Potential and limitations of soil organic matter build-up in dry areas

Semi-arid areas comprise a large portion of the world's agricultural crop land. These areas are characterised by high temperature and low rainfall which hinders soil organic matter accumulation. The reported experiments were designed to study the effect of soil organic matter (SOM) from green manure crops on soil physical properties in two contrasting soils, sandy clay loam and loamy sand under the severe conditions of high temperature and low rainfall that exist in the eastern wheat belt of Western Australia. To remove soil physical and chemical constraints, the soils were deep ripped to 0.4 m after the application of 2.5 t/ha of gypsum. Basal fertilizers were applied according to soil tests to remove possible nutrient deficiencies. Cereals and legume crops were incorporated by disc cultivation into the soils as green manure for four consecutive years. Two plant groups, cereals and legumes were used to provide green manure. Oat (Avena sativa) as a cereal with high C:N ratio (95) and either faba bean (Vicia faba), field pea (Pisum sativum) or lupin (Lupinus angustifolius) were used as legumes with low C: N ratios (12 - 18). These crops were arranged in three treatments for each soil type consisting of different crop sequences. A total of between 14 and 20 t/ha of green material was incorporated into the top soil over four years. All treatments increased soil organic matter significantly compared to the initial soil values but no significant differences between treatments (crop sequences) were observed. Water stable aggregates and soil bulk density were significantly improved by all treatments but no significant differences were observed between treatments. Soil water infiltration was not significantly increased by the green manure treatments after ripping and gypsum addition, though it was significantly different between treatments. The SOM increased from around 1.28 to 1.96%, well below the critical value suggested in the literature for more humid areas to sustain healthy soil, and not as high as expected from the high quantity of green material returned to the soil. However, the levels of organic carbon increase were comparable with those predicted from a CSIRO carbon model. These results point to the dominant role of soil and weather conditions in the low rainfall cropping zone of Western Australia in limiting the build-up of soil organic matter content, regardless of amount or composition of green manure applied.