Integrating biological management practices into standard pome fruit production
systems
Louise van Schoor, ARC Infruitec-Nietvoorbij, Stellenbosch.
Introduction
The increase in the implementation of Good Agricultural Practices (GAP) on South African
fruit farms and the emphasis it places on sound environmental practices have stimulated an
interest in the use of biological soil amendments in standard agricultural management systems
over the past decade. The role of biodiversity in sustaining crop production and soil fertility
and the associated role of soil microbial functions have become clear (Brussard and FerreraCerrato, 1997; Griffiths et al., 2001; Abbott and Murphy, 2003). Optimal fruit production and
sustained yield are associated with minimising stress through maintaining high soil fertility and
good root proliferation. Since microbial activity is generally carbon-limited in agricultural soil,
management practices that provide a range of organic compounds on a regular basis will
maintain an active and diverse microbial population (Magdoff and Weil, 2004). Mulching
provides a favourable environment for microbial activity and fine feeder root development,
especially in surface soil. Biostimulants, such as seaweed extracts and humic substances, as
well as microbial inoculants in the form of compost extracts and effective microorganisms
(EM), are widely used in South Africa although research information is limited. The wide
scope of this field, the nature of amendments and variable orchard conditions make the subject
extremely complex and clear guidelines are lacking.
The effects of organic material and biological amendments also need to be investigated in
optimally managed, modern orchards systems. In high density orchards the controlled
application of fertiliser directly through the irrigation system (fertigation) is becoming
increasingly popular. Fertigation leads to the development of a more restricted root zone,
making it potentially easier to manage the soil-root interface (rhizosphere). Biological
management of these restricted root volumes may lead to improved rhizosphere conditions and
tree performance (Bowen and Rovira, 1999).
The effect of continued applications of organic material, various microbial inoculants and
biostimulants was investigated on tree performance in standard management systems. A field
trial was conducted in the Vyeboom region, Western Cape, in an ‘Early Bon Chretien’ pear
orchard on BP1 rootstock suffering from poor root development in the initial years after
planting. In a high density ‘Brookfield Gala’ apple orchard on M793 in the Greyton region the
application of biological amendments from second leaf was investigated in an optimally
managed, fertigated orchard.
Results and Discussion
In a conventionally managed pear orchard on BP1 rootstock, regular application of compost
extract in addition to annual compost applications over a period of five growing seasons
improved cumulative yield from 29 ton.ha-1 (control) to 44 ton.ha-1. Shoot growth and trunk
circumference were also significantly improved from the first season of application. After the
third growing season the addition of compost extract resulted in a 46% increase in total growth
compared to the control plots. The additional growth when using compost extract relative to
untreated controls may have resulted in an increase in bearing positions and therewith an
increase in yield and fruit number per tree. Biostart® (a Bacillus bacteria inoculant) applied in
combination with a low dosage humate, showed a longer-term response on tree performance
and only improved growth significantly after application over three growing seasons. Most of
the biological amendments (including seaweed extracts and humates) showed a positive effect
on cumulative yield over the two seasons, relative to the control. Application of manure, as
well as EM in combination with molasses, showed the least effect on yield over the trial
period. However, manure was only applied in the first two seasons. No significant effects were
observed on tree performance when using a wheat straw mulch in the tree row possibly
because irrigation was scheduled on un-mulched plots, obscuring positive soil moisture effects.
No consistent effects on fruit quality were found with mulching or any of the biological
amendments over two seasons.
Humate treatment in combination with compost, application of compost extract on its own, as
well as compost extract in combination with compost, resulted in significant positive effects on
yield when integrated into a fertigation system over a three year trial period. These
amendments improved yield from 46 ton.ha-1 in controls, to 55 ton.ha-1 and yield efficiency
tended to increase with all compost treatment combinations. In this study trees were already in
their second leaf when applications commenced. Biological management practices may be
more effective when applied with orchard establishment due to the high fertility regime
required at tree establishment. Pruning mass, as an indication of tree vigour, was highest for
trees treated with both compost and humates, and least for a humate in combination with wood
chips. Generally, treatments including wood chip application, as well as humates applied
without the addition of compost showed no effect on tree performance. In treatments with a
high C:N ratio, such as wood chips, N can be immobilised temporarily in microbial biomass
which initially creates N-deficient conditions. It is possible that wood chip application may
only show positive results over a longer period.
Biological amendments affect plant growth either directly or indirectly via its influence on root
development and soil microbial communities which may lead to improved plant nutrition, crop
protection or changes in plant growth hormone balances. Microbial activity in the rhizosphere
is a major factor determining plant health and productivity. Limited research has been
conducted on the mechanisms by which compost extracts may improve plant performance.
Most research has focused on the use of compost extracts in disease suppression and in
biological control but improved synchronisation of nutrient release and plant uptake may also
improve tree performance. However, more research is needed on the effects of compost extract
quality and the mechanisms involved in a field environment before commercial application can
be fully recommended.
Acknowledgements
The project is funded by the ARC and the DFPT under the Soil Health Programme.
For further information contact Louise van Schoor: e-mail, vschoorL@arc.agric.za; Tel., (021)
809 3367.
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Compost application in a 3-year old apple orchard in the Greyton area.