Agriculture and Environment: Rice

Environmental Impacts of Production: Pollution from Fertilizers

Fertilizers affect water quality. For example, nitrogen absorption is quite low in rice production.

Estimates from the Philippines suggest that only 30% of the applied urea is effectively utilized in rice cultivation. The bulk is lost through volatilisation and denitrification.

Nitrogen from synthetic fertilizers such as urea is oxidized (through nitrification) into nitrate, which in turn is converted to volatile gaseous forms and lost through denitrification. Losses in the form of ammonia are high, contributing to eutrophication of the paddy water, with a resulting high daytime paddy water alkalinity (Witte et al. 1993).

Phosphorous absorption is even lower. Estimates suggest that not more than 10-15% of phosphorous added to the soil is absorbed by the crop (Witte et al. 1993). The rest is often transformed to insoluble forms (a process known as phosphorous fixation), and only under certain conditions can these forms be made available to the crop.

Inefficient fertilizer use not only costs the farmer money and lower profits, it also has a polluting effect downstream. Most of these impacts have not been quantified. It is known, however, that nutrient-rich waters coming from agricultural areas in China are a major cause of the frequent red tides along the coast.

Perhaps most important, the repeated and increasing use of synthetic fertilizers also alters the microbial balance that converts organic matter and dissolved minerals in the soil into forms that the rice plant can use. Over time, the reliance of farmers on synthetic fertilizers tends to lead not only to a slow degradation of soil fertility but also to a reduced ability of the soil to absorb chemical inputs.