THE PROPERTIES OF SOILS 205 



converting the one compound of nitrogen into the other by 

 purely chemical means would be a matter of some difficulty. 

 This brings to the front another aspect of the soil, one difficult 

 to verify by simple personal observations, but one that requires 

 to be appreciated before a proper understanding of some of the 

 simplest problems of soil management can be reached. The 

 soil is not merely a frame-work of sand and clay, in which the 

 plants anchor themselves by their roots, and from which they 

 draw water and a certain amount of nutriment circulating in that 

 water ; it is also a swarming laboratory of minute living agents 

 moulds, bacteria, and kindred organisms some of which are 

 always preparing the food for the higher plants, while others 

 are wasting it or are injurious in other ways. A few simple 

 experiments can be made to illustrate the living nature of 

 the soil ; all that is necessary is the preparation of a litre of 

 a nutrient solution containing 0*1 gram of magnesium sulphate, 

 0*2 gram of potassium phosphate, and 0*1 gram of sodium 

 chloride. 



Thoroughly clean six small flasks, and add 100 centimetres 

 of the nutrient solution to each ; to three of them also add one 

 gram of sugar; to the other three add 0*1 gram of ammonium 

 sulphate ; then to one flask in each set add about half a gram of 

 calcium carbonate. Plug the mouths of the flasks fairly tightly 

 with cotton wool, put them all in a vegetable steamer, and heat 

 them up to the temperature of boiling water for half an hour 

 or so. The heating sterilises the contents, and as the cotton 

 wool plugs exclude all further entry of organisms no change will 

 take place inside the flask, though the air has free access through 

 the cotton wool. When the flasks are quite cold get ready a 

 small quantity of fresh soil, say from a garden, and, lifting out 

 the plug of cotton wool for a moment, drop in with a spatula a 

 piece of soil about as big as a hazel nut and replace the plug. 

 Now take two of the flasks without calcium carbonate, one 

 with and the other without sugar, and heat it up to boiling- 

 point for two or three minutes, repeating this operation 

 on the following day. The double boiling will effectually 

 sterilise the contents of the two flasks, which will now serve 

 as checks containing dead instead of the living soil in 



