Productive agriculture acidifies soils, which increases the concentration of hydrogen ions in soil and causes soil pH to decrease. When newly cleared for agriculture the pH of most of the sandy soils in WA was 5.5-6.0; it is now typically 4.0 or less.
Recent research, largely done using no-till crops, has shown that both molybdenum deficiency and aluminium toxicity induced by soil acidification reduce wheat and barley grain yields. Both are now major problems for no-till cropping on most sandy soils in South Western Australia.
As soil pH values decrease below 5.0 the molybdenum naturally present in the sandy soils becomes increasingly more strongly retained by soil and so is less available for plant uptake. Eventually molybdenum becomes deficient for grain production of cereals.
Also, as soil pH continues to decline below 5.0 the aluminium present in soil constituents (clays, oxides, organic matter) dissolves, increasing the concentration of soluble aluminium ions present in soil solution. Eventually the aluminium in soil solution becomes toxic to plant roots, reducing root growth and uptake of water and nutrient elements from soil by plants, and decreasing grain yields. Soil acidification occurs in the top about 40cm of soil, and is most difficult to ameliorate in the 10-40cm zone, where it is known as ‘sub-surface acidity’.
The solution for both the induced molybdenum deficiency and aluminium toxicity problems is to lime the soil. Growers need to monitor soil pH values in the top 10cm and 10-20cm of soil. If pH in the 10-20cm zone is less than 5.0, then sub-surface acidity is a problem. Sufficient lime then needs to be applied to raise the pH of the top 10cm of soil to 5.5 or greater. Only then will alkali dissolved from lime in the surface soil move down into the subsoil to ameliorate sub-surface acidity.
Adding the lime and increasing soil pH above 5.0 will also reduce the capacity of the soil to retain soil molybdenum, increasing the amount of soil molybdenum that can be accessed and taken up by plant roots.
Adding too much molybdenum fertiliser to soil is not recommended because it can induce a copper deficiency in ruminants (sheep, cattle, goats) grazing pasture, known as molybdenosis. In the rumen the molybdenum reacts with sulfur and copper to form a very insoluble compound and the animal cannot take up copper from the gut into the bloodstream. As long as the stock grazes that pasture, the solution is to inject copper into the blood stream of the animals. So lime the soil. Do NOT apply molybdenum fertiliser.
When soil is limed and soil pH increases the capacity of the soil to retain the micronutrients copper, zinc and manganese increases and deficiency can be induced in crops. In Western Australia, induced deficiencies of manganese and zinc after liming are common. The solution is to tissue test crops before liming to assess the micronutrient status of the crop, or to apply micronutrient fertilisers when liming.
Published, 12 November 2009, Reviewed 14 September 2010