Phosphite fertilisers What are they?

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Phosphite fertilisers - what are they?

By Chris Hollow, Agronomist with Growth Products

Making your life easy - and getting beautiful results - is what our business is all about. It is also all about consumer education, and the science behind our products. Here we offer a brief primer on phosphite and how it differs from phosphate.

Phosphorous (P) is an essent

ial element for plant photosynthesis, root growth, energy storage and protein formation. The most common form of P in fertilisers is phosphate, which is a chemically stable molecule with four oxygen atoms. Because of its rock-like nature, the phosphate in traditional granular fertilisers often becomes bound in soils and is not fully available to plants.

Phosphite is also a form of P, but it has one less oxygen atom than phosphate. Being "lighter" by one oxygen atom than phosphate makes a tremendous difference. Since the fully oxidized phosphate is the most stable P form in the environment, phosphite undergoes a gradual transformation after addition to the soil. Micro organisms are able to assimilate phosphite and release phosphate, gaining energy and nutrients during biological conversion.

Microbes will preferentially take up phosphate for their metabolism before taking up significant amounts of phosphite. However, due to its greater solubility it is more readily available to these microbes and plant roots than phosphate. So, phosphite is less chemically stable, and therefore more agile than its staid cousin. It also has certain benefits that phosphate never dreamed of.

Phosphite is highly water soluble, and when applied to plants is quickly absorbed by leaves, roots, and branches. Once in plants, it is extremely mobile, exhibiting "symplastic ambimobility," meaning that it is uniquely able to move in both xylem and phloem (1).

Years of use in agricultural and horticultural settings have shown that foliar applications of phosphite improve seedling growth, bud formation, blossoming, fruit set, and plant vigour. On golf courses, phosphite improves turf quality and promotes overall growth. Applications of phosphite to turf also significantly reduce summer stress.

Interestingly, phosphite ions, unlike elemental phosphate, have fungi toxic effects on several plant pathogens (1). When PSD approved fungicides with phosphite as an active ingredient are applied to grass, the phosphite moves quickly to the turf's crowns and roots, where it works against root rots such as pythium and phytophthora (1, 2).

Note: Many phosphite fungicides and phosphite fertilisers have the same active ingredient, but only those products with a PSD fungicide label can claim any fungicidal properties or benefits. This discusses published research on phosphite, and does not make any fungicidal claims for phosphite products.
Scientists find phosphites both fascinating and a fertile source of on-going debate and research. The growth-stimulating and other beneficial effects of phosphite are well-documented, even if they are not fully understood (1, 2).

To summarise we can say: phosphite helps fungicides work better; quickly enters through the leaves, crown and roots; highly systemic via xylem and phloem; protection throughout the whole plant including new growth; does not contribute to phosphate build-up in soils; reduces summer stress.

1. Understanding Phosphonate Products, prepared by Peter Landschoot, Professor and Joshua Cook, Dept. of Crop and Soil Sciences, The Pennsylvania State University.
2. Phosphite Fertilisers: What Are They? Can You Use Them? What Can They Do?, by C.J. Lovatt and R. L. Mikkelsen. Better Crops, vol. 90 (2006, No. 4)

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