Topical fertilization methods and compositions
for use therein , United States Patent 4447253
Abstract: | Improved liquid fertilizer compositions for applying urea to the soil surface contain urea-sulfuric acid reaction products having high H.sub.2 SO.sub.4 /urea molar ratios and are free of toxic components such as sulfamic acid normally associated with such products. The use of these compositions reduces or completely eliminates urea volatilization loss normally associated with topical urea fertilization, particularly in alkaline soils. |
Inventors: | Young, Donald C.; |
Application Number: | 318368 |
Filing Date: | 1981-11-05 |
Publication Date: | 1984-05-08 |
Assignee: | Union Oil Company of California (Los Angeles, CA) | ||||
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Primary Examiner: |
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Attorney, Agent or Firm: |
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Claims: |
I claim: 1. A method for fertilizing soil with urea which comprises the step of topically applying to the surface of said soil a fertilizing effective amount of a composition comprising a urea-sulfuric acid reaction product containing about 35 to about 85 weight percent sulfuric acid, about 5 to about 47 weight percent urea, and about 0 to about 35 weight percent water, in which said urea and sulfuric acid constitute at least about 65 weight percent of said reaction product and said reaction product has an H.sub.2 SO.sub.4 /urea molar ratio of at least about 0.7. 2. The method defined in claim 1 wherein said reaction product and said composition contain no detectable amount of a member selected from the group consisting of sulfamic acid, ammonium sulfamate, and combinations thereof. 3. The method defined in claim 1 wherein said composition is applied to the surface of said soil at a rate equivalent to at least about 40 pounds of said reaction product per acre and said composition comprises a solution of about 10 to about 100 weight percent of said reaction product in water. 4. The method defined in claim 3 wherein said reaction product and said composition contain no detectable amount of a material selected from the group consisting of sulfamic acid, ammonium sulfamate, and combinations thereof, and said soil is an alkaline soil having a pH of at least about 7.5. 5. The method defined in claim 1 wherein said reaction product comprises about 25 to about 38 weight percent urea, about 40 to about 80 weight percent sulfuric acid, has an H.sub.2 SO.sub.4 /urea molar ratio of at least about 1, and contains no detectable amount of a member selected from the group consisting of sulfamic acid, ammonium sulfamate, and combinations thereof. 6. The method defined in claim 1 wherein said urea and sulfuric acid constitute at least about 75 weight percent of said reaction product and said composition comprises about 50 to about 100 weight percent of said reaction product. 7. The method defined in claim 1 wherein said reaction product is produced by the method including the steps of introducing concentrated sulfuric acid, solid urea, and water separately and simultaneously into a reaction zone in proportions corresponding to about 35 to about 85 weight percent sulfuric acid, about 5 to about 47 weight percent urea, and 0 to about 35 weight percent water, wherein said urea and sulfuric acid, taken in combination, constitute at least about 65 weight percent of the feed to said reaction zone, the H.sub.2 SO.sub.4 /urea molar ratio of said feed to said reaction zone is at least about 0.7, and said feeds are reacted in said reaction zone at a temperature below about 176.degree. F. and below the incipient decomposition temperature of said reaction product to produce said reaction product containing no detectable amount of a member selected from the group consisting of sulfamic acid, ammonium sulfamate, and combinations thereof. 8. The method defined in claim 1 wherein said soil is an alkaline soil having a pH of at least about 6. 9. The method defined in claim 1 wherein said urea is present in said composition as a member selected from the group consisting of monourea sulfate, diurea sulfate, and combinations thereof. 10. A method for fertilizing soil, which method comprises topically applying to the surface of said soil a fertilizing effective amount of a composition comprising a urea-sulfuric acid reaction product containing about 35 to about 85 weight percent sulfuric acid, about 5 to about 47 weight percent urea, and 0 to about 35 weight percent water, in which said urea and sulfuric acid, in combination, constitute at least about 65 weight percent of said reaction product, said reaction product has an H.sub.2 SO.sub.4 /urea molar ratio of at least about 0.7, and said composition is free of urea-sulfuric acid reaction by-products resulting from the decomposition of a member selected from the group consisting of urea, sulfuric acid, and combinations thereof. 