Zinc is now an integral part of fertilizer recommendations in coffee Agroforestry. It is generally applied along with NPK. The majority of the coffee Planters apply zinc through foliar application. However, Soil application has the advantage of leaving residual effects and thus, permitting a better utilization of applied Zn in a multi-cropping system.
Soil chemists in their review of literature spell out that, of the many interactions of Zn with other nutrients, the most widespread and important to crop production are those with N and P fertilizers on soils with limiting supplies of both Zn and N or P.
Rajendra Prasad et al in their research paper, on the Interactions of Zinc with other nutrients in soils and plants are of the opinion that “In plants, Zn interacts positively with nitrogen and potassium and negatively with phosphorus (P), calcium (Ca), iron (Fe) and copper (Cu). The negative interaction is due to interference of P, Ca, Fe, and Cu in the absorption of Zn on root surfaces or/and its translocation from root to shoot in plants. Zinc interacts negatively with calcium mainly because it competes for the same adsorption sites on soil particles as well as on root surfaces. “They also report in their review that Zinc in soils is present in different forms, such as a soluble, exchangeable, component of secondary minerals, organic matter associated, co-precipitated as secondary minerals, associated with sesquioxides and as a structural part of primary minerals.
Interaction between zinc and Nitrogen
In plants, Zn interacts positively with nitrogen. However, High levels of nitrogen leading to excessive vegetative growth rate may induce Zn deficiency in plants on Zn-deficient soils.
Interaction between zinc and Phosphorus
The interaction of P and Zn has been well-researched and documented in the fields of soil chemistry and plant nutrition. For example, research shows that high rates of P fertilizer without adequate plant-available Zn can reduce Zn uptake by the roots, induce Zn deficiency, and decrease plant growth and yield.
Interaction between zinc and Potassium
Potassium interacts positively with Zn and increases its absorption and translocation in plants. Researchers have observed that with increased levels of potassium, zinc absorption increased significantly. Higher K levels in soil reduces Zinc deficiency in plants that are induced due to phosphorus.
What factors affect zinc availability?
The main soil factors affecting the availability of Zn to plants are low total Zn contents, high pH, high calcite and organic matter contents and high concentrations of Na, Ca, Mg, bicarbonate and phosphate in the soil solution or in labile forms.
In Plants, the interaction between Zn and other plant nutrients does exist and both positive and negative interactions are reported. Phosphorus, calcium, iron, and copper react negatively with Zn and reduce its absorption by roots or/and its translocation to shoot in plants. As regards sulfur, both positive and negative interactions are reported. On the other hand, Zn application reduces boron uptake by plants, and Zn fertilization is recommended for the alleviation of B toxicity in boron-rich soils. Mechanisms responsible for positive or negative interactions between Zn and other nutrients in plants are not well understood and call for further research.
What are the symptoms of zinc deficiency in coffee?
In Coffee, plants, Zn deficiency reduces growth, tolerance to stress, and chlorophyll synthesis. Young leaves are abnormally small and narrow, lanceolate in shape with interveinal chlorosis; leaves bunched at the end of the branch (rosette); short internodes; defoliation and dieback in severe deficiency. The quality of plant products is also adversely affected and plants have increased susceptibility to injury by high light intensity and temperature and to infection of certain microbial-induced diseases.
The total Zn content in soil depends upon the parent rock, weathering, organic matter, texture, and ph. Zn becomes less soluble at pH values greater than 7, therefore, they become less available to plants. Zinc is usually more available as soil pH moves to the acid side of 7.
In soils, pH is the dominant factor determining the availability of Zn. In alkaline and calcareous soils, Zn gets adsorbed or precipitated on hydroxide- (especially those of iron) and carbonate surfaces. Zinc also gets adsorbed or precipitated on negative charges of phosphates. On the other hand in highly Cucontaminated soils Zn can get released in the soil solution. Zn exists as five distinct pools in soils viz., water-soluble, exchangeable, adsorbed, chelated, or complexed Zn. These pools differ in strength (or reversibility) and therefore in their susceptibility to plant uptake, leaching, and extractability. The equilibrium among different pools is influenced by pH, the concentration of Zn, and other cations, particularly iron and manganese.
Zinc (Zn) ions have both beneficial and toxic effects on plant cells. Zinc is usually present in soils in adequate amounts, except where there is an excess of organic matter. There are many factors that have led to the present state of Zn deficiency in coffee soils. The main soil factors responsible for causing Zn deficiency are low total contents of Zn, high pH, high contents of calcite, high concentrations of bicarbonate ions and salts, and high levels of available phosphorus. The mechanisms responsible for the interaction effects of Zn with other plant nutrients in soils and plants are not well understood and call for further research.
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