Dealing with acid soils
Liming paddocks needs to address effects deeper in the soil.
RESEARCHERS estimate around half of Australia's agricultural lands face or have acid soil issues.
Current practice of applying lime to rectify soil acidity, while a strategy in the right direction, is however largely not at sufficient lime rates, nor is it being adequately applied to rectify acid soil layers within the immediate topsoil.
These views are emphasised in a recent paper published in CSIRO journal Soil Research titled "The extent, significance and amelioration of subsurface acidity in southern NSW" (https://doi.org/10.1071/SR20079), by Wagga Wagga NSW Department of Primary Industries (DPI) researchers and soils experts Jason Condon, Helen Burns, and Guangdi Li.
According to the paper two main issues are occurring with current acid soil management. Not sufficient rates are being applied and incorporation is often ineffective. This is leading to formation of acidic layers within the topsoil and pH stratification. Secondly, current soil sampling practices do not identify acid layers.
Commercial and research soil sampling commonly assesses topsoil at depth of 0 to10 centimetre. This sampling either fails to detect or under report the magnitude of pH stratification. Occurrence of pH stratification and the presence of acidic subsurface layers may cause the extent of acidity to be well underestimated.
This has failed to detect that where lime is not adequately mixed into the soil via current minimum tillage systems, (all too often the lime remains in the 0-5cm layer) but there has been inadequate movement into the 5-10cm layer. The 10-15cm soil layer also exhibits an acid soil problem that also has not been impacted by lime application.
The authors stress the importance of soil sampling at 0-5cm soil intervals in the top 20cm to identify the depth and extent of acidity.
Jason Condon and colleagues strongly advocate to target soil pH higher than common targets of around 5.0 (calcium chloride method). They support aiming for 5.5 pH or higher. Higher pH targets help lime movement to greater depths to address soil degradation by subsurface acidification. This is important if lime is surface applied but incorporation gives the soil a head start to improved pH at depth.
The use of offset discs in a Wagga Wagga study were more effective than tined implements or harrows for lime incorporation to adequate depth. Multiple (two to four) passes of tines were required to gain the same effectiveness as offset discs.
Raising soil pH target helps many plants and soils function better. Many crop and pasture legume species have far better rhizobia function, therefore contribution to building soil nitrogen, at high pH. Albus lupins for example are badly impacted at 4.8 pH or lower with damaged shoots, root growth and poor nodulation.
If higher pH is not maintained in the topsoil, acidification of subsurface soils extends further into the profile and eventually requires more expensive operations that mechanically place amendments deep in the soil. Current liming practices largely do not address acidic subsurface layers. Research has demonstrated that it is possible to address subsurface acidity, given time, by liming to maintain a pH target of 5.5 in the surface 10cm. Wagga research has shown that soil pH needs to be greater than 5.5 for bicarbonate from dissolved lime to move below depth of placement.
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The amount of lime needed to get pH above 5.5 is less if preventative liming is used rather than allowing subsurface acidification to the point that productivity is suffering.
Research examining use of organic amendments has shown promise to enhance soil acidity amelioration with depth. However longevity of their effect is questionable. Proactive preventative management of topsoil pH with lime addition to higher pH targets remains the most cost-effective solution, believe the authors.
Part of acid soil management also is crop and pasture selection. Acid sensitive plants should not be grown for 12 to 24 months after application to allow time to react and increase pH. This interval may need extending depending on starting pH, depth of acidic layers and if lime is not incorporated by cultivation.
For further details contact Jason Condon email@example.com
Next week: New winter crop varieties.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email firstname.lastname@example.org or contact 0428 752 149.