Fertilizers are administered to plants, such as crops in the field or in a greenhouse, in order to improve their growth and to enhance the yield of any desirable portions of the plant, such as the fruits. Fertilizers can also enhance the quality of the soil by increasing the concentration of certain plant nutrients, or even to completely supply these nutrients in situations where they would normally be absent, such as in the hydroponic cultivation of plants. One of the most important groups of commercially available fertilizers is the potassium-containing or potassic fertilizers.

The main commercial potassium fertilizers which supply potassium to plants as their main ingredient are potassium chloride, potassium sulfate, and potassium nitrate. Their respective potassium contents, in terms of K2O concentration, are about 60%, 50%, and 46%, depending on the source, producer and purity of the commercial product. However, each type of potassium fertilizer has both advantages and disadvantages.

For example, although potassium chloride is relatively inexpensive and easy to produce, many crop plants are sensitive to chloride, such as tobacco, citrus, and most of the vegetables and flowers grown intensively, mostly as protected crops. Such crops receive relatively large amounts of fertilizers per unit area, and the residual ions like Cl- (chloride) from fertilizers cause specific, undesirable ion effects or else cause damage through salt accumulation simply by their relatively high concentration. Similarly, high concentrations of S04 (sulfate) ions in the environment of protected crops may also cause damage, even though at certain concentrations sulfur is also an important plant nutrient.

Potassium nitrate, which is currently the major fully water-soluble chloride- free potassic fertilizer, does not share these problems related to ion concentration.

Indeed, a good share of potassium nitrate is used as soluble fertilizer applied in solutions mainly for protected crops, where high ion concentrations are particularly problematic, or for"fertigation". The term"fertigation"refers to fertilization through irrigation systems, by first dissolving the fertilizer in water tanks and then pumping the solution into the irrigation system, for example.

Potassium nitrate thus has many advantages as a chlorine-free, fully water soluble, concentrated potassic and nitrate fertilizer; however, it also has some disadvantages, of which the main ones are listed below: First, potassium nitrate is relatively quite expensive overall. In addition, usually both the potassium and the nitrate portions are more expensive than, for example, potassium of potassium sulfate or nitrate of ammonium nitrate, a very common nitrogenous fertilizer. In addition, frequently the mixture of ammonium and nitrate as sources of nitrogen from ammonium nitrate is more desirable, and of course, less expensive, than nitrate alone from potassium nitrate.

A second problem is that potassium nitrate is a very imbalanced plant nutrient source in several respects. For example, the ratio of potassium to nitrogen in terms of % K20: N is about 3.5: 1, which is much higher than most plants require.

Also, potassium nitrate does not contain other necessary plant nutrients which could improve its nutritional balance and its performance. Potassium nitrate is also difficult to dissolve in cold water since the dissolution process is endothermic, causing the temperature of the solution to drop considerably during the process of dissolution and further reducing solubility. Such low solubility is particularly inconvenient for fertilizing crops in northern climatic zones in the winter. Under these conditions, the solution requires a longer stirring time, and in many instances, the water and solution must be warmed, causing significant inconvenience to the grower.

An increasing share of"soluble" (fully water-soluble) potassium sulfate is also used in or through solutions, soluble fertilizers, foliar fertilizers, and the like.

Since the commercial standard potassium sulfate is usually not clean and not fully water soluble,"soluble"potassium sulfate is a purifie crystalline product with smaller particles, usually obtained by re-crystallization of the regular standard fertilizer grade of potassium sulfate. Even so, potassium sulfate is still considerably less expensive than potassium nitrate.

However, potassium sulfate, even the"soluble"product, has a number of disadvantages. Potassium sulfate also has low solubility, even lower than that of potassium nitrate at low temperatures of 0°C-10°C. Although sulfur is a nutrient element, the ratio of sulfur to potassium is very high, often resulting in an excess of sulfate ions which, as mentioned above, have specific ion and/or"high salt" adverse effects on plants, increase the electrical conductivity of the soil solution and cause stress effects on the environment. Thus, for high yield, cash or protected crops only limited amounts of potassium sulfate can be administered for optimal yield and quality of the agricultural and horticultural products.

