5 1.060 0.220

4 22 4.5 1.145 0.243

4 43 3.3* 1.458 0.303

4 64 1.5* 1.588 0.353

5 22 7.5 1.178 0.253

5 43 1.0* 1.930 0.415

5 65 0.8* 1.235 0.228

6 22 6.3 1.503 0.313

6 43 3.5* 1.688 0.368

6 64 1.0* 1.635 0.345

The gall counts marked by an asterisk are considered to be statistically indistinguishable.

All of the treatments are found to be effective against the nematodes; the degree of control which is provided, as measured by gall counts, apparently is direct¬ ly dependent upon the application rate, expressed in terms of the σarbon disulfide σontent.

No signifiσant phytotoxiσity is observed for the stabilized solutions under σonditions shown; strong evi¬ dence is seen that Solution 1 (corresponding stoichiometri- σally to ammonium tetrathioσarbonate) is somewhat phyto- toxiσ at the appliσation rates listed. Further, it should be noted that the stabilized σompositions of the invention exhibit a trend toward aσσelerating tomato plant growth.

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EXAMPLE 8

The nematoσidal effiσaσy of ammonium thioσarbon¬ ate σompositions is demonstrated by appliσation to estab¬ lished grapevines.

In this test. Solutions 1, 2 and 3 from Example 3 are σompared with l,2-dibromo-3-σhloropropane (as a commer¬ cial emulsifiable σonσentrate σontaining 12% DBCP) on grapevines planted seven feet apart, in rows spaσed at ten foot intervals. Single vines, repliσated six times, are treated with nine soil injections spaced six inches apart in a single four-foot band centered on, and eight inσhes from, the vine trunk, paralleling the row. Only one side of the vine is treated.

Soil samples at two depths, upper (10 σm. to 30 cm.) and lower (30 cm. to 60 cm.), are taken at loσations 15 to 20 σm. outside the band, both immediately before, and 31 days after treatment. These samples are analyzed for the numbers of larvae of various nematode genera.

The table shows results whiσh are obtained. Application Rate is shown as the number of liters per hectare, assuming that the vines would be treated equally on both sides of the row. The line for no treatment represents the injection of only water. All values are for the mean values obtained in the six replicates, σalσulated as nematode larvae per kilogram of soil.

Nematode Larvae Population, 31 Days

Treatment Appliσation Root-Knot Stubby Root Dagger Solution Rate Upper Lower Upper Lower Upper Lower

None - 98 93 5 1 16 6

DBCP 44 3 5 0 0 0 0

1 560 71 70 0 0 7 0

1 1120 42 52 0 0 0 3

1 1680 12 31 0 0 0 0

2 560 84 21 0 0 11 0

2 1120 28 17 0 0 2 0

2 1680 15 13 0 0 1 0

3 560 33 33 0 0 2 2

Nematode Larvae Population, 31 Days Treatment Appliσation Root-Knot Stubby Root Dagger

Solution Rate Upper Lower Upper Lower Upper Lower

3 1120 17 12 0 0 3 0 3 1680 12 10 0 0 0 0

The pretreatment nematode σounts per kilogram of soil are as follows: Root Knot (Meloidogyne spp.) 185 at 10 to 30 σm. , 164 at 30 to 60 σm.; Stubby ^' Root (Triσhodorus spp.) 4 at 10 to 30 cm., 6 at 30 to 60 cm.; Dagger

(Xiphineπta spp.) 50 at 10 to 30 cm. , 20 at 30 to 60 cm.

A clear correlation is noted between appliσation rate and nematode population reduction. Also noteworthy is a comparison between Solution 1, corresponding stoiσhio- metriσally to ammonium tetrathioσarbonate, and the stabil¬ ized σompositions of the invention, with regard to the effeσtiveness at greater soil depths. Sinσe the invention results in stabilization against deσomposition, a better movement of aσtive ingredients through the soil σan be obtained for a given appliσation rate.

