AGRI-BRIEFS   AGRONOMIC NEWS ITEMS

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

Spring 1999, No. 4

It is obvious from yield monitor data that there is variability in yield within every field. What causes that variability and how it can be managed are not always as apparent. On the rough cut of nutrient budgets, it follows from yield variability that the nutrients utilized to grow the crop and nutrients removed in the harvest will vary according to the yield levels.

From intensive soil sampling, we often find that the higher yields are obtained from areas with low soil tests relative to the rest of the field. This supports the concept that with fertilizer applications based on field-average soil tests and crop yields, the nutrients applied to the high yield areas of the field have been less than the nutrients removed by the harvested crop. On the other hand, the low yield areas of the field often have high soil test levels because applications based on field-average soil test and crop removal have been greater than the amount of nutrients removed in the harvested crop. In fact, each year that field average management continues will result in a wider variation between the low yield and high yield areas.

This means that when fields are fertilized to meet field-average requirements, the lowest yielding areas of the field will get more nutrients than they need and the higher-yielding areas will get less than they need. This leads to potential for reduced productivity on the best areas of the field. Site-specific nutrient applications will help to bring applications and removal into better balance and create a better opportunity for the high yield potential of the best areas to be realized.

Intensive sampling, yield monitoring, and variable-rate nutrient application may cost more money, but if the information generated allows farmers and their advisers to make better decisions on yield goals and nutrient management, that cost can be offset by higher yields and better allocation of fertilizer dollars. Site-specific nutrient management costs money, but using field-average management may cost even more in lost yield potential and inefficient allocation of fertilizer applied.

With the technology available to implement site-specific management, it is a good idea to take a serious look at how that technology can fit into a plan to improve yields and profits. For many farmers, the answer will be that they cannot afford to continue managing every acre the same. As benefits of integrating site-specific management of other inputs are assessed, the returns to site-specific nutrient management will likely increase.