The Composition and Use of Fertilizers. 279 



(e) Rotation of Crops to Maintain Humus. — The humus of the 

 soil is gradually used up, when hoed and grain crops are con- 

 tinuously grown and this loss of humus may seriously change the 

 physical and plant-food conditions of the soil. The supply of 

 humus may be kept up by use of stable manure and green crops, 

 and this fact should be kept in mind in every system of rotation. 



(/) Use of Farm and Commercial Manures in Rotation. — It is 

 safe to say that economical results will attend the application of 

 farm manures to the land once at least in a rotation of four or five 

 years. Most of the nitrogen needed in addition can be supplied 

 by clover. The balance of feeding will come mostly in the line 

 of phosphoric acid and potash. When commercial fertilizers are 

 used, they will be most beneficial applied to those crops known 

 as weak-feeding. 



*»• 



12. The Use of Lime on Soils. 



Carbonate of lime, slaked lime and quicklime perform four 

 definite functions when applied to soils: (1st) They furnish lime 

 as plant food when it is lacking; (2d) they render available the 

 insoluble forms of nitrogen and potash in the soil; (3d) they 

 lighten heavy soils and compact light soils; and (4th) they neutral- 

 ize the acidity of sour lands. One disadvantage in using quicklime 

 or slaked lime on soils to be used in growing potatoes is that- 

 scabby potatoes are likely to be produced. ' 



How to Ascertain a Soil's Need of Lime. — Put in a cup a 

 tablespoonful or more of soil and moisten with water enough 

 to make a thick paste. After letting it stand 10 or 15 minutes, 

 insert into the soil a piece of blue litmus paper, of any convenient 

 size, say, two inches long and half an inch wide, inserting about 

 half of it into the soil. Litmus paper can be purchased at any 

 drug store. Allow the litmus paper to remain in the soil two 

 minutes or longer. Then take the litmus paper from the soil 

 carefully without tearing, rinse off the soil with water, and ob- 

 serve whether it is still blue or has become clearly red. If the 

 paper has become clearly red, the soil is acid and the. application 

 of lime will be likely to benefit many crops. Be careful not to 

 handle with the fingers the end of the litmus paper that goes 

 under the soil, previous to making the test. 



This test may be supplemented by an actual field trial, using 

 beets for a crop, carrying the work out as directed on page 255. 



