What to do if your soil becomes too acidic

Thursday, June 6, 2013

Low soil pH can cause your plants to suffer from nutrient deficiencies. Doing proper soil testing and applying agricultural lime is the answer

by KEITH REID

We are fortunate that most agricultural soils in Ontario are near neutral in pH – neither too acidic nor too alkaline. As a result, we can usually ignore soil pH and get away with it – until it becomes a problem.  To use a mechanical analogy, it is easy not to check the oil in an engine where the level never drops, until suddenly the engine seizes.

Only about 10 per cent of the agricultural soils in Ontario are acidic. This means that the soil pH is low enough to hurt the growth of some (or most) crops. Unless you are growing blueberries or potatoes, you will probably want to correct the pH in these fields.

How do soils become acidic? Soils naturally become more acidic over time. Pure rainwater is slightly acid, because carbon dioxide from the air dissolved in rainwater forms carbonic acid, so every rainfall contributes (slightly) to soil acidity.

Plant roots excrete acids into the soil, since they balance the electrical charges in the root by releasing positively charged hydrogen ions when they absorb positively charged potassium or calcium. When these plants die and their residues decompose, the process creates organic acids.

Human activity speeds up this process. Air pollution makes the rainfall much more acidic than it would otherwise have been. Widespread use of nitrogen fertilizers also hastens soil acidification, as the conversion of ammonium to nitrate releases acidity. The result has been more rapid declines in soil pH in susceptible soils.

Occasionally, we see soil pH levels increasing (i.e. becoming more alkaline) rather than decreasing. This can almost always be attributed to tillage mixing subsoil with the topsoil, either because erosion has lowered the soil surface, or because new tillage implements are working at a greater depth.

Why are acid soils harmful? Soil pH influences both the chemistry and the biology of the soil, which can, in turn, affect plant growth. The solubility of most nutrients changes with soil pH, and for most macronutrients it declines as pH goes down, causing the plants to suffer from nutrient deficiencies.
The most striking example of this is phosphorus. It is most available at neutral pH, and becomes less available as the pH moves either to more acid or more alkaline conditions.  The drop in availability, however, is faster and more severe on the acid side.  

In contrast, most of the micronutrients (such as manganese, zinc or copper) are more available in acid soils.

Low soil pH may also be associated with a lack of some nutrients in the soil. Calcium and magnesium do not directly affect soil pH, but low pH invariably occurs on soils where those elements have been depleted.

Soil microbes can be highly affected by pH, and many of the beneficial bacteria cannot survive in acidic conditions. We see this when nitrogen fixation by Rhizobia declines, so alfalfa or soybeans display nitrogen deficiency symptoms. Other impacts on soil biology are more subtle, but just as real.

Finally, acid soil conditions will increase the solubility of elements that can be toxic to plant growth. High levels of aluminum or manganese released into the soil solution can damage plant roots, which in turn reduces the ability of the plant to absorb water and nutrients.

There is a range in sensitivity to these harmful elements among different species, so you may be able to grow corn quite successfully in soils where wheat or barley doesn't survive due to low pH. A soil test is the best tool to tell you exactly what your soil pH actually is.

How can you correct soil acidity? If you are faced with the challenge of an acidic soil, add agricultural lime. This material reacts with the excess hydrogen ions in the soil, which causes the acidity, and removes them by tying them up in water molecules. This process, however, takes time, so you will not get the maximum effect from added limestone for a year or more after application.

Some of this time lag comes from the time it takes the limestone to dissolve, but a bigger part is the slow diffusion of hydrogen ions from the clay colloids and organic matter into the soil solution where they can react with the lime. You can speed up how quickly the limestone dissolves by using finely ground materials, but this won't affect the second part of the process.

The process for effective mitigation of soil acidity is to:

Determine if you need to apply lime, using the soil pH test.

Determine how much you need to apply, using the buffer pH test.

Choose the most economical source of lime, based on the cost per acre of applying enough lime to neutralize the buffer pH. (Some limes are more effective than others, so need less product to accomplish the same thing.)

Apply the lime as far ahead of the sensitive crop as you can, usually in late summer or fall for a crop to be planted next spring.

Mix limestone thoroughly with the soil. The limestone particles only influence the soil in very close proximity, so limestone left on the surface or poorly incorporated will not be very effective.

The final choice is whether to use calcitic or dolomitic lime. If your soil test shows that you need magnesium, then you should definitely be using dolomitic lime, since it is the most economical source of magnesium. For soils that don't need magnesium, then either material can be used effectively. BF

Keith Reid is manager, Eastern Canada, Soil Nutrient and GHG Management Agriculture and Agri-Food Canada, Guelph.