Making P and K fertilization decisions
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EmailEditor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Having
current information about the soil pH and available soil nutrient
levels is the passport to effective and economical use of nutrient
inputs. With the cost of phosphorus and especially potassium remaining
high relative to the value of the corn, soybeans and other fields, it is
important to understand the response of crops to nutrient additions in
relation to what is available in the soil. Through years of good
nutrient management nearly two-thirds of Michigan soils contain enough
available phosphorus to grow top corn and soybean yields, but only about
one-third of Michigan soils contain adequate levels of potassium for
producing top yields.
In soils that contain less than the critical level of available P or K
(soil test value which results in 95 percent of maximum yield) nutrient
additions will improve crop yield. In this responsive soil test zone,
significant improvement in crop yield results from the addition of
fertilizer P or K. With each increased increment of P or K addition
there is less increase in crop yield until the point is reached where
the cost of the fertilizer addition is equal to the value of the
increased yield. This is the point of most economical return. The amount
of P and K to add to reach this point varies with the soil test level,
the crop and the soil texture. The critical soil test value for P is 15
ppm for corn and soybeans and is 25 ppm for alfalfa and wheat. The
critical soil value for K ranges from 85 to 125 ppm depending on soil
texture, being higher for soils with more clay.
With resources being tight for buying seed, fertilizer and other inputs,
farmers are wanting to know how best to allocate fertilizer resources.
Decisions need to be made. Let soil test information be your guide. We
know that maintaining soil test values in the adequate range provides
the best opportunity for excellent crop yields from year to year. When
resources are limited, it is best to apply needed amounts of fertilizer
first on fields where the soil P or K levels are less than adequate and
then allocate the remaining fertilizer to fields with adequate levels to
offset crop removal.
When allocating fertilizer to fields at less than recommended rates, it
is better to reduce the application rate a given proportionate amount on
all fields with similar soil test values than to apply the full amount
on some fields and none on others. For example, on fields with similar
soil test values applying fertilizer at 70 percent of the recommended
rate on all the acreage will provide a better economic return than
applying the full rate of fertilizer on 70 percent of the field acres
and none on 30 percent of the field acres. That is because the biggest
yield increase comes from the first increments of fertilizer addition.
