THE STERILISATION OF SOIL 



DURING recent years a good deal of attention has been given 

 to the sterilisation of soil, particularly that used for borders 

 in greenhouses, where the removal of the old soil and replac- 

 ing it with new is necessarily a lengthy and expensive 

 operation. 



Partial sterilisation of the soil is an efficient means of 

 dealing with several pests that live in it, eelworms in par- 

 ticular, and it has the further curious effect of first delay- 

 ing growth for a very short time, then hastening it, so that 

 plants put into such soil quickly overtake those put into 

 similar but untreated soil. The cause of this appears to be 

 as follows : Most of the bacteria are killed, but not all. The 

 minute animals that prey upon the bacteria are also killed, as 

 well as eelworms and so on. The bacteria that remain are able 

 to grow at a rate far exceeding that of those in soil where their 

 animal enemies abound, and in growing they produce from 

 the decaying matter which forms their food a correspondingly 

 greater amount of ammonia. Plants, therefore, in this partially 

 sterilised soil have greater stores of ammonia at their disposal 

 than those in untreated soil, and are consequently-able to grow 

 more rapidly. Other advantages are the prevention of the 

 growth of moss on seed-pans and the destruction of weed 

 seeds. By far the most efficient method of effecting partial 

 sterilisation is by heat. Other methods not so efficient, but 

 able to be applied under certain circumstances, are flooding, 

 drying out, and the use of chemicals. 



Heating by Steam. The most effective way of heating is 

 by the application of steam. Dry heat apparently occasionally 

 causes undesirable changes, which check growth to a very 

 great extent ; but probably this is most marked when a high 

 temperature is reached. As a rule, if a soil mass can be 

 heated to 180 Fahr., that is sufficient to ensure most of the 

 benefits of partial sterilisation. It is obvious that heat such 

 as is required cannot be applied on a large scale outdoors, 

 but on a small scale it may be easily managed. Taking steam 

 first, we may have a box or tank made with pipes perforated 



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