Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 155-161
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Solubilities and Stabilities of the Nitrogen Loss Inhibitors Dicyandiamide, Thiourea, and Phenyl Phosphorodiamidate in Fluid Fertilizers Joe Gautney," Yong K. Kim, and Angela R. Barnard Division ofChemicai Development, National Fertllizer Development Center, Tennessee Valley Author@, Muscle Shoals Alabama 35660
The solubiiiities and stabilities of the nitrogen loss inhibitors dicyandiamide (DCD), thiourea (TU), and phenyl phosphorodiamidate (PPDA) were determined in a w!de variety of fluid fertilizers at room temperature (25 "C). Fertikers studii Included phosphate bases (10-34-0,9-32-0,13-3&0, and 10-30-0), nitrogen bases (31-0-0,36-0-0, and 29-0-0-58), and high-nitrogen N-P20,-K20 formulations (14-14-14, 24-8-0, 18-9-9, and 21-7-7). The results showed that the nitrlflcatlon inhibitor DCD is soluble and stable enough for use with the nitrogen bases and fluid formulations; however, low solubility and some instability (half-lives from 98 to 307 days) will hamper its use in the phosphate bases. Thiourea, a nitrification and a urease inhibitor, can be used with all the fertilizers studied. Phenyl phosphorodiamklate, a urease inhibitor, can be used in the nitrogen bases and fluid formulations, but these fluids must be applied soon after PPDA addition to prevent significant inhibitor losses by decomposition.
Introduction Loss of fertilizer nitrogen from cropland is a worldwide problem. Of the three primary plant nutrients (N, P, and K), nitrogen is used in the largest quantities and with the least efficiency. It is estimated worldwide that only about 40% of the applied fertilizer nitrogen is recovered by crops (Coolie, 1984). Cooke also estimates that doubling the world nitrogen efficiency from 40 to 80% would save approximately 11 billion dollars annually and conserve nonrenewable natural resources. The use of nitrogen loss inhibitors (nitrification and urease inhibitors) is one approach to the problem of low nitrogen efficiency. Three compounds which have recently attracted much attention for use as nitrogen loss inhibitors are dicyandiamide (DCD), thiourea (TU), and pbehyl phosphorodididate (PPDA). Dicyqndiamide is a nitrification inhibitor (Hauck and Behnke, 1981). Phenyl phosphorodiamidate iq a urease inhibitor (Barth et al., 1980;Beber et al., 1979;Byrnes et al., 1983),and TU is bath a nitrification (Malhi et al., 1979)'and a urease inhibitor Walhi apd Nyborg, 1979). As shown in Table I, these three inhibitors have much higher melting points and, as a result, lbwer vapor pressures than the commercially available (trichloromethy1)pyridine nitrification inibitor 2-chlor0-6or nitrapyrin (manufactured by Dow Chemical Co. under $he trade name N-Serve) (Page and Hoffman, 1982;Dow Chemical USA) and the nitrififation inhibitor 5-ethoxy3-(trichloromethyl)-l,2,4-thiad1azole or E" (prevjously manufactured by Olin Corp. under the trade name Dwell; the label and manufacturing rights of ETT were purc%ased by Uniroyal, Inc., on Oct 1,1983.'Uniroyal is not presently marketing Dwell.) (Evrad et al., 1982). Dicyandiamide and TU also uontain substantial amounts of nitrogen, which will be use3 to some extent by the crop after inhibitor breakdown in the soil. No urease inhibitors are currently commercially available. The re@s of Tennessee Valley Authority (TVA) studies to determine the feasibility of cogranulating the nitrogen loss inhibitors DCD, TU, and PPDA with urea were recently published (Gautney et al., 1983,1984). The Wbitors DCD, TU, v d PPDA also have potential for use in fluid fertilizers. Therefbre; the solubilities and stabilities of these inhibitors were determined in a wide variety of fluid fertiliqers at room teknperature (25 "C).
Experimental Section The DCD and TU used in the testa were reagent grade. The PPDA used was prepared by recrystallization or by washing reagent grade PPDA with 95% ethanol. The fertilizers studied included phosphate bases (10-34-0, 932-0,13-38-0, and 10-30-0),nitrogen bases (31-0-0, 36-0-0, and 29-0-0-58), and high-nitrogen N-P,O,-K,O formulations (14-14-14, 24-8-0,18-9-9,and 21-7-7).The compositions of these fertilizer materials are given in Table 11. The 10-34-0and 13-38-0are common phosphate bases. The 9-32-0is a relatively new fertilizer which is currently under development at TVA (Mann et al., 1983). The 1030-0has become popular in recent years because it can be easily prepared at the dealer level by dissolution and ammoniation of granular monoammonium phosphate (MAP). The 31-0-0is urea-ammonium nitrate (UAN) solution which has been in wide use for many years. The 36-0-0 and 29-0-0-5sare UAN and urea-ammonium sulfate suspensions, respectively, which are new nitrogen bases currently being developed at TVA (Jones et al., 1983). The fluid formulations are common fertilizers which are prepared by mixing a phosphate base fluid with a nitrogen base fluid, H20, and KC1 (if potassium is a desired component). The fluid formulations used in our study were prepared using 9-32-0as the phosphate base and 31-0-0 as the nitrogen base. Solubility measurements with DCD and TU were made by preparing fertilizer samples with increasing amoupts of the inhibitors, 'allowing the samples to equilibrate on a shaker for 1 day, and then analyzing a sample of the solution phase for inhibitor. Clear samples of the solution phase were obtained by f i t centrifuging and then filtering the samples. Saturation was assumed when constant values were obtaingd for the samples with increased amounts of inhibitor. The time interval between repeat analyses was at least 1 day. The stability tests with DCD and TU were made with unsaturated solutiqns of the inhibitors. After inhibitor addition, the fertilizer solutions were shaken for 1 day, placed on g shelf, and then a portion of the entire sample (both solid and liquid phases) was analyzed periodically for inhibitor. The procedures for the solubility and stability measurements with PPDA in 31-0-0were the same as those
This article not subject to U S . Copyright. Published 1985 by the American Chemical Society
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Ind. Eng. Chern. Prod. Res. Dev., Vol. 24, No. 1, 1985
Table I. Nitrogen Loss Inhibitors urease inhibitor
nitrification inhibitor
dicyandiamide (DCD)
no
yes
211
66.6
thiourea (TU)
Yes
Yes
178
36.8
phenyl phosphorodiamidate (PPDA)
Yes
no
193-195
10.3
nitrapyrin
no
yes
62-63
6.1
no
Yea
structural formula
name
NH)C=N-CIN
?P> C
% N
NH2
fi
NH~-C--NH~
0 /I/NHZ
e o - .
N -C
'NH~
