Adding Humus

What is humus? Humus is plant and animal remains which is gradually decomposed in the soil. The agents of decay are the millions of bacteria and other microscopic organisms which live in every ounce of earth. They break down dead roots and underground insects as well as fallen leaves carried below the surface by worms. Partially decomposed organic matter with the horde of living and dead bacteria is known as humus.

Soil without humus is nothing more than finely ground rock. Humus is vital because it contains and maintains the vast population of microscopic organisms in the soil. These bacteria are the key to fertility, and have a beneficial effect both before and after death.

When they are alive they produce heat and transform complex organic material into forms which will later be available to the roots. When they are dead they release these plant foods together with colloidal gums. To the soil scientist it is these gums and not plant remains which are humus - the magical material which cements the soil crumbs together.

Under natural conditions there is a rough balance which maintains the humus level of the soil. Under cultivation, however, the humus content declines and so in the garden it is necessary to regularly replenish the supply. Last year's dead plants and dead worms are not enough, you have to add humus-makers. Many types are available, and they all fall into one or other of three basic types.





Raw Humus Makers:
These organic materials contain sufficient readily-available nutrients to stimulate active bacterial growth. Heat is produced and soil structure is improved, hot beds made with fresh manure were producing early vegetables before electricity was discovered.

Common examples are grass clippings, fresh dung, dug-in weeds and seaweed. Using raw humus-makers is a good way of warming the earth and building up the humus content...provided you know the limitations. The sudden increase in the bacterial population robs nitrogen from the soil. Always add some nitrogen when using a raw humus-maker.

As a general rule, humus-makers of this type are used some time before planting or in an area some distance away from plant roots. The generation of heat and the nitrogen-robbing effect can be damaging to nearby roots.

Matured Humus-Makers:
These organic materials do not contain sufficient readily-available nutrients to stimulate active bacterial growth. A raw humus-maker is transformed into a matured humus-maker by composting - a process which produces humus.

Common examples are well-rotted animal manure, properly made garden compost and Bio Humus. The warming effect on the soil is lost, but this is outweighed by the advantages. Matured humus-makers cannot damage tender roots and the nitrogen content of the soil remains available rather than being partly locked up in bacterial bodies.

All matured compost-makers contain the colloidal gums released by dead bacteria during the composting process. Unlike the fibrous humus-makers, they do more than improve aeration and water-holding capacity by physical means, they also immediately provide the agents to produce soil crumbs and so improve the friability.

Fibrous Humus-Makers:
These organic materials are rich in cellulose, but the lack of sugars and simple starches means that little bacterial activity is stimulated. Soil improvement by these humus-makers is basically a physical effect, bark opens up the soil and peat improves aeration and water-holding capacity.

Common examples are peat, pulverised bark and sawdust. Peat is the most inactive material, it is clean and easy to handle but it is hardly a humus-maker in the true sense of the word. It resists breakdown and so crumb-forming gums are not produced.

Sawdust and some bark products are slowly broken down by bacteria. This bacterial activity requires nitrogen, and the soil's supply will be robbed if you don't add a nitrogen-rich fertilizer.