Cover Crops and Green Manure for Tired Soil

Why fields that used to yield well now fall short

If your maize goes pale by the third week, or your cassava roots come out smaller every season, the soil is probably telling you something. Many farms in the Central and Western regions have lost organic matter over years of continuous cropping. The residues that should go back into the ground are burned, carried home for fuel or fed to animals, and very little returns in their place.

Three things drive the decline. Organic matter breaks down faster in warm, wet weather than it can be replaced, so the dark topsoil gets thinner. Nitrogen is taken up by the crop or washed down past the roots when heavy rain falls on bare ground. And the rain itself carries topsoil away when the surface is left open between crops. Sandy soils, which are common across much of the south, drain quickly and hold few nutrients, so they show these losses sooner.

Before you spend money on anything, get a soil test if one is available near you. The district office of the Ministry of Food and Agriculture can point you to the nearest testing service, and the Soil Research Institute in Kumasi, part of CSIR, is the national centre for soil science. A test shows whether the main problem is nitrogen, phosphorus, acidity or organic matter, and that changes what your cover crop should do. For a wider picture of how crops and livestock fit together on a Ghanaian holding, our overview of poultry farming in Ghana is a useful companion.

What a cover crop and a green manure actually are

The two terms overlap, and farmers often use them loosely. A cover crop is any crop sown to keep the soil covered. It shades the ground, slows rain from beating the surface, holds weeds back and protects the soil life underneath. A green manure is a cover crop that you deliberately chop or plough into the soil while it is still green, so its leaves and stems rot down and feed the next crop.

Many legumes do both jobs. A cowpea stand covers the ground and fixes nitrogen, and when you cut it at flowering you can either leave it on the surface as mulch or work it into the soil. The choice depends on what you need most. If the field is badly eroded or the rains are heavy, leave it on the surface. If the next crop is a hungry one like maize and the soil is short of nitrogen, turn it in.

Legumes have a particular advantage. Their roots carry bacteria called rhizobia in small nodules, and these convert nitrogen from the air into a form the plant can use. Some of that nitrogen stays in the roots and leaves when the plant dies back, so it becomes available to the next crop. How much you get depends on the species, how well it grows and whether suitable rhizobia are already in your soil. In good conditions a healthy legume stand can supply a useful share of what maize needs, but it rarely meets the full requirement alone.

Legumes that earn their place on a Ghanaian farm

Five legumes cover most of what a smallholder in Ghana needs. The table compares them, but the timings are rough and change with rainfall, soil and the variety you use.

Choose by what you need. For a quick cover between two maize crops, cowpea is the easiest, because seed is widely sold and the crop is familiar. For heavy biomass on a field that has gone weedy and compacted, mucuna is hard to beat, though it needs a plan for how you will clear it later. Lablab copes better when rain is late or stops early.

Seed quality matters more than many farmers expect. Use clean, well-stored seed from a reliable seller, and if the seed is sold loose, ask which variety it is. Cheap old seed often sprouts poorly and leaves gaps in the cover.

Growing cowpea and mucuna in practice

Sowing cowpea as a cover

Sow cowpea at the start of the rains, or straight after the maize harvest if the soil still holds moisture. For a cover rather than a grain crop, sow fairly thickly, often in the range of 25 to 40 kg of seed per hectare, so the canopy closes quickly. Rows about 50 cm apart, with seeds dropped every 10 to 15 cm, work well with a hoe or by hand. One light weeding in the first three weeks makes a large difference, because young cowpea loses the early contest with weeds.

Growing mucuna without losing control of it

Mucuna performs well on tired, weedy ground. It is usually sown at the start of the rains at roughly 40 to 60 kg per hectare, and it grows fast enough to smother most weeds. Its drawback is that it climbs, tangles and can be hard to clear. Decide the cutting date before you sow. Mucuna pods carry fine hairs that cause severe itching, so wear gloves, long sleeves and a face covering when you pick or cut it. Do not let the pods drop seed into the field unless you want mucuna there next year.

