Use the dry to understand subsoil constraints

Author: Alistair Lawson | Date: 02 Feb 2016

Dr Melissa Fraser in a soil pit at Frances, SA.

PIRSA soil consultant Dr Mel Fraser in a soil pit at Frances, SA.

There are few positives to come out of back-to-back dry seasons - such as that experienced by South Australia’s south east - but growers can use the experience to dig deep below the soil surface to understand what is contributing to crops not fulfilling their yield potential.

Dr Melissa Fraser, a soil consultant with the Rural Solutions SA division of Primary Industries and Regions SA (PIRSA), says a lot of soils in south east SA have a physical or chemical impediment that restricts root growth to the top 10cm of the soil, hindering the plants ability to access moisture and nutrients stored deeper in the profile.

“It’s important to identify what those constraints are and the ways we can overcome them to start utilising and capitalising on soil moisture that is still left at the end of the season,” Dr Fraser says. “Having an understanding of both the water holding capacity of the soil and any subsoil constraints can help growers make management decisions.”

Analysing soils at Frances, SA, Dr Fraser says high bulk density and soil strength were two key physical impediments to root growth. Machinery traffic and grazing animals can cause compaction to occur through the A and B horizons of some soils, inhibiting crop and pasture productivity due to poor root exploration.  

“Soils like this could lend themselves to organic matter incorporation with a green manure crop or other organic matter source and also to deep ripping,” Dr Fraser says.

“The other impediment with this particular soil is its propensity to waterlog. When you have shallow loamy surface horizons overlying very heavy clay subsoils, you find that rainfall enters the surface at a faster rate than it can be absorbed by the subsoil. This causes water to pond at the interface between the two, which can effect plant vigour, especially if the waterlogging is prolonged. These particular soils could also be delved, which would improve infiltration and also increase the water and nutrient holding capacity in the surface.”

Soil tests at Frances also indicate acidic conditions with a pH of 5.05 (calcium chloride) in the topsoil, which drops below pH 5 at a depth of 10 to 20 centimetres.  

“Ideally, soils should be kept at a pH above 5.5 in the surface to maximise nutrient availability. One of the issues associated with low pH is aluminium toxicity, which commonly occurs when the pH falls below 4.8,”

“This is something I’m starting to see a lot of in both crops and pastures in the south east. I’ve seen canola roots only go down a few centimetres and then start growing sideways and weeds such as silvergrass start to outcompete pasture species,” Dr Fraser says. “These are clear indicators of chemical and physical constraints in the soil.”

Dr Fraser is encouraging growers in the south east to dig holes over summer and do some deep soil testing to get a basic understanding of the make-up of the subsoil and to understand what might be holding back yield potential. 

“Traditionally in this region we’ve been able to get away with only farming the top 10cm of the soil because we’ve had regular rainfall and we’ve only been sampling in the top 10cm too,” she says. “But in these dry years it’s time to really start thinking about what is happening below the surface and what the constraints are to overcome to ensure we maximise water use.”


More information

Dr Mel Fraser, Rural Solutions SA soil consultant,

GRDC Project Code CSP00170, DAW00244, DAW00204

Region South