LIME top-dressed and only incorporated by a sowing operation under zero or no till, or even minimal tillage, more than commonly results in surface-concentrated lime and an elevated pH in the 0-5 centimetre soil layer.
Even more importantly, irrespective of liming history, and critical to plant growth, is severe acidic layers within a 5-15cm depth of the soil profile, across a range of soil types. A recent study of extensive surveying reveals this.
For the soil layer at 5-10cm depth to remain acidic despite a history of lime use is a major concern in acidic soil areas attempting to grow acid-sensitive crops such as canola, many cereal varieties, several pulse species and pastures such as lucerne.
A large-scale acid research program, begun in 2015, is investigating legume performance on acidic soils. To date it has included sampling soils from 16 crop paddocks in the medium to high rainfall mixed farming zone of south-eastern NSW. Lime applied in minimal tillage systems in these paddocks remained concentrated in the shallow 0-5cm top soil layer and had minimal effect on pH below this, at 5-15cm.
Helen Burns, Mark Norton and Peter Tyndall (NSW Primary Industries Department) conducted the study and Grains Research and Development Corporation helped fund it.
Sites were chosen to ensure soil types sampled were representative of the most productive regions of the mixed farming zone of southern NSW.
Selected paddocks were understood to be well managed for control of soil acidity. Lime incorporation for most sites relied on mixing by sowing alone, using minimum disturbance systems with knife-point tynes or disc seeders. Soil sampling examined pH (calcium chloride method) to a depth of 20cm, at 5cm intervals.
Although these paddocks had received two or three lime applications since the early 1990s, fine sampling at 5cm intervals detected moderate (pH 4.5-5.0) to severe (pH less than 4.5) acidic layers in the top 5-15cm of the soil profiles.
Mean pH of 5.53 for the 0-10cm bulked samples from sites that received surface-applied lime within the last five years suggest lime had done its job in increasing pH in this vital part of the soil profile. But when assessed in 0-5cm sub lots, pH was 6.12 at 0-5 cm but 4.72 at 5-10 cm.
Mean pH of the deeper 10-20 cm bulked samples from the same soils was pH 4.72. When assessed at 10-15cm pH was 4.49 compared with the 15-20cm layer being 4.84.
This indicates that lime rates used are also insufficient to prevent acidification developing below 10cm. Most soils increased in pH below 15cm layer.
Another group of soils with no lime applied in the last five years showed similar trends, although pH readings were slightly less.
Helen Burns says lime moves slowly into layers below incorporation depth and thorough lime incorporation will hasten lime reaction and increase depth of its effect. Although many producers are reluctant to cultivate, recent studies indicate that well planned strategic cultivation will cause minimal harm to soil structure. Ms Burns also notes sampling soil at standard depths of 0-10cm and 10-20 cm is not detecting pH stratification.
Finer sampling at 5cm intervals is advised to locate acidic layers and guide liming programs and species selection. Incorporation of adequate lime rates to a depth of 10 cm will hasten amelioration of acidic layers at 5-15cm.
Monitoring pH at 5cm intervals also provides a measure of the effectiveness of acidic soil management programs and the confidence to adjust lime rates and frequency.
For further details email Helen Burns – firstname.lastname@example.org
Next week. Crop and pasture recovery post bushfires variable.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email email@example.com or contact (0428) 752 149.