Fertilizer Conference Proceedings
The old issue of MAP vs DAP has resurfaced. Although the renewed interest in this issue stems from manufacturing considerations, it would seem appropriate at this point to review our current understanding of agronomic characteristics of these two materials.
One of the last reviews done on this subject was part of a review of P sources written by Larry Murphy and presented at the 1979 North Central Extension Industry Soil Fertility Workshop, entitled, "MAP, DAP, Poly and Rock". The amount of new research conducted since that review is not extensive, however, some has been performed and the intent of this paper will be to incorporate the new information with the old.
The fundamental chemical differences potentially of agronomic importance between MAP and DAP are caused by the characteristics reported in Table l. The characteristics of monocalcium phosphate (MCP), which is the P form in single and triple superphosphate, are shown for reference. The high pH resulting from DAP hydrolysis and the elevated ammonium level relative to MAP create the potential for agronomic differences via the following mechanisms:
Ammonia formation from DAP. The ammonia present at any given level of ammonium is determined by pH. At a pH of 8.28, the ammonia /ammonium ratio is 0.1 while at 7.28, it is only 0.01. One can easily understand the concern for DAP since both the ammonium level and the pH are at least initially much higher than for MAP. The ammonia formed could cause seed germination problems, seedling injury or possibly interfere with root development in a P band.
Soil-fertilizer reaction product differences. Since the chemical environment of dissolving fertilizer granules of MAP and DAP differ, the minerals formed as the soil and fertilizer react could differ in solubility and result in either short-term or long-term effects on P availability to plants. These effects could be caused by alteration of P concentrations in fertilizer bands, differences in band volume due to varying mobility or by differences in P forms present in the soil solution. The lower initial pH of MAP would favor formation of H2PO4- over HPO4=. Since hydroponic studies show H2PO4 to be taken up more rapidly by plants, this could enhance MAP performance (Fig. 1).
Impurities in MAP. Since lower grade phosphate rock can be used to make MAP, MAP can contain more impurities than DAP. The impact of these impurities on agronomic effectiveness is another theoretical concern.
The remaining sections of this paper will review the literature that addresses these theoretical concerns.
Jackson et al., summarized findings of Pairintra (1973) by stating that spring wheat seedling injury in Montana from ammonium phosphate fertilizers increased in the order ammonium polyphosphate <MAP <DAP <urea ammonium phosphate. Damage from all fertilizers increased as calcium carbonate in the soil increased. Other Montana researchers have reported on MAP and DAP effects on small grain emergence and yield.
Smith et al. (1973) concluded that DAP produced more stand damage of wheat on calcareous soils when compared at equivalent N rates. They also reported lower yields for DAP when sources were compared at constant N rates. However, comparison at the same P level of at least some of their data indicates the sources gave the same yield response and the apparently lower yields with DAP were caused by the lower rate of P applied.
Later studies by Christensen et al. (1977) on irrigated barley grown on soils ranging in CaCO3 content from 0.4 to 15.1 percent again showed greater stand damage for DAP compared to MAP at the same N level.
No yield differences were measured between sources in one year, while in another year that was more conducive to ammonia damage, DAP treatments averaged 2.3 bu/a less than MAP across six locations and four N levels (Table 2). They concluded that damage from banding ammonium phosphates with barley seed could be minimized by not exceeding 20 lb N/A for 12-inch rows when soil moisture is excellent at the time of seeding.
Kansas researchers compared the winter wheat germination and seedling damage potential of MAP and DAP banded with the seed across four site years where yields were measured. Soils were slightly acid to neutral in pH and ranged from medium to very high in soil test P. Since the objective was to measure injury potential, P was broadcast on all treatments to bring the total applied up to 50 lbs/a except where more than that went on in the band. Nitrogen was also equalized at 75 lbs/a with a broadcast application. In spite of potential P response at several of the sites favoring MAP when treatments are compared at equal N rates, no difference was detected between the two sources across the four locations.
Field studies comparing seed placed MAP and DAP at a P205 rate of 20 lbs/a were conducted on two calcareous soils in North Dakota (Dahnke and Swenson, 1981). Neither durum or barley differed in their response to the P sources. Phosphorus increased yield at the site seeded to durum but not at the site seeded to barley that had a higher P soil test level.
