1911} BRIGGS & SHANTZ—SOIL MOISTURE 219 
essential in any critical study of the relation of plant growth to 
soil moisture to be able to determine for any soil the maximum 
amount of non-available moisture. 
In determining the non-available moisture, the permanent 
wilting of the plant has been taken as the criterion of non-availa- 
bility. In making such determinations the following precautions 
are necessary: 
1. The soil used should be as uniform as possible. 
2. The soil should be brought to a uniform water content 
before being used. 
_ 3. All loss of water should be prevented except that due to the 
transpiration of the plant. 
ae sudden PRES in soil temperature should be avoided. 
5. It should be definitely ascertained that the plant cannot re- 
cover turgidity without additional moisture being supplied. 
The method which we have employed consists in growing the 
plants in a small glass pot, evaporation from the soil surface a 
Prevented by means of a seal of wax which is melted and sab: 
over the soil surface. In the case of monocotyledons, this wax - 
can be applied immediately after planting the seeds, and the wpe 
lings will grow readily through the wax, forming a perfect . 
around the stems. In the case of dicotyledons, the wax, sare ee 
usually a mixture of paraffin and vaseline having a low me = 
point and low heat conductivity, can be melted and flowed ees 
the stems of the seedlings without injury. During growth, ge 
pots are kept immersed in a water bath to avoid condensation 
the soil moisture on the pot walls. oo 
The probable error the mean of the determinations ey 2 
Pots or more does not usually exceed 0.1 per cent of actua = 
moisture, which is fully comparable to the accuracy with w 
the soil itself can be defined through its physical properties. ; 
The wax seal method is also particularly adapted to gered 
of transpiration, since all loss of water is avoided except that taking 
place through the plant. 
Bureau or PLant INDUSTRY 
Wasuincron, D.C. 
