General

Amelioration of micronutrient deficiencies involves two approaches. The first approach aims at correction of deficiencies through application of the limiting micronutrients or amendments that increase their availability to plants. The second approach aims at raising crop genotypes with high nutrient efficiency on soils lacking in their availability. Once a micronutrient has been identified to be deficient in a soil and the extent of deficiency has been evaluated by appropriate methods (Chapter 10), it can be applied to the soil (soil amendment) or the plant (foliar amendment) to overcome the deficiency. This approach has been followed since the beginning of cultivation by man and is still the most widely followed method for amelioration of micronutrient deficiencies (Murphy and Walsh, 1991). Where the availability of micronutrients is limited by some soil chemical condition (e.g. high pH), deficiencies can be ameliorated through soil amendments (e.g. liming), causing an increase in the availability of micronutrients through modification of the soil chemical conditions. Micronutrient deficiencies caused by excessive salinity can be ameliorated by application of gypsum (CaS04.7H20), which increases exchangeable Ca2+ and decreases exchangeable Al (Shainberg et al. 1989). Availability of micronutrients (Fe, Zn) can also be increased through biological methods such as VAM associations or microbial siderophores. Rhizosphere microorganisms can affect plant nutrition by influencing the availability of plant nutrients, growth and morphology of roots and nutrient uptake processes (Rovira et al. 1983). Organic manures provide another potential source of micronutrients. They have an edge over the artificial fertilizers in that they also improve soil conditions to favour enhanced availability of native as well as applied micronutrients.

The second approach, that finds favour over the first, because of cost effectiveness, energy costs and sustainability, is the plant-based approach wherein the deficiency constraint is overcome by going in for cultivation of plant genotypes that, by virtue of high nutrient efficiency, can perform well on soils of low micronutrient availability. Use of genotypes, that have high efficiency of absorption, translocation and/or utilization of micronutrients, is particularly advantageous in overcoming deficiencies imposed due to adverse soil conditions such as calcareousness, salinity and sodicity, which are otherwise difficult to correct. During the recent years, greater thrust has been laid on overcoming deficiencies (and toxicities) of micronutrients through use of efficient genotypes (Sattelmacher et al. 1994; Welch, 1995; Graham and Welch, 1996; Pearson and Rengel, 1997; Rengel, 1999; Nielsen and Jensen, 1999). The choice of the approach for management of deficiencies differs with particular micronutrients, soil conditions and crop species.

11.2 micronutrient amendments

The limiting micronutrient(s) can be supplemented in different ways. This may be achieved by application of the limiting micronutrient in a suitable, inorganic or organically complexed form as (a) soil amendment, (b) foliar application or through (c) seed treatment.

Efficiency of soil application of a micronutrient for correction of deficiency depends on the proportion in which the added fertilizer contributes to the labile pool of micronutrients in the rhizosphere. This is influenced to varying extents by the interactions of the added micronutrient carriers with the soil physico-chemical conditions and environmental influences. A large proportion of boron and manganese added to the soil through inorganic carriers may be rendered unavailable, particularly in high pH soils, and this, reduces their efficiency as corrective measures. The effectiveness of foliar application of micronutrients, on the other hand, depends on their phloem mobility. High phloem mobility of molybdenum makes its foliar sprays an effective method for correction of molybdenum deficiency. Seed micronutrient content has an important bearing on seed vigour and viability. When sown to soils deficient in a micronutrient, seeds rich in that micronutrient can establish and support early seedling growth much better than seeds lacking in that micronutrient. Enrichment of seeds in micronutrients either through parental nutrition or seed treatment have been found effective in overcoming their marginal deficiencies. The effects of seed coatings and seed treatments on the establishment of seedlings on nutritionally poor soils are discussed by Scott (1989).

11.2.1 Iron Deficiency

Soil Amendment

Several iron salts find use in correction of iron deficiency (Table 11.1), but they have a common drawback in that a large proportion of iron supplied through the inorganic carriers is rendered unavailable, and this necessities their application at rates that are prohibitive. Amongst the inorganic iron fertilizers, the most commonly used are the ferrous or ferric sulphate. Both are supplied at rates providing 100 to 500 kg Fe ha1.

Table 11.1. Common inorganic iron fertilizers

Fertilizer

Chemical formula

Fe content (%)

Ferrous sulphate

FeS04.7H20

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