Tiie Composition and Use of Fertilizers. 251 



Plant-Food Needs of Soils in Eelation to Previous Crops. 

 — The growing of the same kind of crop continuously on the 

 same land is exhausting in regard to one or more elements of 

 plant food. Thus, wheat grown year after year on the same soil 

 exhausts nitrogen and phosphoric acid more rapidly than potash; 

 hence, phosphoric acid and nitrogen without potash would, in 

 most such cases, give fully as good results as with potash, at least 

 for a time. The continuous growth of grass crops takes large 

 amounts of potash compared with phosphoric acid. Where there 

 has been such a system of farming employed, one element will 

 usually be found to have been removed more extensively than 

 others, and in feeding crops, this lacking element is the one that 

 especially needs to be supplied. 



It often happens that in a system of rotation all elements of 

 plant food are removed in about the same proportion, and hence 

 restoration of high productiveness means addition of nitrogen, 

 phosphoric acid and potash. 



Plant-Food Needs of Soils in Relation to Specific 

 Classes of Plants. — • The food needs of different classes of plants 

 vary in a marked degree and their general requirements should be 

 studied if we try to feed them intelligently. For the purpose of 

 calling attention to the main facts, we can divide most of our 

 common crops into the following classes: 



Root and tuber crops, grasses, leguminous crops, cereals, fruit 

 crops, market-garden crops. 



(a) Root and Tuber Crops include such crops as turnips, beets, 

 mangels, carrots, potatoes, sweet potatoes, etc. While these crops 

 need all elements of plant food, turnips appear to make especially 

 good use of available phosphoric acid in their early stages of 

 growth. Beets and carrots grow less quickly and need nitrogen 

 in readily available forms. Potatoes need liberal supplies of 

 available potash. These crops, owing to their habits of growth, 

 do not make quick use of the insoluble mineral plant food of the 

 soil, especially in their early stage of growth. 



(b) Grasses are mostly perennial, though their length of 

 life depends upon a variety of conditions, such as methods of 

 cropping, kind of soil, etc. They are mostly surface feeders in 

 their early stages of growth. They have the power of using 

 potash and phosphoric acid in its less soluble forms quite easily. 

 It is of advantage, however, to supply them with easily available 1 

 forms of nitrogen which should be fed them quite liberally, be 



