Each individual nutrient species has a
specific role in plant growth. If
one is missing or in short supply, the effectiveness of others is reduced
and growth will suffer. For example, magnesium and nitrogen are necessary
in the production of chlorophyll. However, if magnesium is lacking,
chlorophyll production will be restricted i.e. an oversupply of nitrogen
will not compensate for the undersupply of magnesium. This principle is
known as the “law of limiting factors”.
Successful growth requires that:
+ All
nutrient elements are provided.
+ The
correct species of these nutrient elements are used.
+ Correct
nutrient management to ensure all species are available for root uptake.
Inorganic nutrients
Plant growth depends upon a combination of nutrients being available, and
in sufficient quantity (Chart 3.1). The 6 “macronutrients” are those
needed in the largest quantities. Of these, the 3 most important ones are
the NPK fertilizers - nitrogen (N), phosphorus (P) and potassium (K). The
“trace elements” are also needed but only in small amounts.
Generally speaking, nutrients can only be rapidly absorbed by plant roots
if they are in the form of mineral “ions” – as illustrated in the second
column of Chart 3.1. An “ion” is an atom or group of atoms which has gained
or lost one or more electrons and therefore carries a positive or negative
charge. This “charge” enables ions to either repel or attract other ions
i.e. ions with a positive charge (+) are called “cations” and are able to
exchange places on a root hair with negative (-) ions called “anions”.
In this way, nutrients are brought close to root hairs so they can be
absorbed.
“Inorganic nutrients” or “salts” supply
nutrients in the mineral ion form and, provided pH is within the
acceptable range, are readily available for root uptake.
Some common sources are shown in the “ingredients” column of
Chart 3.1. Note these will often be listed in the derivation statement on
nutrient labels.
Where “inorganic
nutrients” (i.e. salts) come from:
Inorganic nutrients are sourced from seawater
and mineral deposits. Unwanted/nuisance salts are removed by a process
called fractional crystallization. Hence they are “synthetic” only in the
sense that they are purified by man-made process.
Organic nutrients
Pure organic sources (e.g. manure, humic acids, seaweed, guano)
essentially contain only the macronutrients nitrogen, phosphorus and a
couple of trace elements in an organic form.
It is important to realize that “organic”
nutrients must be converted into “inorganic” nutrients before they can be
assimilated by roots.
Hence, organic nutrients are slow to act and
give variable results because their conversion into inorganic form takes
time and also requires the action of micro organisms. For this reason,
inorganic nutrients are preferred - they are a short cut method of
providing plants exactly what they want, when they want.
Organic species are important in ‘soil’ culture because
organic and inorganic species can interact to assist plant nutrition.
Although inorganic’s provide the nutrition, organic’s can increase their
effectiveness through their ability to increase the ‘cation exchange
capacity’ (CEC) of the soil. CEC is a soil’s ability to retain an
inorganic species, and to prevent them from being washed (or draining)
away down through the soil profile. CEC is particularly poor in soils
composed largely of sand. Therefore the distribution of organic matter
(e.g. compost) throughout the soil profile will help increase nutrient
retention. Note that CEC is increased by organic “matter” – it does not
need to be organic “nutrients”.
In hydroponic systems CEC is unnecessary because the root
zone is frequently replenished with nutrients. This is why the use of
substrates of essentially zero CEC (e.g. Rockwool, perlite, clay pebbles)
are perfectly suitable.
Trace elements
It is necessary to “chelate” several of the
trace elements with compounds such as EDTA and EDDHA (Chart 3.1).
Chelates are organic compounds that react
with metal ions to form water soluble “complexes”. These increase the
solubility of the trace elements at higher pH values. Although it is
standard practice to employ chelates for iron, many brands do not use them
for copper, manganese and zinc. This is a risky practice because these
elements can be very unstable at the higher pH values that nutrient
solutions can experience.
Toxic nutrient species
Ammonium:
Its concentration should represent no more than ~20% of the required total
nitrogen concentration. An excess can cause damage to roots and stem
bases, particularly in younger plants, which results in poor
growth/yield. It is generally accepted that plants will not uptake
nitrogen in the ammonium (NH4+) form – it must first
be oxidized into nitrate (NO3-).
Chloride:
Excess chloride will typically cause chlorosis (leaf yellowing) which
results in poor growth/yield. Potassium is sometimes supplied as
potassium chloride – this should be avoided. Several years ago, one
popular nutrient brand contained ammonium chloride which after diluting
100 fold produced ~250ppm of chloride in the nutrient. At this dilution
the chloride content alone would register as 0.3mS (cF 3).
Overview:
Check the label to ensure it contains the full suite of macronutrients
(Chart 3.1). Note, for reasons mentioned later, trace elements might
not be declared on the label. Further, to ensure rapid availability to
roots, confirm the nutrients are supplied using “inorganic” nutrients and
not organics (as per the “ingredients” column in
Chart 3.1).
