CHAPTER XXIV 

 SOIL AMENDMENTS 



Certain substances are sometimes added to soils for 

 the purpose of increasing productiveness through their 

 influence on the physical structure of the soil, and thereby 

 on 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. 



453. Salts of calcium. — Calcium, although essential 

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

 supply the plant directly ; but because of its effect on the 

 soil properties, its use is beneficial to a great number of 

 soils. 



454. 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 calcium carbon- 

 ate. The effect of this structure on tilth has already been 

 explained (par. 120). On sandy soils the carbonate of 

 calcium serves to bind some of the particles together, 

 making the structure somewhat firmer and increasing the 

 water-holding power. It should be used only in small 

 quantities on sandy soils. 



There is a tendency for most cultivated soils to become 



acid, as has already been explained (par. 283). Acidity 



may reach a point where it becomes directly injurious to 



certain plants, but it becomes indirectly injurious before 



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