11. The method defined in claim 10 wherein said composition contains no detectable amount of a member selected from the group consisting of ammonium sulfamate, sulfamic acid, and combinations thereof. 12. The method defined in claim 11 wherein said composition consists essentially of said reaction product. 13. The method defined in claim 10 wherein said composition is an aqueous solution of said urea-sulfuric acid reaction product, which solution comprises about 10 to about 100 weight percent of said reaction product. 14. The method defined in claim 10 wherein said soil is an alkaline soil having a pH of at least about 7.5. 15. The method defined in claim 10 wherein said H.sub.2 SO.sub.4 /urea molar ratio is at least about 1. 16. A method for topically fertilizing alkaline soil with urea and for inhibiting the volatilization loss of the resultant topically applied urea, which method comprises the step of topically applying to the surface of said alkaline soil a fertilizing effective amount of a composition comprising a urea-sulfuric acid reaction product; which reaction product contains about 35 to about 85 weight percent sulfuric acid, about 5 to about 47 weight percent urea, and 0 to about 35 weight percent water, said urea and sulfuric acid, in combination, constitute at least about 65 weight percent of said reaction product, said reaction product has an H.sub.2 SO.sub.4 /urea molar ratio of at least about 0.7, and said composition is free of urea-sulfuric acid reaction by-products resulting from the decomposition of a member selected from the group consisting of urea, sulfuric acid, and combinations thereof. 17. The method defined in claim 16 wherein said alkaline soil has a pH of at least about 7.5, and said composition contains no detectable amount of a member selected from the group consisting of ammonium sulfamate, sulfamic acid, and combinations thereof. 18. The method defined in claim 16 wherein said alkaline soil has a pH of at least about 7.5. 19. The method defined in claim 16 wherein said alkaline soil has a pH of at least about 8. 20. The method defined in claim 16 wherein said composition is an aqueous solution of said reaction product, which solution comprises about 10 to about 100 weight percent of said reaction product. 21. A method for topically fertilizing alkaline soils with urea, which method comprises applying to the surface of an alkaline soil having a pH of at least about 7.5 an aqueous solution of the reaction product of urea and sulfuric acid, wherein the sulfuric acid/urea molar ratio in said solution is at least about 0.7, and said solution is free of urea-sulfuric acid reaction by-products resulting from the decomposition of a member selected from the group consisting of urea, sulfuric acid, and combinations thereof. 22. The method defined in claim 21 wherein said sulfuric acid/urea molar ratio is at least about 1. 23. A method for topically fertilizing alkaline soils with urea, which method comprises applying to the surface of an alkaline soil having a pH of at least about 7.5 an aqueous solution of a reaction product of urea and sulfuric acid, wherein the sulfuric acid/urea molar ratio in said solution is at least about 0.7. 24. The method defined in claim 23 wherein said sulfuric acid/urea molar ratio is at least about 1. |
Description: |
BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the field of topical fertilization with urea and particularly to improved liquid urea fertilizers and methods of topical fertilization, particularly in alkaline soils. The compositions are stable, homogeneous solutions which when topically applied to alkaline soils markedly reduce or completely eliminate urea losses by volatilization. Thus, they enable more efficient use of fertilizer nitrogen values and better control of nitrogen dosage on crops. The compositions and methods also otherwise improve soil characteristics due to the acidifying and nutrient effect of sulfuric acid. 2. Description of the Prior Art Urea is widely used as a topical, subsurface and foliar fertilizer. Topical urea application on alkaline soils is known to result in the loss of nitrogen fertilizer values by volatilization, i.e., the ultimate conversion of urea to ammonia which escapes to the atmosphere. Sulfuric acid has also been widely used in the agricultural industry as a soil adjuvant, a water penetration improving agent, a herbicide for a wide variety of vegetation, and as a selective herbicide on crops resistant to sulfuric acid such as onions and garlic. Previous investigators have observed that urea, sulfuric acid, and, optionally, water can be reacted to form concentrated solutions of urea and sulfuric acid reaction products in which the urea is present as mono- and/or diurea sulfates. However, they did not recognize that this reaction