this is the Agriculture Report.An area known as a dead zone develops every springin the Gulf of Mexico near the mouth of the Mississippi River.It can spread as much as 13,600 square kilometers,extending all the way to the eastern Texas coast.Scientists know what causes the dead zone -- too much nitrogen,but the solution might be hard to accept.Bayani Cardenas is a professor of water studiesat the University of Texas at Austin.He says the movement of rivers usually removes material like nitrates.So he wondered why that natural filtration processdoes not remove nitrates from the Mississippi River.Professor Cardenas says his recent study showsthat more than 99 percent of the river's waterdoes pass through the river's sedimentwhich is on the shore or bottom of the river.But he says the study found that there is so much nitrogenin the river system that is simply can not filter all of it.The water that has nitrogen still in it supports the growth of algae.As the algae dies, it sinks to the bottom of the riverwhere it breaks down or decomposes.It then takes oxygen from the water.This condition is called hypoxia and it is deadly to fish and shrimp.Where does all this nitrogen come from?The answer is farms.The Mississippi River system carries water from 33 American statesand part of Canada to the gulf of Mexico.Along the way nitrogen and other chemicals used in farming enter the system.Farmers say these chemicals must be usedto produce enough food for a growing world population.Aaron Packman is a professor of Civil and Environmental Engineeringat Northwestern University.He says farmers may be able to better controlthe amount of nitrate fertilizer they put on fields."How much fertilizer do you need to give you good yieldsand then how much is maybe a marginal gain from adding lots more fertilizer?There is really a question here:can you maybe and get close to the same level of yieldwithout having such a negative impact?" said Packman.The water in the Mississippi River systemshould be able to clean itself naturally as it flows down stream.But flood controls and other human-made structureshave hurt this filtering process.Storms in the next few month will mix the gulf waterand the dead zone will disappear, but it will return next year.And scientists say it will grow larger in years to come,if something is not done to reduce the amount of nitrogen in the Mississippi River.And that's the VOA Learning English Agriculture Report.For more agriculture stories,go to our website 51voa.com.更多听力请访问51voa.com