On the other hand, research has shown that if the environment is deficient in sulfur, a nitrogen fertilizer is ineffective unless sulfur is applied simultaneously (Wang et al.,"Sulfur Deficiency in Cotton", IRS Res. Inst. ARep. g, 48: 9-30, 1976). Research with cotton plants has also shown that an increased supply of sulfate increases the efficiency of protein production from nitrogen fertilizer in these plants (see Table 8.11, p. 224,"Mineral Nutrition of Higher Plants", Horst Marshuer, Academic Press, London, 1986).

There is therefore a need for, and it would be advantageous to have, a potassium fertilizer which combines the desirable effects of the nitrate and sulfate fertilizers, is fully and easily water-soluble and inexpensive, and which is a much better and more efficient fertilizer and plant-nutrient supplier from the agronomic, technical, economical and practical points of view.

SUMMARY OF THE INVENTION According to the teachings of the present invention, there is provided a fully water-soluble fertilizer for plants, comprising a composition of potassium nitrate and potassium sulfate. Preferably, the potassium nitrate and the potassium sulfate are present at a ratio in a range of from about 85: 15 to about 50: 50 percent weight by weight. More preferably, the potassium nitrate and the potassium sulfate are independently in a form selected from the group consisting of crystalline, compacted, granulated, prilled and powdered. Most preferably, the composition is a mixture of potassium nitrate and potassium sulfate. Alternatively and most preferably, the composition is a double salt of potassium nitrate and potassium sulfate.

According to preferred embodiments of the present invention, the fertilizer further includes an additional source of nitrogen. Preferably, the additional source of nitrogen is urea present in a ratio in a range of from about 10% to about 40% weight per weight. Alternatively and preferably, the additional source of nitrogen is ammonium nitrate present in a ratio in a range of from about 10% to about 40% weight per weight.

According to another embodiment of the present invention, there is provided a method of administering a fertilizer to a plant in solution, comprising the steps of : (a) providing a fertilizer, the fertilizer including a composition of potassium nitrate and potassium sulfate; (b) placing the fertilizer in water to form the solution; and (c) administering the solution to the plant. Preferably, the potassium nitrate and the potassium sulfate are placed in the water sequentially.

Alternatively and preferably, the potassium nitrate and the potassium sulfate are placed in the water simultaneously. Most preferably, the composition is a mixture of potassium nitrate and potassium sulfate. Alternatively and most preferably, the composition is a double salt of potassium nitrate and potassium sulfate.

According to yet another embodiment of the present invention, there is provided a method of production of a fully water-soluble fertilizer, comprising the

step of : (a) mixing dry, water-soluble potassium nitrate and dry, water-soluble potassium sulfate to form a dry mixture. Preferably, the method further includes the step of : (b) grinding the dry mixture to form the fertilizer.

According to still another embodiment of the present invention, there is provided a method of production of a fully water-soluble fertilizer, comprising the steps of: (a) mixing a brine of potassium nitrate and a brine of potassium sulfate to form a solution; and (b) drying the solution to form a dried composition, the dried composition forming the fertilizer. Preferably, the method further includes the steps of : (c) grinding the dried composition to form a ground composition; and (d) mixing the ground composition to form the fertilizer.

According to yet another embodiment of the present invention, there is provided a method of production of a fully water-soluble fertilizer, comprising the steps of : (a) mixing nitric acid, sulfuric acid and potassium chloride to form hydrochloric acid, potassium nitrate and potassium sulfate; and (b) removing the hydrochloric acid to form the fertilizer. Preferably, the hydrochloric acid is removed by a process selected from the group consisting of ion exchange and an extraction process. Also preferably, in step (a), the nitric acid and the sulfuric acid are mixed first to form a mixture, and the potassium chloride is added to the mixture. Alternatively and preferably, in step (a), the potassium chloride and the nitric acid are mixed first to form a mixture, and the sulfuric acid is added to the mixture. Also alternatively and preferably, in step (a), the potassium chloride and the sulfuric acid are mixed first to form a mixture, and the nitric acid is added to the mixture.

According to yet another embodiment of the present invention, there is provided a method of production of a fully water-soluble fertilizer, comprising the steps of : (a) mixing potassium chloride, sodium nitrate and sodium sulfate to form potassium nitrate, potassium sulfate and sodium chloride; and (b) removing the sodium chloride by a process of double-decomposition to form the fertilizer.

