THE CHEMISTRY OF THK 1 -'A KM. 41 1 



be in a poor condition ; while a small dressing of readily available food, 

 as superphosphate or nitrate or sodium, may greatly increase the fertility. 



The nitrogen contained in humus is not in a condition to serve as a 

 general plant food ; cereal crops are apparently unable to appropriate it ; 

 leguminous crops, however, possibly assimilate some humic matters. By 

 the action of a minute Bacterium present in all soils, humus and ammo- 

 nia are oxidised, and their nitrogen converted into nitric acid. Nitrifica- 

 tion only takes place in moist soil, sufficiently porous to admit air. It is 

 also necessary that some base should be present with which the nitric 

 acid may combine : this condition is usually fulfilled by the presence of 

 carbonate of calcium. Nitrification is most active at summer tempera- 

 tures ; it ceases apparently near the freezing point. 



The fragments of rock present in soil, as stones, gravel, and sand, are 



I rule of little value to a plant, the elements of plant food which they 

 contain being in too insoluble a condition to be attacked by the roots. 

 These fragments of rock may however be slowly decomposed by the 

 mechanical action of frost, and by the chemical action of water, and their 

 contents thus gradually made available to the plant. The solvent power 

 of the water in a soil is greatly increased by the carbonic acid, and per- 

 haps also by the humic acid it holds in solution. Water containing car- 

 bonate of calcium in solution is especially capable of attacking silicates. 



If water is allowed to drain through a soil it carries with it a part of 

 readily soluble matter which a soil contains. The substances chiefly 

 removed by the water will be the nitrates, chlorides, and sulphates of cal- 

 cium and sodium. When heavy rain falls these substances are washed 

 into the subsoil, and partly escape by the nearest outfall into tin- sprii 

 l.r.ioks, and rivers. The loss of nitrates from highly manuivd land during 

 a w.-t season is very considerable. When dry weather sets in evaporation 

 tak- irface. (.f the snil, the water of the subsoil is slowly 



brought airain to tin- surface by capillary attraction, and the salts it con- 

 tain- an- concentrated once more in the upper soil, forming in some i 



>f salt upon the surface. Capillai ion has 



15ttl- influence in the case of sandy soils. 



( )f these r.-adily .soluble salts the nit rates are of the --real- tance 



as plant food. The ijuantity f nitrates in a surface soil will vary u'te-'it ly, 



n the richness of the soil in nitrogen, the previous conditions 



nperature and moist mv, t he e\t,-nt of recent washing by rain, and 



ber tli.- soil i. or is not under crop. Where a crop is ^rowin^ the 



will be i :rer the surface, the evaporation of water from a 



Top b.'in- far greater than from a bare soil. The nitrates will 



