For dozens if not thousands of years farmers have inherently known "good soil" from "poor soil." The first and most obvious is the color and smell. Both of these sensory elements relate to the break down of organic matter... decomposing carbon material... dead plant and animal life and manure. This natural compost is biologically active even after losing the original plant shape. Microbial life is busy eating and converting material into energy and releasing nutrients that help new plant life flourish.
There is a lot of work in modern agriculture to refine variable rate nitrogen. Improving the rate and timing of nitrogen is important for improving water quality as it is managing costs and yield on the farm. As one of the single most expensive inputs on the farm, nitrogen is also high on the list for farmers to manage well. However, it is very complex given the mobility and variability within the field.
Rather than explain the entire N cycle, I would like to share and propose the use of soil test OM values to improve nitrogen rate decisions on your farm. At a minimum this approach can provide you some base guidelines for how "good" exactly is your soil. In some more complex treatments, you might also model the nitrogen mineralization from the OM based on soil moisture and temperature properties. This can empower you to tweak timing and understand plant stress at various growth stages.
We work with a farmer that has taken soil samples on 1 acre grids and uses OM to drive his VR seed and VR nitrogen rates. Still others we work with deploy ISNT or other measures. Every farm and farmer is unique so approach this with your own smallest management unit... for one, subdividing a soil type will be enough, while another will want a tighter grid or even some higher resolution soil color sensor.
A simple suggestion to step up from just the basic SSURGO soil type maps would be to subdivide your SSURGO zones for a modified grid soil test. In so doing, you take your soil test in smaller zones which better describe patterns within the field (agree or not?!) Then you would use the OM value from your soil test and assign an anticipated nitrate release through the growing season. When you build out your nitrogen budget you can then deduct this soil contribution!
Piece of cake, right?! Well, not quite... since any field with large variability in soil OM probably should also have variability in yield goals within the field. If that is the case (which is normally so), then you'll want to adjust your gross N needs before deducting soil contributions. In some cases this might even things out, but in many areas, your yield goals might be nitrogen limited because of historical compromises you've made elsewhere. What do I mean by that? Perhaps you've been shorting nitrogen rates in that "medium" textured soil because you didn't want to over do it on the poor soil? You've had enough seed and water, but ran out of N. Not good! So, is the answer applying a higher straight rate across whole field? Not if you want economics or environment to pan out well for your future!
In this way, we believe it is critical to compile ALL aspects of the soil that can effect yield and nitrogen movement. You can make significant strides in cost savings and efficiency by doing a good job creating zone specific yield goals and working backwards from that. However, the true yield gain values are wrought after understanding what of your soil can be managed to optimize performance. That is, what is most limiting? Fix it, then ask what is now most limiting. Fix it, then so and and so forth. (ever heard of Game Theory?!) When this is applied to nitrogen as it is relative to other aspects of crop growth you stand better odds of increasing yields along with improved N efficiency.
If you'd like to know how we use software to do this in our business, please visit http://www.optmzr.co/software.php#soil-test-upload
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