can result in the formation of by-products that are toxic to plants such as sulfamic acid and ammonium sulfamate; the latter of which is a federally registered herbicide. The urea-sulfuric acid reaction is so highly exothermic, particularly when solid urea and concentrated sulfuric acid are used as is necessary for purposes of economy, that it is difficult to control reaction temperature in the large volume production plants required to produce significant amounts of these products. In fact, it is essentially impossible to control reaction temperature and thus avoid formation of undesired by-products, during production of the higher acid content compositions having sulfuric acid/urea molar ratios greater than 0.7, with available methods. Furthermore, previous investigators did not recognize either the magnitude or importance of incipient product and/or reactant decomposition or the temperatures at which such decomposition occurs for products having different urea/sulfuric acid ratios. Their methods were not adequate to avoid incipient decomposition, particularly in the higher acid compositions, and they did not recognize the effect of such decomposition on process control or product quality. These investigators also did not recognize that the use of urea-sulfuric acid reaction products having H.sub.2 SO.sub.4 /urea molar ratios of about 0.7 and higher markedly reduce and, in some cases, completely eliminate urea loss by volatilization even under the most adverse conditions of soil pH and ambient temperature. D. F. du Toit found that urea formed certain compounds with oxalic, acetic, hydrochloric, nitric and sulfuric acids, and that the resulting compounds were stable in contact with their solutions at 20.degree. C. Verslag Akad. Wetenschappen, 22, 573-4 (abstracted in Chemical Abstracts, 8, 2346, 1914). L. H. Dalman expanded on du Toit's work by developing the phase relationships between the solid phase and saturated solutions at 10.degree. C. (50.degree. F.) and 25.degree. C. (77.degree. F.) but, as in the case of du Toit, did not develop or disclose methods capable of handling the high heat of reaction involved in large scale industrial processing. "Ternary Systems of Urea and Acids. I Urea, Nitric Acid and Water. II. Urea, Sulfuric Acid and Water. III. Urea, Oxalic Acid and Water"; JACS, 56, 549-53 (1934). In the article "Adding Plant Nutrient Sulfur to Fertilizer," Sulfur Institute Bulletin No. 10 (1964), the Sulfur Institute discussed the addition of nutrient sulfur to fertilizers and mentioned that urea reacts with sulfuric acid to form two complexes of urea sulfate which are useful fertilizers. Jones, U.S. Pat. No. 4,116,664 discloses what is referred to therein as a tortuous, multistage process of producing combinations of urea and sulfuric acid in which portions of the sulfuric acid are incrementally added to and reacted with the total amount of urea to be reacted in each of several stages until the total amount of sulfuric acid has been reacted with the urea. The resulting product is unstable and requires further processing. Jones preferably adds water later as required to obtain stability and the desired composition. He discloses that the reaction can be carried out at temperatures of 100.degree. to 200.degree. F. and that if the sulfuric acid is added to the total amount of urea at a rate which is too fast the temperature goes to about 200.degree. to 225.degree. F. and that a gas is emitted that causes changes in product characteristics such as solidification. The patent states that temperatures of 160.degree. to 200.degree. F. are preferred and that the products can be used as fertilizers. Although these investigators disclosed several characteristics of urea-sulfuric acid combinations and methods of making those combinations, and that the products were useful soil adjuvants and/or fertilizers, they did not recognize that the methods they disclosed resulted in the formation of products containing toxic reaction by-products or that the use of liquid urea/sulfuric acid reaction products having H.sub.2 SO.sub.4 /urea molar ratios in excess of about 0.7 dramatically reduces or completely eliminates urea volatilization loss associated with topical application of urea on relatively alkaline soils. Those investigators did not appreciate that the incipient decomposition temperature--the temperature at which reactant and/or product decomposition commences--varies with reactant and product composition, or the effect that decomposition has on product composition. It is therefore one object of this invention to provide improved urea-containing topical fertilizer compositions. It is another object of this invention to provide improved urea/sulfuric acid reaction product compositions, which when applied to the soil, even in dilute form, dramatically reduce urea volatilization loss. It is another object of