EVALUATION OF WOOD PRESERVATIVES 137 



cipal and most important factor in the soil-block culture is the moisture 

 holding capacity and content of the soil, rather than its nutrient func- 

 tion. If continued experimentation supports this conclusion it would not 

 be necessary to limit the type of soils used except within rather broad 

 limits. It also appears that the size and thickness of the feeder block 

 now employed introduces enough wood into the culture bottle to mask 

 any minor variations in the soil itself. The all important thing is to have 

 enough water in the soil throughout the test period to keep the air above 

 the soil essentially at 100 per cent humidity and the blocks at about 

 fiber saturation — say about 27 per cent, oven-dry weight basis. 



The soil in use at Bell Laboratories at present is obtained from a plot 

 that has been set aside at the Chester (N. J.) Test Station. This plot 

 has been fallow for twenty-five years. It supports a general grassy flora. 

 The soil is a sandy loam with the following general description: 



pH 4.9-5.0 



Available magnesium 37.5 lb/acre 



Available phosphorus 4.5 lb/acre 



Available potassium 70.0 lb/acre 



Organic matter 3.0 per cent 



The cultures at the Forest Products Laboratory^^ have been made with 

 a silt loam having a pH between 5.5 and 6.0. Bell Telephone Labora- 

 tories' tests have indicated the desirabihty of avoiding soils of either 

 very sandy or very heavy clay types. The soil from the Chester Test 

 Station described above is being used in all cultures, and there is a 

 sufficient layer of top soil on the reserved plot to make parallel cultures 

 for a good many years. Until such time as more definite and positive 

 information on the effect of minor variations in the soil type are deter- 

 mined it is generally agreed that all of the comparative tests in any 

 given series at least should be run on the same soil. Experimental work 

 is now under way to determine the possible advantage of the addition 

 of Krilium* to the soil in the culture bottles to maintain porosity and 

 an even, high moisture holding capacity. 



After the test blocks are placed in the culture bottles and during the 

 course of the ninety-day incubation period the screw caps are left loose. 

 The general technique followed in making up the soil cultures, as far as 

 moisture is concerned, parallels that used at Madison. The moisture 

 content is close to that recommended by Flerov and Popov,^^ namely 

 40-50 per cent of the weight of the soil plus the feeder block, with dis- 

 tilled water added during the test period, if necessary to maintain good 



♦ An acrylonitrile product of the Monsanto Chemical Company. 



