108 SOILS AND FERTILIZERS 



than does a crop of thirty bushels of wheat, including the 

 straw. 



The ability of any kind of plant to secure nutriment from 

 the soil depends on a number of factors which need not be 

 discussed here. According to their ability in this direction, 

 plants have been popularly classified as " weak feeders " 

 and " strong feeders." To the former belong such crops 

 as wheat and onions, which require very careful soil prep- 

 aration and manuring. In the latter class are maize, 

 oats and cabbage which demand relatively less care. In 

 the manuring and rotating of crops, this difference in ability 

 to obtain nutriment must be considered, in order not only 

 to secure the maximum effect on the crop manured, but 

 also to get the greatest residual effect of the manure on suc- 

 ceeding crops. 



130. Substances needed by plants and substances merely 

 absorbed. — Some substances found in soils and absorbed 

 by plants are used for the formation of plant tissue, and 

 hence are indispensable. Other soil constituents, although 

 absorbed by plants to sufficient extent to be found in their 

 ash, are not essential to a normal growth of crops. The 

 substances that are essential are generally present in plants 

 in considerable quantities, because they constitute a part 

 of the plant tissue. 



131. Quantities of plant-food materials removed by crops. 

 — When crops are removed from the land, they carry in 

 their tissues considerable quantities of plant-food materials. 

 The drain on the total supply may be serious if the soil is 

 not well supplied with these substances. The larger the 

 yield of crops the greater the quantities of plant nutrients 

 they are likely to contain. The following table shows the 

 quantities of nitrogen, potash, phosphoric acid and lime 

 removed from an acre of land by some of the common crops. 

 The entire harvested crop is included : 



