What to do with the muck in your garden pond?

The process of organic matter in the bottom of a pool being digested starts with a metamorphosis in part (especially the fish food and fish excreta) into ammonia and ammonia compounds. Ammonia is particularly toxic to Koi, which ironically exude the stuff from their gills during metabolism anyway. Oxygen dependent Nitrosomonas bacteria are hopefully there in the bottom of the pool or in the filter medium breaking this down as quickly as possible to nitrites. Nitrites unfortunately are toxic too, but Nitrobacter bacteria move in to build these up to nitrates. The resulting nitrates can be taken up and used by plants or can further broken down. This is when anaerobic bacteria could come in to play by breaking up the nitrates and releasing the oxygen and nitrogen.

Now if you haul this stuff out of your pond at any stage in the process, it is either going to continue its natural break down outside, or if it has reached the nitrates stage of the process, it will be the equivalent of vegetable rocket fuel. Most plants dont want rocket fuel, and if it is not rocket fuel but still muck continuing its decomposition, there is a danger that it will leech out goodness from the soil to aid the decomposition. So the best place for it is the compost heap. Here it can be diluted and watered in or, if it is thick heavy sludge, it can be mixed in layers between very fibrous and straw types of material. Because this keeps the collection of material open, oxygen is available to complete the rotting down process of the material and it all rots down to a moist friable compost.

As far as the compost heap is concerned, if the compost needs an efficient starter for the rotting down process, nothing can be bought that is any better than pond gunk with its mixture of bacteria, organic compounds and nitrates.

If the worst comes to the worst

If you have nowhere else to put it and it does not smell too bad, spread or water it onto a bed of fairly vigorous shrubs or perennials. Disperse it as much as possible and as it begins to dry, gently prick it in with a garden fork. This ensures that you havent created a suffocating pan over the surface of the soil and opened it up to the air.

In bulk it is on the acid side of neutral. This would make it unsuitable for the vegetable patch without being diluted by garden compost, but may be a nice fillip for an azalea or Rhododendron bed.

Is it necessary?

This bizarre pool and garden called Screen 4 by PRP Landscapes at the Westonbirt Garden Festival in 2003 would have evolved rapidly into a green mess without an efficient filtration system. This was a biological pressure filter hidden under a seat on some decking.

So pond sludge is the inevitable by product of a healthy pond ecosystem. But as these things are never static and are constantly evolving, part of that evolution means ultimately that the pond silts up completely with this muck and there comes a day when it all needs to be cleared out. Certain things bring this inevitability closer. First and foremost; the population of the pond especially the number of fish; the vigour of the plants in proportion to its size and amount of detritus that falls into the pool from outside, like tree leaves; in other words how it fits into the landscape and how it is managed.

The things that delay that ultimate date:

Good management which is what this book is about, and the available technology in the form of filtration. This ensures the pool itself is given extra capacity for containing fish. It takes away a lot of the excess layers of rotting material so that there is constant turnover with nothing having the chance to stagnate in an air less mire. If the filter is a biological filter that actually digests the muck and is of the correct capacity and efficiency, there is no reason that the time for the big clear could not be postponed indefinitely (Why? See filtration). Only the excess growth of the plant life would dictate when a sort-out was due.

But filters are not just about muck and they are not just about green water and algae, the cause for many people to be persuaded into installing biological filters. The main reason for having a biological filter is to improve the water quality. The subject of water quality is the most important as far keen fish keepers and especially Koi keepers are concerned. It is all about how to control the condition of the water by way of dissolved compounds from the environment and the chemicals it contains that come from the fish waste, food and other detritus and that the algae opportunistically feed on. Then once the quality is right, how to maintain it in a stable condition. So if you thought the filter is there so that you can see the fish, it is also for the benefit of the fish, and herein lies the greatest value of a biological filter.

Having a filter means a certain amount of extra maintenance, but this fizzles into insignificance at the filth and upheaval that results from a major pond clean out. Initially a filter needs to be nurtured into activity (see new ponds and filtration: the dreaded new pond syndrome), and this can require a certain amount of patience and application to some fundamental principles, but once in positive action a biological filtration system can be left to fend for itself for most of the year. (See also: Water Problems in Relation to Fish: Can I Avoid Problems from the Start by having a Biological Filter.)