Plants for amelioration of subsoil constraints and hydrological control: the primer-plant concept

In this review, we propose the use of suitable plant species, termed primer-plants, for the primary purpose of preparing soil conditions for the benefit of following crops. Such plants may be used in the temperate agricultural belts of southern Australia, where dryland salinity is a major environmental and agricultural problem that threatens the viability of many crop production enterprises. It is recognised that growing plants that have deeper roots and use more water than the current shallow-rooted annual crops provide a long-term solution for managing the dryland salinity problem. Increased plant water-use is expected to mitigate the rising watertable that transfers salt to the root-zone of crop plants. On medium to heavy textured soils, common in this region, impermeability of the subsoil to roots and water movement is another major impediment to high water-use and productivity by plants, which may lead to other adverse hydrological events in the soil such as water-logging and excessive run-off. Plants that possess the ability to penetrate the dense subsoil and make it porous, in addition to having the capacity for using soil water at high rates, should be effective in combating dryland salinity. These plants normally should have thick roots that grow deep in the soil and are able to modify or withstand the adverse chemistry of the often-saturated subsoil, so that upon the death and decay of their roots, channels or biopores are created. These biopores have greater vertical and lateral continuity and last longer than porosity created through mechanical tillage. In this paper, we argue that potential exists for inclusion of short to medium-term phases of primer-plants in farming systems as a mimic of pre-existing perennial vegetation. We propose that ideal plants for combating dryland salinity should have high water-use and capacity to also improve soil structure and, possibly, nutrition. Examples are presented of soil amelioration that generally supports the viability of primer-plant concept, including the limited work undertaken in south-eastern Australia. We identified key knowledge-gaps, such as lack of well-defined agronomic packages for growing short-phases of Australian native species in mixtures, and our limited understanding of their root dynamics, which need to be addressed before effective implementation of the primer-plant concept.