duction will be more productive,
profitable
and
environmentally
friendly. Nutrient solutions for different greenhouse crops have been
developed at Stellenbosch University, based on Dutch and Belgian information but more research is still
needed to improve these guidelines. In soilless systems, plants
need to be provided with all the
macro nutrients (N, P, K, Ca, Mg
and S) as well as the micro nutrients (Fe, B, Mn, Mo, Cu and Zn) in a
ratio that will optimize the uptake
of each of these elements. This can
be done by using either water soluble fertilizers or controlled release
Figure 1. Absorption zones for mobile and immobile nutrients in the
root zone.
fertilizers depending on the grow-
for his crop, or he can make up his
complexity of some of the dosing
ers’ specific needs.
own mix using a few standard ferti-
systems.
Water soluble fertilizers
Water soluble fertilizers are dissolved into the irrigation water and
applied through the drip irrigation
system with each irrigation cycle.
This system is referred to as continues liquid feeding (CLF) or fertigation. To understand the benefit of
this fertilization method, it is important to remember that only dissolved nutrients, in direct contact
with roots are absorbed. As nutrients are taken up by the roots a
depletion zone develops around the
roots. With frequent, small fertigation pulses the concentration and
ratio of nutrients in the root zone
can therefore be maintained at the
preferred levels.
The grower can either use one of
the many ready-mixes available,
containing all the nutrients needed
lizers. The ready-mixes are convenient and a good option when the
Controlled release fertilizers (CRF)
CRFs are mixed into the substrate
water does not already contain
before planting or top-dressed later
large amounts of dissolved salts.
in the growing season. Growers
When making up your own mix, it
then only need to apply clear water
is easy to customize and adjust the
to the plants unless additional wa-
concentration and ratio of different
ter-soluble fertilizer is needed. The
elements to suit a specific crop at a
minerals in these fertilizers are re-
specific growing stage. As an exam-
leased over an extended interval
ple high ammonium content in the
since the fertilizers are coated with
nutrient solution might be suitable
a substance such as a resin or poly-
for foliage crops, but can result in
mer that prevents the immediate
blossom end rot in fruiting crops,
dissolving of the minerals contained
such as tomatoes, especially during
within. The coating on CRFs actually
the generative growth phase. An
limits exposure of the mineral core
alternative to CLF is applying a
to moisture, resulting in a slow re-
more concentrated solution of the
lease of the nutrients through diffu-
water soluble fertilizer at certain
sion. The polymer coatings used in
times and to just irrigate with water
many of the newer products actual-
in between. One of the disad-
ly allow for flexibility in terms of the
vantages of using water soluble
longevity and release rate. CRFs are
fertilizers can be the large number
more often used for the production
of mixing tanks required and the
of potted plants and an added bo-