SCIENTIFIC AGRICULTURE. 201 



dilion ; and in addition to this, those gases also which the plants 

 extracted from the air during growth. 7. A green crop yields 

 more manure than the same crop could do in any other form. 

 8. A grain crop is greater on the same field when green, than 

 when fermented manures are used. The best plants for 

 green manures are those which grow the fastest, produce the 

 most vegetable matter, and with the smallest expense. 



Sufficient seed should be sown, that the plants may coyer 

 the ground completely; the crop should be ploughed in before 

 the time of full blowing, because the flowers give off nitrogen, 

 which is wasted in the air. Agriculturists agree that a seconJ 

 and third crop of green plants still continue to improve the 

 soil; but there must be a limit, beyond which this practice 

 cannot be carried with benefit and profit Green manuring 

 might perhaps secure a field against barrenness for an indefi- 

 nite period of time, providing nothing was ta!*en off: but if a 

 crop was occasionally carried away, in must of course be im- 

 poverished to the amount of what is taken off in mineral 

 matters. It is probably true that lands in a state of nature, 

 which are covered with forest trees or other vegetation, never 

 become barren. 



The soil may in time become deficient in a particular mineral 

 element which the incumbent plants require ; but when these 

 die out, others immediately spring up by a natural rotation, 

 and, requiring elements slightly different from the first, grow 

 as luxuriantly as they did. Thus one race of plants succeeds 

 another, each in turn exhausting the soil of certain elements, 

 and leaving it richer in others. The question may arise, What 

 becomes of the mineral elements, which are lost, if nothing is 

 taken off the soil, since they do not escape into the air? The 

 probability is, they sink down deeper and deeper into the soil 

 in the form of soluble salts, until beyond the resell of the 

 roots of plants. 



