What is Humus? Humus is plant and animal remains which is bit by bit rotted in the soil. The agents of decay are the millions of bacteria and other microscopic organisms which inhabit every ounce of earth. They break down dead roots and underground insects as well as the fallen leaves carried below the surface by worms. Partly rotted organic matter with the mass of living and dead bacteria is called Humus.
Soil without humus is nothing more than finely ground rock. Humus is significant as it contains and maintains the vast population of microscopic organisms within the soil. These bacteria are the important thing to fertility, and have a beneficial effect both before and after death.
When they are alive they create heat and transform complex organic substance into forms that will later be available to the roots. When they are dead they discharge these plant foods coupled with colloidal gums. To the soil scientist it is these gums and not plant remains that are humus - the magical substance which cements the soil crumbs together.
Under normal circumstances there is a rough and ready 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 frequently replenish the supply. Last year's dead plants and dead worms simply aren't enough, you have to add humus-makers. Many varieties can be found, and they all fall into one or other of three basic types.
Raw Humus -Makers:
These organic materials contain enough readily-available nutrients to stimulate active bacterial development. Heat is created 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 proficient method of warming the earth and building up the humus content...provided you recognize the limits. The sudden increase within the bacterial population robs nitrogen from the soil. So 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 a place a long way 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 enough readily-available nutrients to stimulate active bacterial development. 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 your 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 a lot more than enhance aeration and water-holding capacity by physical means, they also instantly provide the agents to manufacture soil crumbs and so improve the friability.
Fibrous Humus-Makers:
These organic materials are full of 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 ability.
Common examples are peat, pulverised bark and sawdust. Peat is the most inactive substance, it is clean and simple to handle but it is hardly a humus-maker within the true sense of the word. It resists breakdown and so crumb-forming gums are not created.
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.
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