But this applied inorganic phosphate fertiliser is highly mobile and leaches into watercourses. In addition, phosphate rock weathers and is also ultimately washed into the ocean where it either deposits as calcium phosphate or is taken up by marine organisms who also eventually deposit on the ocean floor when they die. Consequently, terrestrial phosphorous doesn’t really disappear, but it can move beyond our reach.
Natural wastage
To complicate matters further, even the phosphorous we can use is largely wasted. Of the phosphorus mined as fertiliser, only a fifth reaches the food we eat. Some leaches away and some is bound to calcium and iron in the soil. Some plant roots have the ability to extract the latter, but not in large enough quantities to retrieve all of it.
In addition to these inorganic forms, phosphate is also converted into cellular compounds, creating organically-bound phosphorus, such as phospholipids or phytate. After the death of an organism, these organic phosphorus compounds need to be returned into the useable phosphate form. How much organically-bound phosphorus is present in soils depends on the number and activity of the organisms that can do this.
Agricultural soils are usually rich in inorganic phosphorous while in undisturbed ecosystems, such as forests and long-term pastures, organically-bound phosphorus dominates. But agricultural land is often depleted of phosphorus during harvest and land management practises such as ploughing, hence the addition of phosphate-containing fertilisers.
Spreading manure and avoiding tillage are ways of increasing microbial abundance in the soil – and so keeping more phosphorus in an organically-bound form.
The risks of peak phosphorous can be countered with some simple solutions. Eating less meat is a start as huge amounts are used to rear livestock for meat. The chances are that agricultural yields are limited by phosphorus availability and will be further stretched as the global population grows.
Humans are themselves wasteful of phosphorous, as most of what we take in goes straight out again. Fortunately, technologies have been developed to mine phosphorus from sewage, but at present are too expensive to be practical.
Peak phosphorus does not mean that phosphorus will disappear, rather that the reserves with mineable high concentrations are depleting. Instead, we are increasing the background concentrations of phosphorus and adding it to the ocean floor. More sustainable phosphorus use requires a greater appreciation and understanding of the many organisms that make up soils – and the part they play in phosphorus distribution – or we may no longer be able to feed the world at an affordable price.
News
Grassroots
Augustus 2017
Vol. 17, No.3
Grassroots
April 2017
Vol. 17, No. 1
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Figure 2: Phosphorous boosts crop yields. Shutterstock
Figure 1: Here be phosphorous. Shutterstock