Phosphorus gets into rivers and streams as particles of soil or animal dung, as dissolved phosphorus released from soil or from animal dung and fertiliser applications that have not yet bound onto the soil. Mechanisms that transport phosphorus in particulate or dissolved form include surface run-off and leaching, in some soils, into drainage water.
Even small amounts can cause problems in streams, rivers and lakes through increased algae growth. When algae decompose the process takes oxygen away from fish and invertebrates causing waterways to become eutrophic.
McDowell advocates farmers identify critical source areas on their farms.
“These are the areas where most of the phosphorus is lost from, but account for a minority of the farm’s area,” he said.
“For example, it could be that 80% of their phosphorus loss is from 20% of their land. Focusing on these areas with targeted mitigation strategies is much more cost-effective than strategies applied farm-wide”.
Critical source areas could be a block of high Olsen P paddocks, steep pastures prone to soil erosion, areas of compaction where dung is washed off the soil surface, or concreted lanes that do not drain into an effluent pond.
McDowell said there are many simple practices that prevent phosphorus getting into waterways from critical source areas.
“Within paddocks we focus on minimising soil Olsen P enrichment and good effluent and fertiliser practice.”
Nutrient budgeting and making sure soils didn’t have phosphorus ranges greater than optimal for pasture production was essential.
“Soil Olsen P concentrations above the agronomic optimum are unlikely to improve production, but will enhance phosphorus losses.
“When spreading effluent, avoid phosphorus losses by not spreading too much effluent onto soil or at a time when the soil is wet and effluent has less chance of soaking in. Decreasing the application rate (mm/hr) with K-lines or other low-flow irrigators is seen as best practice instead of spray guns.
“Similarly, when spreading phosphorus fertiliser avoid doing so when the soil is wet or when a rainfall event is likely to induce run-off or leaching. In most areas doing so can minimise the contribution of phosphorus losses from fertiliser to about 10% of farm losses.”
However, in areas where this could not be achieved, or as insurance against phosphorus loss, McDowell said drip feeding small amounts of phosphorus with lower water-soluble phosphorus products could decrease losses.
Besides managing paddocks, a number of other simple strategies were also applicable, he said.
“For instance, fencing waterways to stop stock eroding banks and defecating in or nearby was one solution and had already been carried out by almost all dairy farmers in New Zealand. “Planting these fenced areas in grasses, shrubs and trees also benefited waterways by lowering water temperatures, shading out weeds, and stabilising banks in times of flood. Buffer zones, such as a strip of grass, on the edge of winter crop paddocks, also help lessen phosphorus loss.”
Farm tracks to and from dairies, areas around water troughs and other areas where stock camped should also be checked as dung build-up in these areas could end up in waterways during rain.
“Farm tracks need to be contoured so run-off from them flows into paddocks and not drains which then lead to waterways,” he said.
“It’s about keeping the phosphorus on the land where you want it to be.”
“Phosphorus is an essential element for plants which extract it from soil water so we can’t eliminate phosphorus losses, but we can mitigate or minimise them.
“Farms are complex biological systems where phosphorus losses are determined by the interactions of management, fixed assets such as soil type and capital infrastructure and chance events like weather patterns.”
So it was not only important what farmers did but when and where they did it.”