Inoculating the seed

Where a field has never grown a legume, the native rhizobia may be few. Some farmers coat the seed with a commercial rhizobium inoculant just before sowing, working in the shade. Never mix it with fertiliser or fungicide, because those can kill the bacteria. Check the label for storage conditions and the use-by date.

Trees that feed the soil: gliricidia and leucaena

Trees give you a supply of green manure that can be cut again and again. Gliricidia, which many farmers already grow as a live fence or shade tree, is the most useful. It roots easily from stem cuttings pushed into moist soil at the start of the rains, and within a season or two you can cut it back for leaves.

Leucaena is another option, but it spreads readily and can become a weed if left unmanaged. Plant it at field edges or in lines rather than across the whole field, and pull seedlings before they establish.

Cut trees back to about knee height or a little higher, and spread the leaves and thin twigs over the bed. Leaves rot within weeks, while thick stems last longer and are better for firewood or stakes. Gliricidia leaves are rich in nitrogen, but fresh foliage can harm some animals if eaten in large quantities. Do not feed large amounts to livestock without advice from a vet or your district livestock officer. Keeping the leaves as mulch avoids that question entirely.

Timing the cover around your main crop

In the Central Region the rains usually come in two spells, a major season from about April to July and a shorter minor season from about September to November. The exact dates shift from year to year, so watch for the first solid rain rather than the calendar. Sow the cover when the soil is wet enough to germinate the seed, not when the first shower falls.

The easiest rhythm is to sow a cover straight after the maize or groundnut harvest, so it grows through the rest of the wet weather, or to sow it between rows at the last weeding of the main crop. Cutting time decides the outcome. Legume leaves hold the most nitrogen around flowering, while the bulk keeps increasing into pod filling. Cutting at flowering gives soft material that rots quickly. Cutting later gives more mass that breaks down slowly.

Leave about two to four weeks between turning in the green matter and planting the next crop. That gap lets the material start to rot and reduces the risk of seedlings being scorched by heat from fast decomposition. In dry soil the wait may need to be longer. If you cannot wait, sow onto a surface mulch rather than burying the green matter.

Mulch: what to use and how thick to spread it

Mulch is the cheapest cover you can make, because it comes from what you already have. Maize stover, groundnut haulm, rice husks, grass cuttings and dry leaves all work. Chop large stalks into pieces about 10 to 15 cm long so they lie flat and rot evenly. Long stalks bridge the rows, trap air and then blow away in the harmattan wind.

Spread the layer loosely, to a depth of roughly 5 to 10 cm. A thin scatter does little, and a dense mat can stop rain soaking in. Keep the mulch a hand's width away from the stems of young plants so the stems do not stay wet and rot. Around established plants you can pile it closer.

Mulch reduces evaporation, keeps the soil cooler in the afternoon and suppresses weeds. Over a few seasons it builds the topsoil, because each new layer adds material as the old one breaks down. The benefit is cumulative, which is why it is worth starting even when the first season looks like little has changed.

Termites, free-ranging animals and bush fire

Termites are the most common objection to heavy mulching, and the concern is fair. Dry, loose residues attract them, especially in the dry season when other food is scarce. They feed mainly on dead material, so the problem is usually limited to the mulch layer, but on a poor field they can move into young plants. Keep the mulch in contact with the soil rather than heaped against the stems, and check for mud tubes on stems or at the base of seedlings. Keeping the layer moist and spread thin makes it less attractive to them.

Free-ranging goats, sheep and cattle will eat a cover crop before it can do its work. A fence, or tethering animals for the few critical weeks, saves the whole effort. Fowls scratch mulch away, so plan how they will be kept off the bed as well.

Bush fire is a separate threat. In the dry season a fire in the grass can destroy the mulch you have built up over a season. Cut firebreaks around plots where the risk is high, and never light fires near the bed. Fire moves fast in harmattan weather, and one careless fire can take out a season's work.

