Fertilizer use differs from country to country, and from too little to too much. Nitrogen and phosphorus can produce big crops. But they can also pollute water and air.

A recent policy discussion in the journal Science compared the nutrient balances of different agriculture systems. Researchers compared the use of fertilizer in three areas that grow maize as a major grain: China, Kenya and the United States.

A man in Amritsar, India places fertilizer on his rice crop last month

A farmer in Amritsar, India, puts fertilizer on his rice crop

By two thousand five, they say, farms in northern China produced about the same amount of corn per hectare as farms in the American Midwest. But the Chinese farmers used six times more nitrogen, and produced almost twenty-three times more surplus nitrogen.

Government policies can have an influence. For example, as China sought food security, its policies increased fertilizer use.

The researchers note that farmers in the Midwest used too much fertilizer on their crops through the nineteen seventies. But improved farming methods later increased their yields and, at the same time, made better use of chemical nitrogen fertilizer.

Farms in western Kenya use just over one-tenth as much fertilizer as American farms. Corn harvests remain small. The researchers say farming methods in Sub-Saharan Africa need to improve or else poor quality soil will increase rural poverty. More than two hundred fifty million people do not get enough nutrients from crops to stay healthy.

Nutrient balances in agriculture differ with economic development. Farmers lack enough inputs to maintain soil fertility is parts of many developing countries, especially in Africa south of the Sahara. But countries that are developed or growing quickly often have unnecessary surpluses.

Ammonia gas released by fertilized cropland is a cause of air pollution. The land can also release nitrous oxide, a heat-trapping gas.

Nitrogen runoffs from farms can create large dead zones, like those in the Gulf of Mexico. Algae microorganisms in the water overpopulate because of the surplus nitrogen. The algae take much of the oxygen from the water. Fish and other organisms die.

Laurie Drinkwater at Cornell University in Ithaca, New York, was an author of the report. Professor Drinkwater says farmers need to think about ways to solve some of the causes of nutrient loss from agriculture. She says different countries need different solutions based on location, environment, climate and population needs.

And that's the VOA Special English Agriculture Report, written by Marisel Salazar. I'm Steve Ember.