Preferably, in step (a), the potassium chloride and the sodium nitrate are mixed first to form a mixture, and the sodium sulfate is added to the mixture.

Alternatively and preferably, in step (a), the sodium nitrate and the sodium sulfate are mixed first to form a mixture, and the potassium chloride is added to the mixture. Also alternatively and preferably, in step (a), the potassium chloride and the sodium sulfate are mixed first to form a mixture, and the sodium nitrate is added to the mixture.

According to another embodiment of the present invention, there is provided a fully water-soluble fertilizer for plants, comprising a composition of potassium nitrate and magnesium sulfate. Preferably, the potassium nitrate and the magnesium sulfate are present at a ratio in a range of from about 85: 15 to about 50: 50 percent weight by weight. Also preferably, the composition is a mixture of the potassium nitrate and the magnesium sulfate. Alternatively and preferably, the composition is a double salt of the potassium nitrate and the magnesium sulfate.

According to another embodiment of the present invention, there is provided a method of production of a fully water-soluble fertilizer, comprising the step of mixing dried potassium nitrate and dried magnesium sulfate to form the fertilizer. Preferably, the dried potassium nitrate is formed by drying a solution of potassium nitrate and the dried magnesium sulfate is formed by drying a solution of magnesium sulfate.

According to still another embodiment of the present invention, there is provided a method of production of a fully water-soluble fertilizer, comprising the steps of : (a) dissolving MgO in a potassium nitrate solution; (b) mixing the solution with H2SO4 to obtain potassium nitrate and magnesium sulfate in a mixed solution; and (c) drying the mixed solution to form the fertilizer.

According to yet another embodiment of the present invention, there is provided a method of production of a fully water-soluble fertilizer, comprising the step of : mixing potassium chloride, magnesium chloride, nitric acid and sulfuric acid to form potassium nitrate and magnesium sulfate. Preferably, the potassium chloride and magnesium chloride are mixed first to form a mixture, and the nitric acid and the sulfuric acid are added to the mixture. Preferably, the nitric

acid is added to the mixture first. Alternatively and preferably, the sulfuric acid is added to the mixture first.

Hereinafter, the term"tap water"refers to water obtained from an ordinary household water supply, without any additional filtration, purification or modification.

Hereinafter, the term"composition"refers to both mixtures and double salts of potassium nitrate and potassium sulfate. The double salts may be either prepared by manufacture as substantially one unit, or by mixture of already manufactured potassium nitrate and potassium sulfate.

Hereinafter, the term"fully water-soluble"is defined according to the usage known in the art of fertigation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of a novel fertilizer composition, consisting of a mixture of potassium nitrate and potassium sulfate as fully water soluble compounds in crystalline, granular, compacted or prilled form. Preferably, the mixture of solid soluble potassium nitrate and potassium sulfate contains these compounds in a ratio of from about 85% potassium nitrate and 15% potassium sulfate to about 50% potassium nitrate and 50% potassium sulfate. In addition, according to other preferred embodiments of the present invention, there is provided a method of using the fertilizer of the present invention to deliver nutrients to plants, in which the fertilizer is dissolved in water and applied to soil or other growth media, and/or directly to plants. For example, potassium nitrate and potassium sulfate could be dissolved simultaneously or sequentially in the same tank of water in order to fertigate or to obtain a solution for plant nutrition.

As described in more detail below, the present invention also encompasses methods for production of the fertilizer of the present invention by dry mixing, by mixing of liquids followed by crystallization and dry mixing, or by simultaneous production of the mixture. Such simultaneous production could be performed

according to one of many possible chemical processes. For example, potassium chloride, nitric acid and sulfuric acid could be combined according to the following equation: 3 KCl + HNO3 + H2SO4 r KNO3 + K2SO4 + 3 HCl Next, the resultant hydrochloric acid could be removed by liquid extraction or by other methods. As another example, if the ratios were altered, the following equation would hold: 5 KCl + 3 HN03 + H2SO4 a 3 KN03 + K2SO4 + 5 HCl Similarly, potassium chloride, sodium nitrate and sodium sulfate could be combined according to the following equation: 3 KCl + NaNO3 + Na2SO4 a KN03 + K2SO4 + 3 NaCl followed by removal of sodium chloride by physico-chemical methods. Again, if slightly different ratios were used, the following equation would hold: 5 KCl + 3 NaNO3 + Na2SO4 r 3 KN03 + K2SO4 + 5 NaCl The reaction producing KN03 is exothermic while that producing K2SO4 is endothermic, thus saving energy during the process of production.

