Waste No Water

Soil Moisture Retention Can Be Improved And A Garden Made More Efficient By Using Environmentally Safe Water-saving Soil Amendments And Products

- Angelo Eliades

The success of any gardening endeavour begins and ends with the quality of the soil. Most plants require a soil that drains well but retains sufficient moisture and nutrients for plants to grow.

Sandy soils tend to dry out more easily but all soil types — composed of various proportions of sand, silt and clay — can dry out completely under severe drought conditions.

When soils don’t retain moisture they don’t keep nutrients, either. Watersoluble nutrients can leach straight through the soil, out of the reach of plant roots, potentially contaminating groundwater sources.

By using soil amendments to improve moisture retention, we can also improve nutrient retention, thereby reducing garden water and fertiliser requirements, while at the same time improving plant growth — a true win-win outcome.

Soil-amendment products work in different ways: they vary in how well they hold moisture and nutrients and how long they remain effective in the soil.

Let’s examine how various soil amendment products stack up in terms of performance.

ORGANIC MATTER: COMPOST & MANURES

Adding organic matter to sandy soils greatly increases both their water-holding capacity and cation exchange capacity (CEC). The CEC is a measure of the soil’s fertility and nutrient retention; more specifically, it’s the soil’s capacity to hold exchangeable cations — positively charged ions such as calcium (Ca2+), magnesium (Mg2+), potassium (K+) and ammonium (NH4+) — in a form that is available to plants.

Soil scientists from the US National Center for Appropriate Technology estimated in 2002 that, for every 1 per cent increase in soil organic matter, soil can hold an additional 75,000 litres of water per acre to a depth of 30cm.