While lime has been widely used for more than 40 years in many districts, and is a vital component of crop and pasture production in the millions of hectares of acid soils, soil acidity, commonly in relatively shallow layers, remains a vital production problem.
A major Grains Research and Development Corporation and Primary Industries Department (DPI) funded study, by Mark Norton and Helen Burns NSW DPI Wagga Wagga Agricultural Institute, has found widespread acid soil layers at depths of five to 10 centimetres.
Helen Burns says the work highlights the extent and severity of pH stratification, even in soils with a long history of lime.
She says acid soil layers can be extremely detrimental to nodulation and crop production of acid soil sensitive legumes such as faba beans, chickpeas, lentils, and even to a degree, field peas. However, the reports suggests growers should also check the impact of acid soil on canola, lucerne, barley and acid-sensitive wheats.
Helen Burns says severe pH stratification identified in the research is only noted by testing in five cm depth bands (especially in the top 0 to 15cm soil layer) instead of the usual 0-10cm.
The current practice of topdressing lime with no incorporation under no-till or zero-till systems, adopted by the majority of growers participating in the study may increase pH in the top five cm layer, but not in the five to 10cm layer. Lime is concentrated in the surface layers (to five cm) with little movement below this.
Ms Burns says under no-till systems, lime topdressing with no incorporation is ineffective in neutralising acidity below about five cm. She said on a Holbrook site receiving four tonnes of lime to the hectare since 2010, incorporation with a Speedtiller® was ineffective in mixing lime below five cm.
At Holbrook the five-10cm layer was pH 4.4 or lower, but was 6.5 in the five cm layer. Pulse crop and soil data from commercial paddocks in 2015 and 2016 showed the detrimental effect of moderately (pH 4.6-5.0) to severely (pH <4.5) acidic layers below five cm on root growth, nodulation and crop vigour.
Faba bean seedlings and rhizobia exposed to a hostile environment had poor nodulation; plants showed severe nitrogen deficiency within three months, had root growth restricted to the surface soil and did not penetrate into the severely acidic soil below five cm. Helen Burns said applying lime with no incorporation and sowing with knife point press wheels or disc seeders confined lime effects to the surface layers.
If severe pH stratification is detected she advocates incorporating lime to a depth of 10cm with full cultivation at least six-12 months before sowing acid-sensitive species. She also suggests using appropriate rates of lime to maintain pH at 5.5 or higher in the entire top 10cm layer, in order to correct pH to 15cm.
Faba bean is “the canary in the coal mine” for detecting acidic layers. While faba beans show dramatic clinical symptoms when exposed to acidic layers, nodulation and root growth of chickpeas is also affected by an acidic layer (pH 4.8) below five cm.
The project’s results reinforce that well nodulated, vigorous pulse crops better withstand stresses, including infection by root diseases and transient water-logging.
Adding to poor growth is residual damage from sulfonyl urea herbicides applied in the previous 12 months. Ineffective incorporation of lime produces an elevated pH in the surface soil and delays herbicide breakdown.
More about this research will be presented at coming GRDC updates. Phone Helen Burns (02) 6938 1947 or email helen.burns@dpi.nsw.gov.au
Next week. Improve accuracy of assessing crop available soil water for crops and pastures.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact 0428 752 149.
