Putting soil pH into perspective

‘Producers who worship at the altar of pH need to understand that good pH does not guarantee a balanced soil,’ warned international soil expert Neal Kinsey at the SA Biofarm course on soil fertility recently presented in Johannesburg. Glenneis Erasmus reports.
Issue date : 01 August 2008

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Many producers want A soil pH of between 5,5 and 7 because they believe this reflects favourable calcium levels in the soil. However, magnesium, potassium and sodium also have an impact on soil pH levels. Excessive levels of potassium and sodium have a much larger effect on soil pH than calcium and magnesium, while excess levels of magnesium have a much larger impact on pH levels than calcium. Magnesium can raise pH levels 1,7 times higher than calcium, according to international soil expert Neal Kinsey.

This is typically seen with the over-application of nitrogen. Excess nitrogen drives calcium out of the soil and while this should result in a drop in soil pH, it seldom happens because magnesium takes over the space that used to be occupied by the calcium. So for every 1% drop in calcium, there will be a 1% increase in magnesium. The soil pH, as a result, might be higher than before the nitrogen was applied, even though there is less calcium in the soil than before.

“What this means is that you might have an ideal soil pH, but nothing grows there because it’s calcium-deficient and either sodium, magnesium or potassium is tying up all the nutrients,” Kinsey explains. The importance of the right calcium levels C alcium deficiency seldom shows up in the field because secondary effects such as high soil acidity usually limit plant growth first. This is why people tend to emphasise pH over calcium levels. If there’s a shortage of calcium in the soil, only addressing this deficiency will help to improve production, but won’t raise the pH.

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Farmers should therefore never use a high pH as an excuse not to apply calcium in the form of lime. Kinsey tells the story of one of his clients who produces almonds. His pH levels measured between 7,8 and 8. The soil was extremely compact, making it very difficult for water to penetrate, and the Albrecht soil analysis indicated high magnesium and low calcium levels. Kinsey advised the client to add calcitic lime to the soil, as it has very low magnesium levels.

Most other soil consultants were horrified at this advice because they felt the pH was too high and that the lime would increase the pH further. But the calcitic lime decreased the magnesium levels by increasing the calcium levels, and soil porosity improved within a season. Kinsey cautions, however, that despite such early improvements, it usually takes up to three years for all the lime to become available to the soil. Understanding pH All this doesn’t mean that pH is unimportant, however. Research over the years has proved that different nutrients become more available to plants at different soil pH levels. Maintaining pH levels between 5,5 and 7, for example, has been found to reduce the concentration of soluble aluminium and manganese – metals that become toxic at low pH levels.

The availability of both soil and fertiliser nutrients is increased in this pH range ). A range of 6,3 to 6,5 seems to be good for most beneficial soil organisms. Fungi tolerate acid soil of a pH below 6 while bacteria thrive at pH levels above 7. It’s also been suggested that different plants perform differently at different pH levels. Blueberries seem to thrive at strong acidic pH levels between 4 and 5. Kinsey explains that this is probably because blueberries require high levels of manganese and iron. These two nutrients can become unavailable to the plants when there are excessive levels of calcium, sodium, magnesium or potassium.

Lucerne, on the other hand, thrives at a high pH because it tends to receive plenty of phosphorous and molybdenum in the higher ranges. But it becomes sensitive to a boron shortage when the pH goes above 7, as boron tends to be tied up at these levels and then becomes unavailable to the plant. Kinsey stresses that soil pH should not be used as a guideline for crop performance. “I once had clients in Pennsylvania who had soil analysis done by a major state university that told them not to plant blueberries on one of their lands because the calcium levels were too high. The Albrecht soil analysis, however, indicated that the calcium levels were in the desired range and that the nutrient levels were relatively balanced.

The end result was that these clients produced some of their best-quality and largest yields from this land.” Blueberries do much better in a soil with balanced nutrients than in an acidic soil where the nutrients are not balanced. This maxim applies to every other crop: most will thrive in soil that has the proper balance of nutrients, regardless of soil pH or soil type. For peak performance, the soil nutrients need to be corrected to the required amounts and fine-tuning can then be done after that to ensure optimal conditions for a specific crop. Read more about liming and correcting calcium and magnesium levels in next week’s edition. Contact SA Biofarm on (012) 333 4222 or visit www.sabiofarm.co.za. |fw