Growing Media for Garden Plants
Choosing the mixture
In an organic garden, plants growing in containers should ideally be fed by the action of micro-organisms releasing nutrients from organic matter rather than directly by chemical fertilizers. This gives a gradual supply of a whole range of nutrients, and should result in healthy, balanced plant growth.
It is therefore important to buy suitable organic mixtures for sowing seeds and potting on plants. These are often called “compost” but are not to be confused with.
Alternatively, you can make up your own growing media. These mixtures require certain properties appropriate to their intended use.
These need to drain freely to allow air into the mixture and enable it to warm up quickly, but they must also retain some moisture. They need to have a fine texture so that they make good contact with the seeds, and they need to be free from weed seeds, pests and disease organisms.
Mixtures that are used for seed sowing when the seedlings are to be pricked out do not need many nutrients as the seed has its own reserves. In fact, a high concentration of nutrients can inhibit the germination of some seeds and damage seedlings. However, if you are sowing into modules, the mixture needs to contain enough nutrients to sustain the growing plants for a short time.
These are used for potting on plants that are eventually to be transplanted, and also for plants that are growing permanently in pots and other containers: houseplants, patio plants and cropping fruits and vegetables such asand , for example.
Potting mixtures need to retain moisture, yet also allow sufficientfor air to get to the plant roots. However, there is less risk of waterlogging with strongly growing plants as these will be taking up a lot of water.
Potting mixtures do not need to be as fine as seed compost, but the texture will depend upon their use: you could use a coarser mixture forin large pots than you would for pot plants, for example.
All potting mixtures need to contain both short-term and long-term food supplies, in greater concentrations for the more vigorous plants. However, too many readily available nutrients can cause toxicities and imbalances. The mixtures should also be free of pests and disease organisms, and generally from weed seeds, although a few weed seedlings are not such a problem in large containers.
Many manufacturers sell “multi-purpose” mixtures, to be used both for sowing seeds and for growing on young plants. These should contain some nutrients, but not such a high concentration that seed germination is affected. They can be useful when you are sowing in modules, but plants potted up into these mixtures are likely to need feeding at an early stage.
The required properties of seed and potting mixtures are obtained by mixing the right ingredients together in the right proportions.
The balance between retaining moisture yet draining freely is determined by the size of the particles that make up the mixture. If the particles are large or fibrous, the spaces between them will be large and the mixture will drain easily. This allows air in but means that the mixture will dry out quickly. Conversely, if the particles are small, the spaces between them will be small and will hold water, meaning the mixture will not dry out so rapidly but there is a risk of waterlogging.
The ideal basic ingredients for a potting mixture should therefore include a range of particle sizes, and hence space or “pore” sizes (see Knowing Your Garden Soil). The mixture must also be stable enough to maintain this structure when packed into pots and continually watered from above. This is one of the main reasons why on its own is not suitable for putting into pots: it becomes solid and airless. Processed peat has an ideal structure, which partly accounts for its popularity in growing media. However, other materials such as bark and coir (see below) can also work well, although leafmould (see below) is one of the best basic ingredients to use in homemade potting mixtures.
Many of the most suitable base materials such as peat and leafmould contain very few plant nutrients. However, these can be added in the form of organic fertilizers such as blood,and bone and/or composted nutrient-rich organic matter like manures or vegetable waste. Proprietary potting mixes are increasingly using what were previously “waste” materials, and in home-made mixtures you can use your own garden or worm compost.
Mixtures containing only organic fertilizers are probably the safest to use for sensitive seeds because composted organic matter can sometimes contain a high concentration of nutrients. However, composted organic matter is a very desirable component of other mixtures, for it provides a long-lasting source of a range of plant foods and there is also evidence to suggest that it can suppress soil-borne diseases. It can also help to improve the structure of the mixture.
The right pH
The pH of most seed and potting mixtures is 5.0-5.5, suitable for most plants. However, in practice, a higher pH (up to 7.0) causes few problems. Acid-loving plants like azaleas grown in pots require a low pH compost. For your own mixtures, measure the pH using a testing kit (see Knowing Your Garden Soil). Peat is very acid, so any mixtures containing peat usually require the addition of lime. Other base materials are less acid, so a correction may not be needed.
