Sustainable Soil Management Is More than What and How Crops Are Grown - Principles of Sustainable Soil Management in Agroecosystems - page 368

and there was less water running to low-lying areas where it contributed to elevated
water table levels. Perhaps three soil degradation challenges are not improved by no-
tillage: nutrient removal, acidity, and water repellence.
Nutrient removal: The soils of South West Australia are highly weathered and
generally have coarse-textured surfaces with low soil fertility and acidity, limit-
ing crop and pasture production (Moore 2001; McArthur 2004). Soil availability
of macronutrients nitrogen (N), sulfur (S), potassium (K), and phosphorus (P) and
micronutrients copper (Cu) and zinc (Zn) have the potential to limit crop and pasture
growth (Moore 2001). While calcium (Ca), magnesium (Mg), manganese (Mn), and
molybdenum (Mo) are important nutrients, they are generally not considered to be
limiting plant production. Soil fertility of Australian soils has been increased by the
application of fertilizers (Weaver and Wong 2011). Nevertheless, cropping results in
significant removal of nutrients, and continued application of nutrients is required to
maintain long-term sustainability and productivity of cropping systems.
Soil acidity: Soil acidification is a natural process enhanced by agriculture. Each
crop that is harvested is essentially alkaline material. Since no-tillage increases
whole farm yields, it is effectively removing more alkaline material from the pad-
docks. Also, the use of nitrogenous fertilizers and the growing of pulse crops cause
acidification. Consequently, soils are becoming more acid through time, especially
when cropped. Lime application is required to maintain the productivity of most
soils, the exception being soils with an alkaline base. Acidification happens more
rapidly in slightly acid sandy soils and where leaching rains are common. Also, these
soils have low levels of organic carbon, less than 1.5%, giving the soil low capacity
for the prevention of soil pH decline.
Some native Australian plants have adapted to these acidic conditions, over many
thousands of years, and they can also fix atmospheric N, further acidifying the
soil. The most common of these species comes from the Acacia genus. After many
years, their N fixation results in the soil becoming very acidic at depth. Such soils
in Western Australia are known as Wodgil soils; however, the area affected by these
naturally very acidic soils is less than 5% of Western Australia's agricultural land
(Gazey and Davies 2009). The result of such strong acidification is severe soil degra-
dation, making the soil unproductive.
The solution to this form of soil degradation is the addition of large amounts of
lime. Under a no-tillage system, the movement of this lime into a 20-cm soil profile
depth can take 4 years (Flower and Crabtree 2011). For a more rapid amelioration of
these acid subsoils, farmers have also used deep tillage or plowing. By doing so, they
are exposing themselves to soil erosion risk. However, large yield responses have
been achieved immediately (Davies 2011), and this has encouraged farmer adoption
of this technique.
Water repellence: Native vegetation can induce water repellence (McGhie 1980).
Nevertheless, Australian sandy soils that contain less than 3% clay are also capable
of becoming water repellent within 10 years of agriculture practices (Crabtree 1983).
The sands develop a wax coat around individual sand particles (Mashum et al. 1988).
The wax is the remnant of plant residue decomposition, and it causes water to run to the
lowest-lying hollows, causing wetting in preferred pathways. Such a phenomenon
exists across several countries. However, Australia has the largest area, with about