Cover crops and green manures: feed your soil for free

What if a crop could fertilise your field for you — and you never even had to sell it? That’s the deal with green-manure cover crops. You grow them not to harvest, but to feed the soil for the crop that follows.

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The crops that pay you back

In southern Africa, three legumes are proven green manures for smallholders (CIMMYT):

Mucuna (velvet bean) — a vigorous smotherer of weeds and a heavy nitrogen-fixer.
Sunnhemp (Crotalaria) — fast biomass and nitrogen.
Lablab — nitrogen plus useful livestock fodder.

Grown in rotation or as an off-season cover, these legumes can leave roughly 50–150 kg of nitrogen per hectare in the soil and lift the following maize crop by adding biomass and organic matter (Mupangwa et al.).

More than nitrogen

Cover crops also smother weeds, shield the soil from erosion, and feed it organic matter as they break down (FAO). In a conservation-agriculture system, you leave the residues on the surface as a mulch — and the lush growth doubles as livestock fodder.

Make them work

Fit them into the rotation — an off-season or relay cover before maize.
Feed the maize, not the legume — the cover crop thrives on residual fertility, so save your inputs for the cash crop (Mupangwa et al.).
Cut or roll before they seed, then leave the residue as mulch.

Compost the surplus, compound the gains

Cover crops build organic matter from the top down; compost and PCAT BLOCK C build it from below. Use the cover crop’s biomass — and your manure — and you’re rebuilding tired, acidic soil from both directions at once.

Do this now

Sow a green-manure legume (mucuna, sunnhemp or lablab) in the rotation.
Don’t harvest it — let it feed the soil.
Leave the residue as mulch before maize.

PCAT BLOCK C — the locally-made, field-proven compost that rebuilds Zimbabwe’s tired, acidic soils.