From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Fall 1996, No. 7


Every year there are some producers who are puzzled by the differences in the amounts of fertilizer recommended for field crops by different sources. Although most people appreciate that soil testing is not an exact science, it is still perplexing to see recommendations that differ by as much as several fold.

There are two fundamentally different approaches used to make recommendations. One focuses on fertilizing the crop and is often called the sufficiency level approach. The other focuses on fertilizing the soil and is referred to as build-up and maintenance (or correction and maintenance). Both approaches are based on sound science, but interpret the soil test in different ways. Depending on the situation, either can be appropriate.

The sufficiency level approach aims to apply the amount of fertilizer which will give the most economic crop yield response within the current cropping season. It is based on a calibrated soil test and may consider other factors such as yield goal and prices of crops and fertilizers. This approach is most appropriate with nutrients that are not retained in the soil over time, such as nitrogen. However, for nutrients such as phosphorus and potassium, whose residual effects extend several years after application, the sufficiency level approach may neglect the long-term benefits of building soil fertility.

The build-up and maintenance approach focuses on building soil fertility to a level that does not limit the yield of crops. The soil test is used to determine whether the soil is above or below this level, which often corresponds with the division between medium and high ratings. Above this level, recommendations are based on the amount of nutrient removed by the crop, so that the level of soil fertility is maintained. Below this level, recommendations are based on the amount of nutrient required to increase the soil test level. The optimal amount of soil test increase can be determined by an economic analysis of the estimated yield benefits over a five to ten year period. Build-up recommendations can substantially exceed those of the sufficiency approach, especially in soils testing in the medium range. The differences tend to narrow, however, after soil fertility has been built up.

The economic benefits of building soil fertility are not realized in the short-term. When the costs of the investment are amortized over a period of five years or more, however, building soil fertility can build profits. In addition, the build-up approach allows greater flexibility in amounts applied in individual years as prices and cash flow fluctuate.

The choice of approach depends on the circumstances of the producer. For those whose land tenure is secure, who have a creative interest in producing high crop yields and who have access to finances for investment, building soil fertility is clearly a wise choice. On the other hand, those who rent land on a year-by-year basis have little incentive to build soil fertility. At lower yield levels and lower overall intensity of management, there is less benefit from build-up of soil fertility.

Evaluate your circumstances, and consider carefully the investment in building soil fertility.

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