Water-repellent (‘non-wetting’) soils are a major constraint to agricultural production in southern and south-west Australia, affecting >10 Mha of arable sandy soils. The major symptom is dry patches of surface soil, even after substantial rainfall, directly affecting agricultural production through uneven crop and pasture germination, and reduced nutrient availability. In addition, staggered weed germination impedes effective weed control, and delayed crop and pasture germination increases the risk of wind erosion. Water repellency is caused by waxy organic compounds derived from the breakdown of organic matter mostly of plant origin. It is more prevalent in soils with a sandy surface texture; their low particle surface area : volume ratio means that a smaller amount of waxy organic compounds can effectively cover a greater proportion of the particle surface area than in a fine-textured soil. Water repellency commonly occurs in sandy duplex soils (Sodosols and Chromosols) and deep sandy soils (Tenosols) but can also occur in Calcarosols, Kurosols and Podosols that have a sandy surface texture. Severity of water repellency has intensified in some areas with the adoption of no-till farming, which leads to the accumulation of soil organic matter (and hence waxy compounds) at the soil surface. Growers have also noticed worsening repellency after ‘dry’ or early sowing when break-of-season rains have been unreliable. Management strategies for water repellency fall into three categories: (i) amelioration, the properties of surface soils are changed; (ii) mitigation, water repellency is managed to allow crop and pasture production; (iii) avoidance, severely affected or poorly producing areas are removed from annual production and sown to perennial forage. Amelioration techniques include claying, deep cultivation with tools such as rotary spaders, or one-off soil inversion with mouldboard ploughs. These techniques can be expensive, but produce substantial, long-lasting benefits. However, they carry significant environmental risks if not adopted correctly. Mitigation strategies include furrow-seeding, application of wetting agents (surfactants), no-till with stubble retention, on-row seeding, and stimulating natural microbial degradation of waxy compounds. These are much cheaper than amelioration strategies, but have smaller and sometimes inconsistent impacts on crop production. For any given farm, economic analysis suggests that small patches of water repellency might best be ameliorated, but large areas should be treated initially with mitigation strategies. Further research is required to determine the long-term impacts of cultivation treatments, seeding systems and chemical and biological amendments on the expression and management of water repellency in an agricultural context.
Natural colloid mobilization and leaching in wettable and water repellent soil under saturated condition
