Concepts of the importance of root growth and function for plant production have changed gradually over the past century from a ‘morphogenetic equilibrium’ between root and shoot growth, which can be summarized as ‘the more roots the better shoot growth’, to a ‘functional equilibrium’ which puts emphasis on the water and nutrient uptake by the root system and not on the size of the root system as such. In fact, separate optima for root growth and shoot growth (dependent on root function) can often be distinguished. To obtain maximum crop production, relatively small root systems can be sufficient, provided the supply of water and nutrients is continuously high. Agricultural interventions aimed at increasing rooting depth by lowering the water-table or by deep soil tillage may be counterproductive for plant growth. Larger root systems may, however, reduce risks under rapidly changing environmental conditions and may increase nutrient use efficiency and hence reduce nutrient losses to the environment. Concepts are discussed in a historical perspective and examples are given of the empirical evidence to support or refute them.
Tag: fields
Infection potential of farm soils as mycorrhizal inocula for Leucaena leucocephala
A greenhouse experiment was conducted to assess the arbuscular mycorrhizal (AM) fungi inoculum potential for Leucaena leucocephala in a range of nutrient-depleted farm soils in western Kenya. Leucaena was grown in 12 uninoculated farm soils with pH 5.0–6.7, with or without rock P and farmyard manure. Root infection, nodulation, and shoot and root weight were determined 25, 46, and 69 days after planting. Spore concentrations in the farm soils at the start of the experiment ranged from 44 to 126 live spores per 100 g dry soil with 6–10 species per soil, principally Scutellospora spp. and Acaulospora spp. Nodulation was absent or poor in all soils, indicating the need for rhizobial inoculation of species belonging to the leucaena crossinoculation group in this agrosystem. Rock-P alone increased final shoot dry weight by a factor of 1.4, manure alone by 1.8, and rock-P plus manure by 1.9, compared with no ameliorant. Root infection with AM fungi was detected in all soils 25 days after planting and increased linearly in the different farm soils to values of 33–65% 69 days after planting. Soil pH and root infection 25 days after planting accounted for much of the variation in final shoot weight among soils with no amelioront (87%). As early root infection increased from 20 to 40% at soil pH 5.0, the predicted final shoot dry weight was doubled, and the response to ameliorants was reduced by two-thirds. The growth responses to increased infection became smaller as pH increased from 5.0 to 6.5.