Leaching is the process by which nutrients are removed from the soil. Rainwater dissolves soluble compounds, such as nitrate ions, and carries them down through the soil, beyond the reach of plant roots and into streams, rivers, ponds and lakes.Leaching is a natural process, but farming makes it much worse. It is most likely when:
• artificial fertilisers are used, because their ions are highly soluble;
• more fertiliser is applied than the crop can absorb;
• fertiliser is spread just before heavy rain, or when crops are not actively growing;
• the soil is sandy and drains quickly.Natural fertilisers such as manure cause less leaching. Their nitrogen and phosphorus are mostly held in organic molecules, which are released slowly as saprobionts decompose them, so fewer free ions are present to be washed away at any one time.Phosphate ions are less soluble than nitrate and bind to soil particles, so they are leached less. However, phosphate can still reach water when soil or manure is washed off fields by surface run-off.Leaching has two costs. For the farmer, the fertiliser that was paid for is lost and the crop receives less of it. For the environment, the extra ions in water can harm aquatic ecosystems (eutrophication), and high nitrate levels in drinking water are a health concern.Leaching is reduced by applying the right amount at the right time, avoiding rainy periods and leaving untreated strips of land beside watercourses.
• artificial fertilisers are used, because their ions are highly soluble;
• more fertiliser is applied than the crop can absorb;
• fertiliser is spread just before heavy rain, or when crops are not actively growing;
• the soil is sandy and drains quickly.Natural fertilisers such as manure cause less leaching. Their nitrogen and phosphorus are mostly held in organic molecules, which are released slowly as saprobionts decompose them, so fewer free ions are present to be washed away at any one time.Phosphate ions are less soluble than nitrate and bind to soil particles, so they are leached less. However, phosphate can still reach water when soil or manure is washed off fields by surface run-off.Leaching has two costs. For the farmer, the fertiliser that was paid for is lost and the crop receives less of it. For the environment, the extra ions in water can harm aquatic ecosystems (eutrophication), and high nitrate levels in drinking water are a health concern.Leaching is reduced by applying the right amount at the right time, avoiding rainy periods and leaving untreated strips of land beside watercourses.