Nutrient loading is often considered the most serious environmental effect on water quality associated with agriculture. To maintain crops of high yield and good quality, farmers have for years added plant nutrients to the soil, mainly in the form of animal manure, plant or animal residues, mineral fertilizers, and the plowdown of legumes. About 3.5 million tonnes of mineral fertilizer and 2 million tonnes of sewage sludge are applied to agricultural land each year. In addition, livestock produce about 300 million tonnes of manure each year in Canada, and most of it is applied to cropland. The difference between the amount of nitrogen applied to the land and that removed by crops can be a surplus or a deficit.
Surpluses can lead to losses of nitrogen to the environment.
This is not the case in Canada, where production of crops with a high demand for nitrogen is on the rise and increasing livestock production is generating more nitrogen in manure. However, Canada's mean nitrogen surpluses are still low when compared with the United States and Europe, as shown in the following table. If the amount of each nutrient in the soil is greater than the amount taken up by the crop, there is a risk that surplus nutrients will move off farmland into groundwater or surface waters. This risk is greatest under wet conditions and if the soil has a low capacity to bind chemicals, as is the case for coarse-textured soils like loamy sands. Areas of intensive livestock production often have high levels of soil nutrients, particularly phosphorus, because of the heavy application of animal manure to the land.