Tabular and graphical data show the effect of organic-matter levels (1-7%) on intrinsic soil structure (lower & upper plastic limit, moisture content atpF2) & actual soil structure (pore space, air content), & on the effect of organic manuring (farmyard manure, town refuse, green manure, ley) on structural properties. An increase in organic-matter content decreased the slaking sensitivity of soils, increased resistance to plastic deformation caused by mechanical forces on heavy clays under wet conditions, & improved actual structure by 0.6-0.7 points (visual estimation) for each per-cent organic matter. Lowest organic-matter content, providing good resistance to mechanical breakdown & giving proper structure for good plant growth, was required on a clay soil in which the 16 micro particles ranged between 15 & 30%, depending on soil treatment. (Abstract retrieved from CAB Abstracts by CABI’s permission)