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ABSTRACT

Antagonistic relationship between phosphorus (P) and zinc (Zn) in soil and plant system is widely known. Excess phosphorus in soil can hinder the zinc mobilization and uptake by plant roots; whereas, higher concentration of phosphorus in plant cells can also interfere the metabolic functions of zinc. Managing the negative interaction is never easy when there is high phosphorus build-up in the soil, and both P and Zn have their prominent roles in plants’ metabolism. Maintaining adequate organic matter, balanced nutrition through soil test based fertilizer application, foliar application of zinc, management of soil pH may be some of the management options to overcome the situation.

Keywords: interaction, phosphorus, zinc, nutrient use efficiency, antagonism

Interaction among nutrients is an important phenomenon which governs nutrient availability and affects the productivity of agricultural field crops. This interaction greatly depends upon different soil physical and chemical factors (soil aeration, moisture, soil temperature, pH, nutrient concentration etc.), plant morpho-physiological factors (root distribution, root density, transpiration and respiration rate, growth rate and age of plant, species and internal nutrient concentration of plants) and climatic factors (temperature, light intensity etc). There is a positive interaction among nutrients within a suitable limit which can help in overall growth and productivity of crop plants. Improving plant growth requires different nutrients to maintain tissue concentration within required limits; mutually synergistic effects among nutrients to promote growth. Among micronutrients, it has been seen that high P application and uptake leads to Zn deficiency in crops such as rice (P-induced Zn deficiency) in marginally Zn-deficient soils. In this Zn-deficient condition, soil cannot fulfil the Zn requirement of crop and antagonistic relationship with P even further worsen the condition. In this situation Zn-deficient soil cannot fulfil the Zn requirements of rapidly growing plants. It was reported that excess application of P results in unavailability of  Zn to the leaves by immobilizing more than 1/3^rd of the total absorbed zinc in the roots and 1/5^th at the nodes of the stem, whereas at the normal rate of P application only 12 and 6 per cent Zn is restricted in the roots and the nodes, respectively (Dwivedi et al., 1975). Application of high levels of P inhibits translocation of Zn from roots to metabolic sites in leaves. Formation of Zn phosphate is considered responsible for Zn immobilization on root surfaces and in leaves (Sarret et al., 2001).

ANTAGONISTIC MECHANISMS

Commonly there are four mechanisms which can explain the P-Zn antagonism: 1. P-Zn interactions within the soil itself, 2. Dilution of absorbed Zn in plant by increased biomass production (dilution effect) in response to P application, 3. Reduced uptake and/or translocation of zinc influenced by added P, and 4. Interference of P to the Zn utilization by plant. Sometimes higher levels of PO₄^3- ions can also reduce the colonization of mycorrhizal fungi which leads to reduction in the absorbing surface area of the roots. Long term improper applications of P fertilizers results in progressive P built up in soil and showed that if application of Zn is not given proper attention or not applied at the ideal time that may lead to severe Zn deficiency. 

MANAGEMENT OPTIONS

Source and method of fertilizer application

Application of highly water soluble zinc fertilizers is the most effective way to address Zn deficiency because of high water solubility and will be absorbed by the plant tissues rapidly due to its mobility and solubility. To reduce the problems related to mobility and translocation, it can be applied in rows for better absorption. Sulphate and oxide of zinc are the most common forms of Zn used for supplying Zn to correct zinc deficiency. Zinc nano-fertilizers, is another upcoming option, may be used for better absorption and reduce the reaction time in soil to reduce the antagonistic effect.

Seed coating and foliar application of zinc

Zinc deficiency in crops is generally observed in the early growth stages. Zinc can be supplied to the emerging plants through seed coating or seedling dipping treatments. Coating of seeds through zinc compounds facilitates supply of Zn at very early stage of crop growth and avoids the interference of phosphorus in soil. If the crop has sufficient leaf area, foliar application at the rate of 0.5% ZnSO₄ can be applied to correct Zn deficiency induced by excess soil phosphorus. Approximately, foliar application of 0.5 to 1.5 kg Zn ha^–1 as ZnSO₄ or 0.15-0.2 kg Zn ha^–1 as Zn-EDTA would help in correcting Zn deficiency and improving crop yields.

 Balanced nutrition and soil testing

Balanced nutrition emphasizes on supplying all the essential plant nutrients in adequate proportion and quantity. With balanced nutrition, crop growth and yield are maximized which also improves nutrient use efficiency. It is witnessed that supplying N, P, K and S in adequate amounts and proportion along with Zn, optimum growth and yield of crops is achieved which  leads to higher nutrient use efficiency. Soil testing is a very useful technique for fertilizer recommendations in field crops and is considered as a successful approach to avoid negative interaction among nutrients arise due to nutrient imbalances. Required amount of P-fertilizer can be applied following the equation-based STCR (Soil test crop response) approach to avoid excess or deficit application and negative consequences. This is beneficial in both ways: correcting nutrient imbalances and saving fertilizer cost.