this invention to provide improved topical fertilization methods employing solutions of urea-sulfuric acid reaction products. Yet another object of this invention is to provide methods for applying urea solutions to the soil surface and minimizing or preventing the loss of nitrogen fertilizer values by volatilization loss. Other objects, aspects, and advantages of this invention will be apparent to one skilled in the art in view of the following disclosure, the drawings, and the appended claims: SUMMARY OF THE INVENTION This invention relates to liquid fertilizer solutions comprising urea-sulfuric acid reaction products particularly suited for topical application to the soil, and to methods of topically fertilizing soils, particularly alkaline soils, with such compositions. The use of these solutions for topical fertilization markedly reduces the loss of urea nitrogen by urea volatilization and improves crop fertilization control. Subsurface urea application usually does not occasion significant loss of nitrogen fertilizer values by urea volatilization, due to the fact that urea decomposition products are adsorbed or otherwise fixed in the soil; thus, they remain available for plant nutrition. However, topical application is often preferred since it is simpler than subsurface application and eliminates the need for injectors or other devices capable of introducing urea below the soil surface. Urea volatilization can occur in soils in which the water in equilibrium with the soil has a pH above about 5. At pH levels of about 5, nitrogen loss is not very significant, however, the rate of nitrogen loss increases dramatically with pH. Thus, the nitrogen loss resulting from urea volatilization with topically applied urea in soils having a pH of about 8 is approximately twice that that occurs in the same period of time in soils having a pH of about 6. The nitrogen loss in very alkaline soils having pH values of about 10 is approximately 100 times that that occurs at pH 6. Most soils in the western United States have pH levels above 7.5 or 8. The topical application of urea on those soils often results in the loss of 25 to 30 percent of the nitrogen fertilizing value of the applied urea and, under extreme conditions, can result in the loss of essentially all of the urea applied topically. The more extreme cases involve more alkaline soils having pH levels above about 8 and relatively high ambient temperatures. In accordance with one embodiment of this invention, unique liquid fertilizer solutions particularly adapted to topical fertilization comprise urea-sulfuric acid reaction products having H.sub.2 SO.sub.4 /urea molar ratios of at least about 0.7 that are essentially free of undesirable by-products normally associated with urea-sulfuric acid reaction products such as ammonium sulfamate and/or sulfamic acid. In accordance with another embodiment of this invention, agricultural soils are topically fertilized with urea-containing fertilizer solutions comprising urea-sulfuric acid reaction products having H.sub.2 SO.sub.4 /urea molar ratios of at least about 0.7. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more readily understood by reference to the drawings of which: FIG. 1 is a ternary phase diagram for the urea, sulfuric acid, and water system illustrating isotherms at several different temperatures, the existence of three prominent eutectics along those isotherms and the urea-sulfuric reaction products useful in the fertilizer compositions and methods of this invention; FIG. 2 is a correlation of the reaction rate constant versus temperature for the two designated urea-sulfuric acid reaction products using prilled urea feeds; FIG. 3 is a correlation of the reaction rate constant with temperature for the two designated urea-sulfuric acid reaction products using granular urea feed; and FIG. 4 is a schematic illustration of an apparatus and process system suitable for use in the manufacture of the urea-sulfuric reaction products described herein. DETAILED DESCRIPTION OF THE INVENTION The topical fertilizer compositions of this invention are clear, homogeneous aqueous solutions containing a fertilizing effective amount of a urea-sulfuric acid reaction product hereinafter defined. The fertilizer solutions usually contain at least about 10 weight percent, generally 10 to about 100 weight percent, and preferably at least about 50 weight percent of the urea-sulfuric acid reaction product. Use of the reaction products without dilution is particularly preferred in many instances to minimize the volume of material that must be delivered. However, the concentrates are relatively viscous at ambient temperatures and their use without dilution requires application equipment capable of handling such viscous solutions. Thus, the dilute solutions of lower viscosity are sometimes preferred in order to obtain more even