Soil tests gauge nutrient needs

Will the 2012 drought continue into 2013 growing season? “We don’t know, but one thing we do know is insufficient rain in 2012 had a significant impact on crop production in some fields and specific areas of fields,” says John Sawyer, Iowa State University Extension soil fertility specialist. “That can be taken into account when deciding nutrient applications for 2013 crops.”

Moving closer to spring planting, if the 2013 season is predicted to have lower-than-normal precipitation, some nutrient management inputs can be adjusted in response to that potential, he adds. A dry 2012 growing season severely impacted corn yields in some fields, so potential for unused or carryover nitrate is high. If dry conditions persist, it’s likely that the N application rate needed will be lower than normal for 2013 corn crops.

Key Points

• Carryover nitrate may be high in fields with severely reduced 2012 corn yields.

• Consider using low end of Corn N Rate Calculator profit range for 2013 corn N.

• A dry fall and lack of rain may cause greater-than-normal variation in P and K.

For corn following soybeans, reduced bean yields in dry 2012 won’t change the effect the bean crop has on the corn’s N fertilization rate requirement. For cornfields that had severely reduced yields that will be planted to corn in 2013, Sawyer says you should consider soil profile sampling down to 2 to 3 feet this spring to determine carryover nitrate-nitrogen.

There is always profile nitrate-N remaining after corn harvest, typically 50 to 60 pounds nitrate-N per acre in a 3-foot profile, he says. You should subtract that amount from any measured carryover nitrate-N. Also, if spring rainfall is high, carryover nitrate that was being accounted for can be lost from the root zone. Thus, you need to monitor early growing season conditions, and if reduced N rates were applied, be ready to apply additional N.

If 2013 appears to be another below-normal rainfall year, consider applying N rates to corn at the low end of the profitable range. Rates can be calculated with ISU Extension’s online Corn Nitrogen Rate Calculator, extension.agron.iastate.edu/
soilfertility/nrate.aspx
. “Split applications of N in 2013 can provide an opportunity for adjusting N rates higher or lower,” says Sawyer, “depending on the spring weather conditions that actually occur.”

Phosphorus and potassium

Reduced crop yields in 2012 may have resulted in lower-than-expected phosphorus and potassium removal. Unused P and K, or recycling from plants with no harvested yield, will be available for 2013 crops, says ISU agronomist Antonio Mallarino.

If drought-damaged corn was harvested for silage instead of grain, P and K removal with silage can be higher or lower than expected with harvest of normal grain yield. This can occur especially for K, as corn vegetative parts have much more K than grain. Consider removal amounts and whether more or less P and K are needed for 2013 application.

Soil sampling and testing since harvest can reflect increased P and K due to less removal. Test results may be lower than should be, as dry conditions slow replenishment of crop-available soil nutrients, as well as reduce nutrient recycling from crop residue due to low rainfall. This is especially an issue for K as it is easily leached from plant material with normal rain. But if rainfall after harvest and until time of soil sampling was low, then the K test would be lower than expected.

“We’ve seen how a spring soil test for K increases compared with an early-fall soil test when the K loss from crop residue increased,” says Mallarino. “On the other hand, dry conditions’ effect on soil K vary greatly across soils and with rainfall amounts, so accurate predictions aren’t possible for specific situations.” Also, soil pH is often lower in dry soils due to increased salt concentration in soil water. Soil pH may be 0.1 to 0.3 unit lower.

Due to issues with soil testing in dry falls, sampling for P and K in spring may offer more reliable results, as there is time for precipitation to help recycle nutrients from crop residue and wet soil.

Source: ISU Extension

This article published in the March, 2013 edition of WALLACES FARMER.

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