The acidity or alkalinity of soil is expressed on a pH scale, based on the total hydrogen ion concentration in the soil water solution. On this scale, neutral condition is represented by a pH of 7, values 7 to 0 are increasingly acidic and 7 to 14 are increasingly alkaline. In practice, the pH of soil ranges from 3 to 9, with the majority between the range of 4.5 and 7.5
EFFECTS OF SOIL pH ON CROP GROWTH
Most soil used for vegetable cultivation become gradually more acidic as calcium is lost due to leaching by rainwater and irrigation. The process is further accelerated by the use of nitrogenous fertilisers such as ammonium nitrate and ammonium sulphate. Vegetables vary in their tolerance to soil acidity. Acidic soil often causes the stunting and yellowing of leaves, resulting in the decrease in growth and yield of crops as the pH levels falls. Additionally, plants grown in adverse pH conditions may be more prone to disease and fungal attack. In general, the most favourable pH range for vegetable is between 5.5 to 6.5 in mineral soil and 5.0 to 6.0 in organic soil
EFFECTS OF SOIL pH ON AVAILABLE NUTRIENTS
The availability of plant nutrients is considerably affected by soil pH. Calcium, potassium, magnesium and sodium are alkaline elements, which are lost with increasing acidity whereas phosphorous is more available in acidic soil conditions. Acidity can also induce deficiencies of micronutrients such as molybdenum, copper and boron, although a deficiency in the latter is more commonly seen in alkaline soils where over-liming has occurred. Other minor elements which may be deficient due to low solubility in high pH includes manganese and iron, the deficiency of which produces a chlorotic condition commonly seen in the young leaves of brassica plants.
EFFECTS OF LIMING
Incorporation of liming materials into the soil to correct soil acidity has shown a number of beneficial effects. For example, it was shown that liming encourages aggregation of soil particles. It also increases the rate of organic matter decomposition by affecting the activities of the soil microorganisms.
The choice of materials for liming includes ground limestones or chalk, hydrated lime, quicklime and magnesium limestone. The latter has slightly higher neutralising value then their calcium equivalents and is particularly useful for correcting acidity of intensively cultivated, sandy soil where magnesium deficiency may also occur. These materials are usually broadcast on acidic soils to increases its pH condition. The increased in pH of the soil can be conveniently measured by a common hand-held pH meter.
Excessive liming, however, can induce trace elements deficiencies. It has been documented that the most prominent nutritional disorder of plants grown in soils with more than 20% of hydrated lime is chlorosis due to elevated levels of bicarbonate concentration. High levels of bicarbonate affect the uptake, translocation and utilisation of iron, among other trace elements in the soil. Severe symptoms of lime-induced chlorosis are correlated not only with the levels of iron content in the leaves but also with severe inhibition of leaf growth and chloroplast development, which in turn results in the decrease in growth and yield of the crops.
The above article is reprinted with the kind permission of the Agri-Food & Veterinary Authority. AVA reserves the rights to this article and no part of which are to be reproduced in any means without written consent from AVA.
*Simple soil pH kits are available from most retail nurseries and are very easy to use. Ang Mo Kio Flora and Landscape have kits priced below S$30.00