SOIL STRUCTURE AND PLANT-GROWTH

Soil structure affects plant growth in many ways. Roots grow most rapidly in very friable soil, but their uptake of water and nutrients may be limited by inadequate contact with the solid and liquid phases of the soil. This contact is much more intimate in hard soil, but then the growth of the roots is strongly inhibited, so that their foraging ability is poor, and the plant may eventually become short of water or nutrients. However, many soils, even if hard, contain continuous macropores that provide niches for the roots to grow in. The Presence of such macropores increases the extent of the root system, but because the roots are clumped within them, the rate at which the roots can extract water and nutrients from the soil between the macropores is considerably slowed. These macropores also provide niches for microorganisms, both symbiotic and pathogenic, so that the response of roots to different tillage treatments may differ markedly on this account alone. Soil structure not only affects the ability of roots to grow and to supply the leaves with water and nutrients; if adverse, it also induces them to send hormonal signals that slow the growth of the shoot, even if they are currently able to take up adequate water and nutrients.