Root growth and distribution in relation to nutrient availability and uptake

Abstract

This paper reviews relevant interactions between soil and roots in terms of effects on root growth, and the consequences for nutrient availability. Changes in rates of root elongation, root diameter, and root branching are identified as the most obvious effects on roots, but the production of root hairs and the state of wall thickening in the endodermis with distance behind the root apex can be influenced as a root penetrates the soil. Uptake of nutrients and their movement, together with imbalances in the uptake of anions and cations can result in a change in the availability of nutrients close to the root. Release by roots of organic molecules, particularly organic acids, can change as roots grow through the soil. The acidic compounds can act as chelating agents within the soil so making nutrients such as iron and phosphorus more available. Physiological and morphological characteristics of roots important for describing nutrient uptake are identified. The uptake of a nutrient at the root surface is characterized by the maximum influx (Imax) and the affinity of a carrier in the cell membranes (Km) for the nutrient ion. Imax tends to reflect the concentration of the nutrient in the shoot. Km may be dependent on nutrient supply, and may vary with the age of the root. We conclude that these physiological parameters are particularly important in agricultural soils when coupling them with morphological parameters of root branching to predict uptake when only a part of a root system is in contact with a localized supply of nutrients. A greater importance could well attach to the value of Imax when mycorrhizae are present.Uptake is particularly sensitive to root growth rate and to the radius of the absorbing root. The effective root radius is enhanced by the presence of root hairs due to their small radius, the increased surface they provide for absorption, and the increased volume of soil accessed. Interand intra-specific differences in physiological uptake parameters may become effective when uptake proceeds mainly through root hairs. Roots compete for nutrients when the radial distance of the depletion zone from the root surface approaches the half distance between roots. When roots are more or less uniformly distributed in the soil, the half distance, b, between adjacent roots depends on the root length density Lv. Competition occurs at greater values of b — and hence lower values of Lv — as ionic mobility increases. For ions moving to the root surface by mass flow, the total root length at the time of maximum water extraction from a given soil layer appears to be an important parameter. More account needs to be taken of non-uniform root distributions, especially in the subsoil. The uptake of water and nutrients are likely to be restricted by the clumping of roots, or poor rootsoil contact. All these root parameters will need to be quantified in developing mathematical models to describe the growth of root systems in the field, if nutrient acquisition is to be predicted accurately.