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Soils - Part 4: Soil pH

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Quality of Agricultural Limestone

Effective Calcium Carbonate (ECC) is evaluated on its ability to neutralize soil acidity, that is, to effect a change in the pH of the tillage layer of the soil within a specified time. Generally a three-year base is used.

The criteria used for determining quality of ag lime are:

1. Purity — neutralizing power as percent calcium carbonate.
2. Fineness of grind — measured by percent passing 8 and 50 mesh sieves.
3. Water content — does not directly influence ECC but it does influence the amount of material to apply.

Purity is determined by an A.O.A.C. (Association of Official Agricultural Chemists) procedure in the laboratory by finding out how much hydrochloric acid is needed to neutralize a known amount of liming material.

Fineness is determined by sieving an oven-dried sample through standard 8 and 50 mesh sieves.

1. Material coarser than 8 mesh — 10 percent effective.
2. Material passing 8, but held on 50 mesh — 40 percent effective.
3. Material passing 50 mesh — 100 percent effective.

Calcium carbonate (CaCO3) is not very soluble in water. One acre inch of water will dissolve about 100 pounds of lime. In order for calcium from lime to replace the exchangeable hydrogen on the soil complex, calcium cations must be in the soil solution. Equation 8 and Equation 9 show calcium coming into solution from lime and the replacement of H+ on the soil clay and organic matter particles.

Solid CaCO3 + 2H2 (dissolved) - Ca++ + CO2 + H2O          (8)

 Clay 2H + Ca--  --> Clay Ca + H-          (9)

The rate at which lime goes into solution depends on surface area of the applied lime and the degree of incorporation into the soil. Research has shown little if any advantage to grinding lime material to pass sieves finer than 50 or 60 mesh. It costs about four times as much to grind material so that 100 percent will pass a 50 mesh screen than to grind so 50 to 60 percent passes a 50 mesh screen. The grinding expense is what makes fluid lime cost more. For equal amounts of ECCE (exchangeable calcium carbonate equivalent) lime, fluid lime material will usually raise the pH slightly faster initially; however, after the second year, there is no difference in pH increase. Remember, the quantity of lime (ECCE) to change the pH of a given soil is still the same, regardless if it is 60 percent ECCE ag lime, fluid lime or pelletized lime.

Water content must be considered when using byproduct or refuse liming materials and ag lime produced from “soft or porous” limestone. Sugar refuse lime may contain 30 percent water when it appears to be dry. Ag lime produced from soft rock may contain 10 percent water, especially if stockpiled for several months. Ag lime produced from hard rock seldom contains more than 2 percent water, even when stockpiled for a long time.

Ag lime laws have been passed by Nebraska, Iowa, Missouri and Kansas. These are basically labeling laws which require the vendor to guarantee the E.C.C.E (effective calcium carbonate equivalent) and moisture. Missouri also requires a guarantee of calcium and magnesium content. Ask for an analysis sheet before you buy lime. For information on Nebraska’s lime law, contact the State Department of Agriculture.

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