2QO PRINCIPLES OF AGRICULTURAL CHEMISTRY 



is soon ploughed in. The third method is applicable to some 

 truck crops. 



The amount of manure applied varies from 3 to 40 tons per 

 acre. In arid JOT dry climates, the manure should be composted, 

 and well rotted. Coarse manure should not be plowed under in 

 the spring in dry sections, as the layer of manure will break the 

 connection between plowed and unplowed soil, and cause the 

 plowed soil to dry out more rapidly, thereby losing water needed 

 for the crops. Coarse manure may be applied best as a top dress- 

 ing on pasture land. 



Well rotted manure may cause wheat to lodge. It can be 

 applied to corn. 



Practice in applying manure varies. In some places, heavy 

 applications are made every four years or more. In other places, 

 it is applied annually in smaller quantities. It is probably better 

 to apply the manure once in a rotation of crops to the crop which 

 does best with it. Manure is valuable not only for the plant food 

 which it contains, but also for its physical and chemical effects on 

 the soil. The lasting effects of manure are shown by experi- 

 ments at Rothamsted and Woburn, England. At Rothamsted, 

 one plot had received manure for 20 years, and none after that. 

 Barley has been grown on this plot for 58 years, and still shows 

 the effect of the manure applied 38 years ago. Thirty years 

 after the last application of the manure, the crop of barley on the 

 manured plot was twice as large as that which had never received 

 any fertilizer or manure. At Woburn a plot which had received 

 manure a few years continued for 25 years to give better yields 

 than one which had received no manure. 



Green Manures and Cover Crops. 1 Green manures and cover 

 crops are planted to be plowed under. It is of course more de- 

 sirable to feed the crop and save the manure, thereby utilizing its 

 feeding value and most of its fertilizing value, but this procedure 

 is not always practicable. 



The objects of green manure and cover crops are as follows : 



(i) To supply organic matter to the soil. 

 1 Farmers Bulletin 278. 



