CONTROL OP ROOT-KNOT. 45 



inches is close enough for all general purposes, and even 2 feet is not 

 too far in deep beds if the sterilization is kept up long enough. The 

 bed should be covered with straw, boards, sacking, or something of 

 the kind to permit the upper layer of soil to become heated through. 

 The pipes or tiles in the soil should be arranged lengthwise in the 

 beds, with the steam inlet in a crosspiece of piping running across the 

 bed, from which the longitudinal rows take their origin. A similar 

 crosspiece at the other end may be used, but is not absolutely neces- 

 sary. There should be no open ends of pipes or tiles; otherwise all 

 the steam will escape out of these and not through the cracks or small 

 holes. Depending upon the pressure of steam used, the time neces- 

 sary for sterilization will vary from half an hour to even two hours 

 when the pressure is poor. 



A method often recommended to determine whether the steam has 

 passed long enough, and one that has considerable merit, is to bury 

 raw potatoes at the surface of the soil underneath the covering of 

 straw, boards, or sacking. When all these potatoes are found to be 

 cooked the steam can safely be turned off. Stone and Smith recom- 

 mend the use of a special boiler so that steam at fairly high pressure 

 can be used, not under 40 pounds per square inch, preferably more. 

 Even 80 to 100 pounds pressure is not too high if obtainable, as it 

 shortens the time necessary and also prevents the soil from becoming 

 as wet as with lower pressure. 



Not only are all nematodes killed by this treatment, but also all 

 insects and other noxious animals, as well as all fungi and their spores. 

 Many bacteria are killed, too, but not all of their spores, the survival 

 of the latter being desirable in view of what we know of the value 

 of soil bacteria. 



This method has some disadvantages. Thus, it can not be used 

 for beds occupied by living plants. Furthermore, care must be 

 taken on the one hand not to leave the soil soggy and on the other 

 not to dry it out too much. T,he latter is, however, a much less seri- 

 ous matter than the former. 



FRESH SOIL. 



For greenhouses, cold frames, seed beds, etc., where a steam-heating 

 plant is lacking and where it would not pay to incur the expense 

 of installing a boiler for the purpose of using it for soil sterilization, 

 the desired results can be obtained by the use of fresh soil each year. 

 This should be taken from some place in the woods or from a field 

 where the nematode is known not to occur. The old soil should be 

 placed where it can do no harm in the way of spreading the disease. 

 If it can be allowed to become perfectly dry for some weeks before 

 taking it out, the danger from the old soil is greatly reduced. The 



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