348 THE PRINCIPLES OF SOIL MANAGEMENT 



For crops to which large quantities of fertilizers are to 

 be added, it is desirable to drop only a portion of the 

 fertilizer with the seed, the remainder to be broadcasted 

 by machinery and harrowed in earlier, and, as is fre- 

 quently better for crops requiring very liberal fertiliza- 

 tion, a later application may be made. 



216. Soil amendments. Certain substances are some- 

 times added to soils for the purpose of increasing pro- 

 ductiveness through their influence on the physical 

 structure of the soil, and thereby upon the chemical 

 and bacteriological properties. These substances are 

 called soil amendments. It is true that they may add 

 essential plant ingredients to the soil, but that function 

 is of minor importance. 



217. Salts of calcium. Calcium, although essential 

 to plant growth, need seldom be added to the soil to 

 supply the plant directly; but, on account of its effect 

 upon the soil properties, its use is beneficial to a great 

 number of soils. 



218. Effect on tilth and bacterial action. On clay 

 soils, the effect of lime is to bring the fine particles into 

 aggregates which are loosely cemented by the calcium 

 carbonate. The effect of this structure upon tilth has 

 already been explained. (Seepage 117.) On sandy soils, 

 the carbonate of calcium serves to bind some of the par- 

 ticles together, making the structure somewhat firmer, 

 and increases the water-holding power. It should be 

 used only in small amounts on sandy soils. 



There is a tendency for most cultivated soils to be- 

 come acid, owing to the formation of organic acids in 

 decomposition and to the greater removal of mineral 



