MSU Extension

Nutrient recommendations for vegetable crops grown in Michigan have evolved over the years, with changes based on observations and controlled field studies (circular bulletin No. 53, Extension bulletin 159 and Extension bulletin E-550). During the 1920s and 1930s, recommendations given for various amounts of various fertilizer grades were based on the crop grown and the management practices being used. The three management practice categories were: no manure or leguminous green manure in the past two years, clover or alfalfa grown within the past two years, and manured within the past two years. Recommendations for muck soils were based on whether it was a high-lime or low-lime muck, and whether it was a deep, medium or shallow muck. In the 1940s, recommendations for the grade of fertilizer to use considered soil texture (sandy, loamy or clayey soil) and whether manure had been applied within two years.

Soil test results began to be considered in making fertilizer recommendations in the early 1950s. Phosphorus and potassium test values were classified as low or high on the basis of the Spurway “reserve” soil test (0.13 N HCl). For phosphorus (P), a soil test value below 50 pounds of phosphorus per acre (lb P/A) was considered low, and above 50 lb P/A was considered high. For soils with a pH above 7.5, the separating value was 100 lb P/A. For potassium (K), the separating soil test value was 150 lb K/A. When rock phosphate had been applied to the soil, the “active” test (0.018 N acetic acid) was used. The separating soil test values for the active test were 25 lb P/A on acid soils, 50 lb P/A on soils with pH above 7.5, and 80 lb K/A. Even when the soil test was high, some fertilizer was recommended because even in the “high-test” soils it was unusual for a lack of an economical response to occur when a balanced fertilizer was applied.

In the early 1960s, the Bray P1 test for phosphorus and the ammonium acetate test for potassium began to be used. Soil test values were divided into very low, low, medium, high and very high categories. In 1963, recommendations for crops grown on mineral soils were given for amounts of P2O5 and K2O per acre in relation to the soil test category. For crops grown on organic soils, the recommendations were given for pounds of P2O5 and K2O per acre on a graded scale according to the actual soil test value. Soon thereafter, nutrient recommendations for all crops grown on mineral and organic soils followed the same format. The tabular recommendations were converted into recommendation equations in 1981.

During the mid-1990s, soil fertility specialists from Michigan, Ohio and Indiana developed a set of common nutrient recommendations for corn, soybeans, wheat and alfalfa (Extension bulletin E-2567). The conceptual model used for those recommendations is followed for the phosphorus and potassium recommendations given in this bulletin for all vegetable crops.

Basis for Recommendations

The growth and development of vegetable crops are influenced by the levels of essential elements (nutrients) available in the soil. Field studies at various locations in Michigan have provided the data for describing growth and yield responses of crops to nutrient additions when available soil levels are less than adequate. Soil testing procedures have been developed to relate extractable nutrient levels to crop growth and yield.

Nitrogen, phosphorus and potassium are the nutrients most likely to be limiting crop growth. The nitrogen status in the soil is quite dynamic, and predicting its availability over time is difficult. The availability of phosphorus and potassium in the soil is fairly stable over time unless major additions are made. Soils in Michigan are naturally quite low in available levels of phosphorus and potassium. Additions of these two elements over time in manures and commercial fertilizers have caused significant increases in the available levels in the soil. In 1962, the median soil test value (Bray-Kurtz P1) for phosphorus in Michigan soils was 12 ppm. This gradually increased over time. Since the early 1980s, the median value has fluctuated around 53 ppm. Similar values for potassium soil test values (1 N neutral ammonium acetate) are 56 ppm in the early 1960s and near 91 ppm in recent years.

Figure 1 illustrates the general relationship between soil test value and crop growth or yield. With each increment of increase in the soil test value, the increase in yield is less (law of the minimum). The point at which yield reaches 95 to 97 percent of maximum is referred to as the critical soil test value. This is also near the point of optimum economic return on investment made in nutrient additions. When phosphorus or potassium is added to the soil, some of it is taken up by the growing crop, some goes to increasing the available level in the soil and some is converted into slowly available forms. Adding more of a nutrient than the crop can take up will result in a buildup of the readily available and slowly available forms. Soil tests have been developed that will extract a portion of the nutrient pool that is available for plant uptake. Soil test values have been correlated with nutrient uptake, growth, and, subsequently, yield. The amount of a nutrient required to enhance crop growth, quality and yield to the maximum is related to the soil test value.

Development of Nutrient Management Programs

Development of a cost-effective nutrient management program needs to take into account the nutrient requirements of the crop being grown and the nutrient status of the soil. The elemental analyses of plants have established the general nutrient requirements of crops. Actual nutrient uptake will vary with crop yield and variety. The nutrient requirement of the crop can be met by nutrients available in the soil and by nutrient additions. Soil tests indicate the ability of soils to supply nutrients. When the soil can supply all of the nutrients required by the crop (the soil test value is greater than the critical value in Figure 1), no additional nutrient inputs are needed to achieve maximum yields. Supplying an amount of nutrient equal to crop removal will maintain the nutrient status of the soil. Field studies have established how much of a given nutrient to add at a given soil test value to optimize yield. Soil tests, therefore, provide the base for building a sound nutrient management program.