The present invention relates to an encapsulated fertilizer with neutralizing capacity and containing a balanced amount of nitrogen, sulphur and liming agents and optionally other components for land cultivation purposes.

When cultivating land, including in particular areas with winter crops, it will be advantageous to distribute only one fertilizer supply a year thereby providing nitrogen to the crop which is important for plant growth, and simultaneously supplying the liming agent necessary to maintain the soil pH (acidity) of the soil.

The supply of a liming agent is most often carried out in the form of a separate liming with agricultural lime, i.e. crushed or comminuted dolomite lime, bryozo- and coral lime, white chalk, silty limestone, marl, etc. Such liming is typically carried out every four years depending i.a. on the solubility of the liming agent supplied.

Liming is desirable since it renders the soil more porous and a number of nutrients become more readily obtainable to the plants. However, a high soil pH also increases the leaching of in particular nitrate ions to water streams, inshore waters and ground water. Moreover, there is an increased risk of percolation of humus. With the state of the art liming practice, where the liming agent is typically supplied every four years, also referred to as "interval liming", there is a risk of the liming effect being too powerful immediately following the supply with entailing leaching of e.g. phosp¬ horus and nitrate. This recent knowledge has resulted in a substantial reduction in the consumption of lime for agricul¬ tural purposes. However, it is not a practical option to supply a sufficient amount of liming agent to cultivated lands once a year as the doses involved are relatively small (about 250 kg per hectare) and it is not possible with the

apparatus available to dosage such small doses correctly. It is therefore common practice to supply about 1 ton of liming agent per hectare every four years.

Dolomite lime is a preferred liming agent for agricultural and horticultural uses due to its high contents of magnesium which is an important plant nutrient in addition to being a potent soil neutralizer. Q

Urea (NH ₂ CNH ₂ ) is commercially available as a nitrogen fertilizer containing 46% of N in the form of amide nitrogen.

Urea is readily soluble and a readily available nutrient for plants, and prior to use it may be dissolved in water and sprayed onto the soil or the crop. Urea may furthermore be distributed in a solid state, e.g. as pellets or granules, directly onto the soil. However, urea is highly hygroscopic since it absorbs moisture from the environment. This renders urea fertilizers more susceptible to fluidization and therefore it is less shelf stable than other solid fertili¬ zers.

From US patent No. 3,684,476 it is known to produce a granular fertilizer containing N, Mg and Ca, wherein solid particulate urea is reacted in the absence of added water with selectively calcinated (burnt) solid particulate dolomite or dolomite limestone. The reaction occurs according to the following equations:

MgO + 2(NH ₂ ) ₂ -CO → Mg(NH-CO-NH ₂ ) ₂ + H ₂ 0 and urea magnesium dicarbamide MgO + (NH ₂ ) ₂ -CO → Mg(NH) ₂ -C0 + H ₂ 0 urea magnesium carbamide

The magnesium carbamides formed are sticky and the calcium carbonate, CaC0 ₃ , remaining from the selective calcination sticks to the magnesium carbamide particles to form a solid protective layer. The water formed in the reaction is to be

eliminated to avoid the formation of Mg(OH) ₂ which may drive off NH ₃ from urea.

Thus, a non-hygroscopic inert fertilizer is obtained wherein nitrogen is protected against leaching and degassing and which may be stored and sown along with grain. Moreover, the fertilizer may be mixed with other fertilizers containing P, K and trace elements. Said fertilizer will be sulphur-free unless sulphur is added separately.

It is therefore the object of the invention to provide a sulphur- and magnesium-containing fertilizer which may be supplied to the cultivated areas once a year and containing a combination of nitrogen fertilizer in the form of urea or derivatives thereof, liming substances and plaster, and which may be produced in a less costly and more resource-friendly process. It is a further object of the invention to provide an encapsulated (coated) urea product which is stable and sprinkable. It is preferred that the fertilizer does not harden or form lumps during storage within normal shelf life.

This object is achieved with the present invention which is characterized in that it relates to an encapsulated particu¬ late fertilizer with neutralizing capacity and consisting of particulate urea or derivatives thereof encapsulated in a combination of a magnesium-containing liming agent and plaster and further optional fertilizer components.

The encapsulated fertilizer according to the invention is preferably prepared from a mixture of from 20 to 80 percent by weight of particulate and substantially anhydrous urea, preferably such urea which contains about 46% of N, pre¬ ferably a granulated or pelleted formaldehyde-treated urea (sometimes referred to as "urea-form") , from 15 to 70 percent by weight of a particulate magnesium-containing liming agent, such as dolomite, dolomite limestone or magnesium limestone with a varying moisture content of from about 3 to about 17

percent by weight of water, and from 4 to 30 percent by weight of plaster, preferably powdered plaster.

