The international system of units and its particular application to soil chemistry1 S. J. Thien and J. D. Oster2 ABSTRACT sentiality of adopting SI in its journals. Using SI in ASA journals would aid communication among scientists The International System of Units (SI) brings uni- within agronomic disciplines as well as among these and form style and terminology to world-wide com- other disciplines, especially internationally and in the munication of measurements. Because the Ameri- pure sciences. Scientists will find that SI does not differ can Society of Agronomy (ASA), Crop Science So- ciety of America (CSSA) and Soil Science Society greatly from the metric system now used, and it in- of America (SSSA) are international, they should creases understanding of the units the authors use. adopt SI in their communications. This paper sum- The General Conference on Weights and Measures marizes SI units, definitions, symbols, and rules of (CGPM) adopted the International System of Units in usage. Discussions of applying SI to agronomic 1960. The International Bureau of Weights and Meas- parameters are included where new concepts are ures (BIPM) and, in the USA, the National Bureau of introduced. Tables of recommended units and Standards (NBS) published rules on its use (2). SI guide- conversion factors are presented so use of SI can lines, besides being used in many foreign countries and begin immediately. It is proposed that a 2 to 3-year international societies, are followed by several Ameri- adaptation period begin with this printing, with authors beginning to use SI and including tradi- can societies and college textbooks (1). ASA members tional units, as desired, in parenthesis. are not expected to adopt this new system immediately. Rather, the ASA SI Units committee (ACS 321.4) Additional index words: Metric, Conversion table, recommends adoption over 2 to 3 years after these Publication style, Measurement. guidelines are published. During the interim, it is strongly recommended that authors begin now to use SI 'Contribution No. 81-483-j from the Agronomy Dep., Kansas E Systeme International d'Unites (SI) is being Agric. Exp. Stn., Kansas State Univ., Manhattan, KS 66506 and the U.S. Salinity Laboratory, AR-SEA, USDA, 4500 Glenwood Drive, L adopted aiound the world. Its purpose is to bring Riverside, CA 92501. Current address of the junior author is Dep. of uniformity of style and terminology in communicating Soil and Environmental Sciences, Univ. of California, Riverside, CA measurements. The world-wide impact of American 92521. Society of Agronomy (ASA) dictates the logic and es- 2 Research soil scientist, and soil and water specialist, respectively. SI UNITS IN SOIL CHEMISTRY 63 Table I. BaseSI Units (2) Table 4. Examplesof SI derived units expressed by meansof special names(2) Quantity Unit Symbol Length meter m Expression Mass kilogram kg in terms of Time second s Quantity Name Symbol SI base units Electric current ampere A Dynamicviscosity pascal second Paos m-~okgos-~ Thermodynamic temperature kelvin K Momentof force newton meter N.m m~.kg.s-2 Amount-of-substance mole mol Surface tension newton per meter N/m kg.s-~ Luminousintensity candela cd Power density, heat flux density, watt per square ~ -3 Table 2. Examplesof SI derived units expressed irradiance meter W/m kg.s Heat capacity, entropy joule per kelvin J/K m2.kg.s-2.K-~ in termsof baseunits (2) Specific heat capacity, joule per kilogram specific entropy kelvin J/(kg.K) m~.s-~.K-t SI Unit Specific energy joule per kilogram J/kg m2.s-~ Thermal conductivity " watt per meter kelvin W/(m.K) m.kg.s-3.K-’ Quantity Name Symbol e Energy density joule per cubic meter j/m m-~.kg.s-2 °’ Area square meter m~ Electric field strength volt per meter V/m m.kg.s-3.A Electric charge density coulombper cubic Volume cubic meter m ~ Speed, velocity meter per second m/s meter C/m m-~.s.A meter per second squared 2 Electric flux density coulomb per square Acceleration m/s 2 Wave number 1 per meter meter C/m m-~.s.A -3. -~. ~ Density, mass density kilogram per cubic meter ~ Permittivity farad per meter F/m m kg s’- A kg/m -’ Current density ampere per square meter A/m2 Molar energy joule per mole J/mol m~.kg.s-2.mol Molar entropy, molar Magnetic field strength ampere per meter A/m * Concentration heat capacity joule per mole kelvin J/(mol.K) m~.kg.s-~.K-’.moF (of amount of substance) mole per cubic meter mol/m’ Exposure (x and coulomb per 3,rays) kilogram C/kg kg-~,s.A Specific volume cubic meter per kilogram m~Jkg -~ Luminance candela per square meter cd/m2 Absorbed dose rate gray per second Gy/s m2*s Table 