Cultivate: but cultivate with care.
That is the message from current NSW DPI research for farmers planning to apply lime their crop paddocks.
This GRDC supported project is investigating the factors reducing the production potential and nitrogen fixation efficiency of legumes grown on acid soils in south eastern Australia.
The shift to minimum disturbance tillage systems and away from lime incorporation can result in soils with severely acidic subsurface layers that are toxic to both legume crops and rhizobia alike, according to research conducted on 25 farms from Cowra to Frances in South Australia by NSW DPI officers, Dr Mark Norton, Helen Burns and Peter Tyndall.
The use of lime as a means of managing soil acidity has been a feature of the farming program for many years; but Helen Burns, DPI development officer - pastures, based at the Wagga Wagga Agricultural Institute said research is showing that the applied lime may not be working as effectively as farmers think it is.
Using traditional nought to ten cm soil tests to check pH and guide lime requirements has been the accepted norm, but Ms Burns points out they continue to be the current rates, except farming practices have changed immeasurably during the past 20 years.
“The guidelines currently used for liming programs were developed at a time when traditional cultivation incorporated the lime to a depth of about ten cm,” she said.
“It’s time to review liming programs: not only have tillage practices changed, but we are also using more nitrogen fertilizer and exporting more product, which in combination result in higher rates of acidification and pH stratification. There is elevated pH on the surface, and if you take a nought to ten cm soil test a pH (CaCl) of 5.2, for example, appears to be suitable for the acid-sensitive pulses. But when we sampled at five cm intervals, in this case, we found we have the lime concentrated in the shallow surface layer (ph of 6.0) with a drop down to an acid pH of 4.4 at the five to ten cm levels.”
It’s time to review liming programs: not only have tillage practices changed, but we are also using more nitrogen
Acidic soils can limit productivity of pulses
Widespread adoption of minimum-till farming has led to pH stratification in the acid soils in the high-rainfall zone, with the low soil pH at depths detrimental to legume and rhizobium growth.
Legumes are a beneficial rotational crop that can be highly profitable, but roots growing in such acidic layers struggle to perform.
Helen Burns, Wagga Wagga-based DPI development officer – pastures, pointed out those low pH recordings are not conducive for root development of the acid-sensitive pulses or the nodulation process.
Current soil tests are not giving farmers an accurate indication of the conditions seedling roots experience and this is a major concern.
“Our recommendation is if you are wanting a ‘rapid solution’ [in cropping paddocks] you really should be incorporating the lime into the acidic subsurface because lime moves very slowly,” she said.
“Also, from the results so far, perhaps the rates we are using are not high enough to counter acidification rates of our current production systems.”
Ms Burns said the research also highlighted the lack of comprehensive paddock records needed to capture change in pH within the subsurface five to twenty cm layers or the effectiveness of lime applied over the past two decades.
“It is a ‘wake-up’ call,” she said. “Farmers don’t have adequate records, even though some are now on their second or third lime application but nobody has been doing a sequence of soil tests actually monitoring how their lime programs have been working.”
It is recommended, where the subsurface is too acidic, to apply adequate rates of fine-grade, high-quality lime, and preferably incorporate at least 12 months before sowing acid-sensitive species.