In recent years, the reduced amount of sulphur in precipita- tion and in the atmosphere due to flue gas purification has resulted in an increased need to supply sulphur to crops, i.a. grain and rape. Plaster is a naturally occuring sulphur compound but it may also be obtained as a side product in industrial processes and flue gas purification. The sulphur content is typically 20% and due to its contents of calcium, plaster has a neutralizing capacity of about 60% which is comparable to dolomite that has a neutralizing capacity of about 90%, cf. Table 1. However, plaster is not applicable for agricultural uses as a liming agent, since it solidifies and forms lumps and crusts. This further renders its contents of sulphur non-obtainable for the crops. An ideal sulphur content in fertilizers for use in conditions such as in Denmark is estimated to be about 2% of S.

Mixing of the particulate magnesium-containing liming agent having a natural moisture content and urea or a derivative thereof produces a loss of heat in the mixture simultaneously with a certain amount of water separating off, which results in a slaking mixture. Whether a chemical reaction occurs and, in the affirmative, how it proceeds is uncertain. However, it may be assumed that, similarly to the reactions disclosed in US-A-3,684,476 between urea and selectively calcined dolomi¬ te, i.e. the MgO-portion, bonding occurs of carbamide from urea to magnesium which is present in the form of MgC0 ₃ in untreated dolomite and the like magnesium-containing mi¬ nerals, resulting in the formation of H ₂ 0. The magnesium- containing liming agent is bonded as a layer around the urea particles and the particles thus produced are moist and sticky thereby allowing admixed particulate, preferably powdered plaster to be bonded to the surface. This also makes the particles dry, and free-flowing encapsulation of the urea particles is obtained. One possible explanation of the

apparent loss of water may be the absorption of free water in the plaster powder as hydrate water. It is consequently preferred that the plaster used is completely or partially dehydrated.

During subsequent normal storage of the fertilizer under pressure no lumping or NH ₃ -evaporation has been observed.

The invention further relates to a method for the production of an encapsulated fertilizer, said method being charac¬ terized in that particulate urea or a derivative thereof is mixed with a magnesium-containing liming agent which is bonded to the urea particles while forming a moist and sticky surface layer optionally by addition of water as granulating liquid, following which the admixture of particulate plaster provides a free-flowing, particulate, encapsulated fertili¬ zer. The desired product may be obtained independently of particular temperature conditions, e.g. at a preferred temperature in the mixture of from 5* to 30*C. The conversion rate is increased at increasing temperatures. It is preferred that the particulate components have a natural moisture or water content of from about 3 to about 17 percent by weight. The mixing is preferably carried out as a drum coating or a drum encapsulation of a pelleted urea or urea-form product with an average particle diameter of from about 1 mm to about 3 mm to obtain a coated or encapsulated product with an average particle diameter of from about 1.5 to about 4 mm and a shell thickness of about 0.5 mm.

The fertilizer according to the invention provides the following advantages:

a liming agent with a neutralizing capacity of up to

70% and nitrate-free nitrogen fertilizer in a product which may cover the total needs of a crop for nitrogen

(N) , calcium (Ca) , magnesium (Mg) and sulphur (S) in a

balanced ratio and in one operation, and minimization of the leaching of nutrients from the soil;

furthermore the use of plaster provides a number of advantages since the admixture of from 4 to 30 percent by weight of plaster as an encapsulating agent along with e.g. dolomite forms a thin micro-porous shell around the urea particles. Hereby the readily soluble urea becomes obtainable for the crop through slow seepage. A porous shell results which will decompose over time while releasing magnesium, calcium and sulphur. Since the plaster powder is finely distributed over the magnesium-containing liming agent in the fertilizer production process it will not form hard lumps upon drying. Furthermore, the plaster in the fertilizer will not solidify in the same manner as will be the case following usual addition of water and drying. Hereby the plaster, which is otherwise a waste product difficult to utilize, is rendered useful for agricultural purposes.

With a combined liming and fertilizing agent according to the invention having a neutralizing capacity of e.g. 41% (cal¬ culated) and having at the same time a high content of N, Mg and S of 24%, about 4% and 3%, respectively, one annual distribution in addition to basic fertilization is all that is needed for grain crops and in particular for winter crops.

Moreover, the fertilizer according to the invention is inexpensive since a liming effect and a supply of Mg and S are additionally obtained at the same cost as a corresponding N-fertilizer with calcium nitrate or calcium ammonium nitrate. Finally a nitrate-free N-fertilizer contributes to building up the humus content of the soil.

Table 1 illustrates different mixing ratios for the fertili¬ zer according to the invention. The neutralizing capacity is given as a theoretically calculated value (non-analytical) .

Definitions:

"Encapsulated" fertilizer means a particulate fertilizer encapsulated in a shell of an encapsulating material. The encapsulating material may occur e.g. as a granulation or an agglomeration.

"Urea" means NH ₂ -C-NH ₂ and corresponding amide compounds and derivatives thereof, including urea aldehyde condensates, such as urea-form (urea + formaldehyde or other aldehydes) which may be reacted with the magnesium-containing liming agent to obtain the fertilizer according to the invention.

