Variable-Rate Fertility Continues Evolution

Dr. Harold Reetz is a well-known agronomic expert who’s been championing precision agriculture for a lot longer than I’ve been at it. As such, he’s served me, and our brand, as an important sounding board for ideas and inspiration.

I was looking for some inspiration for a piece on variable-rate fertilizer, and thought I had found it when I saw the headline below in a web item:

VARIABLE-RATE FERTILIZER FOR BETTER EFFICIENCY

There were two things I did not realize at the time: first, that the piece was written by none other than Harold Reetz. Second was that it was penned back in 1998, when precision was in its relative infancy.

What’s interesting is that a lot of the information and thinking is still very relevant. I just had to send the link to Harold to get his take on his own work of 15 years ago, and he provided some interesting thoughts.

First, here’s the article:

Farmers who can use site-specific management to determine the variability in nutrient needs within individual fields have a unique opportunity to improve their fertilizer use efficiency. To effectively use the new variable-rate technologies, the recommendations should be based upon detailed soil tests, yield maps, and other data sets. If possible, these data should be geographically-referenced using the global positioning system (GPS) to identify the exact location of each data point. Maps showing the variability of each nutrient within the field can be prepared from these data sets, and then used to prepare maps of variability in fertilizer needs across the fields. These are used to develop the database to guide the fertilizer application, adjusting the rates on-the-go, either applying nutrients individually or in combinations of materials.

Efficiency is improved because the amount of fertilizer applied is more closely matched to the estimated nutrient requirement for optimum yield for specific areas within the field. Field-average management tends to over-apply nutrients in low-yield areas of the field and under-apply in high yield areas. With site-specific management, rate adjustments are made on the basis of smaller, more uniform areas. Research has shown that one-acre grids are recommended for most intensive grain production areas with a long history of significant fertilization or manuring. Variable-rate application maps are provided to the fertilizer applicator operator to be used in guiding the on-board controller to adjust rates to match the recommendation as the applicator moves across the field. For best results, a database and map of the amounts actually applied should be generated to verify the rates used and to help build a record of the variable-rate applications made in the field. These actual application maps should closely match the recommendation maps.

All of the maps produced should be cataloged in a geographic information system (GIS) data management package for use in future recommendations and other types of analysis. This is part of the complete site-specific system needed to fully utilize the technology and gain the most advantage over conventional field-average nutrient management. For best efficiency, variable-rate applications of lime and nitrogen should be made as well as potash and phosphate. Initially, most people have focused on phosphorus and potassium, but the best return on the technology investment is often obtained from lime and nitrogen. Recent analysis of a field study by the University of Illinois showed a gross gain of $16.31 and $12.90 per acre, respectively, for two fields studied. Subtracting $8.00 per acre added cost for soil testing, mapping and variable-rate application, the net gain for variable-rate application on these fields was $8.31 and $4.90 per acre, respectively. Approximately 60 percent to 75 percent of this gain came from the variable-rate nitrogen application.

Another study projected that if phosphorus and potassium application rates are based on more precise guidelines than those used for field-average management, the variable-rate application will pay-off in approximately 60 percent of the fields. If variable-rate nitrogen, lime and/or herbicides are also used, the pay-off probably can be dramatically increased. Variable-rate application, based on site-specific data, is not just a good agronomic practice, it also holds substantial profit potential. (To see the original article on line, click here).

“Actually, it is surprisingly current,” Harold said. “The cost/return figures need to be updated, and would make the story for site-specific management even more compelling today.

“I would expand a little on the comments about low vs. high testing areas of the field needing different treatment – which is the main basis of increased efficiency and profit with site-specific management. For example, some areas of a field are naturally less productive for reasons other than fertility, so just adding fertilizer may not be beneficial.

“We always tend to focus on those low end areas and how to improve them,” he continued, “but I think the real potential increases come from what we can do to further enhance the already more productive areas of the field. The more we manage them for average’ production, the more we miss the real potential for increase in their yield. We need to think about putting the high yield management system in place that will take advantage of the good weather years when they come along. That also usually puts us in the best position to handle any stress that occurs.”

I agree with Harold about maximizing the higher yielding areas using VRA, and that’s what I’m hearing from most of the agronomists I talk to these days about strategy. Overall, it’s great to see approaches to VRA evolve, and the growing emphasis on collecting and utilizing field data will only serve to improve the effectiveness of a variable-rate fertility strategy.

 

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