Environment Canterbury biodiversity officer Kennedy Lange (left) explains the benefits of a sediment trap to Catchment group chairman Roger Small. Photo / Sally Brooker
Environment Canterbury biodiversity officer Kennedy Lange (left) explains the benefits of a sediment trap to Catchment group chairman Roger Small. Photo / Sally Brooker
Seeing was believing at last week's Waihao Wainono Community Catchment Group field day.
About 50 people attended the event at Hugh and Liz Wigley's Hook farm, aimed at helping farmers to maximise their soil while meeting land management rules.
Plant & Food Research soil scientist Trish Fraser set out samples of different soil types in clear tubes, to which she added water. Then she showed two samples of the same soil, one that had been in pasture long term and one that had been in crops long term. Water sat on top of the arable sample, whereas it filtered through the pasture sample.
Soil scientist Trish Fraser talks to farmers at the Waihao Wainono Community Catchment Group's field day at Hugh and Liz Wigley's farm. Photo / Sally Brooker
Organic matter could improve soil structure to boost its water-holding capacity, supply more nutrients to plants, reduce erosion risk, feed microbes and aerate crops, Dr Fraser said.
Continued cultivation and ploughing at the same depth created a layer that roots could not penetrate. This could be avoided by cultivating to different depths.
A 14-year Lincoln University study of tillage systems showed pasture produced the best soil structure, followed by no-till, minimum-till, fallow and intensive till.
Catchment group chairman Roger Small speaks at the field day. Photo / Sally Brooker
''When you grow crops, they take up nutrients. Pasture leached the least. When we left plots fallow over winter, they leached a lot more than when not fallow,'' Dr Fraser said.
Much research was being done on which crops would leach least, Dr Fraser said. Plantain was ''looking really positive''.
Nutrient leaching could be reduced with strategic grazing, starting furthest from a sensitive area. Artificial drainage was an option, but not economical on the whole farm.
Earthworms could help damaged soil to recover. Another possibility was mechanical aeration. To test if soil was ready for aerating or cultivating, Dr Fraser suggested rolling a handful into a ''worm'' 50mm long and 4mm thick. If it cracked before the worm was formed, it was ready.
An Otago study showed mechanical aeration of the subsoil in winter forage paddocks had only an 18-month benefit. ''You have to weigh up if it's worth it.''
Visual assessments and earthworm numbers could be used as simple indicators of changes in soil quality.
Agrilink director Andrew Barber said silt or sediment traps would be needed at some point in a cultivation regime. Farmers should allow for 50 cubic metres per hectare.
''Tension time'' was crucial, Mr Barber said. ''Water has to sit for long enough for the sediment to drop out.''
Trials showed the traps were ''incredibly efficient'', removing more than 99% of sediment from water.
Mr Barber has had a web app developed so farmers can work out how much erosion is likely from their slopes and soil types. They can then see what effect mitigation measures such as cover crops or sediment traps would have.
Niwa was building a prediction model for sediment trap sizes, he said.
''It becomes a factor of how often you want to clean out the trap.''
