Reader letters get to the heart of real questions. I received an email from a South Dakota agronomist asking if DAP applied in the spring several days before corn planting will be available when the crop needs it; or is it better to apply in the fall when prices are usually cheaper? "We have customers that like to save an application and put it on with their urea just before they plant their corn," he explained.
Applying urea just before planting corn makes good sense. But is prilled DAP (diammonium phosphate) or MAP (monoammonium phosphate) just as soluble as urea when surface applied (incorporated or not) in the spring?
When we talk about dry fertilizer materials, we think of lime, nitrogen, phosphate and potash. Lime should always be applied in the fall, either as ag lime (spreadable powder) or pell lime (pelletized ag lime). Lime needs time to chemically weather, breakdown and neutralize soil pH. Dry nitrogen products like urea, ammonia nitrate or ammonia sulfate should never be fall applied because there is a high risk that a portion will be lost from leaching or denitrification before the next crop needs it. Dry phosphate fertilizers are also more soluble than lime, but the risk of environmental losses is less than for nitrogen.
The most common phosphate fertilizers used in the U.S. are MAP (11-52-0), DAP (18-46-0) and ammonium polyphosphate (10-34-0) liquid. According to the International Plant Nutrition Institute (IPNI), polyphosphate liquids are 100% soluble, while MAP and DAP are 90 to 100% soluble. Phosphate rock is considered insoluble. The difference between polyphosphate and MAP/DAP is polyphosphate is already a liquid while MAP/DAP are dry materials and do contain some insoluble impurities.
To be available to plants, nutrients must be soluble in the soil solution. The amount of material that dissolves and the soil's water can hold is based on temperature. For example at 32 F, solubility in 100 gallons of water will be 209 pounds of DAP: 358 pounds of MAP: 559 pounds of urea; and only 2 pounds calcium sulfate (gypsum) or 0.05 pound of calcium carbonate (lime). You can find the table listing other nutrients here: http://www.ncagr.gov/…
So, in other words, DAP, MAP and urea are all highly soluble compared to gypsum and lime. Spring applications of MAP and DAP are just as available as fall applications.
MOBILITY AND ENVIRONMENTAL RISK
The fate of fall-applied nitrogen and phosphorus may be a more important question.
MAP (11-52-0) and DAP (18-46-0) both contain nitrogen. If you apply 100 pounds of fertilizer, you are applying 11 and 18 pounds of nitrogen, respectively. If it stays warm and rains a lot in the fall or next spring, a good portion of that nitrogen will be lost to leaching or denitrification.
Phosphorus is trickier. It is generally considered immobile in the soil because it fixes to aluminum and iron in acid soils and calcium in alkaline soils. It doesn't move through the soil with soil moisture to the plant root.
Instead, phosphorus fertilizer dissolves in water or becomes attached to soil or organic matter particles (particulate P). Either dissolved or particulate phosphorus can be carried with runoff and move into surface streams and bodies of water.
While it may be logistically easier and cheaper to apply MAP or DAP in the fall, spring applications are just as effective from an agronomic standpoint. Spring application saves more for the crop and there are some strong environmental reasons to wait.
If you have a question, e-mail Dr. Daniel Davidson at firstname.lastname@example.org
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