Undetected acidity 'choke' a fixable constraint

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Snapshot

Owners: Keith and Debra Lord
Location: Junee, NSW
Farm size: 2068 hectares
Annual rainfall (long-term): 520 millimetres; 800mm in 2016
Soil types: red loams, granite hills, heavy red soils
Soil pH: variable
Enterprises: cropping and sheep (3300 Merino ewes)
Typical rotation: canola/wheat/canola/wheat/canola/wheat or barley/pasture/pasture/pasture/pasture

Soil pH test kit

The colour chart readings in the Manutec® Soil pH Test Kit indicate pH using the 1:5 water method, which is about 0.8 to 1.0 pH units higher than the pHCa measures referred to in this article, calculated using the calcium chloride method. Initial tests with garden soil pH kits should be verified with careful soil sampling and analysis conducted by an accredited laboratory.

Key points

  • Many productive acid soils in Australia’s northern and southern grain-growing regions may not be reaching their potential because of undetected acidic layers 5 to 20cm from the surface
  • A knowledge of soil type and pH variation in the surface layers (0 to 20cm) is essential to make the best decisions about acid soil management and crop choice
  • A shovel and a garden soil pH kit will indicate if you have surface acidity (0 to 10cm soil layer)
  • Further validate this result with laboratory testing, with samples carefully collected at 5cm intervals to a depth of 20cm
  • If lime is required use rates that will maintain pH (measured in calcium chloride) above 5.5 in the top 10cm
  • Subsoil acidity can be avoided by using strategic tillage once every 10 to 15 years to mix lime through the top 10cm
Photo of Keith Lord and Helen Burns.

Keith Lord discusses the results of soil tests with NSW DPI development officer Helen Burns.

PHOTO: Nicole Baxter

A New South Wales mixed farmer was surprised to discover severe acidity in his soil’s surface layers despite having a liming program in place

For Keith Lord, a crop of faba beans has provided a timely alert to acidity in the surface layers of his soils, the remediation of which should provide a healthy lift to grain yields and profits. Keith runs a 2068-hectare mixed farm near Junee, NSW, with his wife Debra, sons Gregory and Andrew, and assistant manager Jeffrey Breading.

A year-in-year-out canola/wheat sequence has been profitable, but Keith wanted to add more diversity to the rotation and so decided to grow faba beans in 2016 on a paddock that had received a blanket lime application in 2011 of 1.3 tonnes per hectare. The lime was incorporated at sowing with knifepoints and press wheels. A follow-up zero to 10cm soil test in 2013 indicated the paddock’s pHCa (measured using the calcium chloride method) was adequate for growing faba beans.

Meanwhile Helen Burns, a development officer at the NSW Department of Primary Industries’ Wagga Wagga Agricultural Institute, was looking for commercial pulse crops to monitor as part of a GRDC project investigating constraints to legume performance in the high-rainfall zone.

She contacted Delta Agribusiness agronomist Tim Condon, who suggested Keith Lord’s faba bean crop. Keith had reported areas with stunted and yellow plants in his 103ha paddock of faba beans.

Detailed soil tests

Helen collected plant and soil samples from areas of ‘good’ and ‘poor’ growth within Keith’s crop. Soil was sampled at 2.5cm increments to 10cm and in 5cm increments from 10 to 20cm down the profile.

Initially, in the paddock she used a Manutec® Soil pH Test Kit to get an indication of any acidity, and later validated her findings using a laboratory analysis of pHCa (Table 1).

The plants collected from the poor growth area were stunted and showed symptoms of nitrogen deficiency. The soil tests showed that their roots had hit a severely acidic layer (with a pHCa of less than 4.5) 5cm from the surface.

“The acidic layers adversely affect root growth, nodulation, plant vigour and the nitrogen fixation potential of acid-soil sensitive pulses, including faba beans, lentils and chickpeas,” Helen says.

