The Redfield ratio is useful as it helps us to estimate the optimal amounts of N and P nutrients for algae. The Redfield N:P molar ratio is 16:1, and this is reflected in a general description of algae given by the formula C₁₀₆H₂₆₃O₁₁₀N₁₆P₁ . For more information click here. On our farm we regularly monitor PO₄ and total ammonia (TAN) . When the ratio of PO₄:TAN is 3:1, it is very close to the N:P molar ratio of 16:1. When we are calculating these ratios it is sometimes very important to get them as close as possible to the 16:1 optimum. A case in point is when we are managing toxic algae. It is also good practice to try at least to be close to the 16:1 ratio for normal pond management as well. This is where Phoslock can be very useful as it gives us a great amount of flexibility in our ability to manage our ponds.

When working out the N:P it is important to not ignore the feed inputs. As a general rule about 75% of the N in feed is returned quickly back to the pond: only about 25% is retained by the animals being fed. About 80% of the returned N is returned as soluble N, the other 20% as faeces. Here is an example of calculating the N input from feed:

• Total feed provided per day: 100 kg

• Protein Content of the feed: 30%

• N content of the feed: 5% or 5kg

• N returned back to the pond: 75% of 5 kg or 3.75kg

• Soluble component of the N returned: 80% of 3.75 kg or 3 kg

In a 1 ha , 1 m deep pond, the 3 kg N returned back to the pond will provide 0.3 mg per litre N, and is equivalent to adding about 6 kg urea. There is a maximum amount of N that a pond can manage each day. Click here for further details.