November 12, 2013

Let’s Stop Treating Our Soil Like Dirt – Part 2

Angela Smith

Angela Smith

Project Adviser

Johns Hopkins Center for a Livable Future

Soil Test ResultsIt’s been a few weeks since my last blog in which I waxed poetic about how wonderful and amazing soil is. I concluded by taking a soil test in my lower field and sending it off to the University of Massachusetts soil testing laboratory in Amherst. The results are not good. I quote: “Both Phosphorus (P) and Potassium (K) levels are extremely high in this soil. Over fertilization can lead to insect problems and plant nutritional disorders. DO NOT [it said in capital letters] add additional P and K.” For those of you who know your soil science, we’re talking 75 ppm of phosphorus and 377 ppm of potassium. The soil scientist at the lab says that ideal levels are 10–14 ppm for P and 160 ppm for K. So how did I end up with levels nearly two to five times too high? Soil tests from an upper field that has been in the Conservation Reserve Program for the last 10 years reveal nutrient levels that are in the low to medium range and of no concern. It’s true that the lower field had been in a conventional corn-soy-corn rotation for the past three years, but before that it had been mostly fallow since the late 1970s when four horses were being boarded down there. The previous owner didn’t know whether the farmer renting the land had used fertilizers, but I know that it’s entirely possible, as the majority of corn and soybean acreage are treated in some way, whether with synthetic fertilizers or manure. Still, even assuming that fertilizers were used, was it possible that only three years of conventional farming could raise my nutrient levels to such excessive rates? That seemed crazy. I decided to do some detective work to find out.

First question: Does either potassium or phosphorus occur naturally in my environment?

Answer: Yes. Potassium (K) is typically found in soil, but 90 to 98 percent of it is in a form that cannot be used by plants. Soil tests measure only potassium that is readily available for plants to use, so my results have nothing to do with the vast majority of the potassium that may be in my field. Phosphorus is a more complicated story. Phosphorus (P) does occur in natively high levels in southeastern Minnesota, where I live. It is naturally present in rock, sediment, soil, and organic matter. Unfortunately, according to a University of Minnesota Extension Educator I talked to, it’s hard to discern what a natively high level of P might be, but around 20 ppm seems to be as good a figure as any. If this is the naturally high end of the scale, then finding 75 ppm in my field must be accounted for by something else.

Second Question: Could these nutrients have come from the river that runs through my property?

Answer: Probably not. Potassium is normally found at rates of only 2 to 3 ppm in rivers. Phosphorus in an undisturbed environment only enters rivers through erosion of rocks and soils, and the decomposition of dead plants and animals. Along my segment of the river, water quality researchers from the Minnesota Pollution Control Agency measured total P levels between 0.08 ppm and 1.59 ppm over a five-month period in 2008. One of their employees told me that even during a flood you would never see levels greater than 3 ppm, and even that high a level would be rare. Another possible contributor may come from the farmer who owns the field above my land. He applies manure to his fields, and his land likely drains into our seasonal stream that flows alongside the lower field. Conceivably, that stream could overflow onto the land, leaving nutrients behind as the waters recede. Even so, in order for this to be the source, we would expect to see lots of sediment and soil piling up as P only travels when bound to soil particles, and that just doesn’t seem to be happening.

Third Question: Did the farmer who rented the land simply put too much fertilizer down?

Answer: It seems likely, and everyone I talk to thinks that this is the explanation. Elevated soil P levels are often caused by a history of excessive manure application, as are really high levels of K. Often this occurs because farmers base their applications on how much nitrogen (N) is needed by a crop, but in doing so can over-apply the P and K that comes along for the ride. I discovered that in 2006 the farmer who rented my land applied for a permit to increase the number of hogs his family raises from 2,000 to 4,000. (They also had around 221 dairy cows at the time.) The request for public comment about the proposed expansion states manure would be collected once or twice a year and sprayed on designated fields. Take as an example just the 2,000 hogs, and we’re talking about 3,600 tons of manure and 10,000 pounds of P annually. Now, it is conceivable that the farmer who rented my land has enough land on which to spray all of this manure, but research tells us that between 1982 and 1997 the U.S. saw a 64 percent increase in the amount of manure that could not be absorbed by our soils. So, it seems a fair assumption that he might have used some of the manure from the swine or dairy operation to help fertilize my fields. Neighbors have told me that he does take soil samples and adjust his fertilizer use accordingly, and I believe them. The simplest way to find out would be to ask him, but I’m not going to do that. I am the new kid on this farming block, and the folks who are farming around me are still my neighbors. So in the end, after all of this, I can’t say for certain where these high nutrient levels came from. But finding the answer is superseded by a more important lesson: the decisions that each of us farmers make about how we manage the land, how we raise animals for meat, and how we prevent nutrients from running off into the waterways add up. We like to think of our properties as self-contained parcels, but nature has no boundaries, and our decisions – perhaps made rationally at the individual level – collectively can cause some serious damage. There is always a downstream, and we need to do better in thinking beyond our own borders. After all, the health of our own land depends in no small part on the health of the whole. [1] I can’t seem to find out whether the expansion was actually permitted, so am using the smaller number. Also, information about the types of hogs being raised (e.g., feeder-to-finish, wean, finishing, etc.) is unavailable, but weights can range from 30 lbs. for a baby to 1,417 lbs. See this chapter from Clemson University for greater detail about different sizes and amounts of manure. Photo: Angela Smith, 2013. << Read Angela’s previous post in this series: Let’s Stop Treating Our Soil Like Dirt – Part 1 >> Read the next installment by Angela Smith, Gone with the Wind (and Water)


  1. Posted by Gary Joachim

    You mentioned 4 horses had been boarded there. How many acres in the lower field and what has been its long term manure history? Very typical in Southern MN that P levels are higher the closer to the building site because in days gone by most everyone had livestock and when the weather was cold, wet, or a combination of the two manure tended to be hauled close to the barn.

  2. Angela Smith

    Posted by Angela Smith

    Good question, Gary. I wish that I knew more about how long the horses had been boarded down there. Best guess would be at least 5 years, maybe more. The acreage is about 2.5, although there are another 7 acres currently in CRP that I could imagine might also have been used for the pasture. Interesting you mention P levels being higher near building sites as I also recently learned that there had been a stable/barn in the middle of the field in question, so perhaps that was a contributor. Do you know if the scenario you’re describing would also account for the excessively high levels of potassium?

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