The role of phosphorus
Why do some plants thrive without it? How do you tell if yours need more… or less? PHIL DUDMAN reveals the power and perils of this essential soil nutrient
Every nutrient has a role to play in the health of plants. Phosphorus (chemical symbol P) is one of the main nutrients needed, along with nitrogen (N) and potassium (K), which together form the well-known trio NPK. You’ve probably heard NPK referred to many times in gardening circles, or seen it on the labels of fertiliser products.
Phosphorus is one of the essential building blocks of life. It helps to transfer the sun’s energy within plant cells during photosynthesis and is fundamental to plant growth at every stage, from stimulating early root and shoot development, to the formation of fruit and seeds.
In many parts of Australia, the soil is inherently low in phosphorus. This is largely because of the age of our ancient landmass, and the years of weathering that has depleted soils of this vital nutrient.
Some of our native plants, such as grevilleas, banksias and other members of the Proteaceae family, have developed fascinating adaptations that allow them to thrive in soils where phosphorus is scarce. If you want to grow plants that haven’t adapted (such as common fruit and vegetables and other introduced plants), then, depending on the nutrient levels of your soil, you may need to add phosphorus to achieve healthy growth.
How can you tell if your soil is low in phosphorus?
A simple way is to look at the plants. Those suffering from phosphorus deficiency are often stunted, particularly during the early stages of development. They mature slowly, produce few or no flowers, and have a weak and poorly developed root system. You may even spot a deficiency in the foliage. Older leaves can be a darker green colour, sometimes purplish, as the nutrient is redistributed towards the new growth, where it is needed most.
It’s worth learning about the general nutrient levels of soils in your local area to determine phosphorus availability.
OPPOSITE PAGE, AND ABOVE
Grevilleas have adapted to low-phosphorus soils; purple colouring in the older leaves of plants may signal a phosphorus deficiency.
According to an analysis of multiple soil samples in the Australian National Soil
Site Data Collation, soils with the lowest phosphorus concentrations tend to be in the western monsoon tropics, south-western Australia, south-eastern South Australia and scattered pockets in eastern Australia.
If you’re serious about knowing the specific levels of phosphorus and other nutrients present in your soil, consider getting your soil tested. A quick online search will reveal soil-testing laboratories in your area. Their experts can also advise on specific nutrients that need to be added to your soil to achieve a suitable base level for the plants you want to grow.
How can you add phosphorus to your soil?
Most synthetic and organic fertiliser blends contain some level of phosphorus. Just check the NPK reading on fertiliser packets to determine and compare the ratio of phosphorus to other nutrients. Of the organic fertilisers, the best sources of phosphorus include blood and bone, and
animal manures, particularly those from grain-fed animals such as poultry. Making compost from your own garden waste and returning it to the soil is a simple way to cycle and conserve phosphorus.
If you’re regularly applying fertilisers and making your own compost, you could already be supplying all the phosphorus your plants need. In cases where greater supplementation is required, you may need to apply phosphorus in a more concentrated form. Superphosphate is a well-known synthetic fertiliser with a high concentration of phosphorus. There are also high-phosphorus organic fertilisers based on guano. As with any fertiliser product, take care not to overuse these amendments. Excess phosphorus can run off and pollute waterways.