Roper suggested that the addition of lime to water repellent soils is a viable and cost-effective alternative to the addition of Kaolinite clay. In particular, far less lime than clay is required to improve wetting – resulting in decreased water requirements for improved cropping potential.
Water repellency occurs in siliceous sands across five million hectares of southern and western Australia. These sands cause many agricultural issues as a result of uneven infiltration and restricted soil wetting. In particular, repellent soils require more rain before seeding than non-repellent soils – potentially reducing yield or resulting in germination difficulty. Wind erosion can also become a problem for these dry soils, exacerbated by reduced crop cover from delays in seeding or patchy germination. Water erosion that would not normally occur with non-repel- lent soils may occur in repellent soils after heavy rain on slopes.
Significant research has taken place to understand the causes of water repellency and reduce its effect on Australian agriculture. When organic matter decomposes it can produce waxes which form a water repellent layer around sand particles. These waxes are generally composed of long chain hydrocarbons, fatty acids and alkanes. Coarse sands are particularly prone to developing repellency, due to their decreased surface area for wax adhesion. There is also some evidence that sheep pastures and blue lupin crops are associated with water repellent soils.
While there is significant evidence to show that the addition of kaolinite clay to water repellent soil can improve wetting, the amount of clay required is significant. The addition of clay to water repellent soils therefore represents an expensive solution for properties without naturally occurring deposits.
There has been some anecdotal evidence from farmers that the addition of lime to water repellent soils can also improve wetting. A recent publication in the Australian Journal of Soil Research (Roper, 2005) tests this theory using both laboratory tests and field studies to compare the repellency of soils after the addition of lime, clay or water.
In both field and laboratory studies, Roper investigated the addition of water to repellent soils, using the MED test (molarity of ethanol drop test, King 1981 Australian Journal of Soil Research 19, 273) to measure repellency. Laboratory samples, taken from a site near Woogenellup in Western Australia, were initially highly water repellent (MED3.9). After the addition of water and a period of incubation at 30 degrees Celsius, the MED of these samples dropped to 2.5, while samples incubated without the addition of water remained severely repellent.
This finding was consistent throughout field study results. At an irrigated field site in Anketell, south of Perth, soils with varying irrigation histories were tested for repellency. Roper found that repellency decreased as the time soils had remained permanently wet increased. In particular, her results showed severe repellency in non-irrigated soils (MED 4.1), but only minor repellency in soils irrigated continuously for seven years (MED 2.4).
The affect of kaolinite clay on the repellency of soils was investigated in the laboratory and at a dryland field site near Woogenellup. As has been shown in previous studies, the addition of clay to the laboratory samples reduced repellency significantly. After sixteen days of incubation, repellency had dropped from MED4.9 to only MED1.8. Thereafter, only a minor decrease in repellency was observed (the MED dropped to 1.3 after 161 days). At the Woogenellup field site, addition of clay to soil was shown to reduce water repellency after a period of one year; however, a degree of repellency returned during the hot dry summer months.
The addition of lime to repellent sands was tested in the laboratory and at both field sites. Confirming anecdotal evidence from farmers, Roper discovered that the addition of lime and water to her laboratory samples reduced repellency after a six month period of incubation. In fact, lime was shown to reduce water repellency from MED4.9 to MED1.8 after a period of 16 days, while repellency dropped to only MED0.7 after 161 days. The affect of lime on water repellency was also evident at the two field sites. In the irrigated site at Anketell, the addition of lime dropped MED from near 1 to 0 after only 35 days; while at Woogenellup, repellency also dropped – al- though seasonal fluctuation was again evident.
Roper proposed that the addition of water, clay or lime to soil can diminish repellency through either a physical or biological interaction. She suggests that the rapid decrease in repellency observed after the addition of clay and lime to laboratory samples indicates a physical interaction - with particles of clay or lime binding to the surface of sand particles and covering their hydrophobic surfaces.
Roper also proposes that lime can reduce soil water repellency by increasing populations of wax degrading bacteria that prefer alkaline soils. The much slower reduction of repellency in the limed samples after a period of sixteen days suggests an additional biological response, caused by the growth in popu- lations of wax destroying bacteria. To validate this theory, Roper tested populations of wax degrading bacteria in a range of samples. As predicted, the ad- dition of lime increased populations of these bacteria tenfold compared with untreated samples.
Roper also proposed that the decline in repellency in the permanently wet samples at the irrigated site at Anketell, as well as in the moist laboratory samples, was due to growth in populations of wax degrading bacteria in the moist conditions. Likewise, the seasonal increase in repellency evident at the dryland site a Woogenellup may have been caused by decreased populations of wax-degrading bacteria in the dry summer conditions. In these months, the addition of lime and clay was shown to have a long-term affect – improving soil wetting in comparison to control samples, despite the dry conditions.
Roper M (2005) Managing soils to enhance the potential for bioremediation of water repellency Australian Journal of Soil Research 43: 803-810. Read Abstract.