October 8, 2026

How leaching soil washes nutrients past the roots

Leaching happens when more water moves through a bed than the root zone can hold. The extra water drains downward by gravity, and it carries dissolved nutrients past the roots on the way down. The usual cause is watering a bed that was already wet enough. The fix is to water less often. Feeding more adds nutrients for the next drain to carry off.

How water carries nutrients past the roots

Soil holds water against gravity up to a point called field capacity. Below that, water clings to soil particles where roots can reach it. Above it, the surplus moves through the larger pores and drains down. Dissolved nutrients ride along in that draining water, so a heavy soak on an already-wet bed pushes nitrate and other soluble nutrients below the roots.

Nitrogen leaches first because nitrate dissolves readily and binds loosely to soil. Sandy and fast-draining beds leach sooner than clay or compost-rich beds, which hold more water before the surplus drains. Raising the organic matter in a bed lifts its water-holding capacity, so more rain stays in reach before it drains away. We cover that in how compost cuts watering.

How much water it takes

A wet bed plus more water triggers leaching. No single number of inches sets the threshold. A bed sitting near field capacity leaches with the next heavy rain or watering. A bed that has dried down soaks up the same water with little loss. That is why watering frequency drives leaching more than the amount you apply in one session.

Rain alone moves plenty of water through a bed. Our garden logged 10.89 inches of rain across July, and one August day dropped 2.46 inches. On days like those the beds were already draining, and any watering we added would have pushed straight through. Our soil probes showed the beds swinging between 32% and 62% moisture across July and August, so there were long stretches where a bed read near its wet end and needed nothing.

What stops leaching in your beds

Two readings tell you when a bed is already wet enough to skip. The first is the bed's own soil moisture. A mapped soil probe reads the root zone directly. Siltwell fires a dry alert only after the two latest readings in a bed both sit under its threshold, 25% by default. A bed well above that number does not need water, and watering it risks pushing nutrients down. See soil moisture monitoring for a home garden for how the per-bed reading works.

The second is rain. Siltwell gives garden-level watering guidance on the Today screen for every garden. It holds watering when 5mm or more of rain is forecast over today and tomorrow, and it marks the garden covered when 5mm or more fell over the last three days. Both rules keep you from stacking water onto soil that is already near field capacity, which is where leaching starts.

If a bed reads wet, skip it and check again in a day or two. When the reading drops toward its dry threshold, water the root zone and stop. For beds that drain fast and lose nutrients between rains, build the soil up over a season rather than watering more, starting with soil water retention when the bed runs dry.