All governments enforce
detailed guidelines on the format and type of information that must be
declared on the labels of horticultural products.
How to interpret
the label analyses of water based nutrients
With the common exception of potassium and
phosphorus, most nutrient labels state the concentration of nutrient
elements in their elemental form – as per the “Element” column in
K2O and P2O5: By convention, many
labels represent potassium (K) as dipotassium oxide (K2O) - see Fig 7.4.
To convert K2O to K, multiply the K2O figure by 0.83. Similarly,
phosphorus (P) is often represented as phosphorus pentoxide (P2O5). To
convert P2O5 to P, multiply the P2O5 figure by 0.44.
Concentration: The concentration of
elements is represented as percentage (%). Specifically this is % m/v i.e.
percentage mass per unit of volume. Multiplying this figure by 10
expresses the concentration in grams per litre e.g. 3.0% equals 30g/L.
Alternatively, multiplication by 10,000 expresses the concentration in ppm
or mg/L. For example, 3.0% equals 30,000ppm.
How to compare nutrient label
analyses of 1, 2 & 3-part labels
Label analysis figures for 2 & 3-part
nutrients are usually based upon the concentration of nutrients in the
individual packs. Therefore, in order to make a meaningful comparison
between a 2-part and 1-part nutrient, it is necessary to first double the
figures quoted on the 1-part. This ensures the contents of equal volumes
are being compared. For example, 2 x 1 litre bottles of the 1-part with 2
x 1 litre bottles of the 2-part (‘A’ and ‘B’) .
To compare a 1-part with a 3-part, first
‘triple’ the 1-part figures.
To compare a 2-part with a 3-part, first
halve the 2-part figures before multiplying them by 3.
This information makes it possible to compare
analyses. Graph 7.30, displays the results of a meaningful comparison
between 1, 2 and 3-part nutrients.
Nutrient element concentration in
the working nutrient solution yielded by a specific dose
To determine the concentration of individual
elements in the working nutrient solution, the individual analysis
specification should first be converted into ppm (or mg/L), then
multiplied by the dose rate (per litre). For example, if part ‘B’ of a
2-part contains 3.0% phosphate as P2O5, then when this part is used at 4ml
per litre, it will yield 53ppm phosphorus as P. That is:
Step 1. “3.0% P2O5” = 30,000ppm as
P2O5 (i.e. 3.0% x 10,000).
Step 2. Convert this figure to P:
30,000ppm as P2O5 = 13,200ppm as P (i.e. 30,000ppm x 0.44).
Step 3. 4ml per litre yields 53ppm P
(i.e. 13,200ppm x 4ml ÷ 1,000ml).