How science can curb leaching
SOIL: A soil scientist has come up with a way to help rivers and boost productivity. It’s all about testing and sound advice.
Farmers keen on reducing nitrogen and phosphate leaching might be interested in a study by Hamilton-based soil scientist Dr Gordon Rajendram. Rajendram has demonstrated how one Waikato farm has improved plant nutrient uptake, pasture and milk production, as well as boosted root growth, earthworm populations, and the soil’s water-holding capacity through a combination of testing and following sound agronomic advice.
“This study is highly relevant, especially to Canterbury dairy farmers as the Government requirement that no more than 190kg of nitrogen per hectare is applied in any one year,” he said. “It is also likely in the future there will be restrictions in phosphate use, as this is more of a threat to the environment if it gets into waterways than nitrogen.
“Farmers are very worried about the ‘190 N rule’, particularly in Canterbury, but you can grow enough pasture with high-quality feed if you get the right advice based on sound scientific principles.”
His case study was was carried on 667ha Belfield Dairies, farmed by Andrew and Richard Grant, with 2400 cows on two milking platforms.
The soil type consisted of Struan stony silt loam, Eyre stony sandy loam and Waterton deep silt loam soils. Before the study, the Grants’ practice was to apply 290-300kg N/ha/year and blanket application of superphosphate (85 per cent watersoluble) up to 45kg/ha.
“We conducted comprehensive soil testing — called soil benchmarking a farm — to determine the nutrient and soil pH status in 2018 when cows were wintered off,” Rajendram says.
Through thorough testing and data analysis, paddocks were identified that were below optimum, at optimum or above optimum for each nutrient. From this, density maps were created for each nutrient or soil parameter. The results showed the farm’s soil had low phosphate and sulphur retention.
The pasture production was monitored to determine fertility of each paddock. The most fertile paddock grew 19 tonnes of dry matter per hectare per year, whereas the worst one grew nine tonnes.
Yearly soil, pasture and cloveronly samples taken from 12 monitor paddocks established on benchmarking to detect any nutrient deficiencies. Extra testing was done on carbon, nitrogen, organic matter and trace elements cobalt, copper, manganese, iron, molybdenum, selenium and zinc. All 16 elements tested are required for pasture growth and animal health.
The paddocks were limed to an ideal soil pH of 6.3 over a three-year programme. “But because the farm soils could not retain phosphate or sulphur, slow-release products such as Di-calcic P and elemental sulphur were applied.”
This not only greatly reduced leaching and runoff but application rates were reduced from 45kg P/ha to 15kg P/ha. To reduce nitrogen application rates to a target of 150kg/ ha/year, they have started to apply it in a liquid form.
The cost of sampling and testing was about $10/ha, making it a costeffective investment.
“The soils were very compacted to help fix this and aerate the soil we used the AgraForum soil conditioning programme,” Rajendram says.
Also, other major and trace elements were applied to overcome any deficiencies.
The Grants have noticed increased earthworm activity “which is a surefire sign of healthy soil”. On top of yearly lifts in grass and milk production, they say the water-holding capacity of the soil has increased and “our fertiliser costs have diminished”.