Why in news?
Scientists have warned that decades of heavy use of phosphorus fertilisers have left soils saturated with so‑called “legacy phosphorus.” This nutrient reservoir no longer benefits crops but instead washes into rivers and lakes, causing algal blooms and dead zones. New research highlights the need for better soil testing and nutrient management to reduce pollution and conserve finite phosphate resources.
Background
Phosphorus is an essential element for plant growth and a key component of DNA and energy‑transfer molecules. Farmers apply phosphorus fertilisers to ensure high yields, but only a portion is taken up by crops; the rest binds to soil particles and accumulates. In many regions, especially in North America and parts of India, fertiliser was historically cheap and soil tests underestimated the nutrient already present. As a result, farmers applied far more phosphorus than their fields needed.
Over time this surplus—termed legacy phosphorus—builds up in soils. Heavy rain or irrigation can transport it into waterways, where it fuels explosive growth of algae. When algae die and decompose, oxygen levels plummet, creating “dead zones” that kill fish and other aquatic life. Cleaning up these waters becomes extremely costly and time‑consuming.
Why it matters
- Water quality degradation: Phosphorus‑laden run‑off causes eutrophication in rivers, lakes and coastal areas. Harmful algal blooms produce toxins that threaten drinking water supplies and aquatic biodiversity.
- Finite mineral reserves: Phosphate rock, the primary source of fertiliser phosphorus, is a non‑renewable resource concentrated in a few countries. Wasting it jeopardises long‑term food security.
- Soil imbalance: Excess phosphorus can hinder uptake of other nutrients such as iron and zinc, affecting crop health. Accumulated phosphorus may remain locked in soils for decades, complicating management.
Solutions and recommendations
- Improved soil testing: Modern spectroscopic sensors and laboratory techniques can quantify plant‑available phosphorus more accurately than traditional methods. Site‑specific testing helps farmers apply fertilisers only where needed.
- Use legacy stocks: In fields with high phosphorus levels, farmers can reduce or temporarily halt fertiliser applications and instead draw on the existing soil reserves. Crop rotations with deep‑rooted plants can bring buried phosphorus to the surface.
- Buffer strips and cover crops: Planting vegetation along field edges and maintaining year‑round ground cover prevents erosion and traps phosphorus before it reaches streams.
- Circular nutrient economy: Recycling phosphorus from manure, compost and sewage sludge reduces dependence on mined fertilisers. Policies encouraging nutrient recovery from waste streams can close the loop.
Addressing legacy phosphorus requires coordinated action by farmers, researchers and policy‑makers. By treating phosphorus as a precious resource and tailoring its use to specific soils, we can protect waterways, save costs and ensure sustainable agriculture.
Sources : Down To Earth
