192 < SOILS AND FERTILIZERS 



inch, it will probably be just as effective as if ground much 

 finer. 



251. Gypsum or land plaster. — In the early agriculture 

 of this country, before ordinary commercial fertilizers were 

 used, gypsum was a popular soil amendment. Its effective- 

 ness has apparently decreased as the soils on which it was 

 used have been longer under cultivation. It has generally 

 been credited with liberating potash, and possibly as the 

 soils have become more acid it has been less effective in this 

 respect. At any rate, it is rarely used at present. 



Gypsum has little effect on tilth and is not in any sense 

 a substitute for caustic lime for that purpose, nor is it of 

 any value to overcome soil acidity, as it contains a strong 

 acid. 



QUESTIONS 



1. How does the need of a soil for lime differ, in principle, from 

 its need for the other fertilizers we have studied ? 



2. Name the forms in which lime is applied to soils. 



3. Which of these are similar chemically and in their effect on 

 soils ? 



4. How is quick-lime made ? Water-slaked lime ? Air-slacked 

 lime? 



5. How does the solubility of lime compare with that of potash, 

 when both are absorbed by soil ? 



6. What is shown by a chemical determination of the lime 

 requirement of a soil ? 



7. What is the effect of lime on some of the bacteriological pro- 

 cesses in soil ? 



8. How does lime affect the availability of certain other plant 

 nutrients in soil ? 



9. What is its effect on certain plant diseases ? 



10. Discuss the use of magnesian limes. 



11. Discuss the use of caustic lime as compared with ground 

 limestone. 



12. How does the fineness of grinding limestone affect its imme- 

 diate usefulness ? 



13. How does gypsum affect soil ? 



