Determination of soil specific surface area by water vapor adsorption I Drying of soil samples

Drying of three mineral soil samples (clay content 4—58 %, organic carbon content I—s °7o) equilibrated at 75.5 % relative humidity was studied. The soils were dried in an oven at +5O°C, +7O°C and + IO5°C for 4 and 8 hours and in a desiccator over pure concentrated H 2 S04 and P2 Os . Drying over desiccants for 8 hours removed less water than drying at + 50°C. Dryingover desiccants for 3—7 days was as efficient as drying at +70°C, for 14—24 days as efficient as 4 hours of drying at + 105°C. Eight hours of drying at + 105°C seemed to be too drastic, because it caused a greater weight loss in the clay sample of 5 % organic carbon content than did prolonged desiccant-drying. Drying at + 70°C removed as much water from fine sand which contained 4 % clay as prolonged desiccant-drying. Index words: oven-drying, desiccant-drying, mineral soils


Introduction
High precision is needed in the determina- tion of soil water content at high pF, e.g. in the estimation of soil specific surface area by water vapor adsorption.In such situations, the method often used for drying soil sam- ples is equilibration in a desiccator over a desiccant like pure concentrated H 2 S0 4 or P,O s (e.g.Orchiston 1953, 1954, Pritchard, 1971, Iwata 1974).A highly hygroscopic desiccant removing the moisture in the air of a desiccator yields a relative humidity near zero.Drying is further enhanced by eva- cuation of the desiccator.However, a relatively long time may be needed for soils to reach constant weights.Equilibration may be accelerated by elevation of the drying temper- ature.Oven-drying is a common method for determination of water content.The aim of this study was to compare oven-drying and desiccant-drying in the determination of soil water content at high pF.

Material and methods
The material consisted of three plough layer samples (       ground to pass a 2-mm sieve.The particle-size distribution of inorganic material was determined by the pipette method (Elonen 1971) and organic carbon content by a modified (Graham 1948) Alten wet combustion method.
Soil, Ig, in a tared weighing bottle was placed in a desiccator over saturated NaCl so- lution at 75.5 °/o relative humidity corresponding to pF 5.6 (Bolt and Frissel 1960).After two weeks of equilibration at + 20°C the soil -(-weighing bottle was weighed.Parallel sam- ples were dried in an oven or over H 2 S0 4 or P 2 0 5 .In oven-drying, the temperatures were + 50°C, + 70°C and 4-105°C.The samples were dried for 4 hours, weighed, dried for another 4 hours and reweighed.In the desiccator, the samples were dried over pure con- centrated H 2 S0 4 or P 2O s and weighed after 8 hours and then occasionally over a period of 3-4 weeks.The drying procedures were carried out in duplicate.

Results and discussion
When the soils were oven-dried at + 50°C, prolongation of drying time from 4 to 8 hours increased the moisture percentage of clay soil No. 3 only (Table 2).Elevation of drying tem- perature to + 70°C increased the moisture percentage of all soils.Prolongation of drying time to 8 hours at + 70°C increased the mois- ture percentage of fine sand sample No. 1. Ele- vation of drying temperature to + 105°C no more increased the moisture percentage of this coarse soil, while the moisture percentage of the other soils increased.Prolongation of drying time to 8 hours at + 105°C increased the moisture percentage of clay soil No. 3 which had the highest organic carbon content.
Drying over H 2 S0 4 or P 2O s for 8 hours removed less water than drying at + 50°C (Table 2).Drying over P 2O s for 8 hours re- moved more water from soil No. 3 than drying over H 2 S0 4 for 8 hours.Drying in a desicca- tor for 3-7 days was as efficient as drying at +7O°C.Desiccant-drying for 14-24 days was as efficient as 4 hours of drying at + 105°C.Eight hours of drying at + 105°C increased the moisture percentage of soil No. 3 more than prolonged drying in a desiccator.
The most frequently used definition for a dry soil is the mass of a soil sample after it has come to constant weight in an oven at a temperature between + 100°C and + IlO°C (Gardner 1965, Baver et al. 1972).How- ever, there exists no unique temperature at which different soil minerals can be heated to drive off all adsorbed water and leave behind only the water of crystallization.Such drying temperatures range from + 100°C to over + 400°C.Organic materials may oxidize at temperatures as low as +5O°C (Gardner  1965, Baver et al. 1972).The non-colloidal particles have a small quantity of adsorbed water which is easily removed from their surfaces by heating (Gardner 1965).Soil No. 1, representing soils of low clay content, seemed to be dried at a temperature as low as + 70°C.Water present in the clay fraction may be considered in two categories, structural and adsorbed water, and it is often difficult to distinguish between the two (Gardner 1965).
In desiccant-drying, soil organic matter is not decomposed and lost from the sample.
Oven-drying with prolonged heating can re- move structural water or cause decomposition of organic matter.Four hours of drying at + 105°C seemed to be sufficient, moisture percentage did not significantly deviate from those obtained by prolonged desiccant-drying.Eight hours of drying at + 105°Cwas possibly too drastic, because it caused additional weight loss in sample No. 3 of about 5 % organic carbon content.It is possible that some decomposition of soil organic matter has occurred.