Soil Test Results - Phosphorus

While we already have talked corn yield (see Figure 2 for reference) I wanted to touch on soil test phosphorus.  The standard narrative is that the corn takes nutrients out of the soil we have to replace.  Once again our phosphorus data (Figure 1) goes against this conventional thought.

 

Figure 1:Soil test Phosphorus Before and after Corn

We took samples (10 samples per treatment) before and after corn, one in December 2015 an one in September 2016 just after the harvest.  In each case we are actually seeing and increase in phosphorus. Corn grain P removal in the grain for 140 bu/ac is ~ 21 lbs/ac P and for 100 bu/ac is ~ 15 lb/ac P.  Let alone the P that's still sitting in the corn stover on the surface of the soil.  Extractant was Mehlich 1, Clemson Soil test Rating for P>81 lb/ac is high. Soil series is an Orangeburg loamy sand, depth to clay ~10 inches.    

In case this experiment was considered a fluke, we have 97 sample (representing about 1,000 acres of corn) points that were sampled pre- and post corn and we looked at them again after corn harvest and we see we went from 81 to 86 lb/ac.  Now I am not saying this is an increase - it's definitely not statistically significant, but  we are not seeing a drop from expected P removal.

By the way, the manganese the folks are telling you to add?  Soil test Manganese increased from 12.5 lb/ac to 21.5 lb/ac.  A statistically significant increase - how do we explain this if we haven't added manganese for 5 years?

Don't get me wrong, P removal is happening, please understand that, but obviously a lot more is going on here, remember soils are deeper than 6".  

Rather than my carrying on, I invite you to search "mycorrhizae" and  "legacy phosphorus".  

Clearly our data are from the Southeastern Coastal Plain, but if you are a farmer, I urge you to rethink the narrative you are being told on phosphorus addition as I am reminded of the Capital One catch phrase: "What's in your wallet?". 

 

Figure 2: Corn Yield by Treatment