The deterioration of structure that accompanies intensive tillage is usually less severe in soils adequately supplied with humus. When humus is lost, soils tend to become hard, compact and cloddy. Aeration, water-holding capacity and permeability are all favorably affected by humus. The frequent addition of easily decomposable organic residues leads to the synthesis of complex organic compounds that bind soil particles into structural units called aggregates. These aggregates help to maintain a loose, open, granular condition.
Water is the better able to infiltrate and percolate downward through the soil.
The roots of plants need a continual supply of O 2 in order to respire and grow. Large pores permit better exchange of gases between soil and atmosphere. Humus usually increases the ability of the soil to resist erosion. First, it enables the soil to hold more water. Even more important is its effect in promoting soil granulation and thus maintaining large pores through which water can enter and percolate downward.