Mind Your pH!

AGRI-VIEWS
by Chuck Otte, County Extension Agent

I remember as a small boy being so excited that our fields were going to be limed. You can imagine how disappointed I was when the big truck came and spread this funny white stuff on the fields. I was expecting green citrus fruits!

One of the most important soil characteristics that impact plant growth is soil pH or how acid or alkaline is the soil. The pH scale runs from 0 to 14 with 0 being very acid, 14 being very alkaline and 7 being neutral. In general, most plants grow best in a soil whose pH is between 6.5 and 7. This neutral to slightly acid range is where we find the best balance of plant nutrient availability. Of course there are some exceptions to every rule. Potatoes and blueberries grow better with an acid soil. Asparagus and many of the legume plants grow quite well in somewhat alkaline soils, but for everything else we should aim for neutral or slightly acid.

Natural weathering processes tend to acidify soil. Applications of nitrogen fertilizers and break down of organic matter both are major soil acidifiers. This is probably a good thing. If soils become too acid we use calcium carbonate containing compounds (limestone) to neutralize this acidity. If soils become too alkaline we need to acidify them using compounds like sulfur and ammonium sulfate. This doesn’t happen too often, fortunately, because sulfur based soil acidifying compounds are much more expensive than limestone.

Soil pH is critical because of needed plant nutrients that are present in soil. The mere presence of a compound, say iron, in the soil does not necessarily mean that it is available to the plant. In soils of neutral and alkaline pH, iron tends to become tied up in compounds that are simply unavailable to the plant. If you are trying to grow a plant with a high iron need the plant will become chlorotic, or the plant equivalent of anemic. If you apply more iron fertilizer to the soil it too becomes bound up in plant unavailable forms. In severe chlorosis the plant dies. The soil needs to be acidified to allow the iron to become plant available. On the other side of neutral, another critical plant nutrient, phosphorus becomes tied up in an unavailable form if the pH drops much below 6.0. And in severely acid soils, such as below 5.0, there can be so much free aluminum in the soil it will literally burn the roots of many plants.

To know what your soil pH is you have to test. We know that in the higher rainfall areas of eastern and southeastern Kansas we are very likely to have acidic soils needing lime. We also know that as you move west across Kansas into lower rainfall zones soil acidity is less of a problem and we start seeing more problems with alkaline soils. We live in an area that has a lot of natural limestone. Yet we also have many soils that are surprisingly acidic. And the soils can change rapidly in pH in just a hundred feet or so. You don’t know until you test!

If you have an alkaline soil it is only practical to use soil acidifying products if it is on a small scale, such as a yard, garden or around a few landscape plants. Lowering the pH on an 80 acre field is simply not economically feasible. If you have an acid soil though, you’re in luck! Liming acid soils is much more economical and practical. If your soil test comes back and indicates an acid soil the test result will also tell you how much effective calcium carbonate you need to add. This recommendation is based on the soils inherent ability to buffer changes to soil pH. Two soils with the same pH may not need the same amount of liming materials. Next week we’ll talk about the different liming materials and which ones are better.

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