Variable Rate Technology Recommendations

With the cost of inputs continuing to rise, it is becoming more and more important to properly manage the application of fertilizer & lime. One way to do this is through the use of Variable Rate Technology (VRT) to apply varying amounts of fertilizer across a field according to the needs of the crop. Many people already do this with their lime, potash, and/or phosphate. Some are beginning to do this with nitrogen. CALMAR can help you make the leap.

Lime, potash and phosphate recommendations are usually made according to soil test results. Amounts of lime and fertilizer are varied depending on the amount needed to bring the soil results to a "target" value. Sometimes this may take multiple applications, sometimes it can be done all in one application. The "target" value can vary depending on many factors. Call CALMAR's Mapping Office to speak with our VRT experts.

In many ways, the use of VRT with nitrogen has lagged behind lime, potash, and phosphate. However, as the cost of nitrogen continues to rise, many are looking to VRT to help manage their nitrogen use and application.

Our VRT nitrogen program starts using a map of the broken into management zones. The management zones are usually the same modified grid zones we use for soil testing, but they can be different. A target yield is assigned for each of these management zones. The target yield can be determined by previous years' yield maps, soil types, fertility levels, or a number of different factors. Yield history is the most common.

The images below show how two previous yield maps are combined with a management zone map to create a target yield map. Click on each one to display a larger image.

Once target yields have been determined for the whole field, one or two soil nitrate sample locations are selected within the field. The locations are determined by the target yields. Usually one sample in a high target yield area and one in a low target yield area. The values from these soil nitrate tests will help determine how much nitrogen is already available to the crop from the soil.

Then target yield values, soil nitrate results, and previous nitrogen applications (including starter fertilizer and manure applications) are used to calculate how much nitrogen the crop will need to produce the desired yield. The result is a variable rate nitrogen recommendation that can be used in the field.

Many variable rate applicators have the ability to record the amount of fertilizer being applied. CALMAR can also map these results for record keeping and billing purposes.


Variable Rate Technology is an effective way to manage your inputs. It may not reduce your overall input bill, but it will reallocate the inputs so they are used more effectively.

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