Each individual nutrient element has a
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. Quite
simply, an oversupply of nitrogen will not compensate for the under-supply
With respect to a plant’s nutrient
requirements, successful growth requires that:
- All nutrient elements are provided.
- The correct species of these nutrient
elements are used.
- The nutrient feed is
managed correctly to ensure
all elements are available for root uptake.
Inorganic nutrients for plants
Satisfactory plant growth requires all
nutrients being simultaneously available and in sufficient quantity (Table
7.20). The 6 “macronutrients” are those needed in the largest quantities.
Of these, the 3 most important ones are the NPK fertilizers -
(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 Table 7.20. 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. Ions with a positive charge
(+) are called “cations” and are attracted to a root hair containing
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. Provided pH is within the acceptable
range, they are readily available for root uptake. Some common sources are
shown in the “ingredients” column of Table 7.20. 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. The unwanted and 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 processes.
Organic nutrients for plants
Pure organic substances such as humic acids
and seaweed can add valuable components to nutrient solutions and soils.
However, they have a limited ability to provide an actual ‘nutrient’
effect. The reason for this is that they typically contain only a narrow
selection of macronutrients and trace elements, all of which are in an
These “organic” macronutrients and trace
elements must first be converted into “inorganic” nutrients before they
can be absorbed by roots. Hence, pure 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 with exactly what they want, as required.
It is worth noting that organic substances
are especially important when growing in soil. They compliment inorganic
nutrients by increasing the cation exchange capacity (CEC) of the soil.
Although inorganic’s provide the nutrition, a high CEC helps prevent the
inorganics from being washed away down through the soil profile each time
the plant is watered. CEC is particularly poor in soils that are composed
largely of sand. Therefore the distribution of organic matter (e.g.
compost) throughout the soil profile will help increase nutrient
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 for plants
It is necessary to “chelate” several of the
‘trace elements’ (Table 7.20) with compounds such as EDTA and EDDHA.
Chelates are organic compounds that “complex”
(or combine) with trace elements enabling them to remain water soluble
within pH range 5 to 7. Although many brands use chelated iron, many do
not use chelates for copper, manganese and zinc. This is a risky practice
because these trace elements can be very unstable even if the nutrient
solution pH exceeds 6.5 for short periods of time.
Although the use of chelated trace elements
is often claimed on some nutrient labels, other brands are unable to
mention this because of the labelling regulations enforced in certain
jurisdictions in which these are sold e.g. USA.
Toxic nutrient species
Toxic species can be present in nutrient
formulations because the nutrient manufacturer has chosen to use an
inferior type or grade of ingredient.
Ammonium: Toxic levels of this species can
occur due to the use of excessive ammonium nitrate in the formulation.
Ammonium’s 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 and 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: Toxic levels can occur due to the
use of potassium chloride and/or ammonium chloride. Excess chloride will
typically cause chlorosis (leaf yellowing) which results in poor growth or
Several years ago, one popular nutrient brand
contained ammonium chloride. This produced ~250ppm of chloride when
diluted 100-fold. At this dilution the chloride content alone would
register an EC of 0.3mS.