As will be demonstrated in the Examples below, the present invention has a number of advantages over prior art fertilizers and methods of application and production of these fertilizers. First, the solubility of the new product at low temperatures is similar or better than that of potassium nitrate alone or potassium sulfate alone, thus enabling better and easier dissolution and application by fertigation, especially in cold climates. Also, the provision of both the sulfate and nitrate ion nutrients provides a more efficient and balanced plant-nutrient source, both in terms of the relative amounts of sulfate and nitrate provided and the ability of sulfate to increase the efficacy of nitrate as a plant nutrient. Indeed, many different ratios of nitrogen, potassium and sulfate are possible according to the present invention. At the same time, the amount of sulfate administered can be controlled and not provided to excess, thereby preventing damages to crops. Also, the amount of expensive potassium nitrate is reduced, so that the final mixture according to the present invention is much more economically viable than

potassium nitrate alone. Nitrogen can be increased to desired concentrations in the final product by the addition of less expensive sources of nitrogen such as ammonium nitrate or urea. Finally, as noted previously, the present invention provides more efficient and energy-saving methods for production of the fertilizer mixture.

The principles and operation of a fertilizer according to the present invention may be better understood with reference to the Examples and the accompanying description.

Example 1 Test of Solubility of Mixture The solubility of mixtures of fully water-soluble potassium nitrate and potassium sulfate at various ratios was tested by adding these compounds, alone or in combination, to tap water and measuring both the amount which dissolved and the resultant temperature of the solution. Essentially, at certain ratios of potassium sulfate and potassium nitrate, the solubility of both substances in the mixture is improved as compared to the solubility of either substance alone. The experimental protocol was as follows.

First, the temperature of tap water alone was measured to be 20°C. Next, 30 grams of commercially available fertilizer grade crystalline KN03 (potassium nitrate, also designated as"KN"in Table 1 below) of formula 13-0-46 (13% N, 0% P205 and 46% K20; product of Haifa Chemicals, Haifa, Israel) were added to 200 cc of tap water and stirred vigorously for 5 minutes to form a solution. The temperature of the resultant solution dropped to 9°C. The amount of potassium nitrate dissolved until saturation measured 28.5 g, or 14.25 g per 100 cc of tap water. Separately, 30 g of commercially available fertilizer"soluble"grade K2SO4 (potassium sulfate, also designated as"KS"in Table 1 below) was added to 200 cc of tap water and the same procedure was followed. The temperature of the resultant solution dropped to 16.5°C and the amount of potassium sulfate dissolved until saturation was 22 g, or 11 g per 100 cc of tap water. These three solutions, including tap water alone, served as control solutions to provide a basis of

comparison with the mixtures of potassium sulfate and potassium nitrate which were tested.

The first such mixture to be tested was of potassium nitrate and potassium sulfate in a 1: 1 ratio. 15 g KN03 and 15 g K2SO4 were mixed together in dry form.

The mixture was then dissolved in tap water as described previously to form a solution. The temperature of the resultant solution was 13°C. The amount of the dry mixture which dissolved in 200 cc of tap water at saturation was 27 g, or 13.5 g per 100 cc of tap water. Thus, the mixture was more soluble in tap water than potassium sulfate alone, but less soluble than potassium nitrate alone.

The next such mixture to be tested was of potassium nitrate and potassium sulfate in a 2: 1 ratio. 20 g KN03 and 10 g K2SO4 were again mixed together in dry form. The mixture was then dissolved in tap water as described previously to form a solution. The temperature of the resultant solution was 12.5°C. The amount of the dry mixture which dissolved in 200 cc of tap water at saturation was 29.5 g, or 14.75 g per 100 cc of tap water. Thus, the mixture of potassium nitrate and potassium sulfate in a 2: 1 ratio was more soluble than the previous mixture, in which the components were in a 1: 1 ratio. In addition, this mixture was also more soluble in tap water than either potassium sulfate or potassium nitrate alone.

