Seedbed: The myth of 'ideal' soil pH ranges

Tuesday, June 3, 2008

Some take these ranges to mean that they should try to reduce a high soil pH,
which is generally futile and has little impact on crop growth

by KEITH REID

A popular chart in many agronomic publications is the list of ideal pH ranges for various crops. The implication is that you shouldn't even try to grow a crop that is outside this range, or that you should work diligently to make sure your soil is within the range for the crops you want to grow.

While I don't want to minimize the importance of soil pH on crop productivity, I find that the implied meanings of these charts greatly exaggerate the reality.

The source of most of the ideal pH charts seems to be agronomists and soil scientists who work in areas with neutral to acid soils. They recommend testing the soil and, ifnecessary, "correcting" the soil pH. In this context, "correction" means liming, period.  And, also in this context, the upper limit on the soil pH range really means, "If you are foolish enough to even think about liming beyond this range, you are wasting your money and you might induce a deficiency of some micronutrients."

Unfortunately, some have taken the range to mean that they should try to reduce a high soil pH, which is generally futile and has little impact on crop growth.
What impact does low soil pH have?

There are a number of changes to soil chemistry under acid conditions which could hurt crop growth, and I think it is fair to say that we don't know which ones have the biggest impact since they all happen together.

Acid soils tend to be low in calcium and magnesium, so a deficiency in these basic cations may be part of the picture. Phosphorus solubility is also much lower in acid soils, so P deficiency can come into play. Some minerals, like aluminum and manganese, are toxic at high concentrations, and these dissolve from soil minerals at low pH.

There can also be a direct impact from the high concentration of hydrogen ions in an acid soil, although this is probably a factor only in very acid conditions. Plants will tolerate much lower pH in organic soils, where the release of aluminum from soil minerals is not an issue.

Plants vary in their tolerance to the conditions found in acid soils because of their sensitivity to deficiencies of the basic cations or to injury from harmful elements. Tolerant plants have developed mechanisms to immobilize these harmful elements, so that they don't build up to toxic levels. In the case of legumes, the rhizobia in the nodules are more sensitive to low pH than the host plant, so the soybean will grow but it will be deficient in nitrogen.

How can I manage low soil pH?

The most effective and most economical way to deal with acid soils is to apply agricultural lime and work it in. The buffer pH in a soil test will tell you how much lime to apply.

What impact does high soil pH have?

For most crops, the impact of high soil pH (alkaline soils) is much less than for low pH. There are a couple of exceptions – blueberries will not grow at all in alkaline soils, and some flower species need low soil pH for proper colours on their flowers – but most crops will grow well in alkaline soils.

There are, however, two challenges with alkaline soils. First, the high soil pH can reduce the solubility of micronutrients to the point of inducing deficiency, and can also tie up phosphorus in the soil.

The second challenge is indirect, as high soil pH is often a symptom of some other problem in the soil (for example, an eroded area with exposed subsoil that is low in nutrients and organic matter) which adversely affects crops. Both of these are addressed by managing the symptoms rather than by trying to reduce the pH in these areas.

How can I manage high soil pH?

Acidifying a highly alkaline soil is futile. The amount of acidity required would be huge to start with, since the pH scale is logarithmic. Each pH unit increase is 10 times less hydrogen ions in the system, so going from a pH of eight down to five would need 1,000 times the acid of changing from six to five.

Adding to this, the alkaline soils have large reserves of free lime (calcium or magnesium carbonate), which neutralize any acidity as soon as it is released. Experiments in South Dakota have shown that up to eight tons of elemental sulphur per acre proved ineffective at reducing the soil pH from 7.6.

Tie-up of phosphorus can be addressed by banding rather than broadcasting phosphorus fertilizers, to minimize the contact between the soil and the fertilizer. Micronutrient deficiencies can be dealt with by applying zinc in the starter band for corn, or foliar application of manganese to cereals or soybeans.  BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture and Food
and Rural Affairs based in Stratford. keith.reid@ontario.ca