EXAMPLE 9

The ability of ammonium thioσarbonate σomposi- tions to σombine with nitrogenous chemical fertilizers is demonstrated by dissolving urea in a solution corresponding to that prepared in Example 1, preparing solutions as tabulated:

Perσent Urea Crys stallization Fertilizer Equivalent

(Weight) Temp . ( ° C.) Designation CS„ (Wt.%)

0 -10 10. -0-0-29.7(S) 14.1

10 -22 14.0-0-0-26.8 (S) 12.7

20 -36 17.6-0-0-21.4(S) 11.3

30 -24 21.3-0-0-20.8 (S) 9.9

40 -1 24.9-0-0-17.8(S) 8.5

50 26 28.5-0-0-13.9(3) 7.1

A minimum crystallization temperature is found at about 20 percent by weight urea, σorresponding approxi¬ mately to a urea-σarbon disulfide mole ratio of 2:1. These solutions have stabilities similar to that of stabilized ammonium tetrathiocarbonate solutions, developing slight pressures of hydrogen suifide over a period of several weeks.

The solutions are useful as providing a means for single application of nitrogen fertilization σombined with fumigation.

EXAMPLE 10

^• Calσium tetrathioσarbonate solution is prepared by mixing 115.8 grams of σalσium oxide with 585 grams water, and adding, with vigorous stirring, 71.6 grams of hydrogen suifide, forming a dark green slurry. When 67.4 grams of sulfur are added, the slurry beσomes dark yellow in σolor; the addition of 180.7 grams of σarbon disulfide produσes a deep yellow solution whiσh is 36.5 perσent by weight σalσium tetrathioσarbonate.

EXAMPLE 11

The utility as nematoσides for σompositions of this invention is demonstrated in a greenhouse experiment with tomato plants.

In the experiment, thirty σontainers are used, eaσh σontaining about 500 grams of sterilized sandy loam soil. Eaσh σontainer σontains one tomato plant. Eaσh σontainer is injeσted with four 5-milliliter portions of extraσt from nematode-infested pepper roots, one inσh below the soil surfaσe, produσing an initial population of 2000 root-knot nematode larvae (Meloidogyne inσognita) per container.

Ten treatments are repliσated three times, eaσh treatment σonsisting of drenσhing the soil with a solution

σontaining the fumigant to provide the dosage of CS ₂ given in the following table. The solutions are .diluted with suffiσient water to saturate the soil. The treatments inσlude σalσium tetrathioσarbonate, stabilized ammonium tetrathioσarbonate, and carbon disulfide at three levels, plus an untreated control. After drenσhing, eaσh σontainer is allowed to stand at ambient σonditions. The plants are harvested after 30 days of growth, and soil is removed from the roots by a gentle washing with water. By use of a magnifying glass, the number of root galls is σounted on each plant.

Results are summarized below, wherein the "Appli¬ cation" represents milligrams of treatment per kilogram of soil, σalσulated as the thioσarbonate salt and the equiva¬ lent carbon disulfide. Gall counts are mean values from the three replicates.

Application , pp Gall Counts

Composition Salt ^a Eq. CS,, __1 _2 _3 Mean

Control 0 0 4 7 1 4.0

(NH ₄ ) ₂ CS ₄ -(NH ₄ ) ₂ S 213 30 5 7 3 5.0

425 60 9 15 6 10.0

638 89 6 5 2 4.3

490 61 11 4 7 7.3

730 91 3 9 9 7.0

CS. 32 31 23 12 22.0

65 28 19 33 26.7

97 27 24 9 20.0

a (NH ₄ ) ₂ CS ₄ » (NH ₄ ) ₂ S applied as a 32.4 percent solution, by weight.

CaCS ₄ applied as a 29.6 perσent solution, by weight.

CS. applied as the pure liquid.

-SO-

fo Number of discrete galls per total root mass.

