The Robinson Garden at Earlscliffe, Baily, Co. Dublin, Ireland
The Robinson Garden at Earlscliffe, Baily, Co. Dublin, Ireland
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Mulches - Alternatives
To Peat And Their Use
Conference On Peat Alternatives
Summary.
Mulching has been practised for hundreds of years and a wide range of materials
can be used as alternatives to peat. Bark chips and plastic film are two
contrasting types of mulch that are increasing rapidly in importance at present.
Mulches can give excellent results when used in the right circumstances, but the
outcome will be disappointing if conditions are unsuitable.
Growers should understand clearly what they want to achieve by using a mulch.
They should also know the properties and cost of candidate materials so that a
suitable choice of mulching material can be made. INTRODUCTION
A
mulch is any material applied to the soil surface to prevent
loss of water by
evaporation, to suppress weeds, to reduce temperature fluctuations or to promote
productivity.
Although mulching is generally regarded as a good horticultural practice,
undesirable side effects can also occur, so that mulches must be used sensibly.
The value of mulching, like many other horticultural practices, depends very
much on local circumstances, such as soil, climate and the type and growth stage
of the plants to be mulched. As far as possible, clear specifications should be
produced both with the regard to the mulching material and also the way it is
used. It is
not surprising therefore that a paper on mulching must contain many qualifiers.
For example a peat mulch can be particularly beneficial to rhododendrons and
many other ericaceous species (Harig and Witt 1984), but is some circumstances
it can be detrimental by absorbing and holding moisture and releasing it again
to the air by evaporation. Mulching is an old horticultural practice used world
wide and so a wide range of alternatives to peat, both organic and inorganic,
are already in use. Organic mulching material is bulky and costly to transport. Consequently
organic mulches are unlikely to be economic unless material is available cheaply
or as a local waste product (Rowe-Dutton 1957). Composted bark has become an
important mulching material in the last decade but a wide range of other
materials are also used. These include organic substances such as spent mushroom
compost (Maher 1990), woodchips, pine needles (Paterson et al 1979), leaves,
sawdust, straw, grass clippings and corn cobs (Gartner 1978); mineral material
such as sand, gravel, stones and granite chippings (Wright 1985), rock wool
(Paterson et al 1979) and manufactured materials such as plastic, paper,
glasswool, metal foil, cellophane and urethane foam (Greager and Katchur 1975).
Proprietary mulches have also been developed e.g. strawdust consisting of resin
impregnated granules of wheat straw. This material is long lasting, sterile and
contains a slow release nitrogen fertiliser (Tuefel 1983).
Mulches are used in horticulture for a wide variety of purposes including
regulation of soil temperature, light reflection and prevention of erosion, but
moisture conservation and weed control are considered to be their most useful
properties. Soil moisture
By
providing a protective barrier at the soil surface a mulch reduces water
evaporation (Scholl and Schwemmer 1982). Suppression of weed growth also reduces
moisture loss through transpiration. This enables the important surface soil
layer to remain moister for a longer period than an unmulched soil surface.
Water
penetration into the soil may be improved by certain mulches such as straw. This
is due to a number of factors including the protection provided by the mulch
against rain impact at the soil surface, reduced soil compaction (Ingle 1981),
and probably to increased activity by soil fauna. Where
moisture conservation is a major consideration, organic mulches such as bark,
straw or wood chips will be preferable to plastic as they tend to retain higher
moisture levels (Ashworth and Harrison 1983). However there are large
differences between organic mulches in their moisture conservation properties,
straw being more effective than manure with hay and wood shavings intermediate
(Harris and Yao 1923). The moisture holding properties of any individual
material is affected by its physical condition. Finely ground bark with particle
sizes up to 25 mm, retains more moisture than coarsely ground bark (75 mm) with
a medium grind (50 mm) intermediate (Gartner 1978).
Plastic and other solid synthetic mulches may prevent water penetration and
cannot be laid over large areas without some means of enabling water to reach
the soil, such as perforating the film. In addition the high temperatures that
develop under clear plastic will also tend to reduce soil moisture levels
(Ashworth and Harrison 1983).
Higher
soil moisture does not always benefit plant growth. On low lying, poorly drained
sites or on soils that do not dry out quickly, excessive moisture under mulches
during wet springs, may result in root asphyxiation (Agulhon 1975). Impeded
drainage and nitrogen deficiency were suggested as the likely causes of the
better response of the moisture-loving alder than sycamore and lime to mulches
of bark, sewage sludge and press board sheets (Insley 1981).
