Generally, fertilizer application is not much of an issue during a drought year. Fertilizers often have been applied before the true extent of a drought is known. If they haven't already been applied, you need to adjust rates based on lowered yield expectancy for the drought year. If little or no production is likely, it may be best to skip an application.
Fertilizer use does become a significant issue the year after a drought, however. Low crop yields during the drought year mean that significant amounts of unused nutrients could remain in the soil at the end of the growing season. Where nutrient carryover is substantial, fertilizer needs for the following year are likely to be affected. Several methods are available to help growers determine nitrogen, phosphorous and potassium carryover and current needs.PHOSPHOROUS AND POTASSIUM CARRYOVER
If phosphorous or potassium was applied but not used because of lower than expected yields, it usually remains in the top few inches of soil. It will not be lost over the winter. Therefore, the unused portion can be credited against nutrient needs for next year's crops.
Drought year application = 75 lb./acre phosphate
= 300 lb./acre potash
Drought year yield goal = 6 tons/acre (alfalfa)
Actual yield = 2 tons/acre
Actual yield/yield goal = 2/6 = 1/3
Therefore, 2/3 of drought-year application is unused
Estimated carryover = 2/3 x 75 lb./acre = 50 lb./acre phosphate
= 2/3 x 300 lb./acre = 200 lb./acre potash
Comparison of the actual yield with the expected yield shows that the drought-year yields were 1/3 of the goal. Under the assumption that nutrient removal is proportional to yield, approximately 2/3 of the phosphate and potash applied in the drought year was not used and likely will be available to the next crop.
Following a drought year, most nitrogen carryover exists as nitrate in the plant root zone. However, the possibility of over winter loss of residual nitrate makes estimation of carryover more difficult than for phosphorous and potassium. The amount of residual nitrogen in the soil at the end of the growing season must be considered, as well as factors affecting over winter loss. Specifically, nitrogen carryover is likely where:
A preplant soil nitrate test should be used to determine how much nitrate has remained in the soil until the next growing season.
Take at least 15 random soil cores from uniform soil
areas no larger than 20 acres.
b) Take separate samples from areas with soil or management practice differences.
c) Sample in 1-foot increments to a depth of 2 feet.
d) Each sample should be placed in a clean container marked for the appropriate depth.
e) Thoroughly mix the soil from each depth and collect a 1-cup subsample. This sample should be sent to a soil testing lab for analysis.
Soil testing and analysis are available from the University of Wisconsin soil testing labs in Madison and Marshfield, and other private soil testing labs. Your county Extension office can provide names and locations of commercial labs performing these tests in your area, as well as more specific sampling instructions and forms. To contact the Madison and Marshfield labs:
& Plant Analysis Lab 5711 Mineral Point Road Madison,
phone: (608) 262-4364
Soil & Forage Lab Marshfield Ag Research Station 8396
Yellowstone Drive Marshfield, WI 54449
phone: (715) 387-2523
Your county agricultural agent, soil testing labs, fertilizer dealers, crop consultants.
"Wisconsin's Preplant Soil Nitrate Test," (A3512);
"Sampling Soils for Testing," (A2100);
"Step-by-Step Guide to Nutrient Management," (A3568);
"Nutrient Management Practices for Wisconsin Corn Production and Water Quality Protection," (A3557).
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