MSU Extension

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.  

Even with higher fertilizer prices investment in nutrients is important for production of high yields of good quality vegetables. A regular soil testing program, annual or biennial sampling, is essential for economic management of nutrient inputs. Soil testing is important for monitoring the soil pH and preventing the soil from becoming too acid. When the soil pH falls below 6.0, the key nutrients, nitrogen, phosphorus, potassium, calcium and magnesium become less available for plant uptake and the return on investment in fertilizer nutrients is less than optimum. When needed, the first best nutrient management investment is in lime. The best time to apply lime is usually in the fall, but lime can be applied at other times of the year as well. When lime is needed, it is important to get it applied.

Vegetable crop growth may be most affected by nitrogen supply. Nitrogen (N) is a mobile nutrient in soils and is subject to loss by leaching, especially in sandy soils on which many of the vegetables in Michigan are grown. To maximize return from applied nitrogen it is important to use split nitrogen applications. During the first four weeks of growth, vegetable crops use less than 20 percent of their total nitrogen requirement. Applying 40 lbs N/acre prior to or at planting and applying the remainder in one two side- or topdress applications results in better N use efficiency. Applying more N prior to planting provides no additional benefit and just increases the risk for loss. Growing legume cover crops (such as clover or vetch), which fix N from the air, can provide additional N. Be sure to consider this N credit when figuring how much supplement N is needed. Growing other types of cover crops can cycle N left in the soil after a vegetable crop is harvested and thereby keep it available for the following crop. This may amount to 30 to 40 lbs N/acre. A presidedress soil nitrate test can be useful in determining how much N is available in the soil and allow determination of how much additional N is needed.

Vegetable crops utilize large amounts of potassium (K), equal to or greater than nitrogen. Potassium has become the most expensive fertilizer nutrient and many farmers are looking for management approaches to reduce fertilizer input costs. MSU Extension bulletin E-2934 Nutrient Recommendations for Vegetable Crops in Michigan provides information on nutrient management. Again, soil test information is the base for good nutrient management.

The MSU phosphorus (P) and K recommendations are based on the “buildup, maintenance, draw down” approach. This approach is built around the critical soil test P or K value, which is the value at which crop yield is near 95 percent of maximum along with good quality. When the soil test value is less than the critical value, there is very good probability of benefit from applying P and K. Above the critical value there is a lower probability of benefit. The critical P soil test value is 30 to 40 ppm for most vegetable crops grown on mineral soils. Maintaining the P soil test value 25 to 30 ppm above the critical value provides the best opportunity for long term production of high yields of good quality produce. For potassium, the critical value ranges from 85 to 115 ppm for most soils used for vegetable production in Michigan. The maintenance range is 30 ppm above the critical value.

When the P or K soil test is at the buildup zone (less than the critical value), the recommendation is to apply an amount of nutrient to buildup the soil test to the critical value over a four-year period plus an amount equal to crop removal. When financial resources are limited, applying only the amount equal to crop removal will result in good economical crop yields. When the soil test is in the maintenance range, the P or K recommendation is equal to crop removal. If financial resources are limited, less than crop removal can be applied without loss of crop yield, but the soil test level will decline some and this deficit will need to be replaced at some later time (perhaps when fertilizer prices are lower). When only half of crop removal is applied, it may take four to five years for the soil test to decline below the critical value, depending on the starting soil test value. If the soil test value is in the high range, allow the crop to draw on the nutrients stored in the soil from past buildup. In this way, P and K stored in the “soil bank” can be used to help reduce nutrient input costs, but it is important to monitor changes in soil nutrient levels by regular soil tests.

Up to 20 percent of loamy sand soils used for vegetable production have low or marginal magnesium (Mg) levels. Be sure to monitor magnesium levels. In sandy soils, 35 ppm or more is adequate. In loams and more clayey soils, 50 ppm or more is adequate. Where lime is needed, use dolomitic lime. In soils not needing lime, apply 10 lbs Mg/acre in band fertilizer or include 50 lbs Mg/acre in broadcast fertilizer.