The calσium tetrathioσarbonate is substantially equivalent to the stabilized ammonium tetrathiocarbonate as a nematocide; however, the σalσium thioσarbonates (as well as the other alkaline earth metal thiocarbonates) are found to be less phytotoxic in that they do not form ammonium thiocyanate upon decomposition during storage, nor, unlike the ammonium ion component of the ammonium thioσarbonates, are the individual σomponents of the alkaline earth metal thioσarbonates (i.e., H ₂ S, S, CS ₂ , and alkaline earth metal ions) phytotoxic.

EXAMPLE 12 The proσedure of Example 11 is repeated exσept that potassium tetrathiocarbonate is substituted for stabilized ammonium tetrathiocarbonate and an in-vitro nematocidal test is used. In the in-vitro test, the nematode larvae are treated in aqueous^ suspension for 1 hour at the σonσentrations of fumigant given in the follow- ing table, washed twiσe with water, and injeσted into the aσtive root zone of the tomato plants. After thirty days the roots are harvested, examined for galling, giving the results summarized below.

^QREΛ

Gall Count

Mean

Control 90 88 89

50 ppm CaCS ₄

6.3 ppm CS ₂ equiv. 185 149 167

100 ppm CaCS ₄

12.5 ppm CS_ equiv. 132 184 158

150 ppm CaCS ₄

18.8 ppm .CS. equiv. 32 66 49

50 ppm K ₂ CS ₄ 6.5 ppm CS- equiv. 33 66 49.5

100 ppm K ₂ CS ₄

13 ppm CS ₂ equiv. 198 145 171.5

150 ppm K ₂ CS ₄

19.5 ppm CS ₂ equiv. 49 22 35.5

10 ppm CS ₂ 64 149 106.5

20 ppm CS ₂ 29 73 51.0

The results show that the σalcium tetrathioσar¬ bonate is substantially equivalent to potassium tetrathio- σarbonate as a nematoσide. However, as desσribed in the following example, the potassium thioσarbonates are less stable to storage as measured by the loss of their ability to generate the aσtive fumigant carbon disulfide.

EXAMPLE 13

Various tetrathiocarbonate salts are evaluated

for storage stability by measuring the loss of the ability of aqueous solutions thereof to generate carbon disulfide upon σontaσt with strong aσid. Aqueous solutions of the salts listed in the following table, having an equivalent of from about 14 to about 16 perσent by weight σarbon disulfide, are stored in air-tight glass σontainers at a temperature of 49° C. As shown by the data below, the σalcium tetrathiocarbonate solution is significantly more stable than the sodium and potassium tetrathiocarbonate solutions and substantially more stable than the ammonium tetrathiocarbonate.

Cation Half-life (months)

NH 0.17

Na 3.0

K + 2.9

Ca ++ 5.0

EXAMPLE 14

Aqueous solutions of alkali metal or alkaline earth metal tri- or tetrathiocarbonates have very high solvency for urea, indicating that eutectiσ σompositions are formed. These σombinations are bioσidal against bacteria, fungi, nematodes, and insects, while providing a wide range of desirable nitrogen and sulfur fertilizer contents. Furthermore, alkali metal and alkaline earth metal cations, in particular, σalcium, magnesium, and potassium, are indispensable plant nutrients. Thus, the compositions described above may be used to provide the major nutrient requirements of crops, while at the same time protecting the crops against pathogens.

To a 41.5 perσent, by weight, aqueous solution of calcium tetrathiocarbonate is added urea until the solubil¬ ity limit of urea is reached. At room temperature, the solution dissolves 122 perσent by weight urea. The result- ing solution is 55 percent urea, 18.6 perσent σalσium tetrathioσarbonate, and 26.3 perσent water, by weight. Thus, the solvency of the aqueous solution of σalσium tetrathioσarbonate for urea is at least as great as that of water alone. Similarly, a 46 perσent solution of potassium tetrathioσarbonate dissolves 100 perσent of its own weight of urea. Similar results are obtained with other tri- and tetrathioσarbonates of alkali metal and alkaline earth metals.

