Zinc is essential for the normal healthygrowth and reproduction of plants, animals and humans and when the supplyof plant-available zinc is inadequate, crop yields are reduced and thequality of crop products is frequently impaired.
In plants, zinc plays a key role as a structuralconstituent or regulatory co-factor of a wide range of different enzymes
in many important biochemical pathways and these are mainly concerned with:
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 isinadequate, i.e. there is a deficiency of zinc, one or more of the manyimportant physiological functions of zinc is unable to operate normallyand the growth of the plant is adversely affected. The changes in plantphysiological mechanisms brought about by a deficiency of zinc result inthe plant developing visible symptoms of stress which might include oneor more of the following: interveinal chlorosis (yellowing of the leavesbetween the veins), bronzing of chlorotic leaves, small and abnormallyshaped leaves, stunting and rosetting (leaves form a whorl on shortenedstems). These different types of symptoms vary with plant species and areonly clearly displayed in cases of severe deficiency. In cases of marginalor 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 remainundetected in crop land for many years unless soil or plant diagnostictests are carried out because there are no obvious signs of stress.
Losses of yield of 40% or more in many zincdeficient soils have a major economic impact on the farmer due to the reducedincome as a result of lost yield. In more intensive types of arable farmingwhere expensive inputs of seed, fertilizers, agricultural chemicals andpossibly irrigation water are involved, the failure of crops to realizetheir potentialyield is a major loss of income to the farmer. In developingcountries, the cost to the nation from significant shortfalls in food productionis also considerable because increased imports of grains will often berequired to make up this shortfall.
Withthe world population continuing to expand and the problems of producingextra food to provide an adequate standard of nutrition for this growingpopulation, it is very important that any losses in production from a causeso easily corrected as zinc deficiency are prevented. This necessitatesidentifying the main areas of zinc deficient soils and crops and treatingthem with zinc fertilizers to correct the shortage in the supply of zincto the crops.
Zinc deficient soils can be identified,or diagnosed, by soil testing, or the analysis of the crop plants (usuallyleaves). The results obtained from soil and/or plant analysis can be comparedwith critical values for zinc in local soil types for specific crops anda decision made on whether or not zinc fertilizer applications to the soilor crops are required.
Very many plant species are affected byzinc deficiency on a wide range of soil types in most agricultural regionsof the world. The major staple crops: rice, wheat, maize and sorghum areall affected by deficiency, together with many different fruit, vegetableand other types of crops including cotton and flax. For rice, which ishighly susceptible to zinc deficiency and grows in waterlogged soils whichare conducive to zinc deficiency, zinc is the third most important nutritionalfactor affecting grain yield after nitrogen and phosphorus. Although wheatis relatively tolerant of zinc deficiency, the soils in many areas in whichwheat is grown have very low concentrations of plant-available zinc andcause widespread zinc deficiency in this crop. In contrast to wheat, rice,maize and beans are highly sensitive to zinc deficiency. However, withinall crop species, individual varieties (or cultivars) can often vary considerablyin their susceptibility to zinc deficiency. It is therefore important toscreen 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 givingrise to deficiencies of zinc can include one or more of the following:
- low total zinc content (such as sandy soilswith low contents of organic matter)
- neutral or alkaline pH
- high salt concentrations (saline soils)
- high calcium carbonate content (calcareoussoils)
- low pH, highly weathered parent materials(e.g. tropical soils
- peat and muck (organic soils)
- high phosphate status
- prolonged waterlogging or flooding (paddyrice soils)
- high magnesium and/or bicarbonate concentrations(and in irrigation water).
Soils with one or more of these propertiescan be found in many areas of the world. Countries with particularly widespreadzinc 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 canbe easily treated with zinc fertilizers to provide an adequate supply ofzinc to crops. Several different zinc compounds are used as fertilizersbut zinc sulphate is by far the most widely used material. Zinc sulphateis most frequently broadcast (or sprayed as a solution) evenly over theseedbed and incorporated into the topsoil by cultivation before sowingthe seed. One application of between 20–30 kg ha–1 of zinc sulphate willoften have an improving effect on the zinc status of the soil which willlast 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 applicationsmay have to be larger and more frequent.
Placement of the zinc fertilizer below andto 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 thefertilizer band to the developing roots. Foliar sprays of zinc sulphate,zinc nitrate or chelated forms of zinc are mainly used on fruit trees andplantation crops but they can also be used to salvage annual crops andreduce yield loss. In all cases of treating zinc deficient soils, regularsoil or plant testing is recommended to determine when additional applicationsof zinc fertilizers are required.
An alternative approach to the problem ofzinc 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 modifyingthe soil to suit the plant. There are zinc – efficient cultivars of riceand wheat which are grown quite widely in areas of soils with a low zincstatus.
An adequate supply of zinc is essentialfor obtaining cost effective yields of crops all over the world. The costto the farmer of lost production is high but the expense of applying zincfertilizer when crop symptoms, soil tests or plant analysis show that theyare required is relatively low. No farmer in areas where soils have beenshown to be deficient can afford not to maintain an adequate zinc statusin his soils.