Weed control
There
are many reports of successful weed control with a wide variety of mulching
materials. Good results have been achieved with many different kinds of organic
material. The depth or thickness of a mulch necessary to control weeds will
depend on the type of material used. A depth of about 100 mm when settled is
required for straw (Bushnell and Weaver 1930) and about 50 - 75 mm for sawdust
(Rowe-Dutton 1976). A deep mulch (100 mm) of hardwood bark chips, sawdust or
crushed corn cobs gave more effective control than a shallow mulch (50 mm) of
the same materials. Provided a 75 mm layer of bark mulch is properly applied,
95% weed control should be achieved over a three year period (Campbell-Lloyd
1986). Apart
from the smothering effect of mulches, chemicals in the materials used can also
affect weeds. Phenols and tannins in coniferous bark and sawdust improved the
degree of weed control and reduced cultivation costs compared with chopped bark
and straw and a mixture of chopped shrub waste and limestone gravel (Kolb et al
1985). Similarly better weed control was obtained in roses with sawdust than
with rockwool (Paterson et al 1979). Although organic mulches can be effective
against annual weeds they usually have little effect on established perennials.
Perennial weeds can emerge through deep layers of
organic mulch and, if present when the mulch is applied are likely to thrive
because of the absence of competition from annual weeds. Opaque plastic
materials give excellent control of annual weeds and are more effective than
organic mulches against perennials. However, some of the more aggressive
perennial species, or those with sharply pointed shoots such as scutch grass (Elymus
repens) can penetrate thin polythene
(38 micron) film (Davison 1983).
Despite the general efficacy of mulches against weeds, problems can arise with
both opaque synthetic and organic mulches. If plastic mulches are torn,
accidentally or to assist water penetration, weed growth through the holes will
be vigorous (Rowe-Dutton 1976). Even with undamaged plastic vigorous weed growth
can occur at the gap around the stem or stake.
Although transparent plastic film results in higher soil temperature than opaque
film and crop growth may be enhanced initially (Agulhon 1975), the vigorous weed
growth that occurs beneath clear plastic is a severe limitation. Consequently
transparent films are little used for mulching of perennial plants. Some organic
mulches (bark and sewage sludge) can break down quickly and be reinvaded by
weeds (Insley 1981). Finely pulverised grades of bark, in particular, tend to be
colonised rapidly (Campbell-Lloyd 1986). Moreover, the weed problem may be
increased by weed seeds introduced in certain mulches, such as fresh manure or
hay cut when seeding. Soil temperature
Each
mulch type creates its own unique soil temperature regime and big differences
have been recorded between organic and plastic mulches. Under organic mulches
soil temperatures are lower during the day and slightly higher at night than
those in bare soil (Paterson et al 1979). Temperature fluctuations are therefore
reduced (Ashworth and Harrison 1983) but organic mulches have less effect on
seasonal temperature variation (Bredell and Barnard 1974). The
effect on soil temperature will depend on the depth and thickness of the mulch
material. A deep mulch (100 mm) of hardwood chips, sawdust, jointer curls or
corn cobs had a better insulation value than a shallow mulch (50mm) (Gartner
1978). Dark
coloured mulches such as black polythene, absorb the sun's rays and may increase
soil temperature considerably especially during sunny periods. In a trial in
Effect of mulches on soil
nutrients and physical properties
Different mulches will affect soil fertility and structure in a variety of ways.
In a comparison of nine mulches, including straw and synthetic materials, the
soil under bark showed the highest pH, organic matter content and potassium
levels (Ashworth and Harrison 1983). Bark mulch also gave a greater increase in
potassium, calcium and magnesium than sawdust or corn cobs (Gartner 1978). Spent
mushroom compost is a rich source of nutrients, an application of 20 tonnes per
acre will provide 36 kg nitrogen, 34 kg phosphorus and 158 kg potassium per acre
(Maher 1990). The
application of mulching materials with a high carbon/nitrogen ratio (> 30:1)
will result in a depression of nitrification at least initially. Mulches such as
fresh sawdust (C:N ratio 500:1) and wheat and barley straw (100:1), normally
will require additional applications of nitrogen to compensate for this
imbalance, but mulches of young grass clippings (12:1), average grass clippings
(19:1) or seaweed (19:1) will not (Wright 1982a). Plants of
Spiraea japonica mulched with conifer sawdust were smaller in size than
unmulched plants after six years, but growth of both unmulched and mulched
plants was greatly improved with applications of ammonium sulphate (Kolb 1984,
Karbe 1984). The growth of rhododendrons was temporarily improved by a mulch of
fermented bark while their growth was adversely affected by unfermented bark
throughout a three year study period (Harig and Witt 1984). Better growth of
trees and shrubs was recorded with a combination of bark mulch plus nitrogen
fertiliser than with fertiliser or mulch alone (Whitcomb 1978).
