KMCU'HlNti SOIL — CO.MMKKCIAL KhSOLKCKS i<7 



5. Amendments 



141*. Substiiuces which contain only traces 

 of the important or available plant- foods often 

 have a beneficial elTect 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 

 l)lant-food already in the soil. Such materials 

 are known as amendments (aS). 



l.'>(). It is often dif!icult to decide, in any 

 partit'ular case, just how an amendment pio- 

 duces its effect. It may be that the meehanieal 

 condition of the soil is improved, its water- 

 h(>ldin«j; cai)acity increase<l, its acidity or sour- 

 ness neutralized, or its plant-food unlocked. 



l.')l. Lime. Soils sometim«\s ])ecome sour, and 

 may then be unsuitable for some plants. One 

 of the i-eason< why plants <io not thi'ive well in 

 sour soils is that it is ditlicult to obtain sutlicient 

 nitmi^en in the form of nitrates. The germs 

 which carry on the i)rocess of nitrification are 

 unable to do their work in sour soils. The soil 

 ncid can l)e neutralized — the soil sweetened — by 

 appiyini: lime (whicli is calcium o.xide, CaO). 



\')'2. Lime may be a])plied in tiie form of 

 water-slaked lime, such as is obtiiiuod by adding 

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

 slaked lime. Quick-lime usually gives the better 



