No. 4.] KEEPING UP FERTILITY. 83 



dissolving phosphoric acid and potash as they go. These 

 constituents enter into the plant, and when this is turned 

 under to decay they become availal)le to the succeeding 

 crops, — more available than are the phosphoric acid and 

 potash, still a part of the mineral particles of the soil. 

 AVhatever has once been a part of a plant is in condition the 

 more readily to become so again. Thus it will be seen the 

 practice of green manuring ultimately increases the avail- 

 ability of the natural stores of phosphoric acid and potash 

 of the soil. 



As regards nitrogen, green manuring may serve two im- 

 portant purposes : it may be made, first, a means of con- 

 serving soluble soil nitrogen ; and second, a means of gather- 

 ing nitrogen from the air and making it a part of the soil 



capital. 



Nitrogen Coxservatiox. 



The first of these two effects — nitrogen conservation — 

 is hardly second in importance to the other. In some of the 

 forms in which nitrogen is applied to the soil, e.g.^ nitrate 

 of soda, it is at once liable to loss by leaching downwards ; 

 in most of the other forms in which it is supplied it soon 

 enters into soluble combinations, and becomes liable to such 

 loss. How to prevent this loss is a question of the first 

 importance. AVe may not be able to do this altogether; but 

 the most effectual means appears to be to keep the soil full 

 of the hungry roots of a growing crop. Wherever the soil 

 is unoccupied, especially in the late summer or autumn, with 

 heavy rains there will be a loss of the soluble nitrogen of 

 the soil which has been accumulating during the warm, dry 

 summer weather, — a loss which could not occur were the 

 soil occupied with a growing crop, for the roots of such a 

 crop would seize upon the soluble nitrogen as fast as pro- 

 duced, it would become a part of the plant, an insoluble 

 part, not again to become soluble until the processes of decay 

 shall break down the new vegetable tissues. Green manur- 

 ing, then, enables the farmer to conserve soil nitrogen. It 

 enables him to adopt a simile, to put the soluble nitrates 

 which his effective handmaids warmth, air and bacteria have 

 been producing under lock and key, and to hold them there 

 during the period (autumn, winter and early spring) when 



