666 



although not in such large quantities as oxygen ; yet it does absorb 

 it, more especially when it is sown with leguminous plants. The 

 method by which the plant absorbs the nitrogen from the soil was 

 firstly clearly denned by Schloesing and Muntz, who showed that 

 it was through the agency of micro-organisms. Carbonic acid and 

 ammonia are partly obtained from the atmosphere being carried 

 into the soil by rain. Soils absorb ammonia direct from the 

 atmosphere, but damp soil? have a greater power of absorbing 

 ammonia than dry soils. Soils greatly differ in their powers of absorb- 

 ing gases. Soils that are rich in organic matter absorb gases to a 

 much greater extent than those poor in organic matter. 



It will now be seen that the improvement of a soil in any one of 

 its physical aspects has also the power of improving it in that of 

 others. The addition of sand and lime to a clayey soil, in conjunc- 

 tion with drainage, improves it ; while the addition of lime and 

 clay, along with drainage, to a peaty soil improves it and brings it 

 within cultivation. 



THE CHEMICAL COMPOSITION OF SOILS. 



We have stated that soils are composed principally of sand r 

 clay, lime and vegetable, or organic, matter. They contain, besides 

 these principal constituents, several others in smaller proportions, 

 such as oxides of iron, alumina, potash, soda, magnesia, phosphoric 

 acid, chlorine and sulphuric acid. 



The first group of constituents, with the exception of vegetable 

 matter, acts mostly as a support to the plant, enabling it to maintain 

 itself in an upright position. The second group, with vegetable 

 matter and lime, supplies food to the growing plants. The combined 

 action of both groups of constituents is that of holder, or fixer, 

 of the plant food, as well as the medium whereby the chemical 

 changes take place that are necessary for the proper preparation of 

 the food of the plants. The principal elements of the soil that 

 enter into the plant food are nitrogen, potash and phosphoric acid, 

 and the-u- are most likely to be deficient in the soil, or in a state in 

 wh eh they are inert, or not immediately available to the plant. 

 Lime, magnesia and other elements of plant food, with the exception 

 of nitrogen, potash and phosphoric acid, are generally to be found 

 in soils in sufficient quantities for the requirements of a crop. 



The sufficiency or deficiency of the three principal constituents 

 of plant food, in a state lit for the plant to absorb them within the 

 time required for its growth, will generally measure the fertility or 

 barrenness of a soil, so far as its chemical constituents are concerned. 

 The climate, as well as the physical state of a soil, has a great deal 

 to do with the fertility of any soil. 



The following analyses of some soils of Scotland, Kngland, and 

 rn Australia will give some idea <>! the chemical Constitution 

 of soils. The late Professor T. Anderson gave the following result 

 of analyses : 





