300 ANNUAL REPORT OF THE Off. Doc. 



breathing. However, at present, our knowledge of this subject is 

 very limited. A proi)erty which depends largely on those we have 

 just been considering is the capacity soils have for absorbing and 

 retaining heat. The temperature of the soil will, of course, largely 

 depend upon the temperature of the air, but this depends on the 

 soil itself. The dark or heat rays strike the soil, a portion is absorbed 

 — which will vary according to the nature of the soil — and the re- 

 mainder is radiated into the air. The temperature of the soil is also 

 due to other sources. When vegetable matter decays in the soil 

 there is always more or less heat generated. Soils which contain 

 a larger amount of vegetable matter are, therefore, w\armer than 

 purely mineral ones. 



The color of a soil has also a striking influence on its temperature. 

 Dark colored soils have a much greater heat absorbing capacity than 

 Those of light color. All are familiar with the fact that a black 

 garment is much w^armer in bright sunshine than a white one. 

 This is because the black surface absorbs the ether waves as they 

 come from the sun to a much greater extent than the white. 



The power soils have of absorbing gases is an important one in 

 connection with soil fertility. The air is largely composed of oxygen 

 and nitrogen. These gases are both absorbed by soils in varying pro- 

 portions. It is well known that a plentiful supply of oxygen in the 

 soil is a necessary condition of fertility, as it has long ago been fully 

 proved that plants absorb oxygen through their roots. Seeds in 

 process of germination require free access to a generous supply 

 of oxygen. This fact emphasizes the enormous importance of pro- 

 viding a good seed bed and not covering the seed too deeply. The 

 soil also receives large amounts of carbonic acid from the air. This 

 is largely w-ashed down in solution w-ith the rain. The various forms 

 of nitrogen as ammonia, nitric and nitrous acid are also absorbed 

 from the air to a considerable extent. Soils containing large 

 amounts of organic matter absorb these gases to a much greater ex- 

 tent than imrely mineral ones. This power soils have of absorbing 

 gases depends largely upon the chemical as well as the physical 

 properties. We shall have occasion to again refer to the absorption 

 of nitrogen when we come to considier the biological properties of 

 soils. 



To recapitulate, the chief physical properties of a soil are, its ab- 

 sorptive and retentive power for w^ater; its capacity for heat and 

 its power of absorbing gases. It will be readily seen how all these 

 physical properties may be influenced by tillage. It would be inter- 

 esting to refer to the impoi-tant influence these properties have on 

 plant growth but time will not permit. As Samantlia would say, 

 *Sve must resume and continue on.'' 