Making your own mixtures You can vary the ingredients of home-made seed and potting mixtures according to the materials available and the plants you are growing, as long as the mixtures meet the same basic requirements. The ingredients must be mixed well in order to obtain a uniform end product.
First, weigh out any fertilizers or limestone required into individual plastic cartons. Mark the levels they reach and label each carton, to save weighing in future. Mix them all together in a larger container. Find an appropriate measure for the bulky materials — a clean 101 (2gal) bucket for example— and put in sufficient bulky material to fill it without any compaction. Riddle coarse materials through a 6mm (Yin) sieve if a fine mixture is required.
Fill the bucket with the first of the materials, then spread the contents out on a clean hard surface — a concrete floor or piece of board, for example — and sprinkle over some of the lime/fertilizer mixture. Repeat with further bulky ingredients until they have all been added.
Mix the ingredients thoroughly with a clean spade or shovel by piling them in a heap, then into another heap, and then a third, by which time the mixture should be thoroughly mixed and of a uniform colour.
This should be good loam. For home-made mixes, soil from an old turf stack is ideal. It is well structured and provides a source of nutrients over a long period. It also has a controlling influence over the supply of water and nutrients to plants. However, it may contain pests and disease organisms, and is likely to contain weed seeds. These can be killed by sterilizing the soil with heat, although this may also harm some beneficial organisms.
If you are going to heat-treat soil, it is wise to use one of the small steam sterilizers available as the temperature and time of heating are critical. There are various methods of sterilizing small amounts of soil in the kitchen using an ordinary stove or microwave oven, but the soil is easily “overcooked”. Whichever method you use, spread out the soil in a thin layer to cool before adding other ingredients.
It is not always necessary to sterilize soil for a potting mixture for large plants, as weed seedlings can be easily dealt with and the risk of troublesome pests or diseases is small.
Well-matured compost that is a dark colour and crumbly in texture is a good source of long-term nutrients. It will also help to hold moisture. It can contain weed seeds and disease organisms, although the risk is less if your compost heap has heated up well (see The Composting Process). Mixtures should not contain more than approximately 50 per cent compost by volume.
Worm compost (see Worm Compost and Worm Composting) is mostly a more concentrated source of nutrients than garden compost, so you need to use less. It also has a fine texture. This makes it a very useful material for home-made mixtures. If you feed your worms only on kitchen scraps and vegetable peelings, the compost should be free of weed seeds. Worm-worked materials of various kinds are included in some commercial mixtures.
As long as they are very well rotted, farmyard or stable manures (see Types of Organic Matter) can be chopped up and included in rough home-made mixtures, but the concentration of nutrients is likely to be high. Manures are included in many proprietary mixtures but they are composted in some way: anaerobically in a “digestor”, for example, or worked by worms. Manures from intensive livestock systems are not acceptable in an organic garden.
Some proprietary mixtures contain up to 25 per cent bark, and you can also buy bags of fine-grade bark suitable for home-made mixtures. This should have been composted or weathered to remove naturally occurring toxins that can harm plants. Avoid coarse grades intended for mulching. Some bark may have been composted with chemical nitrogen, but this is difficult to determine — ask the manufacturers or look for a brand with a recognized organic symbol. Bark tends to make mixtures more open, reducing waterlogging, and there is also evidence that it can suppress diseases such as Pests and Diseases of Seedlings and Young Plants). It has little nutrient value and can deplete the mixture of nitrogen as it breaks down; mixtures containing bark tend to need extra fertilizers or composted manure to allow for this.(see
Peat has an ideal structure for seed and potting mixtures and it is also a stable and sterile material. It has a very low pH. However, peat is not a quickly renewable resource and its extraction is destroying valuable wildlife habitats. It is therefore best to use a substitute such as coir wherever possible.
This is waste from coconut husks after the long fibres have been used to make matting and ropes. Coarse and finely milled grades are available. It is substituted partly or wholly for peat in proprietary mixtures and can be bought for using in home-made mixes, either in bags or in compressed bricks which have to be soaked in water before use.
Coir has a good structure and low nutrient value. Its pH is generally higher than that of peat. Some coir is chemically sterilized, so look for a brand with a recognized organic symbol. One disadvantage of this material is that it has to be shipped from countries such as India and Sri Lanka; local materials are more acceptable and likely to be cheaper.