Pairing poultry litter with green manure

Poultry litter is one of the most accessible fertilisers for a farmer who keeps chickens. It is rich in nitrogen and phosphorus, but fresh litter is hot and high in ammonia. Spread too thickly or too fresh, it scorches seedlings and smells strongly. Composting it for several weeks first, turning the heap a few times, lowers the risk and kills many weed seeds.

Green manure and litter complement each other. A legume adds carbon-rich leaf and stem, and litter adds more nitrogen and a range of nutrients. A layer of chopped green matter mixed with composted litter gives the soil a more balanced feed than either alone. Apply the litter to the bed a few weeks before planting and rake it in lightly.

How much to apply depends on how the litter was stored and how much moisture it holds. Start with a measured amount on one plot, leave an untreated strip beside it, and compare the crop over a season. That comparison is worth more than any general rate. If you are starting a flock mainly for its manure and the household, our guide to starting poultry farming in Ghana covers the basics, and our day-old chicks page explains what we supply and how it arrives. Starting prices are on the chick prices page, and delivery costs are quoted before you pay, with details on the delivery page.

How to tell the soil is recovering

Soil improvement is slow, and most farmers look for a result too early. Organic matter takes years to build, so the signs will be small at first. Look for a darker colour in the top few centimetres after rain, more earthworms in the dug soil, roots that go deeper and branch more, and less surface crusting after heavy showers.

Water tells you a lot. Rain should soak in rather than run off in a sheet, and the soil should hold moisture a few days longer into a dry spell. Plants should look more even across the field, with fewer patches where the soil was once bare and poor.

Keep simple records. Write down the cover crop you used, the sowing and cutting dates, how much mulch you spread and the yield of the next crop. Over two or three seasons the pattern becomes clear, and you have something to show an extension officer if you need advice or a loan later. If you track your costs, the Halaal Farms calculators page has tools for poultry budgets that can sit alongside your crop records.

Keeping the cover alive through the harmattan

The harmattan, a dry dusty wind from the north that usually blows from about December to February, is the hardest period for soil. The air is dry, the wind is strong and the top layer can lose moisture within days. A cover planted late in the rains may die back before it has put on much growth.

Plan the last cover of the year to finish its growth before the harmattan arrives. Lablab and cowpea are useful here, and lablab tolerates dry spells better than most. Leave residues on the surface rather than removing them, and add a layer of mulch on any bare patches. The residue protects the soil, and it feeds the earthworms and other organisms that keep the soil working.

A simple two-season plan for one plot

The sequence below suits a farmer working a plot of about one hectare with maize as the main crop. Adjust the dates to your local rain pattern.

1. After the maize harvest, around August: chop and spread the stover. Leave it on the surface rather than burning or removing it.
2. At the start of the minor rains, around September: sow cowpea or lablab across the field at the recommended seed rate, and weed once in the first three weeks.
3. Around flowering of the cover, roughly October to November: cut the legume and spread the material over the bed. Leave it on the surface if the soil is loose or sloping.
4. Two to four weeks before planting, around February: lightly work the mulch into the top layer, or sow directly into it if you prefer minimal tillage.
5. In the next major season: plant maize with litter or compound fertiliser at the rate your soil test or extension officer recommends, and compare the yield against an untreated strip.

If you keep a flock alongside the crops, the poultry feed calculator helps you plan the feed side of the same budget, and the layer egg profit calculator shows how manure value changes the margins over a year.

Mistakes that cancel out the benefit

• Letting the legume set pods and dry out, then harvesting the pods and removing the rest. You keep some seed but lose most of the organic matter.
• Burning stalks after harvest to clear the ground for the next planting.
• Turning in fresh green matter just before planting in wet soil, which leaves the seedlings in a hot, waterlogged bed.
• Sowing too densely, so the cover competes with the main crop at its most sensitive stage.
• Leaving the ground bare after harvest because the field looks finished. Those bare weeks are when most topsoil is lost.

For broader farm questions and the answers farmers ask most, see our frequently asked questions, or browse the Halaal Farms blog for more practical articles.

Questions farmers ask