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Understanding lime

Although regular application of lime is needed on only about 10 to 12 percent of Ontario soils, it is important for those farming the other portion to know when and how best to apply lime occasionally. “Less-than-optimum soil pH can result in substantial potential yield drops in corn, for example,” says Dale Cowan, a senior agronomist at Agris Co-op, at its Southern Co-operative Services division in Chatham. “Keeping an eye on it is wise.”

Understanding whether soils require lime is a matter of thoroughly understanding soil pH. “Many farmers are confused about the difference between soil pH and buffer pH,” says Keith Reid, soil specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs. “Soil pH is the amount of acidity in the soil solution, which is what plant roots respond to. This test tells you whether the soil needs liming or not.”

If lime is needed, a buffer pH test then indicates how much. “Soil pH test results below 6.2 should be followed by a buffer pH test,” notes Cowan.

Calcium and magnesium levels must be tested also. Where magnesium levels in the soil are low (less than 100), dolomitic lime should be used. If levels are adequate, Reid says either dolomitic or calcitic lime may be used, whichever is most economical. Magnesium deficiency is more common in sandy soils. “Most clay soils have adequate magnesium, but this is not necessarily a given,” Reid explains. “Also remember that clay soils have more ‘reserve acidity’ stuck to the clay minerals and organic matter than sandy soils do, so at the same soil pH, a clay soil will generally need more lime to correct the acidity than will a sandy soil.”

This is why a buffer pH test is so important: it will tell a grower how much more. Reid adds, “Also keep in mind that there are some clay soils that started out acidic from the parent materials that went into forming those soils (e.g., parts of the Niagara peninsula).”
Soil acidity takes time to correct, so lime application should be conducted at least as far back as the fall before a pH-sensitive crop is grown, and preferably before that.

In terms of what lime products to apply, farmers need to be aware of the Agindex rating for lime, which describes the fineness of grind and the neutralizing value. “Not all limes are equal,” Cowan says. “The finer the lime, the faster it will work.”

Although any commercial lime will obviously contain particles of various sizes, lime particles less than 0.25 millimetres in diameter are completely effective in changing soil pH, particles from 0.25 to 2 millimetres in size are only 40 percent effective, and those larger than 2 millimetres are so large they are almost completely ineffective. When a lab recommends the use of lime, Cowan says the lab is referring to a lime product with an Agindex of 75, so growers will need to adjust their application rates up or down for the Agindex of the lime they are applying: more for an Agindex below 75, and less for an Agindex above 75.

Reid adds, “Finely ground lime is dusty, so watch out for applications on windy days. Also, ensure that lime is mixed evenly through the soil.”