How to caculate the amount of Phoslock to add to a pond

• Measure the PO4 in the water. The sample water must be filtered to at least 1 micron. Sample should be taken within 2 hours of sunrise and the pond should be well mixed by aerators.

• Record the result as mg PO4 per litre.

• Estimate the volume of water in the pond in litres. Record the result as litres

• Mutiply the value of the PO4 (mg per litre) x the number of litres in the pond. This gives you the total number of mg of PO4 in the pond. To get this to kg PO4, divide by 1 million.

• Think carefully about how much PO4 you want to remove. If the bloom is about right eg say 40 cm Secchi, then there is no need to have much free PO4 in the water. In this case you would aim at bringing the PO4 down to about 0.05 mg per litre. It is important to retain a small amount of free PO4 because in aquaculture ponds the optimum Secchi transparency is usually 30-40 cm. Removal of all the PO4 can clear a pond of its bloom. If the bloom is thin and you want it to grow some more, then PO4 ought to be in the range 0.15 – 0.25 mg PO4 per litre.

Here are 2 examples.

Example 1

The measured PO4 is 0.25 mg per L. The pond volume is 5 million litres. The Secchi is 40 cm.

The total amount of PO4 in the pond is 0.25 x 5 million = 1.25 million mg or 1.25 kg PO4. Because the bloom is about right at 40 cm, we want to remove most of the PO4 we have measured but leave a small amount, say 0.05 mg per litre, to maintain the bloom. So we want to remove 0.2 mg per litre or 0.2 x 5 = 1 kg PO4, which means we need to add 1 x 30 kg Phoslock.

Example 2

The measured PO4 is 0.45 mg per L. The pond volume is 5 million litres. The Secchi is 80 cm.

Because the bloom is thin, and we need it to grow some more, we should leave about 0.15 mg PO4 per L in the pond. So we want to remove 0.3 mg per litre. This means we need to remove 0.3 x 5 (volume of water in millions of litres) = 1.5 kg PO4. So we need to add 1.5 x 30 kg Phoslock.

Note that use of Phoslock in aquaculture ponds is a process of measuring PO4, adding Phoslock, then measuring again to check PO4 levels, and if necessary, further addition of Phoslock to fine tune the pond. Usually, two PO4 measurements per week is adequate to achieve the control necessary for most aquaculture operations.