RESEARCH assessing the impact of lime or gypsum, or combinations of both, on the rehabilitation of a degraded slightly acidic sodic soil near Peak Hill in the Central West noted continued soil improvements 12 years after application.
Sodicity is known to damage soil condition and subsequent crop and pasture production, therefore it requires careful management.
In the research, lime rates of nil, one, two and five tonnes a hectare, and gypsum rates of nil, 2.5t/ha and 5t/ha, singly and in combination with each other, were reassessed 12 years after application for soil benefits such as chemistry, stability, hydraulics, vegetative growth and soil respiration.
The soil type used was a typical hard setting red loam (Sodosol) with pH of 6.1 (water method) in the top five centimetres and 6.6 in the 5cm to 10cm layer.
The area had a long history of cropping and grazing.
It was a degraded landscape at the start of the research. No previous lime or gypsum had been added.
No mechanical disturbance took place after the research started. Crop and pasture production had been generally poor.
Exchangeable sodium percentage, a measure of soil sodicity and on soils prone to hard setting related to hard setting characteristics, was 6.5 per cent in the surface 0cm to 5cm layer and 10.6pc in the 5cm to 10cm layer, indicating moderate sodicity.
The research was done by John Bennett, National Centre for Engineering in Agriculture, University of Southern Queensland, Toowoomba; Richard Greene, School of Environment and Society, Australian National University; Brian Murphy, Department of Natural Resources, Research Centre Cowra; Peter Hocking (deceased), and David Tongway, School of Environment and Society, Australian National University.
Not only are large farming areas affected by sodic soils, defined as those with more than 6pc exchangeable sodium (as a proportion of the cation exchange capacity), but it is also an issue for mine reclamation where it is a common issue.
The usual method of rehabilitating sodic soils is to apply gypsum (for example 2.5t/ha to 5t/ha in cropping and 20t/ha in mine reclamation) to reduce the exchangeable sodium percentage.
However, the researchers note that gypsum readily leaches on these soils and the positive influence of it on sodicity is commonly brief.
Calcium from lime in the research had a major positive effect on soil health.
Significant effects on soil chemistry were limited to increases in exchangeable calcium and decreases in exchangeable magnesium.
Aggregate stability in water and hydraulic conductivity were improved where 5t/ha lime was applied.
Vegetation, including growth assessments after rain (pasture recovery rates), total nitrogen and carbon were significantly improved where lime had been added at 2.5 or 5t/ha.
Soil respiration, an indicator of soil biological activity was also higher in the 5t/ha lime treatment.
Soils treated with lime generally retained improved aggregate stability better than gypsum treatments.
No lime from that applied could be detected 12 years after application.
It was deduced that lime had acted as a catalyst for increased soil functionality and could be used as one of a suite of tools to reclaim degraded land.
Lime applied at 2.5 and 5t/ha 12 years later on this soil indicated the best response to lowering sodicity.
Full details of the study are reported in the journal Soil Research.
Bob Freebairn is an agricultural consultant based at Coonabarabran. Email Bob Freebairn or contact 0428 752 149.