Stevenson and Bates (1968) studied the effects of N:P atom ratio of ammonium phosphates on the emergence of wheat from an Oneida clay loam and a Fox sandy loam. Both soils were adjusted to pHs of 5.4 and 7.4. Fertilizers were compared at a constant N rate of 60 lbs/a which resulted in 300 lbs P2O5/a from MAP and 150 lbs P2O5/a from DAP in a 7" band spacing. Soil pH had no effect on emergence in either soil nor did N:P ratio in the clay loam soil. However, in the sandy loam soil, early emergence was markedly lower for DAP.
The emergence of winter wheat when DAP, MAP, OSP and CSP were banded with the seed was determined where moisture and temperature were controlled (Baker et al. 1970). The Oklahoma researcher used a Norge loam and a Meno sand for the growth chamber studies (pH not reported).
Detrimental effects of a given fertilizer treatment on emergence increased as temperature increased from 15 to 25C and as moisture tension (stress) increased from 1 to 3 bars. As expected, the detrimental effects were much greater on the sandy soil. MAP and DAP produced more injury than the other sources, especially under warm, dry conditions, but were similar to each other. Under cool moist conditions for the loam soil, rates of 20 and 40 kg P/ha (40 and 80 lbs P2O5/a) as MAP or DAP delayed emergence but did not affect final stand (7-inch rows).
Nyborg and Lopetinsky (1972) studied germination and emergence effects on rape in Alberta when DAP, MAP and TSP were placed with the seed at a rate of 20 lbs P2O5/a in a greenhouse experiment. They concluded that MAP and TSP were essentially equal in effects on rape, while DAP delayed emergence and decreased the number of surviving seedlings.
North Dakota researchers compared MAP, DAP and urea urea phosphate (UUP) using three spread types on a pneuamatic seeder where the seed and fertilizer are applied together (Deibert et al., 1985). The study was conducted on a Bearden silty clay soil with a pH of 7.9. Spread type A was the maxiumum spread and was achieved with a 12-inch sweep and deflector. Spread type B had a 6-inch spread, while spread type C had no spread and was accomplished using a spear point providing approximately a 1-inch band. Spring wheat stand reductions are shown in Figure 2. With the maximum spread, no difference was measured in stand between MAP and DAP at 39 lbs P2O5/a regardless of the urea rate applied. Spread B and C resulted in greater stand reductions due to urea, but differences between MAP and DAP were minor.
An assumption sometimes made is that due to the hydrolysis of DAP in soils, the N in DAP should be treated like urea N when determining the maximum rate for placement with the seed. Research conducted by Creamer and Fox (1980) indicate that such a recommendation may be an over-reaction to the ammonia hazard of DAP at least for acid or near neutral soils. They measured a pH of 9.1 for urea at a rate of 89 lbs N/a applied at a row spacing of 30-inches, while the same conditions with DAP produced a pH of only 7.3.
Also, pH was elevated substantially to a distance of 2-inches for urea while the effect for DAP extended only to approximately 1 inch. The authors pointed out that at a pH of 9.l, 40 percent of the total ammoniacal N would be present as ammonia, while 7.3 free ammonia is only 1.4 percent. One needs to also consider that the DAP rate used in this comparison would contain 121 lbs P2O5/a, much higher than would normally be placed in a band. The authors concluded that the ammonia toxicity potential for DAP is much lower than urea under equivalent conditions, and that there is little or no danger of nitrite toxicity developing around banded DAP.
Ammonia injury to corn from banded MAP and DAP was studied in the laboratory by Allred and Ohlrogge (1964) on a Oaktown fine sand and a Fincastle silt loam. The rates used were equivalent to 254 lbs P2O5/a for MAP and 212 lbs P2O5/a for DAP in 30-inch rows. At these very high rates, root development near MAP bands was more pronounced than near DAP bands. After 10 days, roots were living in the salt zones of MAP bands in both soils while no roots were observed within any of the DAP bands although roots were closer to the DAP bands in the silt loam than in the sand. A distinct ammonia odor was noticed at the termination of the experiment (10 days) around the DAP bands in the sandy soil, while no such odor was detected in the silt loam. Ammonia resulting from DAP hydrolysis rapidly permeated the soil pore space to distances as great at 3 inches from the band.
