Soil is the essential ingredient in any farming enterprise. When it is in poor health, plants cannot grow to their full potential. Contaminated soil is a risk to public health and unstable soil can lead to landslips and dust storms.
Farming contributes to problems such as soil erosion, fertility loss (unless corrected by fertiliser) and compaction.
Farming places a significant demand on the organic and mineral components of soil. Because many New Zealand soils developed under forests, they do not have large stores of vital nutrients. In farming, the minerals needed for plant growth, such as nitrogen and phosphorus, are typically removed more quickly than they can be replaced by natural processes, so farmers add fertilisers. While organic fertilisers are used by some farmers, synthetic fertilisers have been in use in New Zealand since the late 19th century.
Soil erosion is an ongoing concern in New Zealand. Between 200 and 300 million tonnes of soil are carried out to sea every year – a rate well above the global average for soil loss. New Zealand’s hilly topography and active geology amplifies the problem, but farming continues to be a leading cause of erosion. Land clearance and grazing on unstable lands contribute to this problem.
Farmers in drier parts of the world envy the relatively abundant and gentle rains that New Zealand’s farmlands receive. However, local variations in rainfall and topography mean that some farmers in New Zealand are plagued with too much water, while others worry about having too little.
In wetter regions, farmers often drain land to make it dry enough for stock and cropping. However, too much land drainage can lead to the loss of ecologically important wetlands. Important native fauna, like eels and whitebait, depend on wetlands for their survival. Land drainage can also contribute to making floods worse.
In parts of New Zealand, such as Central Otago and the east coasts of both islands, the problem is too little water. Irrigation has increased significantly in these areas in recent years, especially in Canterbury where dairying has expanded. Where farm irrigation is particularly intensive, the depletion of some underground aquifers is an increasing concern.
Farming can lead to the run-off and leaching of nutrients into rivers, streams, estuaries and underground water. The four pollutants of greatest concern are nitrogen, phosphorus, sediment and animal faecal matter. These typically enter waterways as run-off and leaching from farm paddocks. When livestock have direct access to waterways, they can pollute more directly, and can add sediment by breaking down stream banks.
Nitrogen and phosphorus, which can end up in waterways from farms, make some aquatic plants and algae thrive. They then multiply and can damage native eel and whitebait habitats, restrict swimming, boating and fishing, clog drains and hydro dam intakes, and spoil the aesthetic appeal of waterways.
Often, the most immediate water-quality problems can be easily remedied, for example building fences to keep cattle out of waterways. But other problems are more complicated to solve, such as the lag effect in nutrient leaching from soils. For example, the deterioration in Lake Taupō’s water quality is thought to be partly caused by farming activities dating back as far as 50 years.