Fertiliser and the environment

By Bala Tikkisetty

As the soil starts warming up over the next few months fertiliser reps will be kept busy as farmers start preparing to fertilise their paddocks.

But if farmers apply too much fertiliser - or apply it in the wrong place or at the wrong time - this important resource can become a contaminant, polluting groundwater and promoting algal growth in nearby waterways.

Before buying fertiliser, I recommend farmers look at their nutrient budget and talk to fertiliser rep about the strategies I outline below, and other techniques, to ensure they only apply enough fertiliser to grow the pasture needed, and thereby minimise potential impacts on the environment and save dollars.

If farmers don’t already have a nutrient budget, or better yet a nutrient management plan, I recommend they ask their fertiliser rep or consultant to prepare one for them.

There are four particularly significant potential risks involved with fertiliser application, but there are some strategies farmers can use to help avoid problems.

Nitrate leaching
Pasture plants need nitrogen for healthy leaf growth. But nitrogen (N) is an extremely mobile nutrient. If farmers apply more nitrogenous fertiliser than plants can take up, the excess N ends up leaching down through the soil into groundwater. Sometimes N will also be lost as run-off and some is always released back into the air as gas.

An important reason for inefficient N cycling in grazed pastures is the deposition of nitrogen, by the grazing animals, in concentrated urine patches. The amount of N that is deposited is generally much greater than can be taken up by the plants in the area of the urine patch, and the excess N can be lost through leaching. These losses are likely to be highest during periods with excess rainfall and low soil temperatures that limit pasture growth and N uptake – that is autumn, winter and early spring. Dung patches may also contribute, but to a lesser degree, as dung consists mainly of organically bound N that is released slowly.

Farmers can reduce the amount of N leaching from pasture by:

· timing fertiliser application to avoid times when plant uptake of nitrogen will be low, such as when soils are saturated, during heavy rain, colder periods and when soil temperatures are low

· applying fertiliser in split dressings, rather than all at once

· making sure farm dairy effluent is applied to a large enough area

· adjusting fertiliser policy for effluent irrigated areas to account for the nutrient value of effluent

· standing cows off pasture during wet weather and capturing the effluent for later application

· using fenced wetlands and well-managed open drains as nutrient traps.

Nutrient run-off
Phosphorus (P) behaves very differently to N because it binds with the soil and only dissolves slowly in water over time. This means it doesn’t readily leach to groundwater, but it can still pollute waterways through soil erosion and surface run-off.

As is the case with nitrate leaching, farmers can reduce the amount of P running off pasture by ensuring they don’t apply phosphate fertiliser when heavy rain is forecast. Farmers should also:

· follow the NZ Fertiliser Manufacturers’ Research Association Code of Practice for Nutrient Management (which has an emphasis on fertiliser use)

· apply fertiliser when the grass is in an active growing phase

· make sure fertiliser isn’t applied near waterways

· leave a grassed buffer strip between paddock and waterway – the strip filters the nutrients before the run-off reaches the water

· control run-off from tracks, races, feed and stand-off pads.

Excessive Olsen P levels
If soil Olsen P levels are high, it’s likely more P than what’s needed for maximum pasture growth is being applied. It also means there is a greater chance that nutrients are being lost to any nearby waterways.

Soil tests combined with an OVERSEER® nutrient budget is one way to ensure soil fertility isn’t excessive and to reduce the amount of nutrients lost from the farm.

Stock poisoning
If ingested directly by livestock, fertiliser can be poisonous. To avoid this:

· make sure animals don’t have direct access to fertiliser bins or other storage facilities

· don’t graze pasture if fertiliser particles are visible – it’s best to wait until sufficient rain has washed the particles into the soil.

Large nutrient surpluses should be avoided for two important reasons. Firstly, the surplus can in most cases be lost to the environment (N particularly), causing off-site problems, and secondly, nutrient surplus is a waste of farmers’ hard-earned cash. Conversely, a nutrient deficit indicates that farmers may be mining the soil nutrient reserves, a situation that cannot continue indefinitely and will eventually compromise pasture production. A nutrient management plan not only accounts for all the nutrients entering and leaving your farm (this is your nutrient budget) but also identifies what you will do on your farm to both maximize production and minimize losses. Therefore, I strongly suggest farmers all have a nutrient management plan.

Bala Tikkisetty is a sustainable agriculture coordinator at Environment Waikato. For more information about managing fertiliser application, please call him on EW’s freephone 0800 800 401.