Here’s a response to a recent blog post we posted about—well—reading a blog. David Blanchard is an active member on the growers forum and a very insightful agronomist and farmer. This isn’t the first time someone, particularly Mr. Blanchard, has challenged our thinking and we hope it’s not the last.

The December 12, 2019 post on the No-Till Growers blog is titled "John Kempf Has a Blog and You Should Be Reading It…”

The question of whether or not applications of nitrogen and phosphorus in excess of crop demands can lead to problems with soil health and water quality has received a lot of lively debate in this forum. In the wider world of agricultural and environmental science there is no real debate about this, because the reality of the algal blooms and dead zones in places like Lake Erie, Chesapeake Bay, and the Gulf of Mexico is undeniable, as is the strong link between these problems and excessive use of agricultural inputs such as manure and fertilizers.

However, here the debate continues. This post will make a lot more sense if you first read the NTG blog post, follow the link given there to Kempf’s blog post about phosphorus, and finally read the journal article cited there. But to summarize, Here’s a quote from NTG quoting Kempf’s blog:

“… Like this one on the big debate about the mobility of phosphorus in soil, compost, and manure…

“New research just out reports that combinations of rock phosphate and manure increase phosphatase enzyme activity and organic matter1. Increased plant availability, but not water solubility. Which also means not leachable, and produces no water pollution. Perfect.” “

Wow. Apparently we have solved the the agricultural phosphorus problem – which, by the way, consistently makes it into lists of the world’s most serious environmental problems. And he cites a scientific paper to back up this claim, so it’s gotta be true, right? Sadly, wrong. Let’s dig deeper to see why this is so.

First, the idea of mixing manure and rock phosphate to increase the availability of the P in the rock phosphate has been around for decades (it did not originate with Kempf). I myself did it with cow manure back in the 1970s and 80s when I was working in dairy, and it’s a perfectly reasonable approach. So far, so good. And I don’t find any fault with the study itself that Kempf cited, which researched the use of manure, cover crops, and rock phosphate in a maize system in Kenya. There’s nothing startlingly original in it, but it did show that there’s value in these practices for farmers in East Africa, and that they increased soil organic matter and the activity of phosphatase enzymes (which catalyze the mineralization of organic forms of phosphorus into plant-available inorganic phosphate ions). It’s not surprising that when you increase inputs of raw organic matter, both soil organic matter and the enzymes produced by decomposer organisms increase. So, we’re still good. But now, Kempf starts to bend the truth when he states “Increased plant availability, but not water solubility. Which also means not leachable, and produces no water pollution. Perfect.” Actually, there is nothing in this paper about phosphorus availability, the water solubility of phosphorus, the leachability of phosphorus, or water pollution. Or perfection, for that matter. To imply otherwise is extremely misleading, and is unfair to both the authors of the paper and to Kempf’s readers.

In reality, plant roots take up phosphorus ions that are dissolved in the soil water. So to be plant-available phosphorus has to be water soluble. Read the chapter on phosphorus in any soil science textbook and you will find that this is so. Or for a reliable, but not-quite-textbook on soils, get “Building Soils for Better Crops” from SARE, which you can download free here:

www.sare.org/…/or-Better-Crops-3rd-Edition

Read chapters 18 & 19 on nutrient management, nitrogen, and phosphorus.

By all means, read John Kempf’s blog. There’s some interesting stuff there. But do so with your BS detector switched on, because there’s a lot of chaff that needs to separated from the wheat. And practice responsible nutrient management on your farm. One blog post doesn’t make problems with excess phosphorus go away.

- David Blanchard