distribution with available equipment. The urea-sulfuric acid reaction product concentrates are also clear homogeneous solutions containing the reaction products of urea, sulfuric acid, and, optionally, water. Water may be present in very minor amounts or may be omitted entirely in some compositions as illustrated in the ternary phase diagram of FIG. 1. The reaction products contain about 35 to about 85 weight percent, preferably about 40 to about 80 weight percent, sulfuric acid; about 5 to about 47 weight percent, preferably about 25 to about 38 weight percent, urea; and 0 to about 35 weight percent, preferably less than about 25, and more preferably less than about 15 weight percent, water. Urea and sulfuric acid, in combination, account for at least 65 weight percent, preferably at least about 75 weight percent, and most preferably at least about 85 weight percent of the reaction products. The relative proportions of urea and sulfuric acid in the reaction products are such that the H.sub.2 SO.sub.4 /urea molar ratio is at least about 0.7, preferably at least about 1. The reaction products and fertilizer compositions are essentially free of toxic by-products normally associated with urea-sulfuric acid reaction products such as sulfamic acid and/or ammonium sulfamate, and are preferably completely free of such impurities. The reaction products usually have crystallization temperatures of about 80.degree. F. or less, preferably about 50.degree. F. or less, to avoid precipitation during storage and shipment. Although more concentrated solutions having higher crystallization temperatures can be employed in the manufacture of the fertilizer compositions of this invention, since the crystallization temperature can be reduced by dilution to produce the fertilizer composition, the lower crystallization temperature formulations are presently preferred for the reasons discussed above. The four-digit designation for the reaction products used herein, e.g., 18-0-0-17, are conventionally used in the agricultural industry to designate the concentration of nitrogen, phosphorus (as P.sub.2 O.sub.5), potassium (as K.sub.2 O), and a fourth component --in this case sulfur expressed as the element. Thus, the composition 18-0-0-17 contains 18 weight percent nitrogen derived from urea and 17 weight percent sulfur derived from sulfuric acid. Using the atomic weights for nitrogen (14) and sulfur (32) and the molecular formulas and molecular weights for urea (60.06) and sulfuric acid (98.08), it can be readily determined that this formulation contains 38.6 weight percent urea and 52.1 weight percent sulfuric acid. By difference, the solution contains 9.3 weight percent water. The composition of all other products and feed solutions can be determined by the same procedure. The composition and some of the physical properties of the urea-sulfuric acid reaction products are illustrated by the ternary phase diagram of FIG. 1. The phase diagram defines the relative proportions in weight percent for each of the three components--urea, sulfuric acid, and water--at any point within the diagram. At each apex of the triangle the system consists completely of the indicated component. Thus, the urea concentration at the urea apex is 100 percent and diminishes linearly to 0 along a straight line from the urea apex to the midpoint of the H.sub.2 O-H.sub.2 SO.sub.4 boundary line, i.e., the side of the triangle opposite the urea apex. The same is true of the remaining two components: water and sulfuric acid. The diagram also illustrates the isotherms for the system at 14.degree. F., 32.degree. F., 50.degree. F., 77.degree. F., and 150.degree. F. The 150.degree. F. isotherm is illustrated only partially at the lower left-hand portion of the diagram. Each isotherm defines compositions which, if cooled below the temperature indicated for the respective isotherm, will precipitate components of the system. However, the solutions will super-cool dramatically, e.g., by as much as 50.degree. F., or more, under quiescent conditions in the absence of seed crystals, impurities, etc., that promote crystallization. As indicated by the pattern of the isotherms, systems having a fixed ratio of urea to sulfuric acid become more stable at lower temperatures as the water concentration is increased. This is true throughout most of the phase diagram with the exception of the region in the vicinity of the higher acid eutectic in the lower right-hand portion of the phase diagram. Three prominent eutectics are apparent within the region of the illustrated isotherms. Each eutectic represents a discontinuity in the response of the system, e.g., of crystallization point, to changes in solute concentration, and indicates the points of maximum solute concentration for a given isotherm in the regions of the phase diagram associated with those eutectics. As indicated in the legend