Apart
from the effect of mulching material on nitrification, plant growth may also be
affected positively or negatively by chemicals in the mulching material. The
growth of seedlings of
Plant growth and crop
yield
In
view of the complex effect of mulches on many different soil functions, it is
not surprising that many different types of mulch affect plant species in
different ways and that no single type of mulch performs best in all situations
or for all species (Ashworth and Harrison 1983). Good response in terms of
increased plant vigour has been obtained with a wide variety of different plant
materials including pine bark (Insley 1981), hay, black plastic, calcined clay (Litzow
and Pellet 1983), heavy duty green plastic (Ashworth and Harrison 1983), bark
and sawdust (Kolb et al 1983), and plastic and gravel mulch (Werken 1981).
Mulches of black plastic (38 or 125 microns thick) have given consistently good
results on woody ornamentals and fruit plants without adverse effect on any
plant species (Davison 1982). Average increase in the growth of apples and
blackcurrants mulched with black polythene was in the order of 30 - 40% compared
with only 10% for a straw mulch. In these trials polythene-mulched crops made
more growth than those kept weed-free with herbicides or with hand weeding.
However, Whitham (1982) obtained a strong correlation between growth of
Eucalyptus and weed control with most mulching and herbicide treatments ested.
In this work, mixtures of simazine and aminotriazole gave greater growth than
pine bark, grass hay, sawdust, black plastic, scoria, newspaper and hoeing.
The
increased vigour of plants mulched with opaque plastic compared with unmulched
plants may continue for several years. The weights of shoots on mulched vines
were much higher over a five year period than those from non-mulched vines (Agulhon
1975). In
Britain, a mulch or a combination of mulch plus herbicides often gives higher
yields of apples, blackcurrants, raspberries and nursery stock than the use of
herbicides alone (e.g. Davison and Baily 1980). However, in
In
some cases the reduction of yield on mulched plots has been attributed to lower
air temperatures and increased frost injury (Robinson 1966), but the effect of
an organic mulch in reducing soil temperatures during the growing season or in
maintaining excessively moist conditions in the soil may also be responsible.
More
extensive root systems have been recorded under mulches (Agulhon 1975, Allmaras
and Nelson 1971). The total weight of a vine root system in the 0 to 0.60 m soil
layer for a mulched plant was 150% higher than for a non-mulched, cultivated
plant after one year. Initially plastic film tends to encourage shallow rooting,
92% of the roots of a mulched vine rootstock occurring in the 0 - 30m layer and
none in the 0.45 and 0.60 m layer, compared with only 19% of the roots of the
cultivated vines in the 0 - 0.30 m layer. The absence of exploitation of the
deeper soil layers by mulched plants is normally shortlived.
Further studies with vines in
Not
only do plants respond differently to different mulches but different plant
species may respond differently to the same mulch.
Mulching with bark or sawdust significantly increased the growth of Hypericum,
Potentilla and Cotoneaster but not of Spiraea, Arundinaria or Geranium (Kolb et
al 1985). Plant establishment
The
rapid development of an extensive shallow root system on vines mulched with
plastic enabled plants to recover more rapidly after planting (Agulhon 1975).
This was attributed to the greater development of surface roots and the better
balanced root system of young plants. The establishment of ornamental perennials
and shrubs was also improved by organic mulching materials especially bark and
sawdust (Kolb et al 1983). Appearance of mulches
The
appearance of a mulch is more important in landscape situations than in field
grown food crops or nursery stock. Bark, both chips from softwood and coarse
ground and screened hardwood, make decorative mulches (Gartner 1978). Of nine
different mulches tested in
In
prestige situations where cost is not a limiting factor, the moisture-conserving
and weed-suppressing advantage of plastic may be combined with the pleasant
appearance of natural mulches by covering the plastic mulch with a thin layer of
bark or granite chips (Wright 1982a). Economics
The
use of mulches can greatly reduce the labour required for weed control. In
Organic mulches would be used much more widely for weed control if they were not
so expensive. Wright (1982b) calculated that the costs per sq m for a 50 mm
thick mulch were peat £1 - £1.20, bark £0.6 - £0.7 and coarse gravel £0.9 - £1.
The price of black plastic (150 gauge) for a 1 sq m mulch around trees was £0.2
- £0.3 per tree. The
economics of mulching does not depend only on the initial cost but also on the
longevity of the material used. Newspaper and a thin mulch of straw had
virtually disappeared after six months, but chopped bark, sawdust and black
polythene lasted well and were not substantially different after six months
(Ingle 1981).
