How it works

Soil

Layers drain, hold water, feed plants and set the pH the roots feel.

Roots judge the layer filling the top 5 cm: its pH and whether the species likes it.
A thin top dressing does not change what the roots feel.
Organic soil rots slowly, releasing the carbon and nitrogen growth runs on.
Plain sand feeds little, so plants in it grow slowly.
Pressing packs a layer down: fibres to 40% of their loose depth, soils 72%, sand 88%, gravel barely.

Stats