Patchy crop performance could point to soil acidification

image of acid soils — Poor crop performance may point to a problem with soil pH. Photo: Belinda Cay

Grain growers generally see applying lime as a task for late summer, but planning a more effective liming program can begin during the growing season.

Much of South Australia’s productive farmland is either already acidic or is likely to become acidic in the next decade, according to Primary Industries and Regions SA-Rural Solutions SA Principal Consultant Brian Hughes.

Mr Hughes is managing the collaborative project ‘New Knowledge and Practices to Address Topsoil and Subsurface Acidity Under Minimum Tillage Cropping Systems of South Australia’with investment from the Grains Research and Development Corporation (GRDC).

He says assessing the poor performance of acid-sensitive crops such as faba beans and lentils is an effective way of mapping areas to target for soil pH testing in summer.

The assessment can be as simple as looking over the paddock and recording areas of poor growth, or as advanced as mapping biomass with drone or satellite imagery.

“We’ve seen a close correlation between areas of poor growth indicated by satellite images of lentils and faba beans and acid soils,” Mr Hughes said.

“Most crops will perform poorly if they encounter acid soil, however, pulses are especially sensitive and the symptoms are easier to see compared to cereals.

“Lentils and faba beans are very sensitive to acid soils, showing poor shoot and root growth, yellowing foliage, and nodulation failure. Symptoms in the crop look similar to drought stress or nitrogen deficiency. However, those often occur across the whole paddock, whereas soil acidification is typically patchy.”

Once suspect areas of the paddock have been identified, usually from crop inspections, Mr Hughes said growers should test the pH down the soil profile in those zones after harvest. This is because acidity is often stratified in the 5-15 centimetre soil layer, especially if the paddock has already been limed before.

Traditional sampling of the 0-10cm surface soil may not detect this acidity layer but it can still provide an indication of where pH might be low.

“We now recommend growers test their soil pH in layers starting with zero to five centimetres, then 5-10cm, and 10-15cm or 10-20cm,” Mr Hughes said.

“Testing from 20-30cm may also be useful in deeper, often sandier soils.”

A good approach is to dig two or three holes in each zone within the paddock so there is a clean face through the soil profile to the clay layer, then test the soil pH vertically down that face with a pH indicator kit. Alternatively, soil cores can be taken with a Dig Stick soil probe (Spurr Probe) and tested to the appropriate depth.

These tests should be repeated in each zone where the in-season crop assessment indicated possible acidity issues. Growers should also test a zone of good growth from each paddock to confirm that the soil pH is better in those areas.

Mr Hughes said simple soil pH indicator kits available from most hardware stores are adequate for detecting acidic layers in the subsoil. Acidic soils will turn the kit’s indicator powder light green or yellow.

“If testing with a field kit highlights an acidity problem, it is worth following up with laboratory tests to accurately measure the acidity,” he said.

“Soil samples should be carefully collected from each depth level and submitted in clearly labelled zip-lock bags. An agronomist can help with the correct sampling procedure.”

Laboratories will measure the pH in a calcium chloride solution, which is the industry standard and is a more stable method than pH measured in water.

If the pH measured in calcium chloride is below 5.0 in the top 10cm or below 4.8 below 10cm, acidity will be limiting the growth of most crops and lime is required to lift the pH. Liming programs must aim to increase the soil pH to 5.5 or more in the surface and 5.0 in the sub-surface to ensure acidity does not limit crop growth for number of years.

Mr Hughes added that remediation of subsoil acidity with lime is typically a slow process, especially in no-till farming systems.

“Even high-quality, finely ground lime can take years to move through the soil profile unless it is physically incorporated down into the acid layer,” he said.

“Lime top dressed on the soil surface without incorporation moves about 1cm into the soil each year under ideal conditions.

“Growers need to consider the implications of strategic tillage versus the benefits of correcting the sub-surface layer with lime.

“It is very important to detect and treat soil acidification early because it will slowly reduce the productivity of your crops and your profits without you noticing.”