Today we’re going to be looking at using plants to enrich the soil for other plants. Some plants can be grown as green manures which are dug into the soil, some produce leaves which can be harvested regularly and dug into the ground, and some are crops which produce both a harvest and leave nutrients behind once they’ve gone.

Green manures

One of the best ways of covering bare soil and adding nutrients to it is by sowing a green manure. Green manures are plants which grow fast, often ‘fixing’ nutrients in their roots, and are then cut down and dug into the soil, returning nutrients as they rot down.

Their roots are invaluable for improving soil structure, breaking up heavy clods and introducing air.

You can sow long-term manures that last for a couple of years, overwintering manures, and manures that will grow and be ready for digging in within a month. One of the best functions of many green manures is that the plants take in nitrogen through their leaves and store it within their roots. Once dug into the ground, this is returned to the soil. We’ll be looking at key nutrients later this week, but nitrogen is one of the most important, encouraging leafy growth.

Long-term and overwintering green manures

Alfalfa: Sow by mid-summer, and leave for up to a year, including overwinter. Fixes nitrogen and has deep roots which are perfect for drawing up buried nutrients.

Alsike clover: Sow by late summer. Fixes nitrogen, and dislikes dry soils.

Crimson clover: Sow by the end of September. Fixes nitrogen and is very fast-growing, smothering weeds. You can cut this down a couple of times during the growing season and dig the leaves into other beds or put in the compost heap.

Field beans: Sow in the autumn. Fixes nitrogen and is perfect for heavy soils.

Forage pea: Sow by November. Fixes nitrogen and protects the soil over winter.

Fodder radish: sow by the mid-summer if you want to overwinter. Has deep roots which draw up nutrients and has large lush leaves which smother weeds.

Grazing rye: Sow by late autumn. Does not fix nitrogen.

Phacelia: Sow by early autumn. Can be left to grow for a couple of years if necessary, and has beautiful blue flowers which attract bees. Does not fix nitrogen, but produces a large volume of organic matter and smothers weeds.

Sweet clover: Sow by mid-summer for overwintering. Deep-rooted and leafy. Can be left to grow for up to two years.

Trefoil: Sow by late summer for overwintering. Fixes nitrogen. Dislikes acid soils.

Vetch: Sow by September for overwintering. Fixes nitrogen. Dislikes acid soil.

White clover: Sow by August for overwintering. Can be left for up to five years. Fixes nitrogen and can be grown around long-term crops such as raspberries.

Short-term manures

Buckwheat: Sow by late summer. Does not fix nitrogen. Ready in 3 months. Dig in before or during flowering. The flowers are good for beneficial insects.

Fenugreek: Sow by late summer. Does not fix nitrogen and prefers good drainage.

Lupin: Sow by mid-summer. Ready in 3 months. Dig in before flowering. Fixes nitrogen and is deep-rooted.

Mustard: Sow between March and September. Ready within a month. Produces large volumes of organic matter. On heavy soils, dig in the mustard before the winter and allow the frosts to break down the clay.

When to sow a green manure

Clear the soil of weeds and dig it over lightly. Rake the soil over and broadcast the seed evenly.

As a rule (unless stated in the list above), you should dig the green manures in before they flower and set seed, as they can sometimes become a weed in themselves.

Plan to dig in your green manure three weeks before you plant up the bed. Follow the green manure in your vegetable patch with a nitrogen-hungry crop like leafy veg.

Plan to sow an overwintering green manure on any bare beds that you have. Order green manure seeds that will grow quickly in the spring for any beds that are bare until the early summer such as those for tomatoes and pumpkins.

Other plants to dig into your soil

Comfrey is one of the most useful plants in the organic garden. We’ll be looking at some more of its functions tomorrow, but you can harvest the leaves and dig into the soil just like a green manure. Comfrey cannot be used as a traditional green manure as its roots regenerate and will create more and more comfrey plants in the bed you have dug them into, so only use the leaves.

The leaves have a high potassium content and are also rich in nitrogen, so are perfect for digging into soil where you will be growing plants for their flowers, fruits and roots, as these are the functions promoted by comfrey.

You can also line potato trenches with comfrey.

Sunflower plants are rich in nutrients as well. Remove the seedhead and dig the plant and its thick hollow stems into the soil.

Plant some comfrey plants in your garden this autumn so that you’ll have your own supply of nutrients to feed your soil with by the spring.

Beneficial crops

Any member of the legume group (peas, beans) of vegetables will fix nitrogen in the soil, so once the plants have finished cropping, dig them into the soil and follow with a leafy crop of plants to make the most of the nitrogen.

Potatoes and Jerusalem artichokes encourage a better soil structure by breaking up the soil with their busy root systems. The practice of earthing up potatoes with compost is also beneficial as it increases the overall nutrient content of the bed.

Planning crop rotation wisely is another important part of good soil husbandry. It means crops leave nutrients that are useful to the next crop, and that soil diseases and pests don’t build up as quickly.

There are five main groups of vegetables which require rotating:

Legumes: beans and peas.
Brassica: turnips, swedes, radishes, broccoli, sprouts, cauliflowers, cabbages, kales, rocket.
Potato family: potatoes, aubergines, tomatoes, peppers.
Onion family: onions, garlic, leeks, shallots.
Roots/umbellifers: carrots, celery, parley, parsnip, celeriac.

Pumpkins, salads and sweetcorn can be grown wherever there is space. Grow onions and roots together as the scent of their foliage, grown together, confuses pests from the others.

Here is a sample crop rotation plan:

Year 1

Year 2

Year 3

Year 4

If you don’t have a plan for rotating your crops, sit down this weekend and draw one up for next year’s growing.

That’s it for today. You’ve got plenty of sowing and planning ahead. We’ll be giving you advice about how to plan to carry out all the tasks that we have set you on Sunday. Tomorrow we’ll be looking at potting soils and feeds.

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