Harapiak and Beaton (1986) used urea and ammonium nitrate to determine if ammonia formation in a band could affect P availability as indicated by relative 32P activity in barley plant material resulted from MAP dual-applied with N. They measured a depressing effect of the urea.
The authors suggested that too much urea banded with the P source elevated ammonia levels and decreased root penetration into the band. The research of Allred and Ohlrogge and Creamer and Fox is supportive of their theory. The latter observed killed root tips from banded urea and some damage when DAP and ammonium nitrate were banded together at a rate of 121 lbs P2O5/a and 89 lbs N/a. However, these data suggest that the amount of ammonia potential for DAP at typical P rates is not large enough to in itself cause P availability problems due to root exclusion.
SUMMARY: Ammonia injury of wheat and barley from DAP banded with the seed appears to be rare on neutral and acid soils. DAP injury was detected on a sandy loam soil at a rate of 150 lbs P205/a, much higher than would be normally used. Injury from DAP on calcareous soils is more common, however, it can generally be avoided for wheat and barley by not exceeding approximately 20 lbs N/a on a 12-inch row spacing equivalent. Warm, dry conditions tend to increase injury from both DAP and MAP. Yield reductions from ammonia injury are less common than emergence or stand effects.
Some evidence suggests that free ammonia resulting from high fertilizer rates applied in a band, whether from DAP or urea, could temporarily delay root access to the banded P. This does not appear to be a problem at normal P source rates. Guidelines on maximum N rates for placement with the seed that consider local crop, soil, weather and cultural factors have been developed by nearly all states and provinces.
One of the earliest studies on the solubility and mobility of P sources was conducted by Bouldin and Sample (1959) using fertilizer pellets containing 15 mg of P each placed in a Hartsells fine sandy loam (pH=5.2) from Alabama or a calcareous Webster silty clay loam (pH=8.3) from Iowa. After 3 or 5 weeks, both MAP and DAP had moved about 2.5 and 1.5 cm from the center of the pellets with the Hartsells and Webster soils respectively. In the Hartsells soil, water soluble P near the pellets was initially higher for DAP than MAP or MCP, but was similar by the end of five weeks. In the Webster soil, soluble P was markedly less for DAP than for MAP or MCP. Based on these solubility data and on greenhouse uptake studies, the authors gave the following rankings of the fertilizers as P sources. Hartsells: DAP>MAP>MCP; Webster: MCP=MAP >>DAP .
The reaction products of MAP and DAP along with other fertilizer were studied by Lindsay et al. (1962) and reported in a classic paper that led to numerous other investigations. In their experiments, saturated solutions of the fertilizers being studied were shaken with soil for various periods of time. The MAP and DAP studies were done on a Hartsells fine sandy loam (pH=4.9) from Tennessee and on a calcareous Gila loam (pH=8.5) from Arizona.
The major reaction product of MAP in the acid soil was (NH4)3Al5H6(PO4)8*18H2O (taranakite) while in the calcareous soil CaHPO4*2H2O(DCPD) and MgNH4PO4*6H2O (struvite) were the dominate products. DAP produced struvite in both soils.
DAP also formed NH4Al2(P04)2OH*8H20 and a colloidal phase in the acid soil and formed Ca2(NH4)2(HPO4)3*2H2O and colloidal apatite in the calcareous soil. Therefore, they verified that the reaction products can differ between the fertilizer and that the specific products formed are influenced by soil characteristics.
Bell and Black (1970) used a different approach to study the reaction products from MAP and DAP forming in soils ranging in CaCl2 pH from 6.2 to 7.8 and from 0 to 22 percent CaCO3. They believed that field conditions were better simulated if a band of fertilizer was placed in the soil, and the reaction products formed in various zones out from the band determined. The saturated fertilizer solution approach tends to overemphasize the initial characteristics of the fertilizer. Their results differed from those of Lindsay et al. discussed above. After four weeks of incubation with MAP, DCPD was the only reaction product identified for all U.S. soils, and it persisted for the 48-week duration of the experiment.
