Alternatives to peat

Whilst peat remains a popular choice as a compost ingredient, great efforts are being made to find alternatives in order to preserve the wetland habitats where peat is harvested. A list of some of the materials used is given in Table 22.1. Much progress has been made by using suitably processed bark or coconut fibre in composts. Along with several other organic sources they are waste-based and recycling them helps in conserving resources. All such alternatives must be free of toxics and pathogens. Several inorganic materials, such as sand and grit, have always been used in composts, but there is now a wider choice available. Most of the inorganic alternatives are made from non-renewable resources (sand, loam, pumice) or consume energy in their manufacture (plastic foams, polystyrene) or both (vermiculite, perlite, rockwool).

Where possible an environmentally friendly alternative is used, but there is considerable debate about the relative merits of some of those being used because of the associated energy use in their manufacture or transport. However, peat is being replaced successfully by different substitutes, many only available locally, according to the needs of the various sectors of the industry.

Sand, grit or gravel is used in composts, frequently in combination with peat. They have no effect on the nutrient properties of composts except by diluting other materials. They are used to change physical properties. As sand or gravel is added to lightweight materials the density of the compost can be increased, which is important for ballast when tall plants are grown in plastic pots. Sand is also used as an inert medium in aggregate culture. Sand should be introduced with caution because it tends to reduce the air-filled porosity (AFP) of the final mix. It is important that the sands used should have low lime levels; otherwise they may induce a high pH and associated mineral deficiencies (see trace elements).

Pulverized bark has been used as a mulch and soil conditioner for many years. More recently it has been tried in compost mixtures as a replacement for peat. There are many different types of bark and they have different properties. Its problems include the presence of toxics, overcome by composting, and a tendency to 'lock-up' nitrogen (see carbon to nitrogen ratio), which can be offset by extra nitrogen in the feed. When composted with sewage sludge, a material suitable as a plant-growing medium is produced. It is increasingly being incorporated into growing mixes in the attempt to reduce the use of peat.

However, the great variation of barks, especially when they are from a mixed source, makes it difficult to incorporate into growing mixes. Much of the conifer bark tends to be stringy. Consequently the main role of bark is in mulching. The import of bark is strictly controlled by the Forestry Commission to prevent the introduction of pests and diseases. Wood-fibres based on stabilized shredded wood are being used to increase the air-filled porosity of mixes, but they tend to be dusty and not easily dispersed in compost mixes. Sawdust and off-cuts from the chipboard industry are also being tested for use in growing, but there are problems associated with their stability and fungal growth in the freshly stored material.

Coconut wastes such as coir (the dust particles) are proving to be useful in growing mixes. The material has good water-holding capacity,

Table 22.1 Alternatives to peat

Organic materials

Inorganic materials

Pine

Expanded aggregates

Coir

Extracted minerals

Garden compost

Hydroponics

Heather/bracken

Perlite

Leaf-mould

Polystyrene

Lignite

Rockwool

Recycled landfill

Dredgings/warp

Refuse-driven humus

Vermiculite

Seaweed

Topsoil

Sewage sludge

Spent hops and grains

Spent mushroom compost

Straw

Vermicomposts

Wood chips

Woodwastes

Wood fibre

rewetting and air-filled porosity characteristics. It has a pH between 5 and 6, which makes it suitable for a wide range of plants, but it cannot replace peat directly in mixes for calcifuges. It has a carbon:nitrogen ratio of 80:1 which means that allowance has to be made for its tendency to 'lock-up' nitrogen (see p325).

Perlite is a mineral that is crushed and then expanded by heat to produce a white, lightweight aggregate (see Figure 22.3). The granules are porous and the rough surface holds considerably more water than gravel or polystyrene balls. It tends to be used to improve aeration of growing media generally and for the rewetting of peat. It is devoid of nutrients and has no cation exchange capacity. Graded samples may be used in aggregate culture, but it tends to be used to add to mixes to improve the uptake of water.