 Zinc coated fertilizer material

It has been found that zincated superphosphate and diammonium phosphate are as good as zinc sulphate as a source of Zn for many crops while supplying both P and Zn. It is a matter of appreciation that zincated NPK fertilizers are presently produced and marketed in India.

 Use of chelating agents

As an effective strategy to mitigate or reduce the fixation of Zn by high phosphate content in the soil, application of zinc chelates is often recommended. However, only Zn-EDTA chelate is fairly resistant to binding with high P in the soil in comparison with other or cheaper chelating agents. Though, it is an expensive product. For fertigation through drip system (mainly for high value crops) the use of chelate can be recommended and concentration can be reduced to an economically acceptable limit.

Improving soil organic matter

Maintaining sufficient organic matter content in soil through organic manuring should never be ignored in moderating such antagonistic relations, and thereby a balance between different nutrients which are involved into such antagonistic relationship can be maintained. Organic materials are also found promising in reducing P fixation by concealing the fixation sites on the soil colloids and by forming organic complexes or chelates. During the recycling of crop residues, green manuring and farm yard manure can release P and Zn by microbial activities. Soil organic matter can be improved by applying different form of organic manures, adopting appropriate crop rotation, practising conservation tillage, and recycling of crop residues are some of the practicable management strategies. Organic matter mineralization produces various organic acids which may also work as a natural chelating agent, releasing nutrients in slow mode and  help in better translocation of nutrients sustaining the crop demand.

Use of efficient crop species

It is observed that breeding crop varieties for high P uptake efficiency may reduce the capacity of Zn uptake and tissue Zn concentration. Zinc uptake efficiency should also be taken into account while selecting breeding lines with high P uptake efficiency. However, excess application of P affected Zn concentration in the Zn-inefficient but not the Zn- efficient cultivars (Imtiaz et al., 2006). The Zn-efficient varieties maintain P concentrations at a lower level and absorb more Zn²⁺ ions. Landraces and local cultivars of many crops are often reported to be Zn-efficient, but this trait is lost during breeding for commercial traits.

 Symbiosis with mycorrhizae

Association of plant roots with mycorrhizal fungi can improve P and Zn uptake by crop plants with extended mycelial network. Mycorrhizae act as filter by not only improving the uptake of micronutrient elements but also safeguarding plants from excessive uptake and avoid toxicity of these elements. VAM or vesicular-arbuscular mycorrhiza (also known as Arbuscular mycorrhiza fungi, AMF) has been found to improve crop yields of several crop species in low fertile soils by enhanced uptake of immobile nutrients like P, Zn, and Cu through increased water absorption, exudates production and increased forage area of roots (Meding and Zasoski, 2008). However, higher application of phosphorus in soil tends to depress VAM activity associated with plants.

CONCLUSION

High phosphorus supply to soil can induce zinc deficiency in crops. When recommended (balanced) phosphatic and zinc fertilizers are applied into soil, there is a least chance of observing antagonistic effect. Problems mostly arise if Zn content is very low in the soil; simultaneously P is added in high amount leads to reaction between P and Zn in soil and that further restricts the availability. Use of appropriate method, source, and rate of fertilizers are important management practices to avoid P-Zn antagonism in soil system. Additionally, balanced fertilizer application through soil testing is practical approach. Furthermore, maintaining adequate organic matter in soil, seed coating with zinc and deepening roots in the zinc solution, selecting zinc-efficient crop species, improving plant root association with beneficial microorganisms are other important practices to reduce the P- Zn  antagonism.

 

References

 

Dwivedi, R. S., Randhawa, N. S., and Bansal, R. L. (1975). Phosphorus-Zinc Interaction: I. Sites of immobilization of zinc in maize at a high level of phosphorus. Plant and Soil 43(3), 639–648.

Imtiaz, M., Alloway , B. J. , Memon, M. Y., Khan, P., Siddiqui, S. H., Aslam, M. and Shah, S. K. H. (2006). Zinc tolerance in wheat cultivars as affected by varying levels of phosphorus. Communications in Soil Science and Plant Analysis 37, 1689 – 1702.

Meding S. M. and Zasoski R. J. (2008). Hyphal-mediated transfer of nitrate, arsenic, cesium, rubidium, and strontium between arbuscular mycorrhizal forbs and grasses from a California oak woodland. Soil Biol. Biochem. 40, 126–134.

Sarret, G., Vangronsveld, J., Manceau, A., Musso, M., D’Haen, J., Menthonnex, J. J., and Hazemann, J. L. (2001). Accumulation forms of Zn and Pb in Phaseolus vulgaris in the presence and absence of EDTA. Environmental science & technology, 35(13), 2854-2859 .

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