Can Plants Deliver Food, Fibre and Solutions for Anthropogenic Soil Phosphate Problems ?.
Mike J. Hedley, Institute of Natural Resources, Massey Univ, Private Bag 11 222, Palmerston North, New Zealand and Stephen Trolove, Crop and Food Research, 265 Lawn Road, Hastings, New Zealand.
Sustaining food and fibre production to meet the demands of a growing global population remains a major justification for agronomic research into the phosphate (P) acquisition mechanisms of plants and their phosphate use efficiency. Soil P deficiency remains one of the major constraints preventing upland soils being used to produce more food for the economically-challenged but growing populations of developing countries. Overcoming this constraint has focussed researchers on the rhizosphere because the key plant growth limiting process is the diffusion of soil and fertilizer P to the active root surface. Plants capable of rapidly regenerating fine active root in previously unexploited soil volumes and plants that can modify their rhizosphere chemistry to accelerate the mobilization of soil P are of particular interest to researchers. Studies of root systems in situ are technically challenging and new techniques are required for quantifying whether root architecture and function appropriate for efficient P acquisition are being sucessfully achieved. Successful selection of externally and internally P efficient plants, however, is not the complete solution for sustainable food production on P deficient soils. Phosphorus efficient plants can not mine soil organic or inorganic P reserves indefinitely. Phosphorus efficient plants and their rhizosphere processes must be matched to economically appropriate maintainence P inputs, be they low cost fertilizer or recycled wastes. In the developed regions of the world, which have afforded long-term P fertilizer and manure use, the few agricultural soils that continue to have a P limitation are not the prime concern of research funders. Research focus has turned to providing solutions to accelerated eutrophication problems that are caused by P enriched surface runoff and drainage from excessively P fertilized (manured) soils. In efforts to curtail P fertilizer use, demonstration trials illustrating lack of crop response to P are being conducted to convince farmers and advisers that P fertilizer application rates can be reduced or temporarily withdrawn. Phosphorus accumulator plants are being selected and evaluated for riparian borders to allow runoff P to be trapped, harvested and removed from the P enriched catchment area. Land application of P rich, housed-animal manure is a major cause of soil P enrichment. Animal food stuffs with high inositol phosphate (phytate) concentrations, derived from grain and other plant seeds, are the direct cause of P enrichment of animal extra. A partial solution to this problem is the selection of crop plants that produce grains with high carbohydrate but low phytate concentrations. Thus the future challenges for plant scientists are not only to select plants that can grow in P deficient soils and produce marketable food, or fibre, but also select different plants, or cultivars, that can be used in mitigation strategies for minimizing the P enrichment of manures and soils.