But what this means for the soil is still unclear. “The effects of modern irrigated pastoral farming on soil physical properties are not well quantified internationally, particularly for temperate climates,” says Manaaki Whenua senior researcher Dr John Drewry as part of a regional study led by Veronica Penny and Dr Sam Carrick to evaluate the effect of irrigation on soil physical properties across Canterbury.
“Stock and nutrient management practices change as irrigation is expanded to allow land to become more intensively farmed,” says John. “With that comes changing soil physical properties, such as increased occurrence of compaction and its effects on water storage capacity.”
For the study, 24 paired sites in Canterbury were sampled, consisting of a spray-irrigated paddock and an adjoining part of the same paddock that was dryland (unirrigated), with other management the same for each pair. Sites were sampled across the region from Tekapo to Waiau.
Thirteen of the sites were located on dairy farms, with the remainder on farms with sheep and beef, ‘dairy support’, and beef land uses. Sites were selected on deep soils (> 60 cm depth of stone-free soil) so that cores for soil physical measurements could be taken at six increments to 60 cm depth. The 60 cm depth was chosen as this is used for irrigation scheduling and is the depth modelled by the OVERSEER nutrient budget model.
John says the results from the study showed that under irrigation there was a shift towards smaller soil pores. “This was reflected in macroporosity and readily available water capacity being significantly lower under irrigation, while semi-available water capacity (water held between stress and wilting points), and unavailable water (held below permanent wilting point) both increased.”
The study concluded these differences reflect increased compaction under irrigated grazed pasture, which was also consistent with findings in other, similar studies. This study quantified changes in both the topsoil and subsoil but most differences were confined to the top 20 cm of soil. The changes are likely due to grazing when the soil is wet post-irrigation.
For irrigation management, the study indicated the lower readily available water capacity on irrigated pasture is significant, with farmers potentially having to irrigate more frequently. Adopting deficit irrigation could help minimise the impacts of compaction.
Manaaki Whenua researchers are partners in a follow-up project over the next four years to investigate options for co-ordinating grazing management and timing of irrigation, to minimise or mitigate soil compaction under irrigated grazed pastoral systems.
The project was a collaboration between Federated Farmers, Manaaki Whenua, the AgriBusiness Group, IrrigationNZ and Canterbury irrigators. Funding was through the Ministry for Primary Industries Sustainable Farming Fund, and the MBIE Next Generation S-map programme. Further information and results are available from the recent publication at the link below.
