A STOCHASTIC MODEL FOR THE GROWTH OF ROOTS IN TILLED SOIL

Summary A method of defining soil macrostructure, based on observations of the distribution of aggregates and voids along horizontal transects, is combined with a model for the probability of a root either penetrating, or being deflected by an aggregate, after passing through a void. The latter is based on the strength of the aggregate, the angle of incidence, the length of the preceding void, and root properties. Effects of soil macro-structure, aggregate strength and plant species are discussed. A good correlation was obtained between the predicted nutrient uptake per unit length of root and the measured uptake per plant from beds of different-sized aggregates. The inclusion of the effect of aggregate strength on root elongation rate removed all effects of aggregate size on the rate of uptake of non-mobile nutrients. In order to reconcile this result with the correlation mentioned above, it is hypothesized that greater root branching occurs in beds of smaller aggregates so that the nutrient uptake remains the same as it would have been in the absence of rate effects. It is shown that an optimum' soil macro-structure exists for the maximum growth of roots of a given plant species. This optimum structure depends on the strength of the soil aggregates.