SOIL ORGANISMS - Soil Ecology - page 234

plants were, respectively, 2.5, 10.3 and 17.5 m (Callot et al., 1982). Aeration is also
important for root growth and anaerobiosis often limits elongation while the frequency
of pathogenic lesions is increased. In poorly structured and compacted soils, distances
between macropores and cracks may be large and roots tend to concentrate in cracks
(Ringrose-Voase, 1987). Aluminium toxicity may also limit root extension in acid
tropical soils of low nutrient status (Alva et al., 1986).
Specific root length varies with a range of factors (Fitter, 1985) including:
(i) Interspecificvariation(Figure III.16);
(ii) Genotype. Variation from 11.8 to 21.8 has been observed in different maize genotypes;
(iii) Age. Higher values are generally found in young plants, simply because coarse
roots have not completed their growth;
(iv) Soil fertility. Higher values usually occur in poor soils, as shown in Figure III.16
for eight grass species;
(v) Temperature. High specific root lengths are associated with low temperatures
(Veresoglou and Fitter, 1984) whereas moisture responses appear to differ with the species;