It is widely known that root vegetables need phosphorus to grow well, but is it so widely known that phosphorus supplies may be dwindling and that phosphorus fertilizer is a major cause of waterway pollution? Using phosphorus sustainably in gardens, in agriculture and forestry is becoming much more important.

Importance of Phosphorus to Life

Phosphorus is one of those chemicals that are basic building blocks of life.  It is essential in DNA, RNA, the phospholipids that are part of the wall of every plant or animal cell and is in adenosine triphosphate which carries energy through living systems.

While it is essential for growth of all plants1, it is particularly required by root vegetables.  If you read articles about growing them e.g. carrots2, you will see that it is recommended to add phosphorus fertilizer, in contrast to leafy greens where the recommendation is to add nitrogen. This is especially important in Australia where soils are low in phosphorus3.

Phosphorus in soil is present in both organic and inorganic forms, but only the inorganic form, phosphate, is it available to plants1.   Since releasing phosphate by microbes is a slow process, farmers have traditionally added materials which are already rich in phosphate.  Initially guano (bird droppings) was a phosphorus source and then in the mid-1800s, when this source became limiting, sedimentary phosphate-rich rock became the major source.  It can be used as rock dust as a slow release fertilizer, but liquid phosphate fertilizer is obtained by acid extraction of the source material.

Phosphorus Supplies

All but around 10% of phosphorus use worldwide is within the food chain4.  Already by 2010 it was predicted that world phosphorus supplies would last only another 80 years assuming the then population.  But as human numbers are increasing, the supplies would not last that long.

These considerations led to the idea of “Peak Phosphorus” being reached by around 20305 .  However, much debate has followed and it is recognized that this peak really only represents economically available phosphorus because, although there are more phosphorus-containing rock deposits, accessing them becomes too expensive.  Additionally, 84% of these are held by only 5 countries6 but exploration continues for others.

The shortage is mainly because modern agricultural production is a one-way process in which fertilizer is added to soil, excess fertilizer applied to soil is washed into waterways and crops are consumed by animals including humans with waste excreted into sewage.  This process is not only wasteful but creates waterway pollution leading to oxygen depletion which affects aquatic life including the killing of fish7.

How Can We Ensure Future Phosphorus Supplies?

It is time for “reduce, reuse, recycle”!


Minimise Use

This can be done by adjusting fertilizer application to just that needed by crops.  Universities are developing tools to help farmers do this accurately.  Getting the phosphate levels just right is not so easy for gardeners, although kits are available, so we recommend making sure the soil is enriched with compost and manures rather than adding extra phosphate.

Scientific approaches have been developed using pigs for which phosphorus is an essential dietary component for them to grow well.  An enzyme, phytase, which breaks down organic sources of phosphorus, can be added to their diets so that they need less phosphorus supplementation7. Even a genetically modified pig has been developed which produces the enzyme itself.  Both approaches can reduce phosphorus in the food industry8.

Plant Modification

Research on genes that control phosphate transfer within plants may lead to breeding plants better adapted to low phosphate levels9.

Reuse and Recycle

Reversing the one-way pathway of phosphorus is essential.  That really puts the focus on turning all organic waste into material that can be added to soil.  In traditional practice, before the advent of manufactured fertilisers, all material produced farm was kept on the farms.  So composting and using animal and human excrement was the norm.  However, using either urine or faeces is associated with risks of disease caused by pathogenic microorganisms on harvested food or just from working the soil.

Urine is the largest source of phosphorus in cities and their surrounds 6   So methods for extracting phosphorus from urine and wastewater are being developed and there are some commercial products being tested.  These involve the formation of a different form of phosphorus called struvite which actually preforms better than the form commonly used.  It is, however, more expensive.

Solutions for Sustainable Gardening

  • Make as much compost as possible from food and garden waste. Phosphorus concentrations are, however, low and release is slow.
  • Add manures or wood ash phosphorus but release is also slow.
  • Keep chickens and use debris from their run which contains their manure.
  • Grind or crush fish and chicken bones, boil them and cut them into smaller pieces when they become soft and add them to soil.
  • Add clay particles to soil as it helps prevent free phosphate from being washed away.
  • Use a commercially available form of organically certified phosphorus.  These include rock dust, bone meal and kelp products.

In any case, it is vital that we save as much phosphorus-containing material that is “waste” from our food and gardens.

No matter whatever additions you use, it is important that the soil pH is between 6.0 and 7.0 so that phosphorus becomes accessible to plants1.

  5. Prud’Homme, M. (2010). Peak Phosphorus: an issue to be addressed. Fertilizers and Agriculture, International Fertilizer Industry Association (IFA). February 2010.