Use your soil nutrient bank
Editor’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
With the significant increases in costs of phosphate and potash fertilizers since last season and with tight supplies, some reflection on recommended rates is in order. MSU’s recommendations for phosphate (P2O5) and potash (K2O) are based on the buildup and maintenance approach. In this approach, the most important soil test value is the critical level (CL). On average, when the soil test value is at the CL, there is sufficient P or K available in the soil to produce top quality produce and near maximum yield. This is usually very close to the point of maximum economic return. For P, the CL varies by crop. For most vegetable crops, it is 40 ppm. For K, the CV varies with the cation exchange capacity of the soil. At or above the CL applying more fertilizer will usually not improve yield, but if no P or K is added, the available level in the soil will decline.
An adequate soil test or maintenance range has been established over which an amount of P or K equal to crop removal is recommend. For phosphorus, this range is 25 to 30 ppm beyond the CV and for K it is 20 ppm. When the soil test value is beyond the maintenance range, there is no need to apply addition P or K. For most vegetable crops, no P is necessary when the soil test value is over 70 ppm. With K, the soil test value above which no K is needed will range from 105 to 140 ppm depending on soil texture, sandy to clayey. When the P or K soil test value is below the CL, the recommended amount of that nutrient will include an amount to build up the soil test value to the CL over a four year period plus an amount equal to crop removal (maintenance). Crop removal information is available in MSUE bulletin Nutrient Recommendations for Vegetable Crops in Michigan E-2934, page 9. Due to higher prices and limited supplies, farmers should consider applying no more than crop removal. This will provide an adequate amount for top yields and maintain the current available P or K level in the soil.
Many Michigan soils used for vegetable crop production have more than adequate amounts of P. In those situations, no or only a small amount of phosphorus in the planting time fertilizer is needed. Make use of the P in your soil bank. There is a greater potential for benefit from applying potassium (K) than for P. Only 25 to 30 percent of vegetable crop soils contain more than adequate available K. With limited supplies of potash this year decisions may need to be made on how to allocate the potash you can get. Including 20 to 25 lbs K2O in planting time fertilizer can give improved early growth and K use efficiency and allow a reduction in the amount being broadcast. Due to the nature of the yield response to applied K, it is best to apply a reduced amount (e.g. 75 to 80 percent of recommended amount) to all fields needing K rather than applying the full amount to some fields and a small amount to other fields. Soil test results provide a good guide for how best to allocate available fertilizer resources. Those that have soil test information from the past year or two are in a good position to fertilizer allocation decisions. There is still time before planting to collect soil samples and get the soil test information for fields not recently tested.