The graph above shows that :-

• The medium grade peat used in the tests had an air-filled porosity of 10%
• Melcourt Potting Bark has an air-filled porosity of 59%
• The bark containing fines used in the tests had an air-filled porosity of 39%
• The 70/30 mix of peat and Melcourt Potting Bark had an air-filled porosity of 16%
• The 50/50 mix of peat and bark with fines had an air-filled porosity of 16%, ie 20% more bark with fines had to be added to the peat in order to achieve the same air-filled porosity of 16%.

For a 50m³ load of nursery-produced compost, the following peat and Melcourt Potting Bark costs are typical :-

medium sphagnum peat @ £17.00 per m³ Melcourt Potting Bark @ £31.00 per m³ The cost per cubic metre for a 70/30 peat/bark mix is therefore:- medium sphagnum peat £17.00 x 70% = £11.90 Melcourt Potting Bark £31.00 x 30% = £ 9.30 Total = £21.20 per m³ Therefore the total peat and bark cost for the 50m³ load would be £ 21.20 x 50 = £ 1060.00

From the graph it can be seen that to achieve the same degree of compost quality in terms of air-filled porosity, 20 % more bark containing fines had to be used, compared to the fine-free Melcourt Potting Bark.

The increase in cost caused as a result of this, depends on the price paid for the bark. The example below shows what the effect would be if the bark containing fines was the same price as the Melcourt Potting Bark.

medium sphagnum peat @ £17.00 per m³ bark containing fines @ £31.00 per m³ The cost per cubic metre for a 50/50 peat/bark mix is therefore :- medium sphagnum peat £17.00 x 50% = £ 8.50 Melcourt Potting Bark £31.00 x 50% = £ 15.50 Total = £ 24.00 per m³ Therefore the total peat and bark cost for the 50m³ load would be £ 24.00 x 50 = £ 1200.00 i.e. a 13% increase