Model experiments on the behaviour of roots at the interface between a tilled seed-bed and a compacted sub-soil

When elongating seminal roots of developing plants reach the base of a tilled seed- bed, they often encounter a layer of dense, strong untilled soil. At this interface, they may be deflected horizontally and instead of penetrating the sub-soil, they may form a horizontal mat of roots at the base of the seed-bed. If this occurs, the plants are unable to absorb the reserves of water in the sub-soil, and are very sensitive to short periods of drought. Model experiments were done with artificial sub-soil layers having a range of strengths and with artificial seed-beds having sieved soil aggregates in the 4-7.7, 2-4 and 1-2mm size ranges. Roots of wheat were grown through the aggregate beds and the proportions of roots which penetrated into the sub-soft were investigated as functions ol sub-soil strength and diameter of the aggregates in the seed-bed. The proportion penetrating was found to decrease exponentially with sub-soil strength. The rate of decrease was similar for the 2-4 and 1-2 mm aggregates but was greater for the 4-6.7 mm aggregates. It is concluded that, provided that the roots of different plant species behave similarly, the base of the seed-bed should be composed of fine aggregates and that the penetrometer strength of the underlying untilled sub-soil should not exceed 0.4 MPa for plants with a single seminal axis or 3 MPa for plants such as wheat with 4 seminal axes.