简述地膜覆盖的效应。
参考答案：
（1）提高地温白色透明膜夜间最低可以提高地温1.5～2℃；白天最高提高10℃，有利于提早发芽和根系的发生。一般根系生长的极限温度为8℃，根毛发生的界限温度为10～12℃。播种定植时间可以提前7～10天。
（2）保墒性能水分蒸发凝结在棚膜表面，温度低时水珠有流进土壤，防止春季水分蒸发。保持土壤疏松、不板结、物理性状好、免耕。下雨落到地面，不覆盖地膜容易板结。
（3）除草地膜下中午温度可达到60℃以上，可杀死杂草。
（4）省肥地膜下由于高温，固氮根瘤菌等繁殖速度快。因此土壤肥力高的地块覆盖地膜要比土壤肥力低的增产效果好。
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| An eightyearold child heard her parents talking about her little brother.All she knew was that he was very __1__ and they had no money left.When she heard her daddy say to her __2__ mother with whispered desperation (绝望), "Only a miracle (奇迹) can save him now, " the little girl went to her bedroom and took out her piggy bank.She poured all the change out on the floor and counted it carefully. Then she __3__ her way six blocks to the local drugstore (药店). "And what do you want?" asked the chemist. "It's for my little brother, " the girl answered back."He's really, really sick and I want to buy a __4__. His name is Andrew and he has something bad growing inside his head and my __5__ says only a miracle can save him." "We don't sell miracles here, child.I'm sorry, " the chemist said, smiling __6__ at the little girl. In the shop was a __7__ customer.He stooped down and asked the little girl, "What kind of miracle does your brother need?" "I don't know, " she replied."He's really sick and mommy says he needs a(n) __8__.But my daddy can't pay for it, so I have brought my __9__." "How much do you have?" asked the man. "One dollar and eleven cents, but I can try and get some more, " she answered quietly. "Well, what a coincidence (巧合), " smiled the man."A dollar and eleven cents? The __10__ price of a miracle for little brothers.__11__ me to where you live.I want to see your brother and meet your parents." That welldressed man was Dr Carlton Armstrong, a surgeon (外科医生). The operation was completed without __12__ and it wasn't long before __13__ was home again and doing well. The little girl was __14__.She knew exactly how much the miracle cost...one dollar and eleven cents...plus the __15__ of a little child. | ||||
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