5. SupplementarySI Units (2) Table 3. SI derived units with special names(2) Quantity Unit Symbol SI unit Plane angle radian rad Expression Expression Solid angle steradian sr in terms in terms Sym- of other of Sl base Quantity Name bol units units Frequency hertz Hz s-t Sl UNITS -2 Force newton N m.kg.s ~ -~ Pressure, stress pascal Pa N/m m-’. kg ¯ s SI units are divided into three classes: base units, de- Energy, work, quantity of heat joule J N.m m2.kg.s-~. Power, radiant, flux watt W J/s m~.kg.s-3 rived units and supplementary units. Quantity of electricity, Base units are seven, well-defined units which by con- electric charge coulomb C A.s s.A vention are regarded as dimensionally independent: the Electric potential, potential difference, electromotive meter, the kilogram, the second, the ampere, the kelvin, force volt V W/A m2.kg.s-3.A-’ ~ the mole, and the candela (Table 1). Note that lower Capacitance farad F C/V m-~.kg-t.s’.A case is used for unit symbols except where the symbol is Electric resistance ohm i~ V/A m2.kg.s-~.A-~ Conductance siemens S A/V m-2.kg-~.s~.A~ derived from a proper name, then a capital is used (un- Celsius temperature degree abbreviated units are not capitalized). Symbols do not celsius °C K Luminous flux lumen lm cd. sr~" change in the plural and are not followed by a period. Illuminance lux Ix lm/m~ m-~.cd.sr~" Derived units are expressed in algebraic terms of base Activity (of a radionuclide) becquerel Bq s-’ units (Table 2). Some have been given special names and Absorbed dose, specific energy imparted, kerma, absorbed symbols (Table 3), which may be used to express other dose index gray Gy J/kg-~ m~.s derived units (Table 4). ~" In this expressionthe steradian (st) is treated as a base unit. Supplementary units are those not yet classified as either base units or derived units. This class now con- and include traditional units, as desired, in parentheses. tains onIy two units, radian and steradian (Table 5). Each scientific discipline presents unique circum- Punctuation of Derived Units. The product of two or stances regarding unit expressions. Hence, there may more units is preferably indicated by a dot midway in appear cases in crop or soil sciences where strict use of relation to symbol height. The dot may be dispensed SI appears unsuitable. This paper addresses some of with when there is no risk of confusion with another those considerations and the committee intends to symbol, remain active to consider future problems. This paper is one of a series being prepared by mem- for example: Nom or N m bers of the SI Units committee. It first describes SI in general and then discusses its particular application to A solidus (oblique stroke, /), a horizontal line, or ASA publications and the soil science discipline with negative powers may be used to express a derived unit emphasis on soil chemistry. formed from two others by division, 64 JOURNALOF AGRONOMICEDUCATION, VOL. 10, 1981 m for example: m/s, ~-, or m.s-’. Table6. SI Prefixes(2) Multiplication factor Pref’Lx Symbol Only one solidus may be used in combinations of 1~ 000000000000000000 = 10 exa E units unless parentheses are used to avoid ambiguity, 1~’ 000000000000000= 10 peta P 1’2 000000000000 = 10 tera T 1 000000000 = 109 giga G for example: mokg/s3oA, or l~ 000000 = 10 mega M 1~ 000 = l0 kilo k mokg°s-3°A-1, 100 = 102 hecto h 10 = 10’ deka da but not, m°kg/s3/A. 0.1-I = 10 deci d 0.01-2 = 10 centi c -3 Application of SIPrefixes. The prefixes and symbols 0.001 = 10 milli m 0.00001-6 = 10 micro listed in Table 6 are used to indicate orders of magni- 0.000000001 = 10-" nano n tude of SI units. Amongthe base units, the unit of mass 0.0000001300001-1~ = 10 pico p 0.000000000000001 = 10-" femto f (kilogram) is the only one whose name, for historical -1~ 0.0130000000000000001 = 10 atto a reasons, already contains a prefix. Names of decimal multiples and sub-multiples of the unit of mass are formed by attaching prefixes to the word "gram", for Table7. Unitsin usewith S! (2) example rag, but not gkg because compound prefixes Quantity Unit Symbol Definition are not allowed. Time minute rain 1 min = 60 s Prefixes should be chosen to reduce nonsignificant hour h 1 h = 60 min = 3,600 s digits or leading zeros in decimal fractions. A prefix pre- day d 1 d = 24 h = 86,400 s ~ -J ~ ferably should be chosen so that the numerical value lies Volume liter L 1 L = 1 dm = 10 m Mass metric ton (tonne) t 1 t = 10~ kg = Mg between 0.1 and 1000 and the same unit, multiple, or Area hectare ha 1 ha = 1 hm~ =~ 10’ m submultiple should be used throughout a text, including its tables and graphs.