Appendix 5. Units and conversion factors

The International System of Units (SI) is commonly required for reporting laboratory measurements in publications on soil chemical analysis and related sciences. Seven base SI units are given in Table App. 5.1. Prefixes used to indicate fractions or multiples of SI or alternative units are given in Table App. 5.2, while some comparisons with metric, ‘traditional’ and SI units are provided in Table App. 5.3. Conversion factors for some anions are summarised in Table App. 5.4.

Table App. 5.1. Seven base units and associated symbols (abbreviations).

Quantity	Unit	Symbol
Base
Amount of substance	mole	mol^†
Electric current	ampere	A
Length	metre	m
Luminous intensity	candella	cd
Mass	kilogram	kg
Time	second	sec
Thermodynamic temperature	kelvin	K

^†M is commonly used as a symbol for mole throughout this book.

Table App. 5.2. Prefixes, their specifications and symbols used in association with SI and other units.

Prefix	Fraction	Symbol
deci	10^-1	d
centi	10^-2	c
milli	10^-3	m
micro	10^-6	μ
nano	10^-9	n
pico	10^-12	p
femto	10^-15	f
atto	10^-18	a
deca	10	da
hecto	10²	h
kilo	10³	k
mega	10⁶	M
giga	10⁹	G
tera	10¹²	T
peta	10¹⁵	P
exa	10¹⁸	E

Table App. 5.3. Comparisons with metric, ‘traditional’ and SI units used in soil chemical analysis.

Quantity	Description	Symbol	Conversions
Length	metre	m
	centimetre	cm	0.01 m
	millimetre	mm	0.001 m
	micrometre	μm	10^-6 m
	nanometre	nm	10^-9 m
Area	square metre	m²
Area	hectare	ha	10⁴ m²
Volume	cubic metre	m³
Volume	cubic decimetre	dm³	1.0 litre (L)
Mass	kilogram	kg
	gram	g	10^-3 kg
	milligram	mg	10^-6 kg
Density	kilogram/cubic metre	kg/m³
	gram/cubic decimetre	g/dm³	0.1% (w/v)
	milligram/cubic decimetre	mg/dm³	1.0 mg/L
Amount of matter	mole	mol	1 equivalent of monovalent ion
Quantity – concentration of matter	mole/cubic metre	mole/metre³	mol^-3
Quantity – concentration of matter	mole/cubic decimetre	mol dm^-3	1 mol/L
Quantity – matter content	centimole charge of specified ion (i)/kilogram	cmol_c/kg cmol (i^±)/kg	mequiv/100 g for monovalent ion
	millimole of single-charge ionic species (e.g. Na⁺, Ca_0.5⁺)/kilogram	mmol_c/kg mmol (±)/kg	0.1 mequiv/100 g
	millimole of single-charge ionic species (e.g. Na⁺, Ca_0.5⁺)/cubic decimetre	mmol_c/dm³ mmol (±)/dm³	1.0 mequiv/L
	mole/kilogram	mol/kg	1 molal
Quantity – volume fraction	10 × 10^-3 metre³/metre³	m³/m³	1 vol %
	centimetre³/metre³	cm³/m³	1 ppm
	millimetre³/metre³	mm³/m	1 ppb
Quantity – mass fraction	10 × 10^-3 kilogram/kilogram	kg/kg	1 w %
	milligram/kilogram	mg/kg	1 ppm
	microgram/kilogram	μg/kg	1 ppb
Quantity – matter fraction	10 × 10^-3 mole/mole	mol/mol	1 mol %
Quantity – matter fraction	1 × 10^-6 mol/mol	mol/mol	1 ppm
Quantity – time	second	s	1 sec
	60 seconds	s	1 minute (min)
	3600 seconds	s	1 hour (h)
	86 400 seconds	s	1 day (d)
Quantity – pressure	10⁵ pascal	Pa	1 atm
	0.1 megapascal	MPa	1 bar
	133 pascal	Pa	1 mm Hg
	9.81 kilopascal	KPa	1 m H₂O
Quantity–lineic conduction	Seimens	S	1 mho
	deciSeimens/metre	dS/m	1 mmho/cm
	100 milliSeimens/metre	mS/m	1 mS/cm

Table App. 5.4. Factors to convert mg/kg of element to mg/kg of anion species.

Element	Multiplication	Anion species
BO₃^3–-B	5.440	BO₄^3–
NO₃^–-N	4.427	NO₃^–
NO₂^–-N	3.284	NO₂^–
PO₄^3–-P	3.066	PO₄^3–
SO₄^2–-S	2.996	SO₄^2–

Table App. 5.5. Element to oxide conversion and vice versa.

Conversion of oxide to element:
Oxide	Conversion	Element
P₂O₅	0·437	P
K₂O	0.826	K
CaO	0.744	Ca
MgO	0.603	Mg
Conversion of element to oxide:
Oxide	Conversion	Element
P	2.291	P₂O₅
K	1.21	K₂O
Ca	1.339	CaO
Mg	1.67	MgO

Table App. 5.6. Conversion factors for spread-rate to corresponding values in litres/square metre (L/m²).

Imperial unit	Conversion
Gallons (UK)/acre	÷ by 890.184
Gallons (US)/acre	÷ by 1069.066
Gallons (UK)/hectare	÷ by 2199.692
Gallons (US)/hectare	÷ by 2641.721

Imperial or metric unit	Conversion
litres/hectare	÷ by 10 000
millilitres/sq.metre	÷ by 1000
cubic inches/sq.yard	÷ by 51.024
fluid ounces (UK)/sq.yard	÷ by 29.428