Soils, Composts and Fertilizers

As with all plants, aquatics will directly reflect the quality of the medium in which they are growing. Special aquatic planting composts are available, but in most cases carefully prepared garden soil will be quite satisfactory. The only consideration that must be given, over and above the well-being of the plants, is the ecology of the pool. The constituents of the growing medium can have a direct influence upon the water clarity and chemistry of the pool, so it is often better to begin by looking at this aspect, then move towards the plants particular nutritional requirements.

 

A BALANCED GROWING MEDIUM

As far as the ecology of the garden pond is concerned, a growing medium should have a heavy structure, ideally a clay or very heavy loam, which will not disperse readily into the water and will cause problems of clouding. Although particles of clay and loam are quite small, they tend to stick together better than the irregular particles of sandy light soil, which can escape from planting containers and swirl around like aquatic dust.

Organic matter is undesirable, as this will decompose in the same way as an accumulation of fallen leaves, causing a build-up of noxious gases. These will escape freely and harmlessly into the air during the summer, but if the pond freezes in the winter, they will be trapped beneath the ice and will be a danger to the fish. Not only are the gaseous results of decomposition undesirable, but also the unpleasant contamination that can cause the water to turn deep amber or cloudy. If the organic matter is rich in nutrients, it will lead to abundant algal growth.

This is a major difficulty when selecting any growing medium for aquatic plants. You must be sure that sufficient nutrients are available for the plants that will grow in it, but there should not be an excess that will leach into the water and cause a serious algae problem to develop. Simply looking at a growing medium, or even analysing it scientifically, will not indicate accurately whether it contains an excess of nutrients. All that can be done is to choose the soil carefully and apply slow release fertilizers adjacent to the plants, where there will be little chance of leaching occurring.

Proprietary brands of aquatic planting compost vary in quality, but those from leading manufacturers will be composed scientifically and, although comparatively expensive, will produce healthy plant growth with a minimum of nutrients escaping into the water. If you are a newcomer to water gardening and have little understanding of plant nutrition, or your garden soil is of questionable quality for satisfactory aquatic plant growth, buying prepared aquatic plant compost will be a good investment.

 

PREPARING YOUR OWN COMPOST

Aquatic plants benefit from a heavy growing medium, so if your garden contains a clay, or a medium-to-heavy loam soil, there is no reason at all why you should not prepare your own growing medium. If you are uncertain about the nature of the soil, you can carry out a very simple structural test, which will also indicate the amount of organic matter that is present.

Take a small sample of soil from the area that you intend using for making the compost. This should be a part of the garden that has not been dressed recently with artificial fertilizer, as it will escape into the water. What is required is an area where the soil is clean, but preferably impoverished, or at least that has not been treated with fertilizer or animal manure for the past two seasons.

Having removed a good trowelful of typical soil, dry it out until it can be powdered. This can be hastened by putting it in the oven. Place the finely powdered dry soil in an old coffee jar. Add water and shake the jar vigorously until the contents have the appearance of thick muddy water. Then stand the jar somewhere safe where it should remain undisturbed for three or four days.

Provided that the soil sample was dried and pulverized properly, then mixed completely with the water, when it settles out, the various constituents will be clearly visible. Sand particles, being large and heavy, will settle out within a very short space of time. Clay particles may take several days, while organic material will float. The result will be a series of layers in the jar. The bottom layer will be the sand, on top of which will be the clay. Above the clay will be water, and any organic material will float on the surface. Based upon your observations, a decision can be made as to the suitability of the soil: if at least 75 per cent of its content is clay, it should be a satisfactory growing medium for aquatic plants.

When digging up the soil, remove any obvious weeds, old pieces of turf and anything else that is likely to decompose and pollute the water. Discard large stones and pass the soil through a garden sieve placed over a wheelbarrow. This will retain any undesirable debris, like sticks or pieces of glass or pottery, leaving a neat heap of finely graded soil.

The soil should be used in this form without adding any fertilizer, although traditionalists may feel happier if some bonemeal is mixed in. This will do no harm to the water, but it takes such a long time to break down that it is difficult to imagine what value it can have to the plants. A much better idea is to plant aquatics directly into the soil, then administer a balanced, slow release fertilizer. This may be obtained in tablet form or as a small perforated sachet. Both should be pushed into the soil, immediately beside the plant, in the centre of the container. In this way, the plant will benefit from the fertilizer, while there will be virtually no prospect of it seeping into the water.