Thirsty vines Summer girdling Compost teas Organic marketing The ...

The Use of Compost Tea in Kiwifruit
Mike Spiers, HortResearch - Ruakura
Figure 1. Compost tea sprayer in operation in a kiwifruit orchard.
Compost tea has generated interest recently as a product with
potential for improving plant health in a wide range of cropping
systems. However, when applied to soil in organic kiwifruit orchards
in each of three seasons, compost tea had no effect on fruit size,
soluble sugars or dry matter. This study was funded by the MAF
Sustainable Farming Fund, with contributions from ZESPRI, COKA
and four kiwifruit packhouses.
WHAT IS COMPOST TEA?
Compost tea is a water-based extract of compost, produced by
mixing compost with water and sometimes with additional microbial
stimulants, before fermenting for one to seven days. The mixture is
either allowed to stand with occasional stirring (non-aerated
compost tea) or it is agitated continuously by injecting air during the
fermentation process to enhance aerobic conditions (aerated
compost tea). Aerated tea is more common in New Zealand and
was used in this study. The extract is fi ltered to remove the coarse
solid fraction prior to application to crop foliage (to control disease)
or to soil (to improve root health and crop growth). The fi ltered tea
retains soluble nutrients and other organic compounds from the
compost as well as a population of micro-organisms comprising
bacteria, fungi, protozoa and nematodes originating from the
compost, but which may have multiplied during the fermentation
process. This biological content is claimed as the key to the
benefi cial properties of compost tea. Compost tea is also known as
compost extract or compost leachate. Vermicompost/vermicast
(produced by worms) can be used with, or instead of, compost to
make compost tea.
Interest in the use of compost tea among kiwifruit growers in New
Zealand began around 2003, following seminars by Dr Elaine
Ingham of The Soil Foodweb Institute, Oregon, USA. This institute
promotes the importance of the soil foodweb to retain and recycle
nutrients, build soil structure and promote a healthy environment
for plant roots. At that time, compost tea was an unfamiliar term in
New Zealand. However, after attending a seminar and obtaining a
copy of The Compost Tea Brewing Manual (Ingham 2001), many
growers were keen to try this new idea. Several compost tea
manufacturers subsequently set up and began supplying kiwifruit
growers. A Soil Foodweb laboratory and consultancy service,
affi liated to the Soil Foodweb Institute, was also established in New
Zealand.
Among kiwifruit growers, the attraction of compost tea was the
promise of improved soil health at reasonable cost. Organic growers
traditionally use compost to supply plant nutrients and maintain soil
organic matter, but compost application at effective rates is
expensive and laborious. To some, compost tea offered the benefi ts
of compost without the need to spread a bulky product (although
the Soil Foodweb message stresses the need for compost and
does not claim that compost tea is a complete replacement).
Consequently, many kiwifruit growers, especially those using organic
production methods, began to use compost tea, or to show an
interest in its use.
STUDY DETAILS
The study was conducted at eight sites (six ‘Hayward’ and two
‘Hort16A’) in the Bay of Plenty and Waikato over three seasons
from 2004/05 to 2006/07 and was managed at each site by the
orchardist, who chose the tea supplier and the frequency of
application. The compost tea was applied to the soil beneath vines
and not to the vine canopy (Figure 1). At each site, replicated plots
were set up that either received or did not receive compost tea. A
minimum of four applications per season were planned, but on
average only two to three applications were actually applied over
the study period. All other aspects of management of these plots
were kept the same within each orchard.
NZ KIWIFRUIT JOURNAL JANUARY / FEBRUARY 2008
23