Crop sensors measure the relative vigor of the crop in different areas of the field through a set of reflectance measurements that translate into a vegetation index. Typically, the site-specific vigor is referenced to an “optimum” zone in the field, which may be a strip that received a high rate of N fertilizer or an area that is considered to have the best soil conditions in the field. It is generally assumed that N fertilizer is the limiting factor that impacts crop vigor in that case (which may not always be the case). This approach is very different from the one taken by Adapt-N, which simulates nitrogen dynamics in the soil–crop system for an entire growing season. Since the crop sensors are a single time assessment of field conditions, they do not provide the seasonal monitoring capabilities and temporal weather integration capabilities of Adapt-N. But they do offer actual in-field measurements at a critical growth stage which indirectly integrates the effects of soil, management, and weather conditions. This can be useful in combination with Adapt-N’s model simulations to fine-tune site-specific applications.
Crop sensors generally allow you to set the range of N applications for a field through a rate table. This allows for incorporation of Adapt-N information into the crop sensor technology. We have not explicitly tested this method, but believe that the following approach would maximize the benefits of each tool:
First, use Adapt-N in the early growing season to monitor the nitrogen status of the field. It will be indicating whether the crop is deficient in nitrogen and additional applications are needed. If sufficient N is available, use of crop sensing or sidedress N applications may not even be necessary.
If Adapt-N indicates that additional N is required, we suggest entering the Adapt-N recommendations in the rate tables for the crop sensors. These tables indicate the range of N rates to be applied within the field. Adapt-N can estimate this range if you enter multiple zones for a field in the tool, each representing different soil types or other sources of soil variation (organic matter content, texture, etc.). The simplest approach is to enter the highest Adapt-N recommended rate as the maximum rate in the crop sensor software, and the lowest Adapt-N rate as the minimum crop sensor prescription. However, since Adapt-N zones will generally still represent larger field areas, the range may need to be set wider. As an approximate guide, we recommend adding an additional 10 pounds per acre to the maximum prescription and reducing the minimum by an additional 10 pounds per acre. For example, if the “best” zone has an Adapt-N recommendation of 130 pounds per acre, and the “worst” zone has a recommendation of 150 pounds per acre, the minimum prescription in the rate table for the crop sensor would be 120, while the maximum would be 160. The range can be slightly expanded in the case of highly variable fields.
Note, however, that the assumption that areas of the fields with greater crop vigor need less nitrogen may not always be true. Areas with better initial crop growth may also have higher yield potential and need more nitrogen to reach full yields. Therefore, guidance from Adapt-N may help improve the use of crop sensors in ways different than described above. Notably, the nitrogen status in soil and crop estimated by Adapt-N can explain field-scale patterns and help decide which areas need higher or lower N rates.