The importance of humus

Soil without humus is lifeless – it’s dead soil. Humus, the indigestible part of decomposed organic matter, literally converts soil into a living thing because it provides a habitat for beneficial fungi that feed nutrients to many plant families, and microorganisms that keep soil-borne plant diseases under control. It also keeps soil more moisture retentive, yet better drained; improves soil structure; holds nutrients in a form that is easily absorbed by plants; insulates plant roots by keeping topsoil cooler in summer and warmer in winter, and acts as a buffer against extremes in soil pH through a complex exchange of electrically-charged particles in soil. Regular replacement of humus in soil is absolutely essential to healthy plant growth. Humus can be added to soil as compost, green manures, well-rotted, herbifierous animal manures, poultry manures, organic mulches, decaying roots and plants.
The vegetable garden will require the lion’s share of organic matter, but it is very important to fruit trees, and many foreign plants, too. Many Australian natives prefer humus supplied through leaf litter, or leaf mould. Plants from arid soil areas have evolved to require only small amounts of humus.

Applying compost

The best way to add humus to your garden soil is mature compost, which is a mixture of worm castings, beneficial microorganisms, and humus. Compost is not only a fertiliser; it stimulates soil organism activity by providing them with food. Compost made from a variety of sources contains all the nutrient elements required by plants but, for those of us that don’t have an unlimited supply of compost, green manures are an excellent substitute.
In vegetable gardens, a 3–5 cm layer of compost can be applied to beds and mixed into the top 15 cm of soil. If mature compost is in short supply, adding compost to the planting hole or furrow when planting out seedlings or sowing seed will get your plants off to a flying start. Compost can be applied this way to flowering annual and spring bulb beds, too.
Under fruit trees, and around roses and herbaceous perennials, compost should be applied to a damp soil surface under the outer part of the tree or plant foliage canopy – i.e. not close to the trunk. Don’t scratch it into the surface. Many trees and shrubs have feeder roots close to the soil surface and these are easily damaged. If you have plenty of compost, it can replace the tree’s fertiliser, except for an annual application of seaweed tea. If you don’t have a lot of compost, it can form part of the fertiliser application. Every bit of compost that you can spare is very valuable.
The compost should then be covered with 5 – 8 cm of organic mulch to keep the compost damp – also keeping the mulch well clear of the trunk. When compost dries out it loses a lot of its benefits. The mulch will break down to contribute to good garden loam, but its most primary function is to keep the soil surface and compost damp.
If you apply compost annually under your fruit trees and shrubs, you will find that “so-so soil” will steadily convert to dark, sweet-smelling loam full of earthworms – just the type of soil these plants love. Your trees and shrubs will also be more resistant to pests and diseases.

Growing green manures

Green manures are an easy, cheap way to produce organic matter for soil by growing grains until they are knee high, or inoculated legumes* until they start to flower, then slashing them, and leaving them, as organic mulch, on the soil surface to break down, or slashing them into smaller pieces and digging them into the top 10 cm of soil. Suitable green manure crops for each season and climate zone are included in ‘What to grow’ that is posted on this blog each month.
For those unfamiliar with growing green manures, step-by-step guides can be found in my book, Easy Organic Gardening and Moon Planting, or in the March/April 2008 issue of Warm Earth magazine.
If planning to use the bed immediately after slashing the green manure, digging fresh, organic matter into topsoil can cause a temporary nitrogen deficiency, as soil bacteria require nitrogen to break down the materials, then nitrogen then becomes available for your crops. Apply some complete organic fertiliser to the bed so that nutrients are immediately available to your plants.
* Inoculating legume seed enables the legume to fix nitrogen efficiently by introducing the nitrogen-fixing bacteria to soil. Uninoculated legumes can be still be grown as green manures to supply organic matter. (See post on Fixing nitrogen.)