A recent property I visited in relation to helping develop a crop and pasture update program, had several zero to 10 centimetre soil test results indicting pH as low as 4.3, with aluminium levels as high as 50 per cent of the cation exchange capacity.
Applying lime and mixing it well into the top 10 centimetres of soil at 2.5 t/ha of good quality lime, would, from past trials, improve pH on these sandy loam soils to around 5.2 to 5.4 pH and almost eliminate aluminium toxicity in that layer.
ALSO FROM BOB FREEBAIRN:
That would be a good strategy on its own if soil acidity was only restricted to the zero to 10 centimetre soil layer. However, many soils are acid well down the profile and it is very difficult to relatively cheaply increase their pH (and lower their aluminium toxicity) via lime application.
Lime is relatively low in solubility, with quite a bit of research indicating it is best to well incorporate it just to be effective in the top 10 centimetres of soil.
Where soil acidity is mainly only a problem in the top zero to 10 centimetre layer, periodic lime application has for many years proved to be a good strategy, and allows such soils to be used for most crops and pastures like lucerne.
In an annual cropping pasture system, soil acidity gradually occurs if not addressed with products like lime, and gradually also impacts on the subsoil and then can become more difficult to address.
The property (as seen in the top photo) is in an area where most acid soils are also acidic in the subsoil, sometimes right down to several metres.
Applying lime to the top zero to 10 centimetres of soil, even after many years, generally has little impact on improving pH and aluminium toxicity much below 10 centimetres depth.
Where soils are extremely acidic in the subsoil, the best strategy for a cropping or pasture program is to use crops/pastures with varieties that have excellent tolerance to low pH and high aluminium.
There are big variety differences in plant tolerance to severe acidity in some crop species like oats, triticale, rye and wheat.
Almost all oat, triticale, narrow leaf lupin and cowpea varieties for example have excellent tolerance to acid top and subsoils. Saia oats, an old variety, probably has the best tolerance of the oat varieties if extreme acidity exists, but most varieties cover for moderate acidity and aluminium levels.
Bread wheat have several varieties with useful tolerance to aluminium toxicity, but these are still not as tolerant as most triticale or even oat varieties.
For example varieties like Beckom, Coolah, Dart, Sunmax, Condo, Crusader, Gregory, Wedgetail and Longswood, are rated moderately tolerant to tolerant. Yambla and Tulla (older varieties) are the only currently listed barley varieties with useful tolerance.
Where soils are extremely acidic in the sub soil the best strategy for a cropping or pasture program is to use crops/pastures with varieties that have excellent tolerance to low pH and high aluminium.
A major breakthrough in pastures for highly acidic soils has been the discovery of tolerant to very tolerant winter legume species like serradella and biserrula.
These can thrive on soils below 4.5 pH with aluminium levels above 20 per cent. Their rhizobia bacteria (responsible for nitrogen fixation) also are tolerant of these soil conditions.
Other legumes like sub clover have useful acid soil tolerance, but not in the league of serradella and biserrula.
Perennial grasses like the tropical species Consol lovegrass and Premier digit also have excellent tolerance to acidic soils, including those acidic right through the profile.
A number of native grasses also have excellent tolerance. Cocksfoot, kikuyu, and paspalum also have good tolerance.
Several years ago, a NSW Department of Primary Industries' soils investigation across much of the state, noted that soil acidity had gradually developed on many naturally slightly acid soils in farming systems based on annual plants. For example, continuous cropping or cropping and sub clover and even with the occasional perennial plant such as lucerne.
But in perennial pasture systems, even if regularly fertilised to correct deficiencies like phosphorus and sulphur, very little increased acidity had occurred.
Next week: I will write about the NSW DPI's findings in their recently released 'Weed Control in Winter Crops' booklet, which covers herbicide registrations for all winter crops, with detail on aspects such as rate, weeds controlled, spray stage and residual life.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email firstname.lastname@example.org or contact (0428) 752 149.