Trans. Br. mycol. Soc. 49 (3), 367-368 (1966)Printed in Great Britain

SOIL WATER TERMINOLOGY IN MYCOLOGYAND PLANT PATHOLOGY

By D. M. GRIFFIN

School ofAgriculture, University ofSydney, N.S. W., Australia

In soil science, the water content of soil is usually expressed as weight ofwater per unit weight of dry soil, or better, as volume of water per unitvolume of soil in bulk. In mycology and plant pathology, however, suchabsolute values are rarely reported, the water content being given relativeto that under certain specified (or implied) conditions. A brief and by nomeans exhaustive survey of the literature reveals considerable confusion inthe terminology applied to this relative value for moisture content.

Probably the majority of British workers have followed a method similarto that of Keen & Raczkowski (192 I) to determine the amount of watertaken up by unit weight of soil at approximately zero matric suction and insome papers this is called a 'saturated' soil. The water content of apartially-drained soil is then expressed as 'x % saturation' or 'x %saturated' and such a usage seems logical and satisfactory. Others, how­ever, refer to the water content of a soil at zero suction as the 'moisture­holding capacity' and to that of a partially-drained soil as 'x %moisture­holding capacity'. It is unfortunate that this usage has gradually becomepredominant, because the moisture-holding capacity of a soil is notconstant but varies with matric suction applied. As the term' moisture­holding capacity' is now used by mycologists, the phrase 'at pF. 0' mustalways be implied.

Another objection to the use of' moisture-holding capacity' is the pos­sibility of confusion with terms involving the same words, especially' fieldcapacity'. 'Field capacity' is the amount of water held by a soil in the fieldwhen drainage under gravity has become negligible and there has beenlittle loss of water by evaporation or transpiration. For most soils, this stateoccurs at c. pF. 2'0-2'5. 'Normal moisture capacity' (Shaw, 1927), 'fieldwater capacity' (Widstoe & McLoughlin, 1912) and 'moisture equiva­lent' (Briggs & McLane, 1907) all approximate 'field capacity'. Thewords' moisture' and' capacity' therefore occur in a number of terms andthere is evidence of confusion as early as 1925 (McKinney & Davis; cf.pp. 828, 830) and it exists to this day. Thus, Griffiths & Jones (1963) statethat 'the moisture content was always maintained at "field capacity'"(Summary) and 'water was added to bring the soil to 100 % moisture­holding capacity' (Methods section). Both statements cannot be correct.To avoid confusion in the future, I suggest that the water content of soilshould be expressed as 'x % saturation (or saturated)' and not as 'x %moisture-holding capacity'.

Previously (Griffin, 1963) I emphasized the inadequacy of data on

Transactions British Mycological Societymoisture content, however expressed, in defining the moisture regime ofsoil and advocated the use of the moisture characteristic for this purpose.In discussing the moisture characteristic I used the term 'tensiometricsuction', but a Commission of the International Society of Soil Science(Bull. no. 23 (1963)) has recommended that 'matric suction' be used forthis component. Mycologists would be well advised to adopt this stan­dardized usage.

REFERENCES

BRIGGS, L.]. & McLANE,]. W. (1907). The moisture equivalent of soils. Bull. Div. SoilsU.S. Dep. Agric. No. 4S.

GRIFFIN, D. M. (1963). Soil moisture and the ecology of soil fungi. Biol. Rev.38, 141-166.GRIFFITHS, E. & JONES, D. (1963). Colonization of cellulose by soil micro-organisms.

Trans. Br. mycol. Soc. 46, 28S-294.KEEN, B. A. & RACZKOWSKI, H. (1921). The relation between the clay content and

certain physical properties of a soil. ]. agric. Sci. II, 441-449.McKINNEY, H. H. & DAVIS, R.]. (192S). Influence of soil temperature and moisture on

infection of young wheat plants by Ophiobolus graminis. ]. agric. Res. 31, 827-840.SHAW, C. F. (1927). The normal moisture capacity of soils. Soil Sci. 23, 303-317.WIDSTOE,]. W. & McLOUGHLIN, W. W. (1912). The movement of water in irrigated soils.

Bull. Utah agric. expo Stn no. lIS.

(Accepted for publication 4 August 1965)