From mid August through October, fertilizer applicators and grain drills are running across the southern great plains wheat ground. Di-ammonium Phosphate (DAP 18-46-0) is maybe the most popular form of phosphorous (P) utilized in wheat production today. DAP delivers a high content of nitrogen (N) while efficiently binding the toxic aluminum (Al) ions low pH soils. However due to the flooding that occurred throughout the spring the Ports have been closed and DAP could be in short supply. I have heard that many companies and Co-ops have already sourced Mono-ammonium phosphate (MAP 11-52-0) to supplement the lack of DAP for our early planted wheat crop. With this happening, I wanted to share some points about the two sources. In a broadcast incorporated scenario I do not have much preference for one over the other. It is when producers are applying the fertilizer in-furrow or applying to alleviate Al toxicity that source can matter. As described in the post below, in a soil with a neutral pH DAP and MAP perform equally well. So in this scenario I give MAP a slight edge over DAP if the price is the same. I say this as you can run less material per acre with MAP and refill a little less often. Many worry about the drop in N delivered with MAP versus DAP but in my work I see that is the P in the starter that gives us a good response and not the N, which can be delivered with pre-plant or top-dress. However, DAP wins out in soils with a pH below 5.5. The original blog below shows the results when DAP and MAP are banded in acidic soils using the same rate of P per acre.  The reason we see this happen is that when MAP dissolves it forms a slightly acidic solution (approx 4.0) while DAP will dissolve to form a slightly basic solution (approx 7.5). In our work BOTH DAP and MAP increased yield above the check in acidic soils, which goes to show MAP is an affect tool for short term remediation of aluminium toxicity (Band P for Al toxicity Blog). However it might require more MAP per acre to reach the equivalent results. Because of what we saw when comparing the two sources at equivalent rates of P, I would recommend increasing the rate of P2O5 from 30 lbs per acre to 35 or 40. This would be going from 65 lbs of DAP to 67-77 lbs of MAP per acre.

So the take home would be as this wheat season takes off and you find that DAP is hard to locate and you A) Have overall neutral (5.5+) pH levels do not hesitate using MAP. Run the normal amount of material getting a few extra lbs of P or apply less material to get the same amount of phosphate. B) Have a acidic situation and are banding to alleviate aluminum toxicity use the same amount of material or a little bit more.  Keep in mind in acidic soils with a low soil test P level you have to apply enough phosphate to take care of the Al and enough to take care of the P deficiency. Note the results of the NPKS wheat response strip (NPKS BLOG)

Original Post Published July 18, 2016
DAP vs MAP, Source may matter!

Historically the two primary sources of phosphorus have had different homes in Oklahoma. In general terms MAP (11-52-0) sales was focused in Panhandle and  south west, while DAP (18-46-0) dominated the central plains.  Now I see the availability of MAP is increasing in central Oklahoma. For many this is great, with MAP more P can be applied with less material. which can over all reduce the cost per acre. There is a significant amount of good research that documents that source of phosphorus seldom matters. However this said, there is a fairly large subset of the area that needs to watch what they buy and where they apply it.

If you are operating under optimum soil conditions the research shows time and time again source does not matter especially for a starter.  In a recent study just completed by OSU multiple sources (dry, liquid, ortho, poly ect ect) of P were evaluated.  Regardless of source there was no significant difference in yield.  With the exception of the low pH site. The reason DAP was so predominate in central Ok, soil acidity.  See an older blog on Banding P in acidic soils.

Figure 1. The cover of an extension brochure distributed in Oklahoma during the 1980s.

When DAP is applied, the soil solution pH surrounding the granule will be alkaline with a pH of 7.8-8.2. This is a two fold win on soil acidity aka aluminum (Al) toxicity.  The increase in pH around the prill reduces Al content and extends the life of P, and as the pH comes back down the P ties up Al and allows the plant to keep going. However, the initial pH around the MAP granule ranges from an acid pH of 3.5-4.2.  There is short term  pH change in the opposite direction of DAP, however the the Al right around the prill becomes more available and in theory ties up P even faster.

Below is a table showing the yield, relative to untreated check, of in-furrow DAP and MAP treatments in winter wheat.  The N401 location had a ph 6.1  while Perk (green) has a pH of 4.8.  At Perkins in the low pH, both forms of P significantly increased yield, almost 20 bushel on the average.  DAP however was 5 bushel per acre better than MAP. At the N40 site the yield difference between the two sources was 1 bushel.

Relative yield winter wheat grain yield MAP and DAP both applied at equal rates of P (32 lbs P2O5 ac) when compared to a untreated check.

In general it can be said that in acid soils DAP will out preform MAP while in calcareous high pH soils MAP can out preform DAP. So regarding the earlier statement about the traditional sales area of MAP or DAP if you look at the soil pH of samples went into the Oklahoma State University Soil, Water, and Forage Analytical lab the distribution makes since.

Average soil pH of samples sent into OSU soil water forage analytical lab by county.

In the end game price point and accessibility drives the system.  In soils with adequate soil pH levels, from about 5.7 to around 7.0, get the source which is cheapest per lbs of nutrient delivered and easiest to work with. But if you are banding phosphorus in row with your wheat crop because you have soil acidity, DAP should be your primary source.