Many new technologies can be used to manage soil fertility, such as global positioning systems, yield monitors and other sensors, variable rate application, and associated practices like grid soil sampling. However, use of these technologies also requires information about the variability of nutrient supplies-and the effect on crop yields.

As part of an extensive precision farming research program that began in 1995, one project has focused on how soil fertility is evaluated and ways to improve fertilization. Soils and plants are sampled for phosphorus (P) and potassium (K) content, and crop yields are measured with yield monitors or by weight measurements. The information is used to evaluate the relationships between P and K contents in soils and plants, and their effects on crop yields.

Results

Results of these on-farm studies show that the patterns of spatial variation for some nutrients often follow the distribution of soil types or other landscape characteristics. The variability of P and K (and sometimes soil pH), however, often does not follow the distribution of soil types-and patterns usually differ among fields. In addition, often there is little or no relationship between P and K in soils and plant nutrient content or crop yields because soil-test values are higher than levels required by crops. Surveys show that more than 60 percent of Iowa corn and soybean fields test optimum or above in soil P and K. This suggests that the variability for P and K is created primarily by management practices (such as fertilizer and manure applications) and that grid soil sampling is not always more cost-effective than conventional soil sampling methods. The figure shows a soil P map based on intensive grid-point sampling and contouring, compared with a map based on averages by soil type for the same field.

Map of soil phosphorus content in a 50-acre field based on sampling by soil type (or gridpoint sampling, 2 points per acre) and contouring.

Related results

Also significant in this research is that soil P and K variability over many acres often is similar to that observed over areas of two to five acres. Although producers are using a variety of sampling intensities, grid-cell sampling schemes used by many are based on cell sizes that vary from two to five acres. The grid-cell sampling method attempts to represent areas (or cells) within fields by collecting a composite sample from each area. Grid-point sampling, on the other hand, attempts to represent smaller areas (usually 100 to 400 sq. ft.). The more intense sampling per unit area with grid-point sampling (together with use of global positioning systems) may make this method more reliable to estimate changes in soil test values over time. Something to consider, however, is that the grid-point sampling method may require more precise global positioning receivers and that there is more reliance on interpolating algorithms when making maps. These factors should not cause major problems in the future because of constant advances in technology and the decline in equipment and software costs.

General recommendations are difficult

Because the amounts and patterns of variation for P and K vary greatly within fields, it is very difficult to make general recommendations about the best way to get soil samples. Unfortunately, the best scheme and merits of variable rate fertilization can be reasonably estimated only after conducting an intensive (and perhaps expensive) preliminary grid sampling. Some important factors that will determine the cost-effectiveness of these new techniques are variation in nutrient levels in relation to amounts required by crops, the proportion of the field that needs fertilization, expected yield responses to fertilization, and costs involved. Results of this project and others in the area of precision agriculture will provide information that can be used by farmers and dealers to develop more cost-effective site-specific fertilization practices.