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Variable Fertilizer Rate Research Project

Posted May 1, 1997

The Noble Research Institute is helping fund Oklahoma State University's variable fertilizer rate research project. In addition to supplying grant funds, the Noble Research Institute Agricultural Division also furnishes land for variable rate research at both the Headquarters Farm (HQF) and the Red River Demonstration and Research Farm (RRDRF).

The use of variable rate fertilizer application is becoming increasingly popular. Most research at other institutions utilizes grid-soil sampling techniques for site assessment. In order to obtain the resolution necessary to generate good nutrient contour maps, several soil tests per acre can be required, which is expensive when performed over many acres.

The OSU variable rate research project is unique: sensors which remotely sense differences in plant color while the crop is growing are used to obtain spectral radiance readings in the red, green, and near infrared wavelengths. Once readings are made, a relationship called normalized difference vegetation index (NDVI) is calculated. Research conducted by OSU at both RRDRF and HQF sites has indicated that NDVI is correlated with plant forage nitrogen uptake and biomass yield. Variability in NDVI from a bermudagrass pasture at the RRDRF is presented in Figure 1.

This contour map was generated using sensor data taken every 1 inch by 38 inches wide. This kind of variability is typical of that found in the bermudagrass and winter wheat studies evaluated by the OSU team. A variable rate fertilizer application valve is under development which will allow application of the proper rate of fluid fertilizer to optimize the plant nutrient status and obtain optimum yield.

A computer microprocessor marries the two technologies. As the applicator moves through the field, the microprocessor performs all of the necessary calculations based on the spectral radiance readings obtained from the sensor mounted on the front end of the fertilizer applicator. The computer then adjusts the applicator valve mounted on the rear of the vehicle which applies the optimum fertilizer rate to the field element (the area that is both remotely sensed and correctly fertilized based on the real-time sensor data).

Based on work conducted on Noble Research Institute properties, the OSU team has determined that the field element size should not exceed 21 square feet in order to optimize nitrogen fertilizer input in bermudagrass. The OSU research could ultimately result in the creation of much less expensive variable rate technology than systems based on intensive soil sampling that are under development at other major land-grant universities

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