For DAP in the zone of highest pH and P concentration immediately next to the fertilizer Ca(NH4)2(HPO4)2*H2O occurred at four weeks in soils of moderate labile Ca levels. By the end of 16 weeks, this compound had been largely replaced by DCPD and/or Ca8H2(PO4)6*5H2O(OCP) which persisted for the duration of the study. In high Ca soils, OCP formed initially and no ammonium-containing phosphate phase was detected. Since the solubility of OCP is lower than DCPD, its immediate formation in fertilizer bands of high Ca soils could be detrimental to initial early season P uptake. However, one could speculate that its quick formation could decrease the thermodynamic driving force for the formation of even less soluble calcium phosphates such as tricalcium phosphate or apatite and increase the long term effectiveness of the fertilizer.
The mobility of DAP, as well as triammonium pyrophosphate (TPP) and ammonium polyphosphate (APP), was evaluated by Khasawneh et al. (1974) using a Hartsells fine sandy loam form Alabama with a limed pH of 6.0. The distribution profile of DAP at an equivalent rate of 174 lbs P2O5/a at a 30-inch band spacing shows movement to approximately 3.5 cm at one week and 5 cm after four weeks. The researchers also pointed out that for the P sources studied, the extent of movement was influenced more by initial soil moisture content than source of P (greater the moisture content, greater the movement).
SUMMARY: Reaction product investigations do not in themselves make unequivocal statements about likely agronomic performance differences between MAP and DAP. They do suggest that in near neutral and acid soils differences due to reaction products are not likely. In calcareous soils, greater immediate availability is indicated for MAP, while long term differences are harder to predict. If colloidal apatite forms from DAP as indicated by Lindsay et al., effectiveness of DAP would be less than MAP. However, if OCP is the primary phosphate forming as reported by Bell and Black, DAP may actually have greater long term availability than MAP.
Beaton and Read (1963) evaluated the effects of soil-fertilizer reaction period temperature and moisture on short-term P uptake from a calcareous Saskatchewan soil (pH=7.7, CaCO3=6.6 percent) treated with several pelleted P sources. After one week of incubation in their growth chamber study, uptake by oats was greater from MAP than from DAP, however, after seven weeks of incubation differences between MAP and DAP were minor. The advantage of MAP over DAP increased as the temperature of the soil-fertilizer incubation period increased. The investigators attributed the difference in uptake to the assumed formation of reaction products such as colloidal apatite, octacalcium phosphate and struvite by DAP which would be less available in this soil than the DCPD formed by MAP.
Amer et al. (1980) evaluated in a greenhouse study the solubility and plant availability of DAP and monocalcium phosphate (MCP) on two soils from Egypt, a silty clay alluvium containing 4.1 percent CaCO3 and a silt loam desert soil containing 39.0 percent CaCO3. At a rate of 227 lbs P2O5/a, MCP and DAP gave similar P uptake for both soils, but at the 454 lb rate MCP resulted in more P in the plant than DAP. The authors attributed the difference to P immobilization induced by ammonia volatilization from DAP at the high rate.
Lu et al. (1987) compared DAP to single superphosphate (SSP) plus urea in a short term greenhouse study of a Vernon clay subsoil from Texas (pH=8.0, CaCO3=20 percent). Rates included varied from 123 to 981 lbs P2O5/a. The DAP treatments were inferior to the SSP treatments in P uptake and Olsen P whether the fertilizer was broadcast, deep banded or incorporated. Soil test levels for the incorporated placement were substantially lower for DAP, especially at the higher rates.
Zagorodnyi et al. (1970) compared MAP and DAP on irrigated soils of the west central USSR and concluded that under the conditions of their experiments, MAP and DAP were equal in their ability to supply P for a number of crops. Other USSR investigators have drawn the same conclusions in a number of reports published from 1968 to 1976 (Murphy, 1979).
MAP and DAP were compared in a starter band (2 inches below, 2 inches beside the row) for corn for three years in Michigan on a Charity clay at a rate of 200 lbs of material per acre (Yerokun and Christenson, 1989). No differences between MAP and DAP were found in the concentration of N,P, Ca, Mg, Zn or Mn in whole plants at the four-leaf stage or in ear leaves. Also, no yield differences were detected.
