HORTICULTURAL SOILS. 



75 



One essential condition of the nitrification of organic matter 

 is that it only takes place in a moist soil, sufficiently porous to 

 admit air ; hence the beneficial effect of mixing a certain pro- 

 portion of sand, charcoal, or peat to soil composts for potting 

 purposes, and the value of a sufficiency of crocks for drainage. 

 It is also necessary that some chemical base should be present in 

 the soil, with which the nitrates as they are formed can combine ; 

 this condition is usually fulfilled by the presence of carbonate of 

 lime (chalk), nitrate of lime being produced. In leaf moulds and 

 peat moulds rich m humus the nitrification is sometimes 

 rendered difficult by the lack of such a constituent, the alkali 

 bases rendered soluble by nitrification get rapidly used up, and 

 the soils in consequence become overcharged with acidity, to the 

 injury of the growing plants. It is necessary in such cases to 

 apply an antidote, which may be lime, chalk, or wood ashes ; 

 these substances tend to accelerate the nitrification in an extra- 

 ordinary manner.] 



Temperature is another prime factor in determining the rate 

 of oxidation and nitrification in organic materials and soils; the 

 activity of all living agents, whether animal or vegetable, being 

 dependent on the occurrence of a favourable degree of heat, and 

 being confined to certain specific ranges of temperature. 

 Oxidation is consequently found to be far more rapid in summer 

 than in winter, and much more energetic in hot climates than 

 in cold ; accordingly we find it more active in a conservatory 

 than in the open garden. 



The nitrifying organisms in soils may be killed by severe 

 drought. This may probably explain the fact of some plants 

 suffering so terribly from insufficient watering at certain stages 

 of their growth. For instance, the chrysanthemum never 

 thoroughly recovers the ill-effects of excessive drought. 



Eecent investigations have shown that the microderms are in 

 greater or less numbers in all fertile soils, but are most active in 

 soils under cultivation, teaching us the advantage of the frequent 

 use of the hand-hoe and other implements of tillage in the open 

 garden, and of a friable porous soil for potting purposes. The 

 soil should have good capillary action, so that at all seasons it 

 will as near as possible contain that amount of moisture which 

 is present when ground digs well, because this is found to be the 

 degree of moisture most desirable. Soils should also contain 