“An issue to emerge from the faba beans at Keith’s property is that the current standard industry practice of spreading lime with no incorporation and sowing with knifepoints and press wheels or disc seeders largely leaves the lime in the surface soil layers. Also, traditional soil-sampling methods do not detect the pH stratification within the top 10cm like we identified.”

She says they have found moderately (pHCa 4.6 to 5.0) and severely (pHCa less than 4.5) acidic layers at five to 20cm in many paddocks surveyed for the project. These were not detected in soil samples collected at the standard depths of 0 to 10cm and 10 to 20cm.

“Finer sampling at 5cm intervals will help detect pH stratification. If severely acidic layers are discovered, lime needs to be incorporated to a depth of 10cm with a full cultivation at least six to 12 months ahead of sowing an acid-sensitive species like faba beans and up to 18 months before sowing in drier areas,” Helen says.

“Lime deposited on the surface moves very slowly through the soil profile, even in high-rainfall zones, slowing the rate of lime reaction with acidic layers below 5cm and potential crop response, which is an expensive opportunity cost for growers.” Additionally, Helen says relatively acid-tolerant species including barley, canola, lucerne and many wheat varieties are probably suffering a yield penalty from acidic layers with a pHCa of less than 4.7 in the top 20cm.

Table 1 Soil pH (calcium chloride) at two sides on Keith Lord's farm new Junee, NSW1.
Soil depth (cm) Site J1 (good crop growth) Site J2 (poor crop growth)
Soil pH (calcium chloride) Soil pH (calcium chloride)
 Composite sampleSub samplesComposite sampleSub samples
0 to 2.5

5.19

4.81

5.53

4.43

4.37

4.86
2.5 to 5.0 5.44 4.55
5.0 to 7.5 5.15 4.22
7.5 to 10.0 4.63 4.07
10.0 to 15.0 4.60 4.19
15.0 to 20.0 5.02 4.55
Faba bean plant appearance Healthy, vigorous plants with good nodulation, about 35cm tall at the first node growth stage (September). The health and density of the roots, including the finer root hairs, were superior to plants from the J2 site. Plant roots were restricted to the top 10cm of soil. Most plants at this site were yellow and less vigorous, with less root hair development than plants from the J1 site. The plants were 15 to 25cm tall. Plant root growth was concentrated in the top 4cm with minimal root growth below 4cm. Root disease was evident on most plants.

1. Traditional composite soil pHCa tests at 0 to 10cm and 10 to 20cm underestimated the level of pH stratification in the soil profile at two sites on Keith Lord’s farm near Junee, in southern NSW. Soil conditions at each site were reflected in the appearance of the faba bean plants.

SOURCE: Helen Burns, NSW DPI, 2017

A way forward

When Helen shared her discoveries with Keith he felt a mixture of disappointment and relief.

“Although Helen was excited to find an explanation for the problem in my paddock, I wasn’t too impressed,” he says. “Faba beans became like a canary in a coal-mine to us, alerting us to an issue we didn’t know was throttling our grain yields and profits.”

While Keith aims to maintain stubble cover where possible, he now appreciates the importance of using a strategic full cultivation to mix high rates of surface-applied lime through the topsoil.

He says Helen’s work has also demonstrated the value of using more detailed soil tests to guide acid soils management and crop choice.

“I didn’t realise the variation of pH through our soils was such a big issue,” Keith says. “But I’m now expecting that by mixing high rates of lime through our topsoils we will be able to increase our returns.”

Earlier this year, Keith applied lime at 2t/ha. The lime was then incorporated with a scarifier fitted with sweep points.

He is now working with Helen and her NSW DPI colleague Dr Mark Norton to monitor the impact of lime rate and application method on soil pH and subsequent crops.

GRDC Research Code DAN00191

More information:

Keith Lord,
02 6924 1969,
homeview@westnet.com.au

Helen Burns,
0427 721 816,
helen.burns@dpi.nsw.gov.au