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| 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. |