Next, potassium nitrate and potassium sulfate were tested in a 1: 2 ratio. 10 g KNO3 and 20 g K2SO4 were again mixed together in dry form. The mixture was then dissolved in tap water as described previously to form a solution. It should be noted that the temperature of the tap water was 21.5° C, slightly higher than for the previous tests. The temperature of the resultant solution was 15°C. The amount of the dry mixture which dissolved in 200 cc of tap water at saturation was 25.4 g, or 12.7 g per 100 cc of tap water. Thus, the mixture of potassium nitrate and potassium sulfate in a 1: 2 ratio was less soluble than the previous mixtures, in which the components were in a 1: 1 or a 2: 1 ratio. In addition, this mixture was also more soluble in tap water than potassium sulfate alone, but less soluble than potassium nitrate alone. These results are summarized in Table 1 below.

Table 1: Solubili in Tap Water (without heating ! Material Quantity Final Temp. Measured Calculated Solubility Solubility (g/200 cc) (g/100 cc) Tap Water 200 cc 20°C NA NA g9°C28.514.25KNO330 16.5°C22.111.05K2SO430g KN: KS 30 g 13°C 27. 2 13. 6 1:1 KN : g12.5°C29.514.7530 2: 1 KN : g15°C25.412.730 1:2

In an additional test, a wide range of ratios of potassium nitrate and potassium sulfate was examined to determine solubility in tap water. For these experiments, the temperature of the tap water was 23.5° C. 30 g of potassium nitrate alone, potassium sulfate alone and mixtures thereof were dissolved in 200 cc tap water. The results are shown in Table 2 below.

Table 2: Further Determination of Solubility in Tap Water Material Quantity Final Temp. Measured Calculated Solubility Solubility (g/200 cc) (g/100 cc) Tap Water 200 cc 23.5°C NA NA g11°C28.514.25KNO330 g19.5°C24.012.0K2SO430 KN: KS 30 g 13. 5°C 28. 5 14.25 5 : 1 86: 14 KN: KS 30 g 140C 28. 3 14. 15 4: 1 80: 20 KN: KS 30 g 14. 4°C 28. 0 14. 0 3: 1 75: 25 KN : g14.6°C27.413.730 2: 1 66: 34 KN: KS 30 g 15. 5°C 27. 0 13. 5 1: 1 50: 50

From these results, clearly mixtures of potassium nitrate and potassium sulfate are water-soluble in a ratio in a range of from about 85: 15 to about 50: 50.

Finally, two additional tests were performed to examine both the effect of heating the tap water and the effect of increasing the concentration of both compounds on the solubility of the mixture. First, the concentration of the added mixture was doubled, so that 30 g of a mixture of potassium nitrate and potassium sulfate in a 1: 2 ratio was added to 100 cc of tap water and treated as above. The tap water temperature was 22.5 ° C. The temperature of the resultant solution dropped to 13.5°C and 18 g of the mixture was dissolved at saturation, more than the total amount of the same mixture which dissolved when 15 g of the mixture was added per 100 cc of tap water.

Next, tap water was heated to 58° C to assess the effect of heating on solubility of the mixture. 60 g of a mixture of potassium nitrate and potassium sulfate in a 1: 1 ratio was added to 200 cc of tap water at 58° C, and treated as above. The temperature of the resultant solution dropped to 38.5° C and 45.8 g of the mixture was dissolved at saturation, or 22.9 g per 100 cc of tap water. Clearly,

heating the tap water increased the solubility of the mixture. The undissolved residue was not analyzed. However, this residue is known from the literature to be largely composed of K2SO4.

Therefore, although potassium sulfate caused a smaller decrease in the temperature of the tap water, its solubility was lower than that of potassium nitrate.

However, the solubility of the mixture of potassium nitrate and potassium sulfate was even better than that of potassium nitrate alone, even without heating of the tap water, and the drop in temperature of the resultant solution was intermediate.

Thus, the mixture improves the solubility of potassium nitrate and potassium sulfate at certain ratios and temperatures, while being a better and more complete source of plant nutrients.

These data are intended only to confirm the relative solubility of both potassium sulfate and potassium nitrate in local area tap water found in Zichron- Jacob, Israel. The solubilities of both chemicals in aqueous solutions at various temperatures has been researched and data is available in chemical literature and handbooks.

