- There are two major nutrient related environmental problems associated with farming: leaching of nitrogen (N as nitrate) and runoff of phosphorous (P as phosphate).
- The amount of nitrate leaching and P runoff increases as soil fertility increases and land use intensifies. Unfortunately moving towards less demanding extensive land use systems is not an option locally, nationally or globally, from a social and economic perspective. We must find ways to minimize nitrate leaching and P runoff and at the same time maintain productivity. This is not an impossible goal.
- The major source of nitrate leaching is from urine patches. When animals urinate they apply between 300 to 600 kg N/ha in small patches. Managing nitrate leaching is about managing the number of urinations per unit area when the soil is already wet (i.e. in the autumn-winter periods). See Fertiliser Review No 10.
- Unlike nitrate-N, P does not leach. The major source of runoff P is caused by the movement of soil particles containing P. Managing P runoff is about managing the movement of soil across the soil surface and into water ways. See Fertiliser Review No 9.
- In clover-based pastures reducing fertiliser N use will not necessarily reduce nitrate leaching. First the loss of fertiliser N from leaching is very small (see 3 above) and reducing fertiliser will encourage clover growth. Clover ‘fixes’ N from the atmosphere and hence the total amount of N going into the system (fertiliser N + clover N) will be the same.
- Changing to slow fertilisers or organic fertilisers – as distinct from soluble fertilisers, is not a solution. It is not the form of nutrient going into the system which controls nitrate leaching and P runoff but the total amount of N and P in the ‘system’ (see Fertiliser Review No 4, 18).
- Adopting organic practices will not solve these problems. An organic farm and a conventional farm will have the same environmental foot-print (in terms of nitrate leaching and P runoff) at the same level of intensification, because the mechanisms of nutrient loss (refer to 3 and 4 above) are the same (see Fertiliser Review No 4, 5, 6, 18).
- Organic farms are typically less intensive and hence less productive (by about 50%) relative to conventional farming. Because they are less intensive they may have a lower environmental footprint. But if the world ‘went organic’ we would need to approximately double the amount of land under cultivation (think here of biodiversity) or depopulate the planet by about 50% (think socially and economically).
- There are already many practices that can and are being adopted by farmer to reduce avoidable nitrate leaching and P runoff (see Fertiliser Review No 9, 10). These include: controlling soil erosion, retiring less productive land, riparian buffers, wetlands, standoff pads, wintering out of sensitive catchments, the use of nitrification inhibitors and better effluent management.
- Scientists are already researching other potential mitigation practices including: developing deeper rooting plant to intercept leached N, changing the animal diet to reduce the N concentration in urine, developing boluses for animals which deliver nitrification inhibitors onto the urine patch, nutrient socks to absorb nutrients from water ways.