Water repellent soils occupy more than 5 million hectares of western and southern Australia (Roper, 2004). Decomposition of hydrophobic (or water repelling) waxy materials originating from plant residues can coat soil particles preventing the infiltration of water into the soil profile (figure 1; Van Gool & Moore, 1999). Soils with a small surface area (e.g. sand) are more prone to water repellency as it takes less hydrophobic material to coat individual particles, compared to silt or clay.
（source of the image: http://www.soilquality.org.au/factsheets/water-repellency)
The result of water repellence is generally uneven water distribution in the soil profile which leads to patchy and uneven plant emergence (figure 2). Moving from an alternately wet and dry soil can make it difficult to control the depth of sowing causing further problems with establishment (Blackwell, 1996). Water can remain ponded on the soil surface to be evaporated or lost as runoff. Lack of plant cover and heavy autumn or summer rains can result in significant runoff and erosion on sloping sites.
Causes of water repellence
- Native vegetation creates waxy, water repellent residues, so newly cleared agricultural land and native landscapes will be more susceptible to non-wetting.# In arable farming systems, pasture and grain legumes such as clover, medic, lucerne, and lupins which contain higher amounts of plant waxes form water repellent compounds. Cereals do not form water repellent residues so it’s worth breaking a long pasture phase on susceptible soils to prevent build-up of hydrophobic material.
- Fungi can also produce water repellent residues, particularly under perennial pastures such as lucerne.
- As waxy substances in plants are not effectively broken down by their passage through the sheep (Blackwell 1996), sheep camps tend to be more water repellent because of the accumulation of organic matter.
The susceptibility of a soil being or becoming water repellent will be determined not only by the presence of hydrophobic material, but also soil texture (Hunt and Gilkes, 1992). Coarsely textured sandy soils that contain less than 5 % clay are very susceptible to becoming water repellent. In a study done by Harper and Gilkes (1994), it was found that water repellency only occurred in soils with <10 % clay and was most severe for soils with <5 % clay. Although not as common, water repellency can occur in certain soils with a finer texture if the soil has a strongly aggregated structure (Moore, 1998). The repellency occurs when the aggregates become coated in hydrophobic material (Harper and Gilkes, 1994). In Western Australia, soils in coastal sand-plain regions generally have the greatest risk of water repellence.
Even though the breakdown of organic matter creates water repellent residues, there is generally no direct relationship between total soil organic carbon and water repellency. It is the type of organic matter present that influences the soils’ susceptibility to water repellence.
Detecting water repellence
There are two main methods for measuring soil water repellency:
- The time taken for a droplet of water to penetrate the soil.
- The concentration of an ethanol solution needed to penetrate the soil in under 10 seconds. The higher concentration of ethanol needed, the more severe water repellence is.
Hunt and Gilkes (1992) also described visual indicators of water repellence in the field — patchy pasture growth or crop emergence, early growth in depressions where water can pond, shallow wetting of the soil surface and water erosion. Staggered weed germination can also be an indicator of a water repellent soil.
As water repellence is related to organic matter, it is often confined to the upper soil layers, especially in minimum till systems. Standard sampling of the top 10 cm for analysis may “dilute” the water repellent layer, thereby skewing the results. For most accurate determination of water repellence the top 3 – 5 cm should be analysed.