Soil Phos Building. How Much is Enough?

There has been a lot of discussion in ag circles in the last couple of years about phosphorous removal in soils. There is good reason for this. Yields have been high and crop fertility practices are pushing ever harder to maximize yield potential. Nitrogen has been the focus of this push for the most part, and in many if not most cases applied phos has not been scaled up in kind. The ratio of P removal vs P replacement has not been balanced.

There is an inverse relationship between the phosphorous supply of a given soil and the responsiveness of the same soil to applied P. Soil P supply has therefore diminished in these soils that have been “mined”. The responsiveness to applied P would therefore tend to be higher and higher after each year that P removal exceeds replacement.

In my many years of providing crop fertility service, I have consistently observed that the best yielding fields tend to be soils rich in phosphorous – in other words high soil P supply (therefore tend to be *less* responsive to applied P). Targeting low P supply fields for phosphorous building is therefore a smart thing to do to build soil productivity. There is only one way to do this – P applications must exceed P removal. It is not enough to merely meet the current yield targets of the crop.

And so we adopt a P building program…now what? How much P should be applied? How much annual P building is enough? When do we switch from “building” to “maintaining”? I believe it is impossible to guess at this or do it blindly. We have to have a target “ideal” for long term P supply and blind strategies are like shooting in the dark. We have to know where our soil is at and we have to know where we want to go.

The IPNI has a cool website which calculates nutrient removal for different crops. Bookmark that site. Very useful but still only a guide. Soil is much more complicated then a linear calculation of crop removal.  In order to keep moving toward, and ultimately achieve, our optimum soil P supply target we must have strategies based on “ground truth-ed” measurements.

Not every soil needs to have a P building strategy. Some soils are rich in crop available phosphorous. So rich that it can actually be drawn down or “mined” – in some cases for years – by not applying P and still be a P rich soil. Manured soils are the obvious case that comes to mind. We could actually have the same P supply target in these soils but our strategy is the opposite of P building. We allow for crop removal of P to draw soils down to our “target” P supply.

Below is a Western Ag CropCaster image of a field which we would allow P to be “mined”. There is no need to guess or try to calculate when we would switch from a “mining” strategy to a “maintenance” strategy. Annual sampling will show us when we approach our P supply target.

figure 1 cropcaster image

What should our “target” P supply be? The decision rests with each individual client, however I believe that it is pointless to use a maintenance strategy on the above soil. At the very least the target P supply should be allowed to fall to the point where the crop can respond to the applied P. If we set aggressive yield targets of 70 bu canola then current P supply of 117 lbs could draw down to 70 lbs before we would see even a hint of response to applied phos. I suggest that the P supply could be allowed to fall below the 70 lb level, to a range where P maintenance (balance between crop removal and applied) and P responsiveness are at least in the same ball park. Soils capable of at least 25 lbs P supply still have a strong tendency to high productivity and would match up pretty nicely with a maintenance strategy for those pushing aggressively on yield. Using the IPNI calculator the maintenance to cover 70 bu canola would be about 56 lbs. Once we have arrived at the target P supply “space” the P maintenance vs applied P fertility could be fine tuned and reconciled based on continued annual Western Ag sampling and optimized crop fertility planning.







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