Ind. Eng. Chem. Prod. Res. Dev. 1084, 23, 483-489
483
Feasibility of Cogranulating the Nitrogen Loss Inhibitors Dicyandiamide, Thiourea, Phenyl Phosphorodiamidate, and Potassium Ethyl Xanthate with Urea Joe Gautney,' Yong K. Klm, and Patrlck Y. Gagen Division of Chemical Development, Natlonal Fertilizer Development Center, Tennessee Valley Authority, Muscle Shoals, Alabama 35660
Laboratory tests were conducted to determine the feasibility of cogranuiating the nitrogen loss inhibitors dicyandiamide ( E D ) , thiourea (TU), phenyl phosphorodiamidate (PPDA), and potassium ethyl xanthate (PEX) with urea. Tests were conducted to determine the stabilities, solubilities, and dissolution rates of the inhlbitors in urea melts and urea solutions, the effect of inhibitor addition on the rate of loss and qualitative composition of volatiles evoking from the urea melts, the melting point diagrams for the urea inhibitor systems, and the heats of fusion for the urea-inhibitor mixtures. The tests indicate that DCD and TU can be cogranulated easily with urea. Phenyl phosphordiamidatealso can be cogranuiated with urea, but not wlthout small losses of PPDA resulting from PPDA decompositionto phenol. Cagranulation of E X with urea is not feasible because this compound decomposes rapidly in urea meits.
Loss of fertilizer nitrogen from cropland represents an economic loss far both the nation's farmers and consumers. It is estimated that only 45-50% of the applied fertilizer nitrogen is used by crops. Half of the other 50% is converted to soil organic nitrogen, and the other 25% is lost by leaching and denitrification reactions (Page and Hoffman, 1982; Varsa and Huber, 1983; Huber, 1980). In the United States, this loss of nitrogen represents an annual loss of approximately 200 trillion Btu's of energy, with an economic value of 700 million dollars at an average natural gas price of $3.50/million Btu. The leaching and denitrification losses occur when using nitrogen in the ammonium (NH4+)form because ammonia in the soil is oxidized by nitrosomonas and nitrobacter bacteria to nitrite (NO2-) and nitrate (NO3-) forms. nitrosomonas
NH4+
02
nitrobacter
NOz-
0.502 ___+
NOS-
(1)
Since the soil particles are negatively charged, the positively charged ammonium cation is attracted to the soil particles and held in the root zone where it can be used by the crop. Once converted to the negatively charged nitrite and nitrate forms, however, the nitrogen is susceptible to loss by leaching and denitrification reactions (conversionof nitrate to gaseous nitrogen compounds, NO,, which are lost to the atmosphere). Nitrogen losses also occur when using urea or nitrogen fertilizers containing urea (especially surface-applied granular urea) because of rapid hydrolysis of urea to ammonia.
This rapid hydrolysis, which is catalyzed by the enzyme urease, leads to a high pH in the microsite around the urea granule and a resultant loss of nitrogen as gaseous ammonia. Losses of ammonia from surface-applied urea nitrogen can be substantial. Losses of up to 61% have been reported (Terman, 1979). The ammonia nitrogen from urea hydrolysis also is susceptible to losses because of nitrification.
Nitrogen loss inhibitors (nitrification and urease inhibitors) can improve the efficiency of nitrogen fertilizers by delaying the nitrification of ammonia nitrogen and the hydrolysis of urea nitrogen. In addition to conserving nitrogen, use of nitrogen loss inhibitors provides several other potential benefits. Among these benefits are reduced ground water pollution, higher yields, improved crop quality, and, most importantly, use as time management tools, allowing the farmer to apply fertilizer more at his convenience while ensuring that adequate nitrogen is available as needed by his crops. The current trend in farming practices toward reduced tillage and no-till will make use of nitrogen loss inhibitors more attractive in the future. Studies indicate that nitrogen losses as a result of leaching and denitrification will be greater for no-till than for conventional tillage systems (Wells, 1982). In addition, surface application of urea without loss due to ammonia volatilization would certainly be beneficial under reduced tillage and no-till systems. Currently only two nitrogen loss inhibitors, both of which are nitrification inhibitors, are available commercially in the United States. These inhibitors are 2-chlor6-(trichloromethyl) pyridine or nitrapyrin, which is manufactured by Dow Chemical Co. under the tradename N-Serve, and 5-ethoxy-3-(trichloromethyl)-l,2,4thiadiazole or ETT, which was previously manufactured by Olin Corporation under the tradename Dwell. (Label and manufacturing rights were purchased by Uniroyal, Inc., on Oct 1,1983. Uniroyal is not presently marketing Dwell.) The chemical formulas and some of the properties of nitrapyrin and ETT are shown in Table I. Nitrapyrin and ETT are used primarily with anhydrous ammonia. These inhibitors also can be used with aqueous ammonia, liquid fertilizers, and solid fertilizers such as urea, but in moat cases the fertilizer must be applied and incorporated immediately after inhibitor addition to avoid losses due to inhibitor decomposition and volatilization (Dow Chemical USA; Bremner et al., 1978; Emerson, 1982; Evrad et al., 1982). Volatilization losses are more of a problem than losses due to decomposition because both nitrapyrin and ETT have relatively high vapor pressures mmHg at 23 OC and 2 X mmHg at 25 "C, (2.8 X respectively). Volatilization losses are a particular problem
Thls artlcle not subject to US. Copyright. Published 1984 by the Amerlcan Chemical Soclety
484
Ind. Eng. Chem. Prod.
Res. Dev., Vol.
23, No. 3, 1984
Table I. Nitrogen Loss Inhibitors
STRUCTURAL FORMULA NH2
'c=
N-c 5 N
NH2'
s
NH2 - C - NH2
Q
NAME
UREASE INHIBITOR
NITRIFICATION INHlBlTOR
NO
YES
21 I
66.6
YES
I78
36.8
-
16.3
DICYANDIAMIDE ( DCD 1
THIOUREA (TU)
0 ,NH2
-0-P \NH2
:
C2H5-0-C- SK
PHENYL PHOSPHORODI AMIDATE ( P P D A )
POTASSIUM ETHYL XANTHATE ( PEX )
YES
MELTING PT , O C
YES
NO
193 I95
NO
YES
225 226
NlTRAPYRlN
NO
ETT'
NO
-
YES
%N
0
62-63
6.I
