Hidden soil compaction could be costing cereal growers more than they realise, SCRI trials results soon to be reported in Field Crops Research suggest.

Most farmers recognise that their best results come from crops with well developed deep root systems able to withstand the yield-sapping effects of drought, acknowledges researcher Blair McKenzie.

However, new root-restricting experiments supported by the Mylnefield Trust at the institute in Dundee are beginning to unravel the reasons behind this and suggest that some spring barley varieties are more drought-resistant than others.

“It’s early days,” says Dr McKenzie. “But some varieties seem to cope better than others with our root restriction treatments, while some are very susceptible. Further testing is needed before recommendations can be made.”

Deep roots are usually an advantage in cereals but offer little or no benefit when the surface soil can supply all a crop’s needs. However, compaction, a problem often not fully recognised, restricts their growth, he notes.

“When we dig soil known to have compaction problems we find that the few roots that access deeper layers usually do so by following cracks or biopores – channels left by previous crops or created by earthworms.

“To get through compacted layers plants that have the most roots at compaction depth have the best chance of finding a biopore, crack or other way to get through to the water below.”

To test how crops respond to limited access to subsoil water Dr McKenzie’s team needed to find a way of regulating that access.

“We wanted to offer not just full or no access – but a variable amount. The experimental details are fairly simple, but the practice required some effort,” he explains.

Early last year about 100t of topsoil was removed from an area of 30m by 10m. The subsoil was then cultivated to make sure it was uncompacted, and a layer of mesh was spread over most of it, leaving a few areas clear.

“The mesh is permeable to air and water, but prevents root growth.”

Plots with different numbers of holes in the mesh were then created using nails, the topsoil was replaced and five varieties of spring barleys sown in replicated trials.

In all plots the crop germinated and emerged well. But a dry spell from late April to mid-June stressed the plants, especially those growing over the mesh with the fewest holes, and this was reflected in height differences, notes Dr McKenzie.

The rest of the growing season was very wet, but despite that the plots that got off to a poor start never fully recovered.

“Height-wise most managed to catch up. But for other measures, such as total leaf area that require the plant to produce extra tillers, those under most early stress never did so and their yields were lower.”

So what accounts for the apparent differences between varieties in their ability to withstand root restriction and hence compaction?

“It’s speculation and the subject of further research,” says Dr McKenzie. “Perhaps they have more roots in the surface soil and for a short time can use the limited water there better.

“So varieties most susceptible to restricted rooting may be better suited to cultivation systems that loosen the soil to depth.

“Clearly we need more research to understand the interaction between management of soil structure to overcome compaction and to understand the nature of crop roots systems in soils on real farms. Recently we’ve expanded the experiment, moved a lot more soil, and in April we’ll sow 16 spring barley varieties.

“Eventually this might allow us to recommend matching certain varieties to soil conditions.”