78 NITROGENOUS MANURES [chap. 



preliminaries to complete development. A deficiency of 

 nitrogen results in a stunted general growth, in which 

 the grain or seed bears a high proportion to the whole 

 weight of the crop; the plant on analysis, however, 

 shows no marked lack of nitrogen as compared with the 

 other constituents. These other bodies, phosphoric 

 acid, potash, etc., in whatever excess they may be 

 present in the soil, are only taken up by the plant as it 

 can use them — i.e., in quantities proportionate to the 

 growth, which in its turn is proportionate to the 

 nitrogen supply. As the amount of available nitrogen 

 is increased, the development of leaf and shoot increases, 

 their green colour deepens, and maturity becomes 

 more and more deferred, so that a crop grown on 

 land over-rich in nitrogen always tends to be late 

 and badly ripened, and to show a profusion of leaf — 

 characters which, in the case of a grain crop, often 

 result in lodging before harvest. 



But the fact that the primary growth of the plant is 

 up to certain limits almost proportional to the supply 

 of nitrogen, so that an application of nitrogenous 

 manure has a quickly visible effect, not only makes it 

 the leading constituent of a fertiliser, but is apt to give 

 it a fictitious importance in the farmer's eyes. 



On most of our cultivated soils, when the cropping is 

 continued and manure withheld to a point when there 

 begins to be a serious falling off in the yield through 

 lack of plant food, it is the want of available nitrogen 

 rather than of phosphoric acid and potash which 

 determines the yield ; in other words, the soil is much 

 more rapidly exhausted of its available nitrogen than 

 of its available phosphoric acid and especially of its 

 available potash. Thus, while each of these three con- 

 stituents of plant food is equally indispensable to the 

 plant, good crops can often be grown by the aid of a 



