82 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



However, no bad effects Avhatever have been noted from drying 

 fairly sandy soils at the standard temperature. 



After obtaining the air-dry weight of the pan and soil, an amount 

 is removed from the pan sufficient to form a layer one-fourth of 

 an inch deep. The coarser material is excluded from this lot, which 

 is to form a covering for the seeds. With this taken out, the re- 

 maining soil is leveled down with a spoon, the seeds are sown on 

 this smooth surface, and the covering soil is replaced. 



The number of seeds to be sown should be gauged according to 

 known viability, so as to produce about 100 seedlings in a pan of this 

 size. The weight of the seeds is obtained before sowing, and this 

 weight is considered throughout as an addition to the tare. The 

 further assumption is made that the weight of the seedlings will not 

 at any time appreciably exceed the weight of the seeds. 



Having calculated the net dry weight of the soil from the mois- 

 ture content of the dried sample, the moisture content of the soil at 

 any stage in development or wilting of the plants is calculated, after 

 a weighing of the pan, by the equation : 



_.- . , , , „~_ W — (Pan, seed, soil, and paraffin) 

 Moisture percentage equals 100 X — — g -i -• 



The pans are placed in a greenhouse where they may have the 

 necessary light and warmth to induce prompt germination, and for 

 the sake of uniform development and conditions affecting wilting, 

 are preferably kept on a revolving table. 



The soils are watered exclusively with distilled water, both to 

 avoid the introduction of spores and the addition of salts, which, in 

 the absence of drainage, might appreciably increase the wilting co- 

 efficient. Nothing suggestive of a toxic effect from this distilled 

 water has been noted. It is desirable to aerate the water as much as 

 possible before applying. Under ordinary atmospheric conditions, 

 the pans will require 50 to 60 cubic centimeters per day to maintain 

 moisture favorable for germination. 



In working with deeper mineral soils, damping off of seedlings is 

 rarely noted, but surface soils from the forest often contain the 

 damping-off fungi. In fact, this is so common that many observa- 

 tions which ascribed the death of seedlings in the forest to unfavor- 

 able physical conditions may be questioned. Certain it is that 

 damping off in the wilting pans may cause the greatest confusion, 

 if they do not actually vitiate the tests. Soils suspected of contain- 

 ing these organisms should therefore be treated, several days before 

 the seed is sown, with a solution of formaldehyde, as suggested by 

 Hartley (124) for nursery beds. This should be used at the rate of 

 about one-eighth of a fluid ounce per pan, dissolved in sufficient clean 

 water to reach all soil in the pan. Opportunity should afterwards 