Example 2 Test of the Mixture as a Fertilizer The most soluble mixture tested in Example 1, potassium nitrate and potassium sulfate in a 2: 1 ratio, was tested as a source of plant nutrients. The plants were cherry tomatoes [Please give me the scientific name], rooted in two types of soil media. For controls, potassium sulfate and potassium nitrate were tested alone, as were mixtures of potassium nitrate or potassium sulfate and magnesium sulfate. Essentially, similar yields were obtained when either the mixture of potassium sulfate and potassium nitrate, or the mixture of potassium nitrate and magnesium sulfate, was added. However, lower yields were obtained for potassium sulfate alone, potassium nitrate alone or potassium sulfate and magnesium sulfate. The experimental method was as follows.

First, cherry tomatoes were planted in 2 liter pots, containing one of two types of soil media. The first type of soil media was a sandy soil, obtained from Rehovot, Israel. The second type of soil media was a calcareous silty-clay soil, obtained from Zichron-Jacob, Israel. The pots were placed under a high plastic cover and each pot received 2 g TSP (Triple SuperPhosphate 46%) three times during the growth period, for a total of 6 g of fertilizer. Pots containing sandy soil which did not receive magnesium sulfate as a fertilizer had 10 g of Dolomite added per pot after planting, as this type of soil typically lacks magnesium.

There were five treatments, including the control treatment of potassium sulfate alone and each treatment had six repetitions. The soluble fertilizers were applied in irrigation water twice daily, morning and evening; each pot received 400 cc of the solution which brought about the drainage of the excess solution. The treatments are summarized in Table 2 below. In addition, during the growth period, several fungicide sprays [Please tell me which ones] and baits (Safsan, Kedem Chemicals, Israel) against Prudenia [Please give full scientific name] were applied.

Table 2: Summary of Treatments Material Abbreviation K2SO4 (KS)-control KN03 (KN) KN03 + K2SO4 (KN + KS) (2: 1 ratio) KN03 + MgS04 (KN + Mg) K2SO4 + MgS04 (KS + Mg) The overall concentration of nutrients included 150 ppm nitrogen and 200 ppm potassium. The additional nitrogen was applied as ammonium nitrate. In addition, those treatments which included magnesium received magnesium sulfate at a ratio of potassium oxide to magnesium oxide of 4: 1, or 60 ppm magnesium oxide per pot, which is about 180 ppm magnesium sulfate with 45 ppm of sulfur.

Finally, the tomato fruits were picked eight times during the growth period, and were then weighed. The yield results appear in Table 3.

Table 3: Average Tomato Fruit Yields Sandv Soil Sil Soil Treatment Yield Percent Yield Percent (g/pot) Yield (g/pot) Yield KS 900 100 851 100 (control) KN 912 101. 3 900 105.8 KN + KS 1018 113.1 943 110.8 KN + Mg 1031 114.5 954 112.1 KS + Mg 968 107. 6 917 107.8 The results clearly show the superiority of both the mixture of potassium sulfate and potassium nitrate, and of the mixture of potassium nitrate and magnesium sulfate as treatments in both types of soils when compared to the other treatments. However, statistical analysis showed that the differences between the two superior mixtures was not significant. Without wishing to be limited by any hypothesis, the improvement in performance is presumably due to the combination <BR> <BR> <BR> <BR> of soluble S04' (sulfate) alone, or soluble S04'and MgO, with potassium nitrate, at more balanced levels of all ions, particularly in conditions in which probably the soluble ion levels of S04'and Mg++ in the soil and water were not optimal.

At the same time, excesses of sulfate ion presumably caused relatively lower yields of fruits because of both ion and salinity effects. Interestingly, both the mixture of potassium sulfate and potassium nitrate, and the mixture of potassium nitrate and magnesium sulfate, were more effective than potassium nitrate alone. The results are less distinct in the heavy, silty soil than in the sandy

soil, which can be explained by the relative richness of the heavy soil in nutrient elements and the"buffering"power of such soil.

Example 3 Production of Fertilizer Various methods for production of mixtures of potassium nitrate and potassium sulfate are possible within the scope of the present invention, as described in this Example. Briefly, there are several methods of production of potassium nitrate, such as the solvent extraction process of IMI (Haifa Chemicals, Haifa Israel), the Vicksburg process in which elemental chlorine is the by-product, the double decomposition process starting from sodium nitrate and potassium chloride, and a number of ion exchange processes, all of which are well known in the art. Furthermore, potassium sulfate production processes include benefaction of potash ores and mines, treatment with sulfuric acid as employed in the Manheim, Chisso and Climax processes, and treatment of potassium chloride with sulfate salts. Usually"soluble"potassium sulfate is produced by re-crystallization of the standard, fertilizer grade potassium sulfate.

