Improving soil: humus, compost and green manure

Good vegetables depend less on the variety than on the soil. A humus-rich, crumbly soil stores water, supplies nutrients steadily and lets roots work. A compacted, humus-poor soil forces constant watering and feeding — and the result still stays mediocre.

The good news: soil can be improved. Not in one season, but noticeably within two to three years, and the effect lasts.

In short

First work out your soil type — sand, loam and clay need different treatment.
Humus is the common denominator: it helps sandy soils store and clay soils structure.
Compost is a soil improver, not a complete fertiliser. 3 to 5 litres per square metre a year is enough.
Green manure builds humus and loosens the subsoil — without a spade.
Constant digging does more harm than good. Loosen shallowly instead of turning deeply.

First check: what kind of soil is this?

The finger test is enough to start with. Take a handful of moist soil and squeeze it:

Sandy soil runs through your fingers, feels rough and cannot be shaped. It is well aerated but dries out fast and loses nutrients.
Loam can be rolled into a sausage that breaks when bent. It is the ideal garden soil.
Clay rolls into a glossy sausage that bends without breaking. It stores a lot but compacts easily and stays cold and wet late into spring.
pH value: a simple test from the garden centre costs little. Vegetables like 6.0 to 7.0; potatoes a little more acidic, brassicas towards the upper end.

Humus — what this is actually about

Humus is the share of decomposed organic matter in the soil. It works in several ways at once: it stores many times its own weight in water, binds nutrients against leaching, glues soil particles into stable crumbs and feeds soil life.

That is exactly why humus helps both extremes in seemingly contradictory ways — sandy soil, because it holds water and nutrients, and clay soil, because crumb formation loosens it and makes it more permeable.

Using compost properly

Compost is the most important tool in a home garden. It is often misdosed, though — either far too little to have any effect, or in quantities that overload the soil with phosphate.

Quantity: 3 to 5 litres of ripe compost per square metre a year for normal beds. Up to 8 litres for hungry crops.
Timing: in spring once the soil has dried out, or in autumn as a mulch layer.
Working it in: shallowly only, into the top 5 to 10 cm. Compost buried deep rots poorly without air.
Ripeness: ripe compost smells of forest floor; recognisable remains mean it is unripe. That belongs on top as mulch, not in the planting layer.
What compost cannot do: supply nitrogen quickly. Hungry crops also need horn meal or organic fertiliser.

Green manure: soil improvement that works by itself

Green manure is a crop grown not for harvest but for the soil. It shades, roots through, holds nutrients over winter and then becomes humus. For areas standing empty for a few weeks anyway, there is nothing better.

Phacelia: undemanding, flowers quickly, belongs to no vegetable family and therefore fits any rotation. Frost-killed.
Oil radish and mustard: root deeply and loosen compacted soils. Caution: brassicas — do not grow before or after cabbage.
Clover, vetch, lupin: legumes that fix atmospheric nitrogen in the soil with root nodule bacteria. Ideal before hungry crops.
Winter rye: grows on at low temperatures and protects the soil through winter.
Sowing: broadcast after harvest, rake in lightly, keep moist.
In spring: simply leave frost-killed stands lying and work them in shallowly, or keep them as mulch. Do not dig them in deeply.

What harms the soil

Deep digging. It inverts the soil layers and destroys the structure that has grown, along with the soil life. Shallow loosening with a fork or broadfork achieves the same without the damage.
Walking on it when wet. The commonest cause of compaction in home gardens. Lay firm paths and stay off the beds.
Bare soil. Uncovered earth caps over in rain, dries out and loses humus. Always mulch or green it.
Too much lime on good soil. Lime only as needed and after a pH test. Too high a pH locks up trace nutrients.
Bark mulch on vegetable beds. It draws nitrogen from the soil as it rots. For vegetables, use grass clippings, straw or leaves.

Tip

The simplest start for a tired soil: mulch consistently for one season. Grass clippings in thin layers, dried off a little, topped up again and again. After a year you can see the difference in the crumb structure — and the earthworms are back.

Caution

Do not apply fresh manure just before planting. It is too sharp, burns roots and can carry pathogens. Manure belongs on the ground in autumn, or at least six months on the compost heap.

Frequently asked questions

How long until I notice something?

First effects on water retention and crumb structure show after one season of consistent mulching. A real increase in humus content takes three to five years.

Can I apply compost in autumn too?

Yes, and it works well — but then on top as a mulch layer, not worked in. That way no nutrients leach away over winter.

Which is better: green manure or compost?

Both together. Compost brings humus and nutrients in from outside immediately; green manure builds humus on site and loosens the subsoil. They complement each other.

My soil is pure sand — is it even worth it?

Especially there. Sandy soils respond fastest to added humus because they have so little of it. Expect to add some every year, because humus breaks down faster in sand.

Well-built soil pays off with every crop — and sturdy young plants that can root in straight away get the most out of it.