The
transport of bulky organic mulches adds considerably to their costs (Insley
1981). About 500 cu. m of mulching material is required per hectare to give a
minimum mulch depth of 50 mm. Heavy duty plastic can be handled more easily, but
light synthetic mulches are often difficult to lay even in a light wind
(Ashworth and Harrison 1983).
Despite the high price of many organic materials, the cost of mulching was
recouped in one year in
Plastic film
New
technology has resulted in the increased availability of a wide range of plastic
film for mulching (Gilby 1990a). It is now possible to formulate plastics to
control or utilise more effectively the heat and light energy from the sun and
also the heat energy radiated from the soil. When white coloured plastic is
used, more visible light is reflected back towards the crop by the film to aid
photosynthesis. On the other hand, care is needed with dark coloured plastic
where the surface temperature of the mulch may rise excessively under hot
conditions and cause crop injury. Among
the wide range of monolayer mulching films currently available are clear, black,
white, black/white and thermic brown and blue films. The properties of these
films in comparison with uncovered soil is shown in Table 1 (Gilby 1990b). Table 1. Properties of monolayer plastic mulches compared with bare soil
+ =
greater, o = similar, - = less
Clear
polyethylene film (15-50 micron) is used to warm up soil in early spring and
enhance seed germination. This practice is most popular in Mediterranean
countries, but is gaining popularity in
In
contrast to temperate and Mediterranean countries, where clear film is used to
increase soil temperatures, black/white film with the white side uppermost is
useful in tropical countries to mitigate the effects of high soil temperatures.
At the same time weed growth would be suppressed and the exposed surface of the
white film would remain relatively cool.
Experiments are in progress in
Coextruded combinations of black with white or silver are now available. These
have similar energy transmission properties to black films except that the soil
temperature tends to be even lower (Gilby 1990a). The reflection of both light
and heat results in the film remaining cool, but the plants benefit from
additional reflected light. This film can be used to delay the ripening of
late-season strawberry varieties to further extend the season. In
addition, coextrusion makes possible the design of multilayer films in which
different polymers, pigments or different additives with different wavelength
absorption and transmission properties can be incorporated into the various
layers (Gilby 1990a). It is possible in this way to be selective about the
wavelengths that are either transmitted, absorbed or reflected by the film. The
ability to design a mulch to reflect light of any colour is of potential
importance in view of the varying response of crops to different wavelengths.
Specifications
In the
past organic mulches had to be inexpensive because they were bulky to transport
and were often sold without precise specification. Now that there is a growing
market for more expensive bark mulches and plastic film with very different
properties, more detailed specification is needed. For example, a range of
different bark types is available to suppress weeds for several years and
improve the appearance of prestige and other situations. Exact
specification is necessary both with regard to how the material should be laid
and also to the grade of bark to be used. For example, the ground should be
levelled off to a tolerance of 20 mm to remove clods, and edges between the area
for mulching and lawn or paving should be profiled down to 75 mm so that mulch
depths on all edges are no less than 75 mm (Campbell-Lloyd 1986).
Many
different grades of bark are now available depending on source of bark,
composting period, degree of grading and wood content. For example one firm is
currently selling at least nine grades of bark for mulching, ranging in particle
size from 0 - 5 mm for annual plantings up to 45 - 80 mm for large shrubs and
trees.
With
certain organic mulches, information on the nitrogen content is required as
compensatory applications of nitrogenous fertiliser are normally needed if the
carbon/nitrogen ratio is above 30:1 (Wright 1982a). Discussion
Mulches can give excellent results when used in the right circumstances, but the
outcome will be disappointing if conditions are unsuitable. With the growing
emphasis on reducing the use of pesticides, mulches provide a useful
non-chemical means of weed control. Because of their attractive appearance, bark
chips are useful for ornamental plantings but plastic is more practical where
aesthetics are not of great importance. The
benefits of a mulch are, however, usually much greater than weed control alone. The
improved plant growth and vigour often recorded may be due to the conservation
of soil moisture by reduced evaporation and run-off, increased photosynthesis,
the provision of a more favourable soil temperature regime, improved water
infiltration, increased availability of mineral nutrients, enhanced
nitrification, additional nutrients and organic matter derived from a
decomposing mulch, or the preservation or improvement of soil structure. In
addition, mulches may increase the density and spread of roots by increasing the
moisture content of the surface soil.
Where
mulches result in decreased plant performance, this effect too may be due to a
variety of factors, including reduced nitrification, retention of excessive soil
moisture, or the reduction of soil temperatures below optimum for heat-loving
plants.
Mulches are used in so many different situations and can affect so many
different interacting soil factors that it is not usually possible to identify
only one factor as being the main reason for good or bad results.
Literature Cited
Further Reading
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