EXAMPLE 15

It has been found that the stability of dilute aqueous solutions of alkaline earth metal thioσarbonates (as measured by rate of deσomposition to yield carbon disulfide) increases with the pH of the solution. There¬ fore, in irrigation applications, wherein dilute solutions are utilized, it is desirable to provide a base to increase the pH of the irrigation solution. A suitable base may be selected from the group consisting of the alkali metal hydroxides and carbonates, e.g. KOH, NaOH, K ₂ CO-, Na ₂ CO_ , etc. The base may be added to the water of dilution utilized in making up the irrigation solution or can be incorporated in the aqueous alkaline earth metal thiocar¬ bonate solution. Suffiσient base is added to provide an irrigation solution having a pH of at least about 7 and preferably at least about 8. Most preferably, the amount of base added will provide an irrigation solution having a pH of at least about 9.

To demonstrate the effect of pH on evaporative losses of CS ₂ from thioσarbonates, solutions are injeσted into a σlosed bottle σontaining well stirred σitrate- phosphate buffers, giving a solution σonσentration of 125

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milligrams per liter of thiocarbonate ion. Pure carbon disulfide is also injected, for comparison.. A syringe is used to periodically sample air in the bottle, and the air is analyzed by gas chromatography. Half-life times for production of carbon disulfide are summarized in the following table.

Half-life (minutes)

_£_! cs ₂ αra ₁ ) ._cs ₁ . (NH ₄ ) ₂ S CaCS .

5.2 1 1 1 6.0 1 1 1 .8 7.0 1 2. 1 2. 7 8.0 1 9. 2 8 .0 9.0 — 26 . 1 11 .3

Results for calcium tetrathiocarbonate at pH values above 7 in this buffer system are unreliable, since calcium phosphates tend to precipitate, causing more rapid dissociation of the thiocarbonate. It is apparent, howev¬ er, that decomposition for these two compounds proceeds at similar rates.

EXAMPLE 16

Thioσarbonate solutions are used in a test of their fungicidal properties. Cultures of four plant pathogenic fungi are grown upon potato dextrose agar at room temperature, in diffuse natural lighting. After one week, square blocks having 2 millimeter sides are cut from the edges of actively growing mycelia spots on the agar.

The bloσks are immersed in sterile deionized water, as a σontrol, or in dilutions of thioσarbonate solutions using the sterile water, in a σlosed σontainer. Subsequently, the bloσks are removed and plaσed upon agar

in σlean plates, and myσelia are allowed to grow for one wee .

Radial growth of myσelia colonies is measured for each of the six to eight replicate plates used for a particular fungus, and average colony radius is σalσulated. Perσent σontrol is defined by the following equation:

_ . __. i Perσent σontrol - l-< _{Avera e} radius of σontrol plates ¹⁰⁰

Results are summarized in the table whiσh follows. Conσentrations given for solutions used to treat the agar bloσks are expressed in grams of thioσarbonate solution per liter of diluted solution. These results show that the σompositiόns have aσtivity against fungi.

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Perσent Control

Treatment 2/1 Fusarium Phytophthora Verticillium Sclerotiui oxysporum σinnamomi dahliae rolfsii

^K 2 ^CS 4 100 76 100 100 100

(9.43%CS ₂ ) 10 10 68 15 8

1 8 56 27 42

κ ₂ cs ₄ 100 74 100 100 100

+6.1%NH ₃ 10 83 100 41 59

(8.21%CS ₂ ) 1 87 100 46 45

κ ₂ cs ₄ 100 92 100 100 100

+10.7%urea 10 6 97 53 100

(8.17%CS ₂ ) 1 0 30 77 48

Na ₂ CS ₄ 100 100 100 100 100

(10.6%CS ₂ ) 10 6 37 26 100

1 4 37 23 54

a ₂ CS ₄ 100 100 100 100 100

+6.1%NH 10 14 _.-.* 59 100

(9.52%CS ₂ ) 1 2 __* 37 48

a ₂ CS ₄ 100 94 100 100 100

+10.7%urea 10 30 __* 20 100

(9.69%CS ₂ ) 1 8 __* 8 50

CaCS ₄ 100 100 100 100 100

(2.8%CS ₂ ) 10 18 56 22 62

1 3 56 13 46

(NH ₄ ) ₂ CS ₄ - 100 100 100 91 100

(NH ₄ ) ₂ S 10 100 74 81 93

(13.0%CS ₂ ) 1 70 97 41 49

* σontaminated σultures

EXAMPLE 17 The effeσt of various appliσation rates of thioσarbonates for pest σontrol is shown in a series of experiments.

