Soil Acidity and Liming
Soil pH is a measure of the acidity or alkalinity of a soil. It usually varies from pH 5.0 (acid), where crops will fail due to high levels of aluminum and/or manganese around the roots, to about 8.0 for overlimed or high limestone soils.
Crops differ in their sensitivity to soil pH. In addition, the optimum use of fertilisers containing nitrogen and phosphorus is obtained when soil pH is between about 6.2 and 7.2. The availability of some trace elements especially manganese and boron is decreased when pH is above 7.0. A deficiency of manganese is quite common in cereals, which follow sugar beet in the rotation due to the over use of lime for the beet.
Although potatoes can grow very well below pH 6.0 it is necessary to maintain the soil at a pH suitable for the rotation. The pH levels to aim for in mineral soils are as follows; –
pH 6.3 Grass.
pH 6.5 Cereal
pH 7.0 Beet, Beans, peas
For peats, more than 15 cm (6″) in depth aim for a pH of 5.5.
In grassland, on soils high in molybdenum it is important not to raise the pH above about 6.2, as increasing the pH increases the availability of molybdenum and this induces copper deficiency in the animal by reducing copper absorption. High-molybdenum soils occur on carboniferous black shales and calp limestone.
The lime test used at Johnstown Castle is based on the change in pH of a buffered solution. This takes soil texture into account. The recommendations are given in tonnes ha-1 and tonnes ac-1 of ground limestone required to raise the pH to 7.0 for tillage and 6.8 for grassland. If beet is not grown in the rotation, the recommendation for tillage should be reduced by 1 tonne ac-1. In grassland, if there is frequent soil sampling and liming e.g. once every 5 years, then our recommendations should be reduced by 1 tonne ac-1.
In tillage, if the recommended dressing exceeds 7.5 t ha-1 (3t ac-1), half should be applied pre-ploughing and the other half post-ploughing and worked in. For grassland, 3 tonnes ac-1 should be applied initially and the remainder after two years. As lime takes up to two years to have its full neutralising effect on soil acidity it should be applied well before sowing for acid sensitive crops such as beet. However, if a crop if failing due to acidity it is often worthwhile applying a time top-dressing of 3t ac–. Lime should not be applied within two years preceding a potato crop because it can increase the risk of common scab. Surface acidity (top 5cm or 2″) often occurs in grassland due to our high rainfall and the use of nitrogenous fertilisers. This reduces the availability of fertiliser phosphorus. For this reason it is better to have frequent small applications of lime than one large application. Keeping the pH above 6.0 in grassland improves nitrogen recycling and reduces total nitrogen requirements.