To prevent the deserts coming near, China has planted billions of trees---to replace destroyed forests and as barriers against the sand. This isn't a cure, though, say experts, as thirsty trees can make the problem worse by taking in groundwater. "Planting tress is one way, but it isn't that simple. It doesn't solve the basic issue of water resources," says Wu Bo, a professor. "We need to calculate how much water the trees will absorb, or else it could have a negative effect." Villagers in Zhengxin have taken on this challenge, with limited success. When the irrigation channels began to run dry, Lu Xianglin switched from wheat to cotton on his land. He also planted trees to protect his fields from sandstorms. He says he still gets good yields  using flood irrigation and earns a good income for his family. Other farmers haven't stuck it out : about one in three have left Zhengxin in the past 10 years after their wheat crops died. Young people who can find jobs in the towns rarely return. Last week, Mr Lu joined the other men in his village on a government-arranged trip to see the land that has been set aside for their relocation, nearly 40 miles to the south. The next day, he was back, shaking his head at the plan. The idea of uprooting his family troubles him, as does the idea of giving up the land that fed his forefathers. He prefers to stay and keep up the fight. "With enough water, this problem can be solved," Lu says. "We can plant trees and grass, and they will grow bigger. That will stop the desert." Experts say that farmers could switch to drip irrigation  to lessen their water intake for growing crops. Elsewhere in the region, farmers have built brick greenhouses as part of a plan to grow vegetables using less water. Roadside signs urge farmers to "Save Water, Protect the Environment". The negative effect of planting trees in deserts is that   _  .
it can make groundwater become less it can prevent the sand moving freely it can stop crops growing well it can get the soil to become poor