Don’t Forget Zinc When Applying Phosphorus to Your Farm
Introduction
Phosphorus (P) fertilizer is frequently used to increase crop productivity; however, farmers should not forget the role of micronutrients like Zinc (Zn). High amounts of P fertilizer, for example, without adequate plant-available Zn, could restrict Zn uptake by the roots, generate Zn shortage and diminish plant growth and production.
Causes of Negative P-Zn Interaction
Formation of Insoluble Zn-Phosphate Complexes
One of the primary causes of Zn deficiency in soils treated with high amounts of P fertilizer is its reaction with soluble Zn in the soil. This reaction produces insoluble Zn-phosphate complexes, which influences the availability of Zn for plant uptake over time. A potential outcome is a reduction of Zn availability leading to Zn deficiency and reduced plant growth.
Role of Mycorrhizae in Causing Zn Deficiency in Plants
Mycorrhizae are beneficial fungi that form a symbiotic association with roots. Mycorrhizal fungi play an important role in plant nutrient uptake (particularly P and Zn) by extending Hyphae (e.g., a fungi-produced root-like structure) horizontally and deeper into the soil where plant roots normally cannot reach. These fungi allow plant roots to have a greater surface area for increased water and nutrient uptake capacity. In terms of Zn, Mycorrhizae may be responsible for up to 50% of the total Zn uptake in crop plants. When high rates of P are applied, Hyphae become “lazy” and the overall root footprint is decreased, resulting in a reduction of Zn uptake by the plant. High P application rates may, therefore, reduce Zn uptake mainly due to the reduced Mycorrhizae-dependent Zn uptake.
Situations Where Negative P-Zn Interaction Occurs
When soils receive high P application rates or have extremely low plant-available Zn, negative P-Zn interactions can ensue. A greenhouse study, for example, employing a calcareous soil with low plant-available Zn revealed that high P treatment rates generated Zn shortage and impaired plant development. Plants that received adequate Zn, on the other hand, did not demonstrate any Zn deficit and responded positively to higher P rates.
Soil Test Zinc
Soil test Zn is commonly measured using a DTPA extraction. Extensive calibration tests conducted across a diverse set of soils in the US has found that DTPA-Zn concentration in soil is very well correlated with the root uptake of Zinc. Soil test laboratories often recommend the application of Zn in soils where values fall below 0.7-1.0 ppm. Many high-yield systems where high-rates of P are applied to match removal values may find a benefit from applications of Zn even when soil test Zn values are above these critical levels.
Conclusion
The addition of P fertilizer without adequate Zn can reduce crop growth and yield. Research suggests that two of the key reasons are: 1) Negative P-Zn interaction and the formation of insoluble Zn-Phosphate complexes and 2) Reduced Mycorrhizal activity. The negative effect is magnified under conditions where soil Zn supply is very low and P application rates are high. Improved plant growth response to P was realized when the Zn supply was sufficient or when Zn was co-granulated with P fertilizer.
Source:
https://www.cropnutrition.com/