The various mixtures of soluble potassium nitrate and potassium sulfate, which are the subject of this invention, can be produced according to several different methods. For example, one method includes thorough mixing of the crystalline or powder potassium nitrate at the desired ratio with the crystalline or powder"soluble"grade potassium sulfate. If necessary and desired, the resultant mixture can be ground and further mixed. Alternatively, the brines of the two compounds can be mixed prior to drying. Also alternatively, the brines can first be dried to form two dried substances, which are then either directly mixed, or else first ground and then mixed.

Another possible process is the extraction process of IMI, or Haifa Chemicals, with the necessary modifications. This process could be used to extract a mixture of potassium nitrate and potassium sulfate by treating potassium chloride with the desired ratio of HN03 + H2SO4. One of the advantages here is

that the process of formation of potassium nitrate is exothermic while that of the formation of potassium sulfate is endothermic overall, thereby enabling energy to be saved while controlling the temperature of the solutions. Adaptation of the solvents and the usage of catalysts may be required. One advantage of this method is the possibility to directly achieve a fully water-soluble mixture or double salt.

The disadvantage is that it will not be practical to produce in this way more than 1 or 2 mixtures with different ratios.

Also, such mixtures could be produced by reacting KCI + HNO3 + H, S04 and removing the hydrochloric acid by physico-chemical methods. The mixtures can be granulated and compacted for easier usage and handling. Alternatively, the mixture of salts of NaN03 + Na2SO4 could be reacted with potassium chloride.

This step is followed by separation of the sodium chloride, and purification of the resulting mixture of potassium sulfate and potassium nitrate The most preferred ratios for potassium nitrate and potassium sulfate (KNO3 : K2SO4) and the resulting fertilizer formulae are given in Table 4 below.

The resulting formulae are given according to the international convention, as the ratio of percent elemental nitrogen, to percent phosphorous as Pros, to percent potassium as K20. Since none of the compositions contain P205, the percentage of that substance is zero for all formulae.

Table 4: Preferred Ratios Approximate Ratio (w/w) Resulting Formula K2SO4(%N-%P2O5-%K2O)KNO3: 85:15 11-0-47/11-0-48 80: 20/76: 24 10-0-48/10-0-47 70:30 9-0-48 66:34 9-0-49 64:36 9-0-49 62:38 8-0-49 54:46 7-0-50/6-0-50

These preferred formulae have special economic, commercial and agri- technical advantages. First, the total nutrient contents, in terms of percent elemental nitrogen and percent K2O, are quite advantageous. Second, the final solubility in water is enhanced. Third, the content of sulfate ion is controlled for administration of a desirable level of that ion. Fourth, the production process is simplified and saves energy. Fifth, the final cost is lower. Finally, a double salt of potassium nitrate and potassium sulfate, either as formed directly or else as formed from a mixture of these two substances, is highly desirable.

Example 4 Additional Formulae Additional sources of nitrogen can be added to the mixtures of the present invention in order to adjust the nitrogen content or to produce a more efficient or more complete fertilizer. Although a number of different nitrogen-containing compounds could be added, certain non-limiting illustrative mixtures are described herein, along with methods for their production.

For example, ammonium nitrate could be added to a mixture of potassium nitrate and potassium sulfate having a formula of 8-0-49 according to the international convention described above, so that the final ratio of potassium nitrate and potassium sulfate to ammonium nitrate in the mixture is 45.3: 54.7.

This mixture has a formula of 22-0-22. The mixture must either be done with granulated products, or be granulated by itself.

As another example, urea in crystalline form was added to the above mixture of potassium nitrate and potassium sulfate. The compounds were then thoroughly mixed together to form a final mixture. The final water-soluble mixture had a ratio of potassium nitrate and potassium sulfate to urea of 47: 53, with a formula of 28-0-28 (28% N: 28% K20). The same can be done with the granular forms of these compounds.