Soil solution P concentration (Cli) and diffusable P (Csi) were measured by Ernani and Barber (1989) in a Raub silt loam at pH 5.6 where MAP and DAP had been incubated moist for 30 days at a rate of 100 mg/kg soil. Values of Cli an Csi did not differ for the two sources. The researchers used the Barber uptake model to predict plant P uptake during the 30 day period and again found no differences between the sources.
SUMMARY: Growth chamber studies tend to support the predictions from the reaction product studies. In calcareous soils, especially at very high P rates, P availability from DAP has been less than MAP or MCP. However, in field comparisons with lower P rates, studies discussed in this section as well as those form the ammonia section tend to show similar plant availability for MAP and DAP.
Hossner and Richards (1968), working with a soil at pH 7.0 observed no difference in Mn uptake between DAP and MAP when no Mn was added to the P band, however, when 25 ppm Mn was added, both MAP and DAP increased Mn uptake but MAP had a greater effect than DAP.
Randall et al. (1975) compared the effects of MAP and DAP on soil Mn availability to soybeans on a Sebewa loam at pH 6.1 and, unlike Hossner and Richards, noted that DAP was more effective than MAP in correcting Mn deficiency. The higher pH, higher P rate (50 ppm P) and shorter growth period of the Hossner and Richards study might explain the difference in outcome.
Voth and Christenson (1980) conducted several experiments evaluating the effects of fertilizer sources on Mn availability. They conducted an incubation study using a Tappan loam soil and measured pH and DTPA extractable Mn in bands where MAP or DAP, KCl and Urea had been applied. After three weeks, sufficient nitrification had taken place to significantly depress pH in the band for both treatments below that of the check. In this study, MAP and DAP did not differ in reaction zone pH at three, five or seven weeks of incubation. Mn was elevated for both sources throughout the incubation including DAP at the three-week sampling. In other work by the researchers, sugar beet and soybean field studies at sites testing near a pH of 8.0 showed that tissue Mn concentrations were higher five to six weeks after planting when MnSO4 was band applied with MAP than when applied with DAP at a rate of 50 lbs P2O5/a (Christenson, 1981).
SUMMARY: Band application of either MAP or DAP generally increases Mn availability to crops. Whether one source is better than the other at increasing Mn appears to be unresolved.
The greater level of impurities in MAP compared to DAP has led to recent investigations on the impact of MAP impurities on agronomic properties. Sikora et al. (1989b,c) studied the water-insoluble fractions of three MAP (11-52-0) fertilizers. The fertilizers were produced from North Carolina,, Florida and Idaho phosphate rock. In greenhouse comparisons, they found that the water-insoluble fractions were inferior P fertilizers, however, no difference was found between the total P fertilizers nor did they differ in bioavailability from reagent grade MAP.
The investigators also characterized the impurities present in the three MAP fertilizers. In a later paper, the solubility products of the five impurities identified were determined (Sikora et al.,1989a). The order of P solubility at pH 6.5 was AlNH4HPO4F2 > MCP > MgNH4PO4 = FeNH4 (HPO4) 2 > MgAl(NH4)2H(PO4)2F2 > variscite > = strengite > DCPD = DCP > hydroxyapatite.
This sequence indicates that the impurity compounds in MAP are more soluble than the compounds that normally form in soils such as DCPD, DCP, variscite and strengite. Therefore, the impurity compounds may not be as adequate a source of P as MAP or MCP, but they will supply P to plants over time as the P reverts to less soluble soil compounds.
Mullins and Sikora (1989) in a field study evaluated nine MAP sources that varied from 81 to 97 percent water-soluble P. Their findings were similar to the greenhouse investigations since no meaningful differences in yield or P uptake between the sources were detected.
SUMMARY: It does not appear that MAP impurities pose any agronomic problem.
Availability of MAP and DAP, under the field conditions in which they are normally used, appears to be similar. As indicated by Murphy in 1979, more information on the residual effects of various P materials, including MAP and DAP, over long time spans could be useful data. The impact of organic matter on long term source differences might also be of value.