Coarse sand or grit
This can help drainage and make the compost heavier, which is useful if you have pots outside exposed to wind. Use a material with a particle size of 3.0 – 1.5mm (1/8 -1/16in). If it is too fine it will fill up air spaces and impede drainage. It is best to buy “horticultural” sand and grit from a garden centre, as materials from builder’s merchants may have a high lime content.
Composted crop wastes
Products such as brewer’s waste and milled straw are sometimes used in proprietary composts.
Composted municipal waste This is an ingredient of some proprietary composts. Problems of using general waste include the possibility of contamination by heavy metals, glass and plastic fragments and disease organisms, and the fact that it is mixed makes it a very variable commodity. Separate collection of green waste is much to be preferred. A few local authorities may sell bags of composted waste to gardeners.
Two or three-year-old leafmould (see How to Compost – Making Leafmould) is one of the best basic ingredients for homemade mixtures. It is stable and holds air and water well, although it tends to drain more quickly than peat. The older it is, the better it holds water. It generally contains few weed seed’s or pests and disease organisms, and contains very few nutrients. For fine-textured composts, use leafmould that has passed through a 6mm (1/4in) sieve.
A mixture of chopped comfrey and leafmould left to break down (see Comfrey Uses) is a very useful ingredient for home-made mixtures. The comfrey supplies a good range of nutrients and the leafmould gives the mixture the required Physical properties. The concentration of nutrients can be increased by adding organic fertilizers or it can be decreased by adding extra leafmould.
Perlite and vermiculite
These are sometimes used like sand to aid drainage but they are much lighter. They are avoided in organic mixtures because their manufacture requires a lot of energy.
This is a waste product successfully used in some proprietary mixtures and available for homemade mixes. It can give the same problems of nitrogen depletion as bark.
Mixtures of organic fertilizers (see Organic Matter for the Garden) can be used to boost the nutrient content of growing media. They generally release nutrients slowly enough to be used safely in mixes without jeopardizing seed germination or growth of seedlings.
Limestone/dolomite lime/calcified seaweed Any of these materials (see Knowing Your Garden Soil) can be used to raise the pH of mixtures if necessary.
Using new mixtures
Try to find out the ingredients of any product you buy before using it. If possible, choose a brand of growing media which displays a recognized organic symbol, and check that mixtures from garden centres are new stock and have not been stored outdoors in the sun and rain. Do not sow or plant everything into an unfamiliar mixture at once — try a few sample seeds and plants first in case it is unsuitable. You may have to adjust your watering: water little and often at first, as it is easier to correct underwatering than overwatering. If the mixture is dry on the surface, poke your finger or a plant label in it to see if it is wet underneath.
Small black sciarid or mushroom flies are sometimes attracted to mixtures that contain organic matter, particularly if they are over-watered. The larvae of some species feed on young roots, weakening plants and sometimes killing seedlings. Repot affected plants into new compost, destroying any maggots that you see. A biological control similar to that used against vine weevil (see Plant Pests and Diseases – Biological Control) is also effective.
All seed and potting mixtures are best used fresh, particularly with those containing organic matter and organic fertilizers. Micro-organisms continue to work when the mixtures are stored, and in airless conditions this can cause the build-up of substances harmful to plants.
It is best to make up your own mixtures as you need them — ideally not more than two or three weeks in advance. Put them in clean plastic bags or covered bins which have some air holes. Store all mixtures in a cool, dry place.
If you have old mixtures, tip them out into a thin layer for a day or two before using them.
Potting up plants
Before potting up, water plants thoroughly. Loosen them by gently knocking the side of the pot on a firm surface or with a hammer.
The new pot should be large enough to allow about a 2.5cm (1 in) gap all round the rootball of the plant. It should also be tall enough to allow a space between the top of the potting mixture and the rim of the pot so that you can water without spillage.
Place the plant in the new pot and fill the space around it with moist potting mixture, tapping the pot gently and lightly firming the mixture with your fingers. Water to settle it in.
Plants newly transplanted into a nutrient-rich mixture may have difficulty in taking up water at first, so give them shade in sunny weather and make sure they do not dry out.