Citrus trees are treated with a 32 perσent by weight solution of (NH CS ₄ » (NH ₂ ≤ applied evenly to soil around the trunks using a sprinkler can, and thoroughly watered in with flood irrigation. Soil samples taken 30 days following treatment are counted for citrus nematode larvae, giving results summarized below, where the applica¬ tion rate is expressed in liters per hectare.

Appliσation Larvae/500 σσ.

0 2887

470 325

940 521

1870 1739

Using a drip irrigation system, grapevines are treated with (NH ₄ ) ₂ CS ₄ « (NH ₄ ) ₂ S at a rate of about 43 kilograms per heσtare, using three equal treatment appliσa- tions made at three day intervals. Total σontrol of σitrus nematode larvae is obtained over a three month period. In a laboratory test, it is found that a single appliσation of the composition produσes 96 percent to 100 percent control of the larvae at an application rate of about 655 kilograms per hectare. Sugar beets, infested with sugar beet σyst nematodes (Heterodera spp.) , are treated by appliσation to the soil of about 94 kilograms per heσtare of CaCS ₄ , dissolved in irrigation water. Counts of nematode larvae in the soil, following treatment, remained high, but the larvae were not viable, due to parasitism by other soil organisms.

In petri dish tests of CaCS ₄ against the fungus Fusarium spp. , control with solutions containing less than about 10 grams per liter of the compound, in both potato dextrose agar and potato dextrose broth is obtained using the solution when the broth also contains another fungus, Trichoderma spp.

The results of these tests indicate that control of soil-borne plant parasites can be obtained by applying sub-lethal doses of biocide, that is, amounts which are insufficient to substantially eradiσate the pests, but which σan weaken the pests and thereby faσilitate their σontrol by natural predators in the soil. Deσreased long-term_ σontrol is obtained by higher appliσation rates of bioσide, sinσe the higher rates σan stimulate an in- σrease in the reproduσtive effort of an organism; a better initial kill will be followed by, for example, a muσh larger egg hatσh _r yielding an aσtual net inσrease in parasite population. Very high appliσation rates will effeσtively eradiσate susσeptible pests, but may lead to rapid proliferation of less susσeptible pests, whiσh may also be undesirable.

Another useful appliσation method initially utilizes only suffiσient pestiσide to stimulate a large reproductive effort, followed by a high dosage, immediately after the egg hatch, to obtain a maximum pest mortality.

EXAMPLE 18

The effeσt of multiple appliσations of lethal doses of thioσarbonates is shown in a series of experi¬ ments. In the experiments, two or more small, but lethal, doses are applied to the soil repetitively, beginning at a time prediσted to σorrespond to a seasonal inσrease in population of a susσeptible phase in the life σyσle of a pathogen. Suσh appliσation permits the use of minimum quantities of non-persistent pestiσides. In the

-=gJRE

experiments, the thioσarbonate is a 32 perσent by weight solution of (NH ₄ ) ₂ CS ₄ -(NH ₄ ) ₂ S.

Soil systems, σontaining all life stages of reniform nematode and used for pineapple σrops with drip irrigation, are treated with thioσarbonate solution. One soil reσeives only a single preplant treatment. Another area reσeives the same quantity of thioσarbonate, but applied with the irrigation water in six monthly doses (one-sixth before planting and the remainder in five equal doses) . A third area reσeives no treatment. Soil samples are taken at fixed intervals following the first treatment and σounts made of nematodes per 300 milliliters of soil. Results are summarized in the following table for tests at two treatment levels.