Vermiculite is a mica-like mineral expanded to twenty times its original size by rapid conversion to steam of its water content. The finished product is available in several grades, all of which produce growing media with good aeration and water-holding properties (see Figure 22.3). There is a tendency for the honeycomb structure to break down and go 'soggy'. Consequently, for long-term planting, it tends to be used in mixtures with the more stable peat or perlite. Some vermiculites are alkaline, but the slightly acid samples are preferred in horticulture. Vermiculite has a high cation exchange capacity, which makes it particularly useful for propagation mixes. Most samples contain some available potassium and magnesium.

Rockwool is an insulation material derived from a granitelike rock crushed, melted, and spun into threads. The resulting slabs of lightweight, spongy, absorbent, inert and sterile rockwool provide ideal rooting conditions with high water-holding capacity and good aeration. Shredded rockwool can be used in compost mixes (see Figure 22.3). Its pH is high but is easily reduced by watering with a slightly acid nutrient solution. It is frequently used in tomato and cucumber production and film-wrapped cubes are available for plant raising and pot plants. It is necessary to use a complete nutrient feed (see aggregate culture). It has some buffering capacity, but this is very low on a volume basis. The main problem areas lie in calcium and phosphorus supply and the control of pH and salt concentration. Some rockwool has been formulated with clay to overcome some of these problems. This increases its cation exchange properties, making it very suitable for interior landscaping. Rockwool is also available in water-absorbent and water-repelling forms. Mixtures of these enable formulators to achieve the right balance between air-filled porosity, water-holding and capillary lift. Rockwool is available as granules that provide a flexible alternative for those who produce their own mixes. However, it is most usually supplied as wrapped slabs,

Perlite Alternatives
Figure 22.3 Growing media. Top to bottom: rockwool, perlite, vermiculite, expanded clay aggregates

cubes, propagation blocks and plugs that are modularized to create a complete growing system.

Pumice is a porous volcanic rock that is prepared for use as a growing medium by crushing, washing (to remove salt and 'fines') and grading. It is most commonly used to grow long-term crops such as carnations in troughs or polysacks.

Expanded polystyrene balls or flakes provide a very lightweight inert material, which can be added to soils or composts as a physical conditioner. It is non-porous and so reduces the water-holding capacity of the growing medium while increasing its aeration, thus making it less liable to waterlogging when over-watered. This has made it an attractive option for winter propagation mixes. However, it is less popular than it might be because it is easily blown away and sticks to most surfaces.

Plastic foams of several different types are becoming popular for propagation because of their open porous structure. They are available as flakes and balls for addition to composts or as cubes into which the cuttings can be pushed.

Chopped straw has been used with some success. Generally the main types available, wheat and barley, break down too easily and a practicable method of stabilizing them has not yet been found. Stable, friable material has been derived from bean and oil seed rape straws, although care is needed in mixes because of the high potassium levels.

Lignite is very variable soft brown coal formed from compressed vegetation; often found at the base of the larger peat bogs. The dusts, 'fines', have been used as carriers for fertilizers and the more granular material can be used to replace grit in mixes, often bringing an improved water retention.

Absorbent polymers have the ability to hold vast quantities of water that is available to plants. However, this is considerably reduced in practice because water absorption falls as the salt concentration of the water increases and the release patterns appear to be very similar to that of some compost ingredients, such as sphagnum moss peats.

Wetters, or non-phytotoxic detergents, are included in mixes to enable water to wet dry composts. They reduce the surface tension of the water, which improves its penetration of the pores. This speeds up the wetting process and maximizes the water-holding capacity of the materials used. Wetters should be selected with care because the different types need to be matched with the peat in the mix and above all must not be harmful to the plants.

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Responses

  • adelinda
    Where to buy organic peat moss?
    6 years ago
  • ursula
    Does perlite eat alkalinity?
    5 years ago

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