THE PEINCIPLES OF MANURING 



193 



consideration. Still it is true, in general, that those 

 which contain the largest quantities of fertilising in- 

 gredients require most manure, and that any crop which 

 contains an exceptionally large proportion of any par- 

 ticular constituent usually gives a larger return for 

 those kinds of manure which supply that constituent. 



Division of Crops into Groups. The average com- 

 position of the ordinary crops has been given in Ibs. per 

 acre (p. 169). It will be seen on reference to the table, 

 that crops which belong to the same class resemble each 

 other more or less closely in this respect and may be 

 arranged in groups accordingly. It is shown, perhaps 

 more clearly in the diagram (Fig. 28), that the grain crops 

 and grass all contain similar quantities of nitrogen, phos- 

 phoric acid and potash ; clover, peas and beans each con , 

 tain about the same quantities of these ingredients ; so 

 also do turnips and swedes ; potatoes and mangolds cannot 

 be referred to any of these groups nor can they be classed 

 together. 



In round numbers the quantities of nitrogen, phos- 

 phoric acid and potash in the crops of the several groups 

 are as follows : 



PER ACRE. 



It will be seen that all the crops contain considerably 

 larger quantities of nitrogen and potash than of phos- 

 phoric acid, but it does not follow that they stand less 



S.M. o 



