Soil pH
To correct soil acidity, broadcast ground limestone and work it into the soil at rates determined by a soil test. Table 1, Guidelines for lime application to Ontario crops, shows the pH values below which liming is recommended, and the target soil pH to which soils should be limed for different crops. In Ontario, most crops grow quite well at pH values higher than the target pH to which liming is recommended. The soil pH measures the amount of acidity in the soil solution and indicates whether liming is necessary for crop production. It does not measure the amount of reserve acidity.
Table 1. Guidelines for lime application to Ontario crops
| Soil Type | Crops | Soil pH below which lime is beneficial | Target soil pH1 |
|---|---|---|---|
| Coarse- and medium-textured soils (sands, sandy loams, loams, and silt loams) | perennial legumes, oats, barley, wheat, triticale, beans, peas, canola, flax, tomatoes, raspberries, strawberries, all other crops not listed below | 6.1 | 6.5 |
| corn, soybeans, rye, grass, hay, pasture, tobacco | 5.6 | 6.0 | |
| potatoes | 5.1 | 5.5 | |
| Fine-textured mineral soils (clays and clay loams) | alfalfa, cole crops, rutabagas | 6.1 | 6.5 |
| other perennial legumes, oats, barley, wheat, triticale, soybeans, beans, peas, canola, flax, tomatoes, raspberries, all other crops not listed above or below | 5.6 | 6.0 | |
| corn, rye, grass hay, pasture | 5.1 | 5.5 | |
| Organic soils (peats/mucks) | all field crops, all vegetable crops | 5.1 | 5.5 |
Buffer pH
Buffer pH measures the amount of reserve acidity held on the clay and organic matter particles in the soil, which will dictate how much lime is needed. Different amounts of reserve acidity will mean that two soils at the same pH value will need different amounts of lime to raise pH to the desired level. The greater the amount of reserve acidity, the lower the buffer pH and the more lime is required to raise the pH. For soils needing lime, Table 2, Lime requirements to correct soil acidity based on soil and buffer pH, may be used to determine the amount of lime required to reach different target soil pH values.
Table 2. Lime requirements to correct soil acidity based on soil and buffer pH
| Ground limestone required1 – t/ha (ton/acre) | ||||
| Buffer pH | Target soil pH | |||
| 7 | 6.5 | 6 | 5.5 | |
| 7.0 | 2 (0.9) | 2 (0.9) | 1 (0.5) | 1 (0.5) |
| 6.9 | 3 (1.3) | 2 (0.9) | 1 (0.5) | 1 (0.5) |
| 6.8 | 3 (1.3) | 2 (0.9) | 1 (0.5) | 1 (0.5) |
| 6.7 | 4 (1.8) | 2 (0.9) | 2 (0.9) | 1 (0.5) |
| 6.6 | 5 (2.2) | 3 (1.3) | 2 (0.9) | 1 (0.5) |
| 6.5 | 6 (2.7) | 3 (1.3) | 2 (0.9) | 1 (0.5) |
| 6.4 | 7 (3.1) | 4 (1.8) | 3 (1.3) | 2 (0.9) |
| 6.3 | 8 (3.6) | 5 (2.2) | 3 (1.3) | 2 (0.9) |
| 6.2 | 10 (4.5) | 6 (2.7) | 4 (1.8) | 2 (0.9) |
| 6.1 | 11 (4.9) | 7 (3.1) | 5 (2.2) | 2 (0.9) |
| 6.0 | 13 (5.8) | 9 (4.0) | 6 (2.7) | 3 (1.3) |
| 5.9 | 14 (6.2) | 10 (4.5) | 7 (3.1) | 4 (1.8) |
| 5.8 | 16 (7.1) | 12 (5.4) | 8 (3.6) | 4 (1.8) |
| 5.7 | 18 (8.0) | 13 (5.8) | 9 (4.0) | 5 (2.2) |
| 5.6 | 20 (8.9) | 15 (6.7) | 11 (4.9) | 6 (2.7) |
| 5.5 | 20 (8.9) | 17 (7.6) | 12 (5.4) | 8 (3.6) |
| 5.4 | 20 (8.9) | 19 (8.5) | 14 (6.2) | 9 (4.0) |
| 5.3 | 20 (8.9) | 20 (8.9) | 15 (6.7) | 10 (4.5) |
| 5.2 | 20 (8.9) | 20 (8.9) | 17 (7.6) | 11 (4.9) |
| 5.1 | 20 (8.9) | 20 (8.9) | 19 (8.5) | 13 (5.8) |
| 5.0 | 20 (8.9) | 20 (8.9) | 20 (8.9) | 15 (6.7) |
| 4.9 | 20 (8.9) | 20 (8.9) | 20 (8.9) | 16 (7.1) |
| 4.8 | 20 (8.9) | 20 (8.9) | 20 (8.9) | 18 (8.0) |
| 4.7 | 20 (8.9) | 20 (8.9) | 20 (8.9) | 20 (8.9) |
| 4.6 | 20 (8.9) | 20 (8.9) | 20 (8.9) | 20 (8.9) |
Adjustments to lime recommendations
Limestone material:
Calcitic limestone consists largely of calcium carbonate, and dolomitic limestone is a mixture of both calcium and magnesium carbonates. Use dolomitic limestone on soils with a magnesium soil test of 100 ppm or less, as it is an excellent and inexpensive source of magnesium for acidic soils. On soils with magnesium tests greater than 100 ppm, use calcitic or dolomitic limestone.
Agricultural Index:
The Agricultural Index is an indicator of limestone quality developed in Ontario, combining the neutralizing value and fineness rating into a single value. The higher the Agricultural Index of the liming material, the more effective it is on an equal weight basis. A more effective liming material is also worth proportionately more than a lime with a low index because it may be applied at a lower rate.
Guidelines from the OMAFA-accredited pH and buffer pH soil tests are based on limestone with an Agricultural Index of 75. If the Agricultural Index is known, a rate of application specifically for limestone of that quality can be calculated using the following equation:
For example, if there is a limestone requirement by soil test of 9 t/ha (4 ton/acre), and the most suitable source of limestone from a quality and price standpoint has an Agricultural Index of 90, then apply 7.5 t/ha (3.3 ton/acre) from calculation using the above formula: (7.5 = 9 x 75/90).
Tillage Depth:
Lime amounts presented in Table 2 should raise the pH of the top 15 cm (6 in.) of a soil to the listed target pH. If the soil is tilled to a lesser or greater depth than 15 cm (6 in.), proportionately more or less lime is required to reach the same target pH. Where reduced tillage depths are used, reduce rates of application proportionately. More frequent liming will be needed. For no-till with surface applied fertilizer, soil sample the top 10 cm (4 in.) to determine pH and apply two-thirds the rate prescribed in Table 2.