386 YETEEINAEY HYGIENE 



to give porosity and adhesiveness, but to fully bring this 

 about the clay must be in a flocculated condition, or it 

 would impair the porosity of the soil. This flocculated 

 condition is brought about by the mixture of lime with clay 

 which produces a marl ; such clay is no longer impervious 

 to water, while by mixing with sand it produces a granular 

 state of soil. 



A marl is a clay containing from 5 to 20 per cent, of lime 

 carbonate ; if the lime is in excess of this the soil is termed 

 calcareous, while sand containing lime is called a calcarene. 



A soil to be fertile requires something more than these. 

 The mould such as is seen in a garden is a loam, but 

 its dark colour is due to a substance known as humus. 



Himms is the bacterial outcome of the decay of animal and 

 vegetable matter in the soil, it is brown or even black in 

 colour, and chemically is very complex, consisting, among 

 other things, of certain acids of which humic and ulmic 

 are best known. 



An excess of humus in a soil is a serious matter, such 

 soils are practically infertile, for example, peaty and boggy 

 soils, in which the humus exists in considerable quantity. 

 This is due, among other causes, to the liberation of the 

 above acids under the influence of the water in the soil. 

 The soil becomes too acid for anything but a poor form of 

 shallow-rooted vegetation, such as sedges, rushes, mat 

 grass, Yorkshire fog, etc., but in moderate amounts, 2 to 9 

 per cent., the humus in a soil carries material easily capable 

 of being converted into plant food, while if the amount of 

 natural humus in the land gives an indication of failing, 

 the farmer replaces it in the form of manure. 



Soils consist of both an organic and inorganic portion, 

 and from both of these the plant derives its food supply. 

 In all cases the plant food must be in solution, hence water 

 in the soil plays an important part as also do certain micro- 

 organisms, the function of which is to render the nitrogen 

 of the soil soluble. 



The considerable differences which various soils present 

 depend entirely on the relative proportions in which the sand. 



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