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Topics : Soil Fertility for Forage Crops : Pre-Establishment



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Soil pH and Nitrogen Nutrition of Legumes

Soil fertility management for forage crops is a continuous process that begins well before the forage crop is established. In the pre-establishment phase the soil conditions are adjusted to provide optimum soil fertility when the crop is established. At establishment the fertility program should deal with any last minute small adjustments in soil fertility and any requirements for getting the plants established such as a starter fertilizer. Finally, once the crop is established the fertility program will focus on maintenance of good fertility levels in the soil for the life of the forage stand. This soil fertility management time-line is illustrated in Figure 1.

Fig. 1. Soil Fertility management time-line for corn-forage rotation.

The goal in soil fertility management before forage crop establishment should be to have the soil pH in the optimum and the phosphorus and potassium levels in the soil in the optimum to high range as indicated in Table 1.

Crops vary in their sensitivity to soil pH. Generally alfalfa is the most sensitive forage crop followed by the other legumes and then by the grasses (Table 1). With the legumes, the pH sensitivity is related primarily to the nitrogen fixation process. At low soil pH the bacteria responsible for nitrogen fixation are not very active and thus the crop can suffer from nitrogen deficiency. This effect on nitrogen is illustrated in Figure 2. With nitrogen fertilizer applied, soil pH has little or no effect on the yield of alfalfa. However, with no nitrogen applied there is a very dramatic alfalfa yield increase as soil pH is increased. Thus, it boils down to adding lime to maintain the proper soil pH or adding nitrogen fertilizer. Economics greatly favor applying limestone over adding nitrogen fertilizer. A typical requirement of two tons of lime for 3 or 4 years would cost about $10 per year compared to 250 lbs. of nitrogen required every year which would cost about $60 per year.