Module 6:

6.3 Soil science

6.3.1 Plant nutrition

1   What are the figures on a fertiliser label?

A fertiliser label must comply with the Fertilisers Regulations 1991. The main nutrients are actually quoted as Nitrogen, N; Phosphorus Pentoxide (P₂O₅); and Potassium Oxide (K₂O); along with the elements Phosphorus (P) and Potassium (K) being quoted in brackets after the first mention of the relevant nutrient. Phosphorus Pentoxide is known as Phosphate and Potassium Oxide is known as Potash.

Whenever commenting about fertiliser requirements, or determining a fertiliser programme, for turf surfaces it is important to be clear whether it is the element or compound oxide which is being considered as this can have a significant effect on how much nutrient is actually applied.

2  

The following factors (described in Schedule 2 of the Fertilisers Regulations 1991) are used to convert the oxide numerical values to the elemental form.

• Phosphorus Pentoxide (P₂O₅) × 0.436 = Phosphorus (P).
  [To convert P to P₂O₅, the value is P x 2.29]
• Potassium Oxide (K₂O) × 0.83 = Potassium (K).
  [To convert K to K₂O, the value is K x 1.20]

Example

A fertiliser with an analysis of 15:10:5 (N: P₂O₅: K₂O) has the following elements:

• 15% of Nitrogen.
• Phosphate (P₂O₅) is 10%, so Phosphorus content is 10% × 0.436 = 4.36%.
• Potash (K₂O) is 5%, so Potassium is 5% × 0.83 = 4.15%.

Many people when discussing fertilisers will typically refer to these figures using the element name only, so the 10% phosphate in this example will often be referred to in a discussion as the 10% phosphorus, which is actually incorrect, so just be cautious and clarify exactly what is being talked about.

Other nutrients, where present, will often also be included on a label, however, we are only looking at the three main (or primary) nutrients in this example.

3   How are these figures arrived at?

P and K are single atoms, whilst P₂O₅ and K₂O are molecules. The weight of a single atom is less than the weight of a molecule. The Periodic Table identifies all the elements and their atomic weight (Relative Atomic Mass). From these values can be calculated the weights of any molecule.

Element     Atomic weight

Phosphorus     31.0

Potassium     39.1

Oxygen       16.0