on FIG. 1, the left-hand eutectic on the 50.degree. F. isotherm corresponds to the formulation 29-0-0-9. The middle eutectic on the same isotherm corresponds to the composition 18-0-0-17. The right-hand eutectic on the 14.degree. F. isotherm corresponds to the formulation 9-0-0-25, and the formulation intermediate the 50.degree. F. and the 77.degree. F. isotherms between the middle and right-hand eutectics indicated by a triangular designation corresponds to the formulation 10-0-0-19. The 29-0-0-9 eutectic and, for that matter, all formulations to the left of line B on the ternary phase diagram are not encompassed by the definition of reaction products within the scope of this invention, since they do not significantly reduce or completely eliminate nitrogen loss by volatilization, particularly when applied to relatively alkaline soils, which is a principal objective of this invention. The bold lines within the diagram generally define the boundaries for formulations for which the methods of this invention are uniquely suited. Bold lines parallel to one side of the trilinear diagram define a fixed concentration of the solute designated at the apex of the triangle opposite the side to which that line is parallel. Thus, the higher horizontal line in FIG. 1 borders the area of formulations containing 35 percent water or less and varying amounts of urea and sulfuric acid. The area below the lower horizontal line defines formulations containing 25 weight percent water or less. Vertical line A in the center of the diagram intersecting the urea-sulfuric acid line at the half-way point defines compositions having a one-to-one weight ratio of sulfuric acid to urea and an H.sub.2 SO.sub.4 /urea molar ratio of 0.61. If extended vertically, line A would intersect the water apex. Line B on the phase diagram represents reaction products having H.sub.2 SO.sub.4 /urea molar ratios of 0.7 and weight ratios of 1.143. It intersects the urea-sulfuric acid boundary at the point corresponding to 46.66 weight percent urea and 53.34 weight percent sulfuric acid and, if extended, would pass through the water apex of the diagram. All formulations to the right of Line B have H.sub.2 SO.sub.4 /urea molar ratios of about 0.7 or greater. Line C intersects the urea-sulfuric acid boundary at the point corresponding to 37.98 weight percent urea and 62.02 weight percent sulfuric acid and represents reaction products having H.sub.2 SO.sub.4 /urea molar ratios of 1 and weight ratios of 1.633. If extended upwardly, Line C would also intersect the water apex. All formulations to the right of Line C on the phase diagram have H.sub.2 SO.sub.4 /urea molar ratios of 1 or greater. The urea-sulfuric acid reaction products useful in the compositions of this invention can be produced by either batch or continuous processes as described in my copending applications Ser. No. 318,343, filed Nov. 5, 1981, and Ser. No. 318,629, filed Nov. 5, 1981, both of which are incorporated herein by reference. Those processes can be used to accurately and consistently produce urea-sulfuric acid reaction products of predetermined composition and crystallization temperature essentially or completely free of decomposition products such as sulfamic acid and/or ammonium sulfamate. Generally the reaction products can be produced by separately and simultaneously feeding urea, sulfuric acid and, optionally, water as required into a reacting liquid phase contained in a reaction zone in proportions corresponding to the relative proportion of each respective component in a predetermined product composition within the ranges discussed above. The urea and sulfuric acid react within the reaction zone under controlled conditions in which reaction temperature is always maintained at a point below about 176.degree. F. and below the incipient decomposition temperature of the predetermined product. Even minor decomposition of the reactants and/or product during manufacture or otherwise results in the formation of known toxic materials including ammonium sulfamate and sulfamic acid. Thus, adequate temperature control is imperative to prevent decomposition which, once commenced in a large volume of inadequately cooled material, can lead to very rapid temperature escalation, e.g., up to 600.degree. F. and higher, and to the literal explosion of the reactor and associated processing facility. The magnitude of the reaction exotherm and incipient decomposition temperature variations are illustrated in the following table: ______________________________________ Incipient Heat of Reaction Composition Decomposition Temperature BTU's per Ton ______________________________________ 29-0-0-9 158.degree. F. 73,600 18-0-0-17 176.degree. F. 173,400 9-0-0-25 176.degree. F. 149,500 10-0-0-19 176.degree. F. 195,500 ______________________________________
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