Table 1. Characteristics of saturated solutions of three phosphate fertilizers at 250C (Lindsay et al., 1962).
Table 2. MAP produced slightly higher irrigated barley yields then DAP one year in Montana but gave the same yields in another year
Figure 1. Influence of pH on solution orthophosphate form and relative uptake of Paz from a nutrient solution by field bean shoots (Hendrix, 1967).
Figure 2 . Spring wheat stand reduction as influenced by N and P source and rate for three spreader types (Deibert et al., 1985).
Numbered as 4 in print version
Allred, S.E. and A.J. Ohlrogge. 1964. Principles of nutrient uptake from fertilizer bands. VI. Germination and emergence of corn as affected by ammonia and ammonium phosphate. Agron. J. 56:309-313.
Amer, Faith, M.S. Shams, K.M. Awad, and M.A. Khalil. 1980. Immobilization of diammonium phosphate and monocalcium phosphate in calcareous soils. Soil Sci. Soc. Am. J. 44:1174-1178.
Baker, J.M., B.B. Tucker, and L.G. Morrill. 1970. Effects of sources of phosphorus under varying soil temperature and soil moisture regimes on the emergence of winter wheat. Soil Sci. Soc. Am. Proc. 34:694-697.
Beaton, J.D. and D.W.L. Read. 1963. Effects of temperature and moisture on phosphorus uptake from a calcareous Saskatchewan soil treated with several pelleted sources of phosphorus. Soil Sci. Soc. Am. Proc. 27:61-65.
Bell, L.C. and C.A. Black. 1970. Crystalline phosphates produced by interaction of orthophosphate fertilizers with slightly acid and alkaline soils. Soil Sci. Soc. Am. Proc. 34:735-740.
Bouldin, D.R. and E.C. Sample. 1959. Laboratory and Greenhouse studies with monocalcium, monoammonium, and diammonium phosphates. Soil Sci. Soc. Am. Proc. 23:338-342.
Christensen, N.W., J.D. Franklin, and C.M. Smith. 1977. Comparisons of different ammonium phosphates--influence on seedling injury, growth, and yield of irrigated barley. Proc. 28th Ann. Northwest Fert. Conf., Twin Falls, Idaho. p. 121-131.
Christenson, D.R. 1981. Unpublished data. Dept. of Crop and Soil Sciences. Michigan State University, East Lansing, MI.
Creamer, F.L. and R.H. Fox. 1980. The toxicity of banded urea or diammonium phosphate to corn as influenced by soil temperature, moisture, and pH. Soil Sci. Soc. Am. Proc. 44:296-300.
Dahnke, W.C. and L. Swenson. 1981. Comparison of diammonium and monoammonium phosphate. Soil Science Dept. Annual Rept., North Dakota State University.
Deibert, E.J., D.A. Lizotte and B.R. Bock. 1985. Wheat Seed Germination as influenced by fertilizer rate, fertilizer source, and spreader type with one-pass pneumatic seeding-fertilizing. North Dakota Farm Research 42(6):14-20.
Ernani, Paulo and S.A. Barber. 1989. Unpublished data. Dep. of Agronomy, Purdue University, West Lafayette, IN.
Halvorson, A.D., M.M. Alley, and L.S. Murphy. 1987. Nutrient requirements and fertilizer use. _In Wheat and wheat improvement, Agronomy Monograph No. 13 (2nd Edition). ASA-CSSA-SSSA, 677 Segoe Road, Madison, WI 53711.
Harapiak, J.T. and J.D. Beaton. 1986. Review Phosphorus fertilizer considerations for maximum yields in the Great Plains. J. Fertilizer Issues 3:113-123.
Hendrix, J.E. 1967. The effect of pH on the uptake and accumulation of phosphate and sulfate ions by bean plants. Amer. J. Bot. 54:560-564.
Hossner, L.R. and G.E. Richards. 1968. The effect of phosphorus source on the movement and uptake of band applied manganese. Soil Sci. Soc. Am. Proc. 32:83-85.
Jackson, T.L., A.D. Halvorson, and B.B. Tucker. 1983. Soil fertility in dryland agriculture. in Dryland agriculture, Agronomy monograph no. 23. ASA-CSSA-SSSA, 677 Segoe Road, Madison, WI 53711.
