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Effingham, IL | I feel like this is an important topic so I started a new thread to keep it fresh. I hope nobody minds me doing it this way.
Ron, you ask some excellent questions regarding the practice of reducing the rate of fertilizer in a banded situation. The questions you ask get to the heart of the controversy on whether to reduce rates or not.
The theory in reducing rates is based upon the fact that a corn plant can more effectively utilize the nutrients applied when placed in a band. This theory does have some basis in fact (but not completely) but you have to go back to basic soil fertility to understand the reasoning behind the theory.
We know that when we broadcast apply phosphate and potassium containing fertilizers, somewhere between 20-40% of the applied nutrients are available to the crop in the year of application. The remainder is "tied up" by the soil and released at a later date. In other words, 60-80% of the nutrients go into the soils reserves.
In a banding situation, we increase the amount of applied nutrients which are available to the crop. So instead of only 20-40% being available under broadcast, approximately 50-60% is available in a band. This is where the "efficiency" comes from, but this "efficiency" is very often misunderstood. Some have interpreted this to mean that the crop does not take up as much nutrient when the fertilizer is banded. This is a false assumption. The harvested grain still removes the same amount of nutrient. We've only reduced the amount of nutrient in which the soils reserves must supply to the plant which brings us to Dr. Rehms research.
Dr. Rehm has shown that under low and very low soil testing conditions a 50% rate of banded fertilizer is equal to or better than a full rate broadcast application. Why is this so? Again, you must go back to basic soil fertility for the answer.
We know that when a soil tests low in nutrients, the soils ability to supply the amount of nutrients required by the plant is greatly reduced, thus the crop cannot uptake the amount of nutrients needed for maximum yield. Broadcast fertilizer has a limited ability to correct this situation because we know that the majority of what is applied is unavailable to the plant. Banded fertilizer however, increases nutrient availability in the year applied, so the net effect is that when you add up what is released by the soil and what is available from the applied fertilizer material, a 50% rate in a band is equal to a 100% broadcast rate as far as total nutrients available to the crop - thus yields are equal in both situations. We need to be careful though when we apply this same strategy to a soil that tests adequate in nutrients.
There is no doubt a 50% rate in a band in a soil that tests adequate will yield as well as when we use a 100% rate. But over time you will slowly deplete the soils nutrient reserves which in turn slowly reduces the soils ability to produce maximum yields. In other words, you might start out with a soil that has the ability to produce a maximum of 180 bu. corn, then over a given period of time (because you have slowly depleted the soils reserves) you might end up with 100 bu. maximum potential. So the question becomes, is it worth the cost savings in fertilizer over the long run? What it boils down to is that it depends on how quickly that maximum potential is reduced, and quite frankly, I do not know that anybody has a good answer to that question.
I do think that in high testing situations this is a no-brainer. Reducing the rate is certainly a wise decision, especially if one is not comfortable cutting off nutrient applications altogether like the universities and the soil labs recommend.
You also asked what everyone is using as a basis for the reduced rates. In my experience most people who are doing this simply take the removal rates for their chosen yield goal and cut them in half. So if your yield goal was 170 bu. and the removal rate was 65 lbs. P2O5 and 46 lbs. K2O then you would apply 1/2 that rate.
Another question you asked is why someone would want to "mohawk" the corn's roots by band applying fertilizer. The short answer to that question is that the roots do not coalesce around the banded fertilizer. There's really only two factors that determine where a root grows. This is 1) the path of least resistance and 2) where the moisture is located. Fertilizer placement has no bearing on this whatsoever.
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Reduced Rates of Fertilizer - In reference to Ron's posts
