Introduction to Soil and Crop Nutrition
Zinc is essential for the normal healthy growth and reproduction of plants, animals and humans and when the supply of plant-available zinc is inadequate, crop yields are reduced and the quality of crop products is frequently impaired.
In plants, zinc plays a key role as a structural constituent or regulatory co-factor of a wide range of different enzymes
in many important biochemical pathways and these are mainly concerned with:
a) carbohydrate metabolism, both in photosynthesis and in the conversion of sugars to starch,
b) protein metabolism,
c) auxin (growth regulator) metabolism,
d) pollen formation,
e) the maintenance of the integrity of biological membranes,
f) the resistance to infection by certain pathogens.
When the supply of zinc to the plant is inadequate, i.e. there is a deficiency of zinc, one or more of the many important physiological functions of zinc is unable to operate normally and the growth of the plant is adversely affected. The changes in plant physiological mechanisms brought about by a deficiency of zinc result in the plant developing visible symptoms of stress which might include one or more of the following: interveinal chlorosis (yellowing of the leaves between the veins), bronzing of chlorotic leaves, small and abnormally shaped leaves, stunting and rosetting (leaves form a whorl on shortened stems). These different types of symptoms vary with plant species and are only clearly displayed in cases of severe deficiency. In cases of marginal or moderate deficiency,
plants can often have greatly reduced yields (<40% reduction) without obvious visible symptoms. This is often called ‘hidden’, ‘latent’ or ‘sub - clinical’ deficiency. These deficiencies may remain undetected in crop land for many years unless soil or plant diagnostic tests are carried out because there are no obvious signs of stress.
Losses of yield of 40% or more in many zinc deficient soils have a major economic impact on the farmer due to the reduced income as a result of lost yield. In more intensive types of arable farming where expensive inputs of seed, fertilisers, agricultural chemicals and possibly irrigation water are involved, the failure of crops to realize their potential yield is a major loss of income to the farmer. In developing countries, the cost to the nation from significant shortfalls in food production is also considerable because increased imports of grains will often be required to make up this shortfall.
With the world population continuing to expand and the problems of producing extra food to provide an adequate standard of nutrition for this growing population, it is very important that any losses in production from a cause so easily corrected as zinc deficiency are prevented. This necessitates identifying the main areas of zinc deficient soils and crops and treating them with zinc fertilisers to correct the shortage in the supply of zinc to the crops.
Zinc deficient soils can be identified, or diagnosed, by soil testing, or the analysis of the crop plants (usually leaves). The results obtained from soil and/or plant analysis can be compared with critical values for zinc in local soil types for specific crops and a decision made on whether or not zinc fertiliser applications to the soil or crops are required.
Very many plant species are affected by zinc deficiency on a wide range of soil types in most agricultural regions of the world. The major staple crops: rice, wheat, maize and sorghum are all affected by deficiency, together with many different fruit, vegetable and other types of crops including cotton and flax. For rice, which is highly susceptible to zinc deficiency and grows in waterlogged soils which are conducive to zinc deficiency, zinc is the third most important nutritional factor affecting grain yield after nitrogen and phosphorus. Although wheat is relatively tolerant of zinc deficiency, the soils in many areas in which wheat is grown have very low concentrations of plant-available zinc and cause widespread zinc deficiency in this crop. In contrast to wheat, rice, maize and beans are highly sensitive to zinc deficiency. However, within all crop species, individual varieties (or cultivars) can often vary considerably in their susceptibility to zinc deficiency. It is therefore important to screen crop varieties so that the more tolerant (or ‘zinc - efficient’) varieties can be grown on soils of lower available zinc status.
The soil conditions most commonly giving rise to deficiencies of zinc can include one or more of the following:
- low total zinc content (such as sandy soils with low contents of organic matter)
- neutral or alkaline pH
- high salt concentrations (saline soils)
- high calcium carbonate content (calcareous soils)
- low pH, highly weathered parent materials (e.g. tropical soils)
- peat and muck (organic soils)
- high phosphate status
- prolonged waterlogging or flooding (paddy rice soils)
- high magnesium and/or bicarbonate concentrations (and in irrigation water).
Soils with one or more of these properties can be found in many areas of the world. Countries with particularly widespread zinc deficiency problems include: Afghanistan, Bangladesh, Brazil, China, India, Iran, Iraq, Pakistan, Sudan, Syria, Turkey, Australia, Philippines, many states in the USA and parts of Europe.
Once identified, zinc deficient soils can be easily treated with zinc fertilisers to provide an adequate supply of zinc to crops. Several different zinc compounds are used as fertilisers but zinc sulphate is by far the most widely used material. Zinc sulphate is most frequently broadcast (or sprayed as a solution) evenly over the seedbed and incorporated into the topsoil by cultivation before sowing the seed. One application of between 20–30 kg ha–1 of zinc sulphate will often have an improving effect on the zinc status of the soil which will last for around five years before another application is required. However, this will vary in different areas; in some of the most deficient soils, such as those with a high content of calcium carbonate, zinc applications may have to be larger and more frequent.
Placement of the zinc fertiliser below and to one side of the seed at sowing is also frequently used. In this case, lower application rates are used because of the close proximity of the fertiliser band to the developing roots. Foliar sprays of zinc sulphate, zinc nitrate or chelated forms of zinc are mainly used on fruit trees and plantation crops but they can also be used to salvage annual crops and reduce yield loss. In all cases of treating zinc deficient soils, regular soil or plant testing is recommended to determine when additional applications of zinc fertilisers are required.
An alternative approach to the problem of zinc deficiency is to select and/or breed crops which are ‘zinc - efficient’ and able to tolerate low available concentrations of zinc in the soil. This approach is one of matching the plant to the soil, rather than modifying the soil to suit the plant. There are zinc - efficient cultivars of rice and wheat which are grown quite widely in areas of soils with a low zinc status.
An adequate supply of zinc is essential for obtaining cost effective yields of crops all over the world. The cost to the farmer of lost production is high but the expense of applying zinc fertiliser when crop symptoms, soil tests or plant analysis show that they are required is relatively low. No farmer in areas where soils have been shown to be deficient can afford not to maintain an adequate zinc status in his soils.
For a detailed view on zinc in soils and crop nutrition please refer to Professor Brian Alloway's book in our publications section titled: Zinc in Soils and Crop Nutrition. To view a Table of Contents of this publication, and to download individual chapters, click here.
