Agricultural Lime - pH Decline
Interpretation of soil pH
The degree of soil acidity or alkalinity is measured by what is known as the pH scale.
A figure of pH 7 represents a materials relationship to the neutral position of pure water at pH 7.0. Figures below 7 indicate increasing acidity and above 7 increasing alkalinity. The optimum for general cropping is between pH 6.5 and 7.0 (5.8 on peaty soils). For permanent grassland the optimum pH is slightly lower between 6.0 and 6.5 (5.3 on peaty soils).
The soil pH is the measure of the hydrogen ions in the soil and is what determines how acid or alkaline the soil is. To cure an acidic soil a recommendation of aglime is required which is produced once a pH soil sample has been taken and mapped on each field. For information on how this is done please see below or click here.
In-field measurements using pH indicator on some soils where free chalk or lime particles exist may give lower values than laboratory results for the same field. This is because grinding the soil for laboratory analysis pulverises any chalk/lime particles and the pH as measured is increased.
Unless steps are taken to redress the balance of soils by applying a liming material there will be a natural reduction in the lime status of most soils. This results in a natural increase in acidity and in many cases a reduction in soil fertility and damage to soil structure.
Over the past 2 decades a decline in lime application has resulted in an increase in the proportion of UK soil samples exhibiting pH values below optimum levels. This means that the full agricultural potential for these soils has not been realised.
Although, ‘The British Survey of Fertiliser Practice’* suggests that the steady decline in agricultural land area receiving lime in Britain appears to be reversing.
However, the survey also concludes that this area is still considerably less than that calculated to require liming. In addition, it states the principle causes of acidification have not lessened, and it is reasonable to believe that significant areas of arable land, and more particularly grassland, are at a pH level which could limit productivity.
*Organised and jointly funded by the Fertiliser Manufacturers’ Association (FMA), the Department for Environment, Food and Rural Affairs (DEFRA) and the Scottish Executive Rural Affairs.
Influence of soil pH on plant nutrient availability & plant growth
The availability of different nutrients at the different pH bands is indicated by the width of the white bar: the wider the bar, the more available is the nutrient (redrawn for PDA from Truog, E. (1946). Soil reaction influence on availability of plant nutrients. Soil Science Society of America Proceedings 11, 305-308.).
Agricultural Lime - Losses
LEACHING | CROPPING | FERTILISING | POLLUTION
AVERAGE LIME LOSSES CAN AMOUNT TO 1250 Kg CALCIUM CARBONATE PER HECTARE PER YEAR
With Intensive arable and livestock rotations, there is a natural reduction in the lime status and pH of the soil, with lime losses occurring from:
- Organic and inorganic acids formed during decomposition of crop residues (acid forming)
- Plants remove calcium and magnesium from the soil (acid forming)
- Deep tillage may bring acid subsoil into the root zone (acid forming)
- Plants release hydrogen ions (H+) to the soil (acid forming)