By Harry Brook
Last week I talked about soil compaction and the effects it has on crop growth. I also mentioned gumbo soils. What is commonly called a “gumbo” soil is actually a soil type with structural problems. These soils are also known as Solonetzic soils and they have a heavy clay, impermeable or semi-impermeable layer just under the topsoil. The reason they are a problem is that these clay layers swell when wet and prevent roots from penetrating deeper into the soil. This causes crops to be shallow rooted and susceptible to drought. There are a lot of soils in Flagstaff County that are affected by this problem.
In a dry year it is easy to identify these soils. You’ll see crop with height variability. Wherever the crop is short that is where the topsoil is thin and the drought more severe.
Why does the clay swell? It is because the clay particles have high amounts of sodium attached to the clay particles. There are a few things you can do to improve the soil quality and tilth but they aren’t cheap or fast.
Back in the 1970s there were a number of field experiments using a deep, one-bottom plow with a scraper. The scraper would scrape off the topsoil, then the plow would invert the heavy clay layer with the parent material underneath it. The plow would then scrape the topsoil back on top of the inverted soil. This worked, as long as the lower soils were high in calcium. Once inverted, it destroyed the impermeable clay layer and rains would wash the calcium down the soil profile. Sodium on the clay particles would be replaced with calcium and prevent the swelling clay layer from reforming.
On the fields where this was demonstrated, it improved the soils and rooting depth of the crop for the long term. The one drawback was the cost of doing this. It was just too expensive to be done affordably. The plow had to go down to 3 feet to reach the calcareous parent material. There are other techniques that can be used. One is applying large amounts of limestone to the soil. Do not try this on soils with a salinity problem as that will make the salinity worse. It takes longer, and the limestone should be soluble. Rain will take the calcium into the soil and, once again, replace the sodium on the soil particles. It is expensive and takes time for rains to dissolve the calcium and move it down into the soil. An added benefit of using some limestone products is that it will also increase the soil pH. Chemical fertilizers tend to acidify soils over time, as your soil test will show, especially if you zero till.
Some people have used deep ripping or paratilling to try to break up the clay layer. The soil has to be dry to fracture but you only see an improvement for a year or two and the benefit does not last long. Once again, cost significantly outweighs the benefit.
There have been attempts to use crops to try to fracture the hard clay layer. Tillage radishes were tried in the county a few years ago. Unfortunately, their roots could not penetrate the clay barrier. They hit the hard clay and went sideways.
Poor soils due to the clay hardpan is also associated with low organic matter. You can improve growth on Solonetzic soils by increasing the organic matter, as organic matter has the ability to retain more moisture, leading to better plant growth. Using perennial forages can increase the organic matter, over time.
Unless you have large pots of money to spend, the main tools for improving these soils involve a good mix of crop types, both annual and perennial, organic matter application and some soluble calcium. That and good rain can make a difference. Building up organic matter with zero tillage is another gradual way to improve yields. Regardless of the approach, don’t expect sudden improvements.
For more information, watch the video below.