Organic matter in the root environment - Principles of Horticulture - page 160

The majority of fungi (see p. 62) live saprophytically on
soil organic matter. There are many that are tolerant
of acid soils and they are responsible for much of the
decomposition of organic matter in these conditions.
Some fungi species are amongst the few organisms
that can digest and breakdown wood. The white rot
fungi are very unusual as they are able to digest lignin
in wood, leaving the white cellulose, such as bracket
fungi, Bootlace or Honey fungus ( Armillaria spp., see
p. 261). Soft rot fungi attack and break down the
cellulose in wood and along with the brown rot fungi
(formerly 'dry rot') can damage wooden structures.

gardeners depend on composting as a means of
recycling their garden and kitchen waste to maintain
organic matter levels in their soils. Many councils
now collect 'green waste' and supply composting
equipment to encourage householders to recycle
garden waste as well as their paper, glass and metals.
This is not as environmentally friendly as home
composting as there is significant transport involved.
Horticulturists are increasingly concerned with the
recycling of waste and attention is being given to
composting methods to deal with the large quantities
of material generated on their units. Composting
is fundamental to successful organic growing (see
p. 10).

Mulching
Mulches are materials added to the soil surface in
order to provide one or more of the following:
X decorative fi nish (e.g. chipped bark on borders)
X weed suppression
X moisture retention
X protect the soil surface and reduce erosion
X maintaining/increasing soil organic matter
X stimulating benefi cial soil organisms
X modifying soil temperatures (insulates)
X protecting edible crops from soil contact/splash
(e.g. straw under strawberries).

Composting is the decomposition of organic
matter (including plant residue) in a heap before
it is applied to soils.

Conditions for successful composting must be
favourable for the decomposing organisms (see
p. 157):
X Air (oxygen) - the benefi cial organisms are
aerobic (see p. 114) so they require well aerated
conditions throughout.
X Water - the decomposers require a moist material
to eat and to live in, most are inactive when too
dry. If too much water is added, the air (oxygen)
is driven out (in the waterlogged conditions the
anaerobic organisms take over leading to poor
compost). Once the heap is moist enough, a roof/
cover is advantageous by keeping out rain (excess
water) and retaining heat.
X Organic matter mix - in order to achieve a
mix that provides the right balance of nutrients,
water and aeration, it is convenient to distinguish
between 'green' (leafy/succulent/tender) and
'brown' (fi brous/woody/tough) materials and
combine them in approximately equal measure
(Table 13.1).
X Accelerators or activators can help when there
is not enough 'green' material to supply the full
range of nutrients especially nitrogen. They are
essentially nitrogen fertilizers and not normally
needed in garden composting. Also available but
even less likely to be needed are materials that
make good a shortage of 'brown' material, such as
sawdust (see Table 13.1).
X Shredding - the rate of decomposition is speeded
up by reducing the size of the material put on the
compost heap. Shredding increases the surface
area so making more of the material accessible to
the organisms. The degree of shredding should be

Mulches are materials applied to the surface
of the soil to suppress weeds, modify soil
temperatures, reduce water loss, protect the soil
surface and/or reduce erosion.

Many organic materials are used as mulches including
farmyard manure, garden compost, mushroom
compost, composted municipal waste, leaf mould,
chipped bark, composted straw and green manure.
There are several non-organic materials used for
mulches including minerals (pebbles, slate, stone
chippings), tumbled glass and sheets such as woven
polypropylene/fibre, polythene, paper and old carpet.
Except for the sheet mulches (woven fibre, polythene,
paper, carpet), the materials need to be laid thickly to
be effective. Most can be applied at any time when
the soil is moist in accordance with their function, but
those that are insulatory should be applied after the
soil has warmed up. Care should be taken to remove
perennial weeds before applying and to keep mulch
away from the base of woody stems.

Composting
Compost is a dark, soil-like material made of
decomposed organic matter (see Figure 13.1). Many