Increasing organic matter

Good levels of organic matter can help the structure and nutrient levels of the soil.

Methods to increase organic matter

There are various practices that can be carried out to improve the organic matter in the soil including:

adding compost
green manuring
crop rotation
returning crop residues
nil cultivation
spreading dairy effluent
applying animal manures.

Adding compost

Compost is made from all sorts of organic materials including old plant material and animal manure. The organic material is left to decay and be broken down by micro-organisms to form humus. The humus will release nutrients, which can be taken up by the plant roots in the soil water.

Most plants or animal material can be used. This is a good way to improve soil structure.

Compost improves the soil structure and also adds nutrients.

Compost helps the soil retain moisture and can also warm the soil, as the dark colour absorbs heat.

Well-decomposed compost can be added to soil during cultivation and before transplanting plants or sowing seeds. Compost can also be used as mulch as in the case of a no-dig garden.

Green manuring

This involves fast growing crops that are planted in the winter and then put back into the soil before they set seed. While they are growing, they help prevent nitrates being leached from the soil and protect the surface from the effects of winter rain. The roots make channels, which improve water infiltration and soil aggregation, and help earthworms burrow. When they are dug into the soil, these crops will increase the organic matter in the soil, and decompose to release nutrients.

The crops need to be leafy and grow fast. Oats and lupins can be used for this purpose.

You can see the nodules on this legume. The nodules are able to fix nitrogen in the soil.

Legumes such as lupins can be grown as a green manure crop. They will also add nitrogen to the soil, which will reduce the need for fertiliser.

These crops are planted in the late summer and autumn and dug into the soil to rot down during the winter. Before the plants start to flower and use too many of the nutrients that have been made in their nodules, they are trampled into the ground and chopped up with a sharp spade. The green manure is dug into the ground and left for 4–6 weeks to decompose before spring planting.

Returning crop residues

Any parts of a crop not harvested for sale can be returned to the soil to help rebuild the organic matter levels. This type of crop residue is sometimes called stubble. Crop residue will protect the soil surface and help water infiltrate into the soil.

Stubble has been worked into the soil.

When soil has been cultivated the stubble is returned to the soil.
Returning the old plant material to the soil will add nutrients and provide food for soil animals like worms.

Nil cultivation

With nil cultivation/no tillage the weeds or unwanted plants are sprayed but no cultivation is carried out. Seeds can be sown directly into the soil without reducing the organic matter in the topsoil.

Organic matter and soil animals

Greater levels of organic matter will increase earthworm numbers. Earthworms help the soil by churning up the soil organic matter and helping the soil's physical properties. They form channels that help water infiltration, and promote soil aggregation and aeration. Earthworms turn over topsoil, helping bring up deeper soil to the surface, and burying organic matter.

Crop rotation

If the same type of crop is grown on the same piece of land year after year, the nutrient and organic levels will gradually decrease. The same range of nutrients is being drawn from the soil each year and so soil fertility will decrease. When specific soil elements are used up the soil becomes deficient.

When broccoli is removed from the land, the fertility will be reduced.

When broccoli (part of the brassica or cabbage family) crops are harvested and removed from the land, the fertility will be reduced.

Crop rotation is where the type of crop grown in a particular area is changed in a planned order. Each crop that is grown should be as different as possible from the previous crop. Different types of plants either use nutrients or return them to the soil.

Plants to use

The aim of crop rotation is to grow a variety of crops that take different elements from the soil. Crops with similar growth needs can be grouped together. Each crop will have roots that grow to different depth and so will drag nutrients from that depth.

Grow legumes to add nitrogen. Legumes are crops like peas and beans.
Leaf crops with stalks, roots and leaves will add lots of organic matter.
If a deep-rooting crop follows a shallow-rooting crop, nutrients are used from different levels in the soil.

Crop rotation is also used to prevent the build-up of soil pests and diseases by ensuring that a crop from a different family is planted in the same area in succession. Crop rotation allows the soil to remain disease-free.

Here is a list of plant families of some common food crops that are used in horticultural crop rotations:

Leguminales family: peas, beans, lupins (used as a green manure)
Solanaceae family: tomatoes potatoes, peppers
Brassica family: cabbage, broccoli, kale, turnips, cauliflower and mustard (used as a green manure)
Gramineae family: corn, oats (used as a green manure) and pasture grasses
Allium family: onions and leeks
Umbelliferae family: carrots; parsnips, parsley and celery
Cucurbitaceae: zuchinni, pumpkin, cucumber
Compositae: lettuce.

In an ideal crop rotation:

the legume family is followed by leafy crops because leafy crops need high levels of nitrogen which legume crops add to the soil
leafy crops can be followed by roots crops or deep rooting crops because their roots move down deeper into the soil profile. Root crops bring nutrients back up to the soil surface that the leafy crops would not have been able to access
all crops can be followed by a green manure crop so long as the green manure crop chosen is not from the same family of plants that has just been in the soil.

Activity 4A Test your understanding of how to increase organic matter.

To improve the soil organic levels there are several things that can be done.

Drag the method on the right to its correct description on the left.

Complete Activity 4b in your workbook

Key points

Organic matter helps the soil structure by improving the pore spaces and the amount of water and air in the soil.
Organic matter improves soil structure, increases water-holding capacity, raises soil temperature, increases aeration and improves the friability of the soil.
Small amounts of nutrients are also added to the soil as the organic matter decomposes. The type and amount of nutrients added will depend on the type of organic matter applied to the soil.

Compost: This is made from all sorts of organic materials including old plants and animal manure. The organic material is left to break down and decay. It then can be added to the soil. This will help the soil structure and also add nutrients.

Green manuring: Fast-growing crops that are grown and ploughed back into the soil before they seed.

Returning crop stubble/residue: Part of a crop not harvested for sale can be returned to the soil.

No tillage: The soil is not cultivated, but seeds are sown directly into the soil.

Crop rotation: Involves growing different types of crops in the same area (legumes, leaf crops, root crops). This maximises the use of the nutrients in the soil, often without the need for additional fertilisers. It also reduces soil-borne disease and pests, as they are not able to build up because most pathogenic organisms affect only one type of plant.

What's next?

Go to: 5 Lime.