When should we think about phosphorus availability?

The other day I had the opportunity to provide a guest lecture to a class of juniors and seniors in Agricultural Systems Technology here at Iowa State. The class is about animal production systems and I was asked to provide a lecture on “manure.” It is always interesting when you get general requests like this as there are literally hundreds of issues about manure and it seems like you could pick almost any of them. However, in cases like this I usually talk about methods for determining appropriate manure application or in a case like the how to fill out an Iowa Manure Management Plan. There are lots of skills that I think would be useful, but when it comes down to it, making sure they understand the steps in determining manure application rates is the one I’d really like them to know.

As we talk about it, we use the check book analogy, i.e., there are two things you need to keep track of, the ins and the outs and we want to try to keep those in balance. When I do this for manure the two things we want to know are the demand side (how much of this nutrient do we need) and the supply side (how much of that nutrient is our manure going to provide). We do this first for nitrogen and talk about how losses might occur during application and how this might cause you to adjust your rate and what nitrogen availability means. Then we do this for phosphorus, talking about situations when your manure application rate might be limited by the amount of phosphorus you put on (based on the Phosphorus Index here in Iowa). Normally I do nitrogen first as its more complicated due to volatilization losses and having to worry about availability that it makes phosphorus seem easy.

However, the other day I got a good question… it was, “why don’t you worry about phosphorus availability when you do this?” I thought for a second, it seemed I done these calculations so often and usually just glossed over the phosphorus availability assumption that it was second nature to me, but now with an interested student in my office asking why I’d get a chance to explain it. So I answered, well usually we think of the fields getting manure as one that get repeated applications year after year, and since typically our manures supply more phosphorus than the crop needs we have built up phosphorus levels in the soil so we aren’t really concerned about availability since there will be enough there to satisfy our crop.

I’ve always thought this approach was fair, and was quick to accept it, but the student’s question got me thinking… is this still true? Recently we’ve seen lots higher N:P ratio’s in swine manure; this has occurred for numerous reasons including improved availability of phosphorus in DDGS as compared to more traditional ingredients as well as the inclusion phytase into the swine diet to improve phosphorus digestibility, but it might mean that we start to care more about phosphorus availability in swine manures. And just the other day I was writing about how far you could move manure if you were getting it to fields were there was a need for this phosphorus. What about these cases, i.e., fields with low soil test phosphorus, what does phosphorus availability mean in these instances?

Well, in general, a large fraction of the phosphorus in manure is considered to be plant available immediately after application (often 60-90%). The fraction that isn’t immediately plant available is often an organic fraction that will become available as microbes in the soil break apart those organic molecules and free up the phosphorus. The other part of phosphorus is very much soil dependent, there are lots of things (iron, aluminum, calcium) that can react with the phosphorus and make it more difficult for our crops to obtain. This is why soil sampling is so important, it gives us an estimate of the potential of seeing a yield effect with phosphorus application. Fields in the high or very high category see little to no response in yield with additional phosphorus, so considering phosphorus availability has no effect. In fields with very low or low soil phosphorus concentrations though we have much greater potential for a yield impact, and in these cases it would be important to make sure we are applying sufficient available phosphorus to supply what the crops need the first year or two. So if you are applying your manure to fields with low soil test phosphorus, you just might want to look at some of those first year phosphorus availability numbers to make sure your crops have what the need. For more information on phosphorus recommendations in Iowa see PM 1688: A General Guide for Crop Nutrient and Limestone Recommendations in Iowa.