Last month, I discussed yield variation that occurs across almost every crop production field. This yield variation occurs through a combination of geological, biological, environmental and cultural variables. In the September and October 1998 Today's Farmer issues, I discussed the Natural Resources Conservation Service (NRCS) soil productivity index (PI) and how it affects crop production and management. Modern technology allows you to combine yield potential, yield mapping and PI. By doing so, you can better understand and manage yield differences that occur throughout a given field.

This article will only address the native soil productivity variability that occurs within a field. Please remember that other factors such as pest control, variety selection, crop rotation, weather, tillage, long-term management and crop nutrition all interact with native soil productivity to determine crop yield.

Figure 1 represents an actual soil map and PI-based corn yield potential from an Audrain County, Mo., field. It demonstrates how soil variation across a field can be managed to positively affect your bottom line.

The 74.6-acre field contains 4.9 (6.57 percent) acres of Putnam silt loam, 29.9 (40.01 percent) acres of Leonard silt loam and 39.8 (53.35 percent) acres of Mexico silt loam soils. The NRCS has assigned PIs of 60, 72 and 76 for the Leonard, Putnam and Mexico soils, respectively. MFA's Agronomy Technical Services has assigned realistic corn yield goals of 135, 157 and 160 bushels per acre for these soils (Figure 2).

Let's look at some very crude economics associated with this information. By concentrating solely on nitrogen fertilizer inputs, you can get an idea of how this information can be used to improve your agronomic management. For this example, assume that yield variation in this field is only influenced by soil type and nitrogen management. And assume that it takes 1.2 pounds (Midwest average) of nitrogen to produce 1 bushel of corn. The weighted average yield potential for this field is 150 bushels per acre. Treating this field as a single unit, we would then recommend 150 x 1.2 = 180 pounds of nitrogen fertilizer per acre. Multiplying this number by 20 cents (average nitrogen cost per pound) across 74.6 acres, you get a total nitrogen fertilizer cost of $2,679.63.

Would it be profitable to variably manage nitrogen inputs into this field based upon soil productivity? If you believe in PIs, the answer is yes.

By managing nitrogen for soil type alone, you could expect the following savings. The Mexico silt loam soil has 160 bushels per acre yield potential. This is 10 bushels per acre more than the field average. Multiply the 10 bushels per acre by 39.9 acres of Mexico silt loam and $2.25 per bushel corn value to get an increased income of $895.50. The extra 10 bushels of corn per acre over 39.8 acres would require 1.2 pounds of nitrogen per bushel at $0.20 per pound. This would equal $95.52 added nitrogen cost to the field. Therefore, you would net an additional $799.98 by managing according to soil productivity. A similar process for the Putnam silt loam would net you $68.95. The Leonard silt loam corn yield potential would require 15 bushels/acre times 1.2 pounds nitrogen per bushel, which equals18 pounds of nitrogen per acre less than the field average. At $0.20 per pound of nitrogen fertilizer over 29.9 acres, this amounts to a savings of $107.64. Adding the savings (Leonard soil) and increased income potential (Putnam and Mexico soils) derived from variable PI-based nitrogen management on this field would produce a net return of $976.57 or $13.09 per acre compared to using whole field average management.

Of course, the example given is based purely on empirical logic. Having spent most of my career working as a field agronomist, I realize that there is nothing empirical nor logical about farming. However, the concept of managing crops based upon variable native soil productively does follow sound agronomic principle. By combining these principles with modern technology such as precision farming and using the most competent agronomy technical services available, you are increasing the odds of a profitable farming operation.