ARCHIVE: Fertiliser Manual (RB209)

Soil Acidity and liming


(See Section 6 for additional soil pH and liming recommendations for Fruit, vines and hops, and Section 8 for recommendations for grassland).

Soil pH is a measure of acidity or alkalinity. It can be measured in the laboratory using a soil sample taken from the field or directly in the field using a portable soil test kit. When determined in the laboratory, pH is usually measured in a soil/water suspension. The natural pH of a soil depends on the nature of the material from which it was developed. It ranges from about pH 4 (very acid), when most crops will fail, to about pH 8 for soils naturally rich in calcium carbonate (lime) or magnesium carbonate. For soils with a pH lower than 7, natural processes (e.g., rainfall, crop growth and especially leaching of calcium in drainage water) and some farming practices (e.g., use of large amounts of some nitrogen fertilisers) tend to acidify soil. Acidifying processes can cause soil pH to fall quite quickly and regular pH checks are advisable. Such acidifying processes rarely affect the pH of calcareous soils except perhaps in the top few centimetres where the soil is undisturbed. If problems are suspected, soil pH should be checked.

The optimum availability of most plant nutrients in soil occurs over a small range of soil pH values. Unfortunately the range for each nutrient is not the same but there is sufficient overlap in the ranges to decide the best possible compromise for each cropping system and soil type and these are shown below.

  Optimum soil pHa
  Mineral soils Peaty soils
Continuous arable cropping 6.5b 5.8
Grass with occasional barley crop 6.2 5.5
Grass with occasional wheat or oat crop 6.0 5.3
Grass with an occasional grass/clover swards 6.0 5.3

 

a The optimum pH is based on soil that has been correctly sampled (see Appendix 3). In some soil samples containing fragments of free lime, analysis of the ground soil sample in the laboratory can give a misleadingly high value for pH. The pH is measured in a soil/water suspension.
b In arable rotations growing acid sensitive crops such as sugar beet, maintaining soil pH between 6.5 and 7.0 is justified.

Maintaining the optimum pH level in the topsoil in all parts of the field is important to achieve optimum yields and consistent quality. Not correcting soil acidity can cause large yield losses, but over-use of lime is wasteful and costly and can create problems with the availability of some micronutrients.

Lime recommendations

For each field the amount of lime to apply will depend on the current soil pH, soil texture, soil organic matter and the optimum pH needed. Clay and organic soils need more lime than sandy soils to increase pH by one unit. A lime recommendation is usually for a 20 cm depth of cultivated soil or a 15 cm depth of grassland soil. The table below gives examples of the recommended amounts of lime (t/ha of ground limestone or chalk, neutralising value NV 50-55) required to raise the pH of different soil types to achieve the optimum pH level shown in the table above.

Initial
soil pH
Sands &
Loamy sands
Sandy loams &
silt loams

Clay loams
& clays

Organic
soils
Peaty
soils
  Arable Grass Arable Grass Arable Grass Arable Grass Arable Grass
          t/ha          
6.2 3 0 4 0 4 0 0 0 0 0
6.0 4 0 5 0 6 0 4 0 0 0
5.5 7 3 8 4 10 4 9 3 8 0
5.0 10 5 12 6 14 7 14 7 16 6

 

Where soil is acid below 20 cm, and soils are ploughed for arable crops, a proportionately larger quantity of lime should be applied. However, if more than 10 t/ha is needed, half should be deeply cultivated into the soil and ploughed down with the remainder applied to the surface and worked in. For established grassland or other situations where there is no, or only minimal, soil cultivation, no more than 7.5 t/ha should be applied in one application. In these situations, applications of lime change the soil pH below the surface very slowly. Consequently the underlying soil should not be allowed to become too acid because this will affect root growth and thus limit nutrient and water uptake, which will adversely affect yield.

Liming materials

The effectiveness of a liming material depends on its neutralising value (NV), its fineness of grinding and the hardness of the parent rock. The NV is the relative effectiveness of a liming material compared to that of pure calcium oxide (CaO). Lime recommendations are usually given in terms of ground limestone or ground chalk (NV 50-55), but other liming materials can be used provided allowance is made for differences in NV, fineness of grinding and cost.

The application rate is adjusted to take account of differences in NV and fineness of grinding of the materials because this affects the speed of reaction in the soil. The Fertilisers Regulations (see Section 9) give details of the meaning and required declarations of different named liming products. In addition, materials such as sugar beet factory lime and lime treated sewage cake contain a useful amount of lime. Appendix 7 gives typical NVs of some common liming materials. The booklet Agricultural Lime – the Natural Solution (Agricultural Lime Association) gives more information on liming materials.

The cost of different liming materials can be compared by calculating the cost per unit of NV but allowance should also be made for any differences in particle fineness.

Example:

Ground limestone has an NV of 50 and costs ?20/t delivered and spread. An alternative liming material (A) has an NV of 30 and costs ?17/t delivered and spread.

Ground limestone costs (20 x 100) / 50 = 40 pence per unit of NV.

Liming material A costs (17 x 100) / 30 = 57 pence per unit of NV.

Provided the two materials have the same physical characteristics, the ground limestone is the more cost-effective liming material.

 

Some liming materials contain other useful nutrients and this also should be taken into account when deciding on which to use. For example, magnesian limestone (dolomitic limestone) contains large amounts of magnesium and is effective for correcting soil magnesium deficiency as well as acidity. However, many years of using magnesian limestone can result in an excessively high soil Mg Index and excess magnesium in the soil may induce potash deficiency in crops.

Lime application

It is important to maintain the appropriate soil pH for the cropping system and soil type and soil pH should not vary by more than ±0.5 pH unit from the optimum. However, when to apply lime can be fitted in with the crops being grown. For example:

  • Potatoes can tolerate acid soils and applying lime immediately before potatoes are grown should be avoided because of the risk of increased levels of potato scab.
  • Sugar beet and barley are sensitive to soil acidity. Lime should be applied before these crops are grown.
  • Clover is more sensitive to soil acidity than are many grass species and soil pH should be maintained to encourage a clover-rich sward.

A liming material should always be well worked into the cultivated soil because it can take some months to have its full beneficial effect in increasing pH throughout the topsoil. It is unwise to grow a crop which is sensitive to acidity immediately after liming a very acid soil. If it is important to try to achieve a rapid effect then the use of a fast acting liming material could be considered.

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