ENRICHING SOIL COMMERCIAL RESOURCES 97 



5. Amendments 



149. Substances which contain only traces 

 of the important or available plant- foods often 

 have a beneficial effect on soil. Lime and salt 

 are examples. Though they may not add to the 

 soil any needed plant -food, the plants are en- 

 abled by their presence to utilize more of the 

 plant -food already in the soil. Such materials 

 are known as amendments (58). 



150. It is often difficult to decide, in any 

 particular case, just how an amendment pro- 

 duces its effect. It may be that the mechanical 

 condition of the soil is improved, its water- 

 holding capacity increased, its acidity or sour- 

 ness neutralized, or its plant- food unlocked. 



151. Lime. Soils sometimes become sour, and 

 may then be unsuitable for some plants. One 

 of the reasons why plants do not thrive well in 

 sour soils is that it is difficult to obtain sufficient 

 nitrogen in the form of nitrates. The germs 

 which carry on the process of nitrification are 

 unable to do their work in sour soils. The soil 

 acid can be neutralized the soil sweetened by 

 applying lime (which is calcium oxide, CaO). 



152. Lime may be applied in the form of 

 water- slaked lime, such as is obtained by adding 

 water to quick- lime till it crumbles, or by air- 

 slaked lime. Quick-lime usually gives the better 