Example 5 Magnesium Sulfate Mixtures In addition to the mixtures of potassium nitrate and sulfate, magnesium sulfate can also be used as a source of sulfate. In particular, mixtures of potassium nitrate and magnesium sulfate also have improved solubility at lower temperatures when compared to potassium nitrate alone. Currently, some of the commercially available potassium nitrate fertilizers have relatively small amounts of soluble magnesium sulfate added to the extent of 2-4% nominal MgO, which is not an effective amount. According to certain preferred embodiments of the present invention, considerably higher amounts of soluble magnesium sulfate would be added to potassium nitrate to form mixtures according to the present invention.

These mixtures would have improved solubility at lower temperatures, as well as sufficient amounts of the important plant nutrients magnesium and sulfur. The most preferred ratios of magnesium sulfate to potassium nitrate would be from about 15% to about 50%. For example, the desired ratio of K20: MgO for tomatoes is about 5: 1, meaning about 79% potassium nitrate and about 21% magnesium sulfate in the mixture.

As for the previous Examples for potassium nitrate and potassium sulfate, solubility tests were performed on mixtures of potassium nitrate and magnesium sulfate. The tested mixtures are given in Table 5.

Table 5: Tested Ratios of Potassium Nitrate (K and Magnésium Sulfate (M) %MgSO4%MgOFormulaK:M I 5 : 1 16.7% 5.56% II 3 : 1 25% 8.33% III 2 : 1 33.3% 11.11% IV 1 : 1 50% 16.66%

A similar procedure was followed as for Example 1.30 g of each mixture I- IV, as well as 30 g of each compound alone, were dissolved in 200 cc of tap water at 28° C and the solubility was determined at saturation. The results are shown in Table 6.

Table 6: Solubilitv of Mixtures I-IV Compound or Mixture Final Temp-° C Insoluble Residue-g K219° M (MgSO4) 1 (5: 1) K: M 25. 5° 0. 5 (3: 1) K: M 25° 0. 3 (2: 1) K: M 28. 5° 1 (1: 1) K: M 33° 0. 5 From these results, clearly the addition of magnesium sulfate at any ratio of potassium nitrate to magnesium sulfate enhances the solubility of the mixture beyond that of potassium nitrate alone. Interestingly, certain ratios, such as 5: 1 or 3: 1 potassium nitrate to magnesium sulfate, also were more soluble than magnesium sulfate alone. Furthermore, the dissolution of the mixtures was less endothermic and/or more exothermic than the dissolution of potassium nitrate alone. Thus, these mixtures display greater solubility than potassium nitrate alone.

However, such solubility is even more important at lower temperatures, such as those experienced in more northern climates in the winter season. To test the ability of mixtures of potassium nitrate and magnesium sulfate to dissolve in relatively cold water, mixtures of potassium nitrate and magnesium sulfate were prepared, and 30 g of each mixture, as well as of potassium nitrate and magnesium sulfate separately, were mixed in 200 g of tap water at 6.5° C. The solubility was measured as before and the results are given in Table 7.

Table 7: Solubilitv in Cold Water Compound or Final Temp-° C % MgO in Compound Mixture or Mixture K (N03) 5° C M (MgSO4) 28° C 7.33: 1 (K: M) 50 C 4% 5: 1 (K: M) 7°C 5.56% 2: 1 (K: M) 10°C11. 11 %

Although the undissolved residues were not directly measured, clearly the final temperatures of the resultant solutions were closer to that of the solution of potassium nitrate alone than that of magnesium sulfate alone. Tables of dissolution of KN03 + MgS04 appear in the chemical literature.

Mixtures of potassium nitrate and magnesium sulfate can be produced by several methods. For example, fully water soluble crystalline potassium nitrate can be mixed with crystalline soluble magnesium sulfate at the desired ratios, as is currently being done by the relevant producers. Dried material used for the mixtures could also be granulated or compacted. In a second method, powdered MgO could be dissolved in a potassium nitrate solution. The resultant solution would then be mixed with H2SO4 at the desired ratio to obtain potassium nitrate and magnesium sulfate in solution, which could then be crystallized and dried to form the dried fertilizer. Alternatively, solutions of potassium nitrate and magnesium sulfate could be mixed, crystallized and dried. Optionally, potassium nitrate and magnesium sulfate could be produced from potassium chloride, magnesium chloride, HN03 and H2SO4.

It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the spirit and the scope of the present invention.