Fertility and the NPK group – Phosphorus

Phosphorus is usually associated with root growth. Plants grown in soils deficient in Phosphorus are usually stunted. Adding phosphate fertilisers to new plants is a common practise that produces rapid lush growth, however it does not increase the final yield in many cases. For this reason grazier farmers add superphosphate (or super as it is called in the agricultural community) to their fields when regrowth begins in spring. Unfortunately much of it is wasted and ends up in the water run off with disastrous results in the aquatic systems. Usually applied as phosphate in horticultural applications, it produces dramatic improvements in growth because the added root growth gives plants more access to nutrients. Unfortunately it is often overused in agriculture, to the point where it leaches into rivers, creating algal blooms and choking aquatic weed growth. Each year the nitrogen and phosphate rich run off from farms in the Mississippi valley runs off into the Gulf of Mexico where the nutrient rich water reacts with sunlight, creating a massive algal bloom that covers 16,000 square kilometres. When the algal bloom dies off, it leaves an area the size of New Jersey, so deprived of oxygen it is lethal to all fish.

Satellite view of Gulf of Mexico

The read region around the mouth of the Mississippi River shows water with oxygen levels below 2 parts per million due to phytoplankton blooms. Courtesy of NASA

Phosphorus deficiency in young plants can appear similar to a potassium deficiency because the roots are stunted and cannot take up enough nutients, making the leaves appear yellow at the edges. As the plants grow sugars can accumulate and cause anthocyanin pigments to develop, producing a reddish-purple color in the leaves. Before you decide there is a shortage of phosphorus, check your plants do not typically have a red/purple tendency. Generally this is rare in most soils except those in tropical regions where monsoonal rains and flooding can leach phosphates out of the soils.

Tomato vine leaf with purple undersides

A tomato vine leaf showing signs of phosphorus deficiency – a purple shade under the leaves.

Animal manure is generally high in phosphorus. It is best aged or steeped in water and the resulting water used to water the plants.

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