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In the Netherlands the Pw value, based on an extraction of soil P with water, is used as a basis for P-fertilizer recommendations for arable crops. Using a simple, mechanistic model of P transport in the soil the Pw value required for adequate P uptake by crops can be calculated on the basis of daily uptake requirements, root area index, P-adsorption isotherms and total amount of P taken up during a growing season. Calculated Pw values for adequate uptake are in the same range as the present recommendation scheme based on field experiments. Possible refinements of the model are discussed. For each soil the Pw value can be calculated that corresponds to the P concentrations in the soil solution according to standards set to reduce environmental pollution. The model predicts that, unless the root area index of non-cereal crops is considerably improved, these standards cannot be met in the plough layer without affecting crop production levels. Calculations show that the present method of determining the Pw value yields a reasonable compromise between a measurement of intensity and capacity of P supply in the soil. (Abstract retrieved from CAB Abstracts by CABI’s permission)

The hypothesis was tested that the avoidance of acid subsoil by the velvet bean Mucuna pruriens is based on a mechanism acting on the whole root system rather than on individual roots. In a split-root experiment with circulating nutrient solution the growth of plants with Al-containing (+/+) or Al-free (0/0) solution on both sides of the root system was compared with that of plants which had a choice (0/+). Two levels of Al (110 and 185 μM) were tested at two levels of Ca (50 and 1250 μM). In the 185 μM Al treatment the concentration of monomeric Al varied between 53 μM, directly after refreshing the solution, and 5 μM at harvest time. An external Al concentration of 110 μM had no effect on shoot and root dry weight, while 185 μM Al applied to both sides of the root system (+/+) increased root dry weight and reduced shoot dry weight and shoot/root ratio, compared with the 0/0 control. Application of 185 μM Al to half of the roots, ied to a significant shift in root growth in favour of the control side; this response is described here as Al avoidance. On the basis of total root length, root dry weight and root surface area, the ratio of 0/+ roots was 3.1, 2.8 and 2.4, respectively. Al avoidance at 185 μM Al was confirmed in another experiment, in which root response was measured to a local P source, supplied in a third compartment containing only KH2PO4. A significant increase in root length and dry weight in this compartment was observed, when other roots of the same plant were growing in the presence of Al. This result indicates that Al avoidance by Mucuna roots is related to P preference.

The relation between nitrogen fertilization and root development is studied for several crop types. Also the root development in intercropping of maize and cassave is studied

The role of roots in obtaining high crop production levels as well as a high nutrient use efficiency is discussed. Mathematical models of diffusion and massflow of solutes towards roots are developed for a constant daily uptake requirement. Analytical solutions are given for simple and more complicated soil-root geometries. Nutrient and water availability in soils as a function of root length density is quantified, for various degrees of soil-root contact and for various root distribution patterns. Aeration requirements of root systems are described for simultaneous oxygen transport outside and inside the root.Experiments with tomato and cucumber are discussed, which were aimed at determining the minimum root surface area required in an optimal root environment. Experiments on P-uptake by grasses on various soils were performed to test model calculations. Model calculations on the nitrogen balance of a maize crop in the humid tropics suggested practical measures to increase the nitrogen use efficiency.