Gardening glossary
Leaching
Leaching describes water carrying dissolved nutrients downward and out of the root zone. In a vegetable bed, that means nitrogen — particularly nitrate — washed out by winter rains. In a houseplant, it means accumulated fertiliser salts flushed out of the pot when you water heavily from the top.
What leaches easily and what does not:
- **Easily leached.** Nitrate, sulphate, chloride, sodium, boron, calcium and magnesium in small quantities. These are highly soluble and move with every heavy watering. - **Moderately leached.** Potassium and magnesium in sandy soils; almost nothing in clay soils, where they bind to the negatively charged clay surface. - **Rarely leached.** Phosphorus, iron, manganese, zinc, copper. These bind tightly to soil particles and stay in place.
Leaching as a problem. In an outdoor vegetable garden, autumn and winter rains can wash 30–60% of applied nitrogen straight out of the root zone. This is why heavy autumn nitrogen applications are wasteful — the rain takes most of it before the next crop can use it. Cover cropping with rye, oats, or vetch over winter captures that leaving nitrogen and holds it in plant tissue until you turn the cover crop in the next spring.
Leaching as a tool. In container growing, every watering with synthetic fertiliser leaves behind a small residue of salts. Over months, those salts accumulate, the soil EC (electrical conductivity) rises, and roots start to suffer from osmotic stress — leaf tips brown, growth slows, salt crystals appear on the pot rim. The fix is to *deliberately* leach: every 6–8 weeks, water the pot with about three times its volume of plain water and let it all drain through. The high water volume dissolves and flushes accumulated salts out of the bottom of the pot.
For hydroponic and indoor growers, periodic leaching is a maintenance task as routine as cleaning a filter. For outdoor gardeners, the goal is the opposite — cover crops, mulch, and incorporated organic matter all slow leaching by holding nutrients in living biomass and clay-humus complexes until the next crop needs them.