Final Lab Note, Soybean Yield after Wheat - Biggest Surprise

This is the last the lab notes from Farmer Carl and me, for this project – turns out that because we nursed our funds a bit and had some help from a lot of volunteers, we were able to get the yield data for soybeans following wheat and a set of soil test results.

One of the things that is common practice and one can see this from our state fertilizer recommendations is that if you double crop soybeans behind wheat, you always make allowances to ensure your soybeans get enough P and K. The P2O5 and K2O applied in this experiment were all done in November 2014, so we are talking the bean yield taken in November 2015.

Here are the soybean yield results for 2015 arranged by fertilizer treatment (n=10 per treatment):

 

Needless to say, it doesn’t take a statistician to say that there is no difference in soybean yield between treatments here. The BIG take-home is that the P2O5 and K2O applied in the fall made no difference at all. So, for this situation, it makes zero economic sense for the farmer at apply P2O5 and K2O. While we can’t rush out and say this applies to all situations, we can say that the conventional wisdom of making allowance for P and K to soybeans after wheat may have to be reexamined. Underlying this conventional wisdom is the assumption (obviously unspoken) that commercial fertilizer is the only source of fertilizer the farmer ought to count on (the corollary to this is that yes, there is organic fertilizer etc., but these make no practical difference). Clearly this assumption needs to be revisited under various circumstances.

Another nail in the coffin of this assumption is the change in soil test P and soil test K between October 2014 and November 2015 (see table below). While we see a drop in soil test P (lb/ac) the input of fertilizer P in the last treatment barely seems to have been an influence – as the differences between treatments is zero. Note that soil test K actually increased and there appears to be no influence on the amount of soil test K from the 125 lbs/ac added in the first treatment.

 

So, in short to answer the question “how much fertilizer do we really need”, in this case we can say, yes we still need nitrogen, but maybe not as much. As for P2O5 and K2O, nope, not really. Again, this is for coastal plain soils in South Carolina, let’s not ge carried away in saying we don’t need P and K fertilizer, but at least this research suggests “hey, let’s go back and see if those assumptions still hold true”!

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