Soil Acidity, Lime and Pasture Growth
An inevitable consequence of plant growth in soil is ‘acidification’ which decreases
soil pH. The rate of acidification in pasture soils requires between 100 and 200 kg
lime/ha/year to neutralise it to maintain a constant soil pH and calcium content.
Application of nitrogen fertiliser increases the rate of soil acidification and the lime
Pasture fertiliser programmes are aimed at providing phosphorus, sulphur and other
nutrients required for clover growth. Clover has the ability to ‘fix’ atmospheric nitrogen
and convert this to protein. Organic material derived from clover plants is part of the
‘organic matter cycle’ in pasture soils and is a source of nitrogen for the grass
component of the sward, the basis for low input cost pasture farming systems
however this system does not always function effectively.
Where the soil pH is less than 5.6 there is increased probability that clover growth will
be retarded and at pH less than 5.5 ryegrass growth is also affected due to poor root
growth. However it is important to remember that soil test results from farms
represent an average for the area sampled so where a soil pH of 5.6 is reported, half
of the area will have a lower pH. The minimum soil test pH for optimum clover and
ryegrass growth which allows for normal onfarm variation should be up to 6.0.
Low soil calcium levels are also associated with poor soil physical structure with low
water holding capacity which may affect late summer and autumn seasonal pasture
production by up to 30%. Microbial activity is reduced affecting the organic matter
cycle and reducing the supply of nitrogen to grass and development of a ‘thatch’
layer is common. Ryegrass/clover pastures suffer a reduction in clover content and
ultimately reduction in ryegrass content as the pasture reverts to ‘low fertility’ grass
species with slow growth rates. The alternative to maintain pasture productivity is to
apply high rates of nitrogen fertiliser on a regular basis, this is not a low input cost
farming option and may not be sustainable.
Application of lime to achieve adequate soil calcium and pH is required and then
regular applications should be made to compensate for the rate of acidification.
Failure to do this will compromise the investment that farmers make in fertilisers,
seed and other inputs aimed at achieving optimum pasture production.
Lime in the Soil Environment:
The effectiveness of lime is related to application rate, the ‘acid neutralising power’ or
carbonate content and the particle size or range of particle sizes.
Lime (calcium carbonate) has 2 effects in the soil environment:
1 The initial effect is breakdown of carbonate, increasing the pH of soil moisture.
There is a significant effect on the soil environment while this process is happening.
The particle size of the lime applied and the level of soil acidity are factors that
strongly influence the speed of this reaction in moist soils. Small lime particles react
quickly at a speed related to the surface area/volume ratio of the lime product used.
2 Soil pH is controlled by the balance of exchangeable acidic and basic cations in
the soil, the dominant basic cation is calcium. The effect of lime application after
carbonate decomposition has finished is related to the amount of calcium applied,
this forms the basis for calculation of lime requirements.