Time r Nematode Counts

Mont] IS Preplant Monthly Untreated

Trial A - total 280 liters per hectare

0.5 63 73 150

1 50 23 104

2 164 47 128

3 67 15 88

4 340 228 59

5 248 101 136

Trial B - total 560 liters per hectare

0.5 50 176 150

1 51 54 104

2 61 28 128

3 68 28 88

4 1972 64 59

5 713 158 136

Grape vines are treated with monthly applications of thiocarbonate, at a rate of 190 liters per hectare, applied in furrow irrigation water. Soil samples, taken after the first treatment and at monthly intervals

thereafter, are counted for root-knot nematode, giving results summarized in the following table:

Time, Nematode Counts per Kg. Soil

Months Treated Soil Untreated Soil

0 265 350

1 68 135

2 30 110

3 7. .5 36

4 77 95

5 270 460

Soil used for growing potatoes, and known to contain several active bacterial and fungal plant patho¬ gens, but no significant nematode population, is treated with thiocarbonate solution. The following table summa¬ rizes results of the experiment. In the table, the treat¬ ment on 25 May is before planting. Yield is shown in metric tons per hectare, for both the total potato harvest and those potatoes of the largest size (U.S. Number 1) . When no treatment is given to a particular plot, a urea- ammonium nitrate fertilizer solution is applied, in an amount which will provide a total amount of nitrogen equivalent to ammonium ion in the fumigant applied to other plots.

Liters per Hectare Applied Yield

25 May 7 July 15 August No. 1. Total

374 374 374 14.2 38.1

1122 0 0 14.1 33.2

748 0 374 19.4 38.8

0 748 374 20.1 42.4

0 374 748 26.8 50.8

0 0 0 13.1 31.5

-gJ E

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EXAMPLE 19

Experiments are performed to demonstrate the advantages of applying thioσarbonates to moist soils. A sandy loam soil is plaσed in 1-liter glass bottles, fitted with stoppers having fluoroσarbon liners and siliσone rubber septa, to give a soil depth of about 4 σm. Water is added to the soil, in quantities to obtain 50 and 100 percent soil saturation. Thiocarbonate solution or carbon disulfide is injected near the bottom of the soil layer, the bottles are promptly sealed, and the air space in the bottles is sampled at intervals with a syringe, for gas chromatographiσ analysis of CS-. Results are summa¬ rized below, wherein degradation time is the number of hours required to aσhieve the maximum CS ₂ σonσentration in the air spaσe.

Soil Moisture Degradation

% of Saturation Compound Time, hours 0 CS ₂ 3.5

(NH ₄ ) ₂ CS ₄ -»(NH ₄ ) ₂ S 2 K ₂ CS ₄ 2

CaCs ₄ 4

50 CS ₂ 3.5

(NH ₄ ) ₂ CS ₄ »(NH ₄ ) ₂ S 3

K ₂ CS ₄ 5

CaCS ₄ 5

100 CS ₂ 3.5

(NH ₄ ) ₂ CS ₄ .(NH ₄ ) ₂ S 48 K ₂ CS ₄ 48

A σotton field, having furrows about 195 meters in length, is irrigated, requiring a total of 5 hours for filling of the furrows. Three days later, another irriga-

OMPI

tion was conduσted, using water whiσh σontains 217 milli¬ grams per liter of σalσium tetrathioσarbonate, but only 25 minutes is needed for filling the furrows. Applying the same total amount of the thioσarbonate, about 95 kilograms per heσtare, would have required a σoncentration in the original water of about 18 milligrams per liter, probably a conσentration too low to be effeσtive for fumigation.

From these experiments, it is apparent that the deσomposition of thioσarbonates is substantially retarded when application is made to moist soils, particularly when the soil contains water at more than 50 percent of satura¬ tion. Thus, the fumigant can penetrate soil to a greater depth for performing its function. Also, applying the compositions to moist soil, in irrigation water, permits the use of higher conσentrations, for a given appliσation rate.