Fertilizer Application By Management Zone

Soil color, landscape position, and past management experience are keys to developing management zones for fertilizer application.

Developing accurate variable rate technology (VRT) fertilizer application maps is critical in implementing precision farming technology. Intensive grid soil sampling has traditionally been used to develop application maps. However, the cost and labor associated with grid sampling has led us to conclude that another approach, such as production level management zones may be more feasible. A production level management zone is a region of a field that has similar yield limiting factors. Therefore, a specific rate of fertilizer, or other input, may be appropriate for this management zone.

The question is how do we identify these different production level management zones in a field?

We have known that soil color in bare soil photographs can be used to identify different soil organic matter (SOM) levels. We also know that topography affects production with higher yield generally being obtained in lower positions in the landscape, with lower yields on eroded side slopes and hills. One of the best estimates of yield differences in a field is obtained from the producer. Producers know which areas of a field produce good yield and which areas are low in production. It is logical that nutrient needs are different among these areas.

Based on these relationships, we have developed a research and extension program to determine if management zone technology could be used to develop accurate VRT application maps based on aerial photographs showing soil color, landscape position, and past management experience.

Figure 1. Field 1 - farmer developed management zones; (low productivity = white, medium productivity = light gray, high productivity = dark gray).

Aerial photographs were taken and used as an initial template in developing production level management zones. In this procedure, a gray scale image of a bare soil photograph was enhanced using Adobe Photoshop® to contrast color differences. The farmer drew vector lines using commercially available software (Agri Trak Professional®) to establish the individual management zones (high, medium, and low productivity areas) based on soil color, topography and his/her management experience with the field (figure 1). Management zone maps were developed for two center pivot irrigated corn fields in northeast Colorado. The fields were sampled using a 1.4 acre grid. Samples were analyzed for nitrate, organic matter, phosphorus, potassium, zinc, pH, and texture. Electrical conductivity and yield were also mapped on both fields.

Evaluation

Using the soil and yield data we have begun to evaluate if the farmer-developed management zones represent areas of high, medium and low productivity and have the potential for developing Variable Rate Application maps. We found that soil nutrients and grain yield followed the trends indicated by management zones. Levels of soil organic matter, potassium and grain yield were significantly different in all management zones.

Soil texture data show similar trends. Clay and silt levels were significantly higher in high productivity zone, intermediate in the medium zones, and lowest in the low productivity zones. Sand followed the opposite trend. These soils are sandy and higher productivity in areas of lower sand and higher clay levels would be expected due to the higher water holding and cation exchange capacity of the more clayey soils.

Electrical conductivity values also followed trends indicated by the management zones and were significantly different across all management zones. In these soils, texture probably has the greatest influence on conductivity values with clay having the highest conductivity values and sand the lowest.

Results from field No. 2 indicated that the soil and crop parameters were significantly different across the management zones, however, they did not always follow trends indicated by the zones. This indicates that our method of delineating management zones is consistent . However, the zones need to be ground truthed to classify them into various productivity levels. Electrical conductivity data collection appears to be a cost-effective alternative to soil sampling to ground truth management zones.

Practical implications

Grid soil sampling to the intensity required to generate accurate Variable Rate Application maps may not be feasible because of the time and expense required. Management zone technology may be a more economical method of developing Variable Rate Application maps.

by Kim Fleming and Dwayne Westfall
Dept. Soil and Crop Sciences
Colorado State University and
D. W. Wiens
CenTrak L.L.C.