Khasawneh, F.E., E.C. Sample, and Isao Hashimoto. 1974. Reactions of ammonium ortho- and polyphosphate fertilizers in soils. Soil Sci. Soc. Am. Proc. 38:446-451.Khasawneh, F.E., E.C. Sample, and Isao Hashimoto. 1974. Reactions of ammonium ortho- and polyphosphate fertilizers in soils. Soil Sci. Soc. Am. Proc. 38:446-451.
Lindsay, W.L., A.W. Frazier, and H.F. Stephenson. 1962. Identification of reaction products from phosphate fertilizers in soils. Soil Sci. Soc. Am. Proc. 26:446-452.
Lu,D.Q., S.H.Chien, J. Henao, and D. Sompongse. 1987. Evaluation of short-term efficiency of diammonium phosphate versus urea plus single superphosphate on a calcareous soil. Agron. J. 79:896-900.
Lundquist, M., P. Gallagher, M. Wagger, and D.E. Kissel. 1980 MAP-DAP-UAPP Band applied to winter wheat. Kans. Pert. Res. Rept. Prog. 389:26-27.
Moore, W., C.W. Swallow, M. Lundquist, M. Wagger, and D.E. Kissel. 1979. MAP-DAP-UAPP band applied to winter wheat. Kans. Pert. Res. Rept. Prog: 42-44.
Mullins, G.L. and F.J. Sikora. 1989. Field evaluation of commercial monoammonium phosphate fertilizers. Pert. Res.: Submitted
Murphy, L.M. 1979. MAP, DAP, POLY and Rock. Proc. North Central Extension Industry Soil Workshop. St. Louis, MO.
Nyborg,M. And K. Lopetinsky. 1972. Effects of phosphorus sources on germination and emergence of rapeseed. Univ. of Alberta. Report to Western Co-op Fertilizers, Ltd. (Mimeographed).
Pairintra,C. 1973. Influence of NH3 from phosphate fertilizers on germination, seedling growth and small plant yield of wheat (Triticum aestivum L.).Ph.D. Dissertation. Montana State Univ., Bozeman. (Diss.Abstr. 73:27,490).
Sikora,F.J., J.P. Copeland, and G.L. Mullins. 1989a. Solubility products of impurity P compounds in monoammonium phosphate fertilizers. Pert. Res. :In preparation.
Sikora,F.J., J.P. Copeland, G.L. Mullins and J.M. Bartos. 1989b. Phosphorus dissolution kinetics and bioavailability of phosphorus in the water-insoluble fractions of monoammonium phosphate fertilizers. Soil Sci. Soc. Am. J. (in preparation).
Sikora,F.J., E.F. Dillard, and J.P Copeland. 1989c. Chemical characterization and bioavailability of phosphorus in the water-insoluble fractions of three mono-ammonium phosphate fertilizers. J. Assoc. Off. Anal. Chem.: In press.
Smith, C.M., C. Pairintra, ad E.O. Skogley. 1973. Comparisons of different ammonium phosphates -- influence on germination, seedling injury, and yield of wheat. Proc. 24th Ann. Pacific Northwest Pert. Conf., Pendleton, OR. p. 115-121.
Stevenson, C.K. and T.E. Bates. 1968. Effect on nitrogen to phosphorus atom ratio of ammonium phosphates on emergence of wheat (Triticum vulgare).Agron. J. 60: 493-495.
Voth, R.D. and D.R. Christenson. 1980. Effect of fertilizer reaction and placement on availability of manganese. Agron. J. 72:769-773.
Yerokun,O.A. and D.R. Christenson. 1989. Use of urea and ammonium phosphates as starter fertilizers. J. Fertilizer Issues 6:12-16.
Zagorodnyi,G.P. and Z.M. Magomedov. 1970. Comparative effectiveness of different forms of phosphorus fertilizers on irrigated meadow-chestnut soils of the Dagestan plain. Fert. Abst. 5:1752.
1Paul E. Fixen is Northcentral Director, Potash & Phosphate Institute, Box. 682, Brookings, S.D.
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