Sandra Taylor

Lincoln PhD student has found stones can hold a surprising amount of moisture, throwing doubt on long-held assumptions about the water holding capacity of stony soils.

Balin Robertson, from Lincoln University, has been studying water storage in stony Canterbury soils and says there is a lack of science on these soils. As a result, irrigation and nutrient models such as Overseer are relying on the assumptions that field capacity (upper drainable limit of a soil) is the water content at a suction of 10 kPa and that rocks do not hold water.

Field work was carried out when paddocks were at field capacity, the soil moisture state after a saturated soil has drained for two days. Results showed that field capacity occurred in stony soils at suctions lower than 10 kPa, which suggests that they retain more moisture than had previously been predicted.

Balin says his work, carried out at 52 soil pits at 24 pastoral sites across Canterbury, found rock fragments in the subsoil below 40 cm could contribute to a substantial proportion of the water retained at field capacity.

“On average, rock fragments account for one third of the water held in the subsoil.

“So, while it may not be possible to get blood out of a stone, you can get water.”

Stones accounted for 10% of the water retained within the soil to a depth of 60 cm, which is equivalent to around 11 mm of water.

Size does matter when it comes to rock fragments. Small fragments of between 2mm and 20mm appear to have a greater ability to absorb water than fragments larger than 20mm because of their higher relative surface area.

He says while it is difficult to measure the properties of stony soils, the work does need to be done.

He admits he has been surprised by his findings, which suggest best management practices may be conservative and there could be a buffer for drainage and nutrient leaching targets.

However, it has yet to be determined just how dynamic the water held in the rock fragments is and how much of the water retained by rock fragments is available to plants.

Balin says he is addressing this question now in an experiment that had to be relocated to his garage rapidly at the time of Covid-19 lockdown.

“Globally, stony soils are very understudied.”

In trying to correct this Balin has spent many hours in the laboratory quantifying the relationships between a number of soil properties, including the water retention of fine earth and rock fragments at field capacity, rock fragment size classification and content, particle size distribution, and carbon.

This information will be used to provide data to guide both management and regulation on Canterbury’s stony soils.

Sam Carrick from Landcare Research says Balin’s research is crucial for S-map, which is used extensively to supply soil data to the farming industry but has had little quantified data to draw on for the widespread stony soils of Canterbury.

“Farmers have been asking for this knowledge for a number of years and now, through Balin’s sweat and dedication, they have the data upon which to base decisions.”