Dan Bloomer1, Alex Dickson1, Alec Mackay2, Alan Palmer3
1 LandWISE, 21 Ruahapia Rd, RD10, Waipatu, Hastings
2 Farm Systems & Environment, AgResearch, Tennent Drive, Fitzherbert, Palmerston North
3 Farmed Landscapes Research Centre, School of Agriculture and Environment, Massey University, Palmerston North
dan@landwise.org.nz
This presentation covered work completed by LandWISE, AgResearch, Gisborne District Council, Massey University and others. The collaboration came together in the aftermath of Cyclone Gabrielle, aware that very limited data had been collected or published immediately after other events. The core team included Alex Dickson (coordinator) and Dan Bloomer from LandWISE, Alec Mackay of AgResearch, Alan Palmer from FLRC at Massey University, and Bryce McLoughlin and David Suter from Gisborne District Council.
Following Cyclone Gabrielle, one of the key challenges for growers was managing sediment deposited on some of the country’s most productive soils. The critical question was how best to return these low fertility, low organic matter soils back to full production.
Historical precedent existed for addressing this challenge. Long-term studies completed after the May 1948 North Island storm in the Gisborne region and the February 2004 Southern North Island storm floods in the Manawatu/Rangitikei regions examined effective management practices for sediment revegetation and developed guidelines for farmers. These studies produced a decision tree based on factors like depth and texture to guide optimal sediment management. After Cyclone Gabrielle, growers who followed these established guidelines achieved good outcomes where conditions were suitable.
In the immediate aftermath of the cyclone, we consulted the science community including Plant & Food (now Bioeconomy Science Institute), AgResearch, Massey University and independent horticultural consultants for guidance. The collected information was compiled to help affected growers plan immediate responses, then uploaded to the LandWISE website and presented at grower meetings.
Stage one baseline sampling was designed to provide growers with current site-specific information for management decisions. Conducted one to three months after the cyclone, this sampling effort collected 151 samples from 111 sites across all major catchments, sediment types, and enterprise types, plus 14 additional samples for contamination testing. Assessment parameters included sediment depth, texture, bulk density, structure, fertility, and earthworm presence along 50m transects. Both planned and actual field actions were recorded. The baseline project engaged 65 farmers and growers across the impacted areas.
The initial sampling filled a major gap in knowledge of the behaviour of soils and sediments in the weeks and months immediately following a storm event. Sediment deposition on the Hawke’s Bay and Tairawhiti varied in depth (< 5 cm to > 100 cm), texture (sand to silty clay loam), volumetric moisture content (10 – 80%), bulk density (0.75 – 1.65 g cm-3), nutrient fertility including pH (5.5 – 8.5), Olsen P (2 – 30 μg ml-1), exchangeable potassium (2 – 16 MAF units), sulphate sulphur (2 – > 200 mg/kg) and in its biology (12 – 70 earthworms m-2). The physical condition of the sediment as assessed using the Visual Soil Assessment methodology varied from poor to moderate.
Initial concern regarding chemical or biological contaminants in the sediments was not supported by any of 14 samples taken from sites in Hawke’s Bay. The initial survey as recorded actions growers took, or were intending to take, where significant amounts of sediment (5 – 20+ cm) were deposited on their highly productive land as there is little or no documented information on best management of sediment impacted sites with high value crops on elite soils. Previous studies have been almost exclusively of re-grassing pastureland.
Our broader goal was establishing a multi-year longitudinal study focused on cropland and orchards, building on the 1948 and 2004 research. The baseline sampling was carried out collaboratively by LandWISE, Massey University, AgResearch, Plant and Food Research, Hawke’s Bay Regional Council and Gisborne District Council, with support from Horticulture NZ, and the Ministry for Primary Industries.
Earlier studies focused primarily on pastoral remediation and contained limited information directly relevant to highly productive orchard or cropping land. The proposed stage two aimed to follow baseline sampling with continued monitoring to improve knowledge for site remediation responses on very high-value land for future similar events. Ultimately, stage two was only able to visit a small number of sites on and around the Heretaunga Plains.
PowerPoint presentation