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Coldwater, Michigan | Well Ron, I'm going to dive into this with both feet! First of all, as has been written in the soils books for years (Modern Corn Production), P is only 5-20% available to the crop in the year of application. K is somewhere around 40-60% available in year of application.
Why is this so? Partly because of the chemistry of the element. K is a more mobile element within the soil. If a root doesn't hit it, it can draw the element a short distance to the root and take it up. On the other hand, P is essentially a non-mobile element. If the root doesn't hit it, it will not be taken up! Therefore you have differences in how much will be taken in within the year of application strictly due to chemistry. End of Chemistry lesson!
Broadcast vs Banding is a "Physics Lesson"! When we broadcast an element we expose a small concentration of element( P& K) to a large amount of soil. Thus it is easy for the soil to "fix" the element before the element is taken into the root system to nourish the plant. If instead of broadcasting, we chose to Band an element we change the equation a bit. We are now applying a concentrated element in a small amount of soil. Because of the more limited exposure of the element to the soil particle, it takes longer for the element to become "fixed" by the soil. Thus we have a longer availability of the element to the root (or plant). Now that part was actually "Chemistry" and "Physics".
Now for the "Physics" lesson. Because the banded is close to the root system where "most" of the roots are concentrated early on, you have more roots taking up elements before they those elements become fixed. It is easy to see why you can have a more efficient uptake of nutrients by banding. This is the case whether a soil is high testing, or low! You don't see the effect as much in high testing soils because the soil has so much fertilizer in it that we literally flood the root system with fertilizer and we have plenty of it. Therefore, additional applications of banded often shows no additional response.
Now the question most don't want to ask! At what expense did we flood the soil? It took a potload of fertilizer to get the soil to the point that no matter where we plant that crop it has a flood of fertilizer around it. What if we would have been content to fertilize the total to a lower level, though enough to provide adequate fertility to the crop, then used a band of concentrated, more utilized nutrients to get the system started faster in the spring?
The way we have done it for years assures that we have elements that will never give most of us availability for years to come. Most soils people will admit that we only get about 1% of total nutrients to show on a soil test at any given time.
At one time, when I was starting in the fertilizer business, we tried to never broadcast P because it was an immobile element. We did however broadcast K because the mobility within the soil was so much better. Place the immobile elements close to the root system, the others can be placed anywhere. No-till or strip-till does present some issues of their own because we are not mixing those elements within the soil as we do with conventional till.
While some of the basics remain the same, some things need a slightly different approach. If we no-till, or strip-till how do we get more of those immobile nutrients, or elements, to the plant if we don't band? Even the fairly mobile elements like potash have become a bit more of an issue when we don't disturb all of the soil. The ridge-till farmers ran into potash problems years ago, even though they had high soil tests. I think if we don't disturb most of the soil with our farming systems, we must do a certain amount of banding to maintain or increase yields.
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Reduced Rates of Fertilizer - In reference to Ron's posts
