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Pond Nutrients
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Managing Nutrients to Control Algae

Larry Laitner - Click here or otherwise item separations will be distorted (1k)
Solving the algae problem in a pond is not exactly rocket science but it involves several factors that must be understood in an integrated manner. Actually it is just basic agricultural science. Put as simply as possible the problem is phosphorous. Those of us that have been paying attention to environmental rhetoric know that phosphorous that gets into our streams, rivers, and lakes causes water quality to deteriorate.

The same is true of small artificial aquatic systems except that the phosphorous doesn't come from external sources, it comes from the fish themselves. The problem is that small artificial ponds do not have the same range of inputs and outputs so that the phosphorous fish produce can accumulate in the system. When the phosphorous content becomes high enough planktonic algae grows rapidly and the water turns green. It will stay green until the phosphorous content of the water decreases.

A pond system that can tie up dissolved phosphorous as quickly as the fish produce it will be clear or nearly so. The key to removing dissolved phosphorous from the water is to have vigorous growth of vascular plants. Water hyacinths are among the best choices for this task. The catch phrase here is "vigorous growth," since it is quite possible to have lots of hyacinths in a pond and still have algae. What happens here is that the hyacinths do not have all the other nutrients to support growth, so that the phosphorous will not be removed. In this case the hyacinths will do poorly and the planktonic algae will thrive.

So what nutrients do the hyacinths need to have vigorous growth? Ok, here's the agricultural science part. The three main nutrients are nitrogen, phosphorous, and potash. If any one of these is missing the hyacinths will not grow. There are also some micro-nutrients that we will ignore for now.

Nitrogen and phosphorous are produced by fish and we can safely assume that any pond that has fish will have a continuous supply of these nutrients. The source of these nutrients is the food that the fish eat and excrete as waste products.

Potash, which we also know as potassium, can come from the mineral content of water that flows onto natural bodies of water. Unfortunately, potash is often deficient in artificial ponds. If potash is deficient, vascular plants will not grow well and thus will be unable to utilize the phosphorous and nitrogen. When this happens planktonic algae will bloom and your pond will look like pea soup..

What usually happens in an artificial pond is vascular plants will grow until one of the three nutrients is exhausted. The other two nutrients will remain in solution, unusable until the missing nutrient is replenished. Then growth will proceed slowly at a rate that is controlled by the rate that the missing nutrient is produced. The key to clear water is to make sure that the nutrient that runs out first is phosphorous. Once this state is achieved the phosphorous the fish produce will be utilized almost immediately by the plants that are awaiting its production.

Now the task is to manage the nutrient load in the water so that nitrogen and potash are always available. Nitrogen might be deficient in a pond that has no fish but will normally be plentiful if fish are present. Potash must usually be added to ponds, sometimes in substantial quantities. A pond that has had an ongoing "green water problem" may accumulate a substantial burden of phosphorous over time. When this occurs any added potash will be quickly used by the plants that have been starving for it for some time, so several additions of potash will have to be made over a period of time.

How much potash should be added? How often? Tetra Pond makes a product called Flora Fin that is a very expensive 6% potash solution. They recommend initially adding 165ml of this solution to 800 gallons of water followed by one half this amount every two weeks. My experience is that this amount is often not enough, especially if the initial load of phosphorous is substantial. I would recommend this full dose at least once a week until the water clears. A very cheap potash solution can be made from muriate of potash (potassium chloride - KCl), which is available at any garden supply. Add 50 grams of muriate of potash to 500 ml of water to create a 6% potash solution at a cost of about ten cents, or you can just buy Flora Fin for about ten dollars a bottle. (Buy several, it doesn't go far) At the beginning of this discussion I said we would ignore micro-nutrients but it is possible that one of these may also be lacking.

The micro-nutrient most likely to be lacking is iron. One way to supplement iron, and the other micros as well, is to add a very small quantity of Ironite. Ironite is intended as a lawn fertilizer so it comes in a big bag that will last several lifetimes. It also contains nitrogen in the form of urea which is toxic to fish if you add too much, so go easy. Be particularly cautious with this if your pond hasn't been in operation long enough to have colonies of the various de-nitrifying bacteria that turn urea into the relatively harmless nitrates that your plants will consume. If you are not sure about this you can test your water for ammonia.

If you are successful in managing your nutrients your water will be clear, the plants will be a lush dark green color, and hyacinths will develop long root structures as they attempt to collect the phosphorous as quickly as it is produced. As winter approaches try to leave the hyacinths in the pond as long as you can so the inevitable winter phosphorous build-up will be minimal. During the winter continue to add your potash solution once a month or so as long as your pond isn't frozen over. During the winter you will probably have some filament type of algae grow on the sides of the pond as the various nutrient processes continue at reduced pace. The filament algae will tie up phosphorous at least temporarily (until the fish eat it) but that will suffice since fish do not excrete much phosphorous in the winter. In the spring get the hyacinths in the pond as early as frost allows and boost the potash again at that time.

It is imperative that the hyacinths be removed from the system either when the frost comes if you live in an area where winter conditions occur or on an ongoing basis in frost free areas. The hyacinths tie up the phosphorous; only you can remove it. If you leave the hyacinths to rot, the phosphorous will be returned to the water.

Your fish probably don't care much about the color of the water but most humans like the water clear so they can see the fish. If you manage your pond's nutrients you should have clear water most of the year with only a slight murkiness in the spring during the time between the awakening of fish metabolism and the arrival of your hyacinths.

End of Nutrients

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