Agriculture Reference
In-Depth Information
High concentrations of free nutrients in fresh crop residues inhibit the production of
enzymes required for parasitism by biocontrol agents such as
Trichoderma
spp. (Hoitink
et al
., 1993).
Composts must be suffi ciently stable and colonized to a degree that allows microbio-
stasis to prevail. Immature composts frequently contain toxic compounds which affect
the growth of crop plants and pre-dispose them to attack by pests and pathogens (Hoitink
et al.
, 1993; Hoitink & Boehm, 1999). The point during development of maturity at which
most of the readily soluble sugars and other easily metabolized nutrients have been used
up often coincides with the development of disease suppression.
On the other hand, excessively humidifi ed organic matter, such as dark sphagnum
peat, cannot support the activity of biocontrol agents. For example, the addition of older,
more humidifi ed peats to composted bark (a common practice in container production)
reduced or eliminated its suppressiveness (Boehm & Hoitink, 1992). The maturity of
bark compost has also been shown to affect the degree of suppressiveness (Nelson
et al
.,
1983). Composts that contain organic matter with properties in between the two extremes
of decomposition are likely to best support biocontrol. There are several commercially
available diagnostic tests available to determine compost maturity based on respiration
rate (O
2
uptake or CO
2
evolution), C:N ratio and ammonium-N:nitrate-N ratio or a mix-
ture of these. Others have determined the decomposition level of organic matter present in
composts using nuclear resonance spectroscopy (NMR) or Fourier Transformed Infrared
(FT-IR) procedures with the aim of relating it to the level of disease suppression recorded
(Inbar
et al.
, 1989; Boehm
et al.
, 1997). However, researchers do not yet agree on practi-
cal guidelines that defi ne the critical stage of decomposition. The temperature zone within
the compost pile from where the composted hardwood bark is taken has also been shown
to affect suppressiveness of the container medium (Chung & Hoitink, 1990).
5.7.4.3
Chemical and physical attributes of the compost
Several chemical and physical compost properties (the values of which are largely derived
from the feedstock type (s), the composting system, the level of compost maturity and
the degree of decomposition of the organic matter) are known to affect crop growth, crop
health and the degree of compost suppressiveness (Hoitink
et al
., 1993). These include
cellulose and lignin content, C:N ratio, nutrient (especially N) content, electrical conduc-
tivity (content of soluble salts), pH, the presence of toxic compounds, particle size and
air-fi lled porosity (especially in container media), moisture content and possibly also the
chloride ion concentration.
Suffi cient information has now become available on the disease-suppressive properties
of composts to allow predictable biological control of diseases in some crop production
systems. In particular, reliable control of
Pythium
and
Phytophthora
spp. can be achieved
in container production systems, where the optimum chemical and physical properties of
growing media have been documented and tested in detail (Nelson
et al
., 1983; Hoitink &
Fahy, 1986; Ownley & Benson, 1991). It has also been shown that the use of composts
prepared from heterogeneous wastes that vary in salinity, N availability and degree of
decomposition can lead to marked increases in disease incidence and severity, therefore
quality control of composts is of prime importance where they are to be used as part of a
disease control strategy.
