Understanding soil pH Part II

Growers know soil pH is an important soil characteristic, but may not understand what pH really is and what it does. pH is a standard characteristic measured by a soil test, but how does it influence soil productivity and eventually plant growth and yield?

In my previous article, “Understanding soil pH Part I,” I discussed what pH measures, what the numbers mean, how pH affects nutrient uptake and, therefore, plant growth. But pH affects plant growth through more than just making necessary nutrients more available for uptake by the plant. This article continues the pH discussion looking at different soil characteristics influenced by pH.

Soil micro-organisms

Bacteria are generally more prevalent at higher pH and fungi at lower pH. Not surprisingly a soil pH between 6 and 7 will be the best environment to maximize both. When it comes to soil microbes, diversity is better. By making soil friendlier to good microbes it can help work against the bad ones. Microbes are also responsible for making soil nitrogen available through the nitrification process. There is a lot going on underneath your feet!

Knowing that pH affects microbial activity can help combat some diseases like potato scab (Streptomyces scabies) and club root (Plasmodiophora brassicae) of Brassicas. Both diseases are fungi whose severity is related to soil pH. Potato scab is less of a problem at pH 5.0 to 5.2, while club root is decreased at pH 7.0 to 7.3.

Heavy metals

Heavy metals like lead, mercury, copper, zinc, aluminum and cadmium become more mobile at lower pH. This generally occurs outside the normal range of crop production but there may be certain situations (application of sulfur or ammonium sulfate) where certain areas of the field may have a temporary pH low enough to allow release of these metals. If this happens, the biggest concern is that they would leach into ground water or run off into surface waters. This should not be a problem if a soil test is obtained and the proper amounts of product applied in the proper manner.

Pesticide interactions

pH affects pesticides in a couple ways. At lower pH, certain pesticides will bind more tightly to clay particles and, therefore, will be less effective. However, lower pH can reduce pesticide-leaching potential. This characteristic can also cause problems when soils are limed and pesticides liberated. When this happens, pesticide carryover problems can appear years after a pesticide was applied. pH also has an indirect effect on pesticides since pH affects microbial activity and microbes are responsible for degradation of some pesticides and greater microbe activity will lead to quicker pesticide breakdown. Organophosphate insecticides and other pesticides are more readily degraded at higher pH. Read the label for any precautionary concerns.

I hope after reading this and my previous article you have a better understanding and appreciation on the importance of soil pH in the performance of your crops. Do yourself a favor and conduct periodic soil tests to determine nutrient levels as well as pH. You should have a soil test done at least every three years, more often on sandy soils since they can change more quickly, and more often if you have a problem site you are trying to correct. For a general reference on soil acidity and liming go to, refer to MSU Extension Bulletin E1566 Facts About Soil Acidity and Lime.

For more information on commercial vegetable production, contact Ron Goldy at 269-944-1477 ext. 207.

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