"Magnesium-containing liming agent" as used herein means a particulate product with a mesh size curve (particle size distribution) authorized for land cultivation uses, e.g. a product wherein 100 percent by weight of the particles are 4,0 mm or below, about 50 percent by weight thereof being 0,1 mm or below, and containing varying amounts of CaC0 ₃ , CaO, Ca(OH) ₂ , MgC0 ₃ , MgO and Mg(0H) ₂ and optional trace elements. The liming agent may thus be selected from among e.g. dolomite, dolomite limestone, magnesium limestone and combinations of calcium carbonate-containing minerals and products, such as bleaching chalk, bryozo lime, coral lime, white chalk, Thomas slag and precipitated calcium carbonate, and magnesium-containing minerals and products, such as kieserite, carnallite, epsomite and kainite, preferably MgC0 ₃ - or MgO-containing minerals and products. The liming agent is preferably non-treated except from optional milling or grinding.

"Plaster" as used herein is used to designate completely or partially dehydrated plaster, viz. naturally occuring or synthetic calcium sulphate hemihydrate (CaS0 ₄ «ijH ₂ 0) and an¬ hydrite (CaS0 ₄ ) , including by-products, such as flue gas plaster from desulphurization plants and phosphorous plaster

from the production of phosphorous acid. The plaster used in the invention is preferably powdery and capable of absorbing and bonding free water.

Further fertilizer components include all natural and synthetic inorganic and organic compounds exhibiting fertili¬ zing performance, including trace elements. In particular such trace elements include the elements Mn, Fe, Mo, B, Co, Cu, Zn and salts or complex compounds thereof.

"Neutralizing capacity" as used herein means the calculated percentage of calcium carbonate on the basis of a chemical standard basicity determination of a weighed sample of the liming agent.

"Land cultivation uses" as used herein means every activity in connection with the cultivation and maintenance of agricultural, silvicultural, horticultural and park areas, including also private gardens as well as nurseries.

The encapsulated fertilizer according to the invention preferably consists of rounded or spherical particles with a preferred particle size distribution of from about 1.2 to about 4 mm. This may readily be obtained by the method according to the invention for producing the fertilizer.

The fertilizer may be distributed manually, but for large areas mechanical distribution is preferred, e.g. using a centrifugal-type spreader for which the fertilizer is particularly suitable due to its granular form.

The fertilizer according to the invention may be used for all crops and for garden, grass and park areas and for all possible land cultivation uses as it replaces known in- dividual fertilizers. Moreover, the fertilizer according to the invention may form a part of an NPK-fertilizer whereby an

NPK-fertilizer with neutralizing capacity may be obtained. Such an NPK-fertilizer may be mixed mechanically.

Example 1

Mixing of 60% of urea (supplied by Landbrugets Storkøb A/S, DK-9260 Gistrup) , 30% of dolomite with about 10% of Mg and 10% of plaster (stucco plaster, supplied by P. Brøste A/S, DK-7000 Fredericia) yields a fertilizer having the following contents: N=28, Mg=3, S=2 and a neutralizing capacity of 34%. This fertilizer will be able to replace nitrate-containing individual fertilizers, such as calcium nitrate, and calcium ammonium nitrate.

Analysis result:

Total nitrogen, N (2,6,1) (1) 28,4%

Ammonium nitrogen, N (DS 230/241) (1) 0,2%

Nitrate nitrogen, N (DS 230/241) (1) 0,2% Amide nitrogen, N (2, 6, 2) (1) 28,0%

Total magnesium (Mg) (8,1-7) (1) 2,7%

Total sulphur (S) (8,1) (1) 2,3%

Neutralizing capacity (CaC0 ₃ ) * 29,8%

*Neutralizing capacity is given as the mean value of 10 determinations, of which the lowest was 24,2% and the highest 33,0%.

(1) Laid down by the Danish Ministry of Agriculture,

Example 2

Mixing of 50% of urea, 37,5% of dolomite and 12,5% of plaster yields a fertilizer having the following contents: N=23, Mg=4 and S=3 and a neutralizing capacity of 42%. This fertilizer will be suitable for mechanical admixture to produce an NPK- fertilizer with neutralizing capacity, e.g. NPK 23-3-7.

Example 3

Mixing of 33% of urea, 57% of dolomite and 10% of plaster yields a fertilizer having the following contents: N=15, Mg=6 and S=2 and a neutralizing capacity of 58%. This fertilizer is a suitable replacement for the nitrate-containing calcium nitrate with the same nitrogen content.

Example 4

Mixing of 75% of urea, 15% of dolomite and 10% of plaster yields a fertilizer having the following contents: N=35, Mg=2 and S=2 and a neutralizing capacity of 20%. This fertilizer will be able to replace pure urea having N=46 and will be very suitable for continuous wheat-growing wherein attacks of take-all disease are avoided.