102 AGRICULTURE 



third of a ton of straw per acre. It cannot 

 be said, therefore, that the application of 

 manure is in all cases necessary to produce 

 crops, although in productiveness such crops 

 may be much below the average of those 

 grown upon land otherwise treated. 



Another answer that may be given to the 

 question " Why do we manure ? " is that 

 manure is used in order to supply crops with 

 plant food. As regards their requirements 

 in the matter of food, farm crops show 

 marked variations. A crop of cereals, grain 

 and straw, will, on the average, remove 

 from an acre of land about 50 Ib. of nitrogen, 

 and 20 Ib. of phosphoric acid ; while a crop 

 of turnips, bulbs and leaves, will similarly 

 remove about twice as much nitrogen and 

 fifty per cent, more phosphoric acid ; whereas 

 mangolds take from the land about fifty 

 per cent, more nitrogen, and an even larger 

 proportion of phosphoric acid than a crop 

 of turnips. As regards potash, the variations 

 are also very wide, turnips, for instance, 

 removing about 150 Ib. per acre, as con- 

 trasted with less than 30 Ib. in the case 

 of wheat. It is possible, by ascertaining 

 the weight of soil upon an acre of land, 

 together with a chemical analysis, to deter- 

 mine approximately the quantity of plant 



