Low pH and Wet Soil

This is the best piece I have read on Crop Talk in  a long time.

"Yes there is a connection. Wet soils allow for the growth of organic matter but don't let it mineralize as much due to the lack of oxygen and cooler soil temperatures. This leads to an accumulation of organic matter over time. Lots of deep black dirt with OM levels over 4%.


Organic matter is composed of elements held together by energy captured during photosynthisis. Generally, the larger the organic molecule assembled, the more energy that is stored and the lower the PH. (and the blacker the dirt) That high energy/ low PH breaks apart minerals in the soil like a leaky battery on a concrete floor eats away at the concrete or makes your manure spreader rust in record time. It makes nutrients available to plants by assembling nutrients into organic form that otherwise would be bound up in the inorganic form in the lattice of particles of silt and clay.

The problem is that increasing amounts of organic matter never breaks down (mineralize) to release the nutrients back to the plants, and new organic matter keeps forming and pulling more and more nutrients out of the declining mineral portion of the soil and incorporating them into the organic portion.

These long chain organic compounds are like long magnets with nutrient exchange sites lined up along their length. Once the OM production outpaces the nutrient supply, the result is lower PH. Empty exchange sites are filled with the only thing left, Hydrogen. You need to fill up those exchange sites with Ca+, K+, Mg+, Nitrate...? and the Hydrogen H+ will disappear. The other thing you can do is help decrease your OM by doing things to help it mineralize such as Tile and tillage. This would decrease the number of exchange sites, and raise the PH.

PH does not cause it to be wet, but wet soils start this cycle of high OM accumulation and low breakdown. OM long chain molecules are like long chain polymers that retain water much like a sponge. Rising OM levels increases the water holding capacity of the soil. And as I mentioned above, OM accumulation leads to low PH. So there is a connection between the PH, but the PH is a symtom and not a cause.

Lastly about the high P levels. A soil test does not tell what is in a soil sample. Repeat A soil test does not tell you what is in a soil sample. It tells you what is available in the soil. High OM leads to high energy organic acids making lots of P available in the sample. There might not actually be any more P in the soil on that high testing 40 than on your worst testing clay hillside, but one is available due to OM and one is not available due to mineral lattices. (its in the middle of a rock). If you really want to know what the posibilities of P in your soil are, I suggest splitting samples and running half at the lab as a manure test on both this field and your lowest testing P field. The results may shock you as they did me."

That is a good explanation for many soils.  However, we have a lot of old Illinoian Glacial Till in this region that is low organic matter with acid topsoil and very acid subsoil.  One percent organic matter is common with a pH of 6 in the topsoil and 4-6% in the subsoil.  Farmers struggle to get enough lime on these soils.

That's why farmers in this area will be liming and tiling these beautiful Midwestern soils for maximum productivity!

Ed