The subject of plant nutrition is huge. This section is about
all of the essential macro and micro mineral nutrients, the
several ways these materials are assimilated and utilized by the
plant, and the effects of over or under application. This
section is also about the different materials that are used as
fertilizers and their application.
The essential mineral macro nutrients are: nitrogen, phosphorus
(measured as an ingredient of phosphoric acid or a phosphate),
potassium, sulfur, and calcium. Note that nitrogen is not a
mineral in the true sense of the word and is not present in the
soil particles, per se. Nitrogen as a nutrient must come from
organic matter, air, or synthetic chemical sources rather than
from mineral or rock particles.
The essential mineral micro nutrients or essential "trace
elements" are: magnesium, iron, copper, zinc, manganese,
boron and molybdenum. Certain plants also require: chlorine,
aluminum, sodium, silica, or cobalt.
It seems ironic that in an FAQ on organic gardening that so much
attention needs to be placed on soil chemistry but plants do
feed on mineral nutrients and these are chemical materials.
There is a problem in semantics. Chemicals are so often equated
with toxic and environmentally destructive products, and in many
cases, this is true, but all material things are chemical.
When a plant assimilates and uses an essential nutrient, it does
not differentiate whether that nutrient came directly from a
natural or synthetic source. The problem with synthetic
fertilizers isn't that they contain toxic chemicals, per se. It
is because they contain high levels of nutrients and they
release those nutrients much too fast.
Commercial ammonium sulfate, for example, contains about 21%
nitrogen and releases all of this nitrogen over a period of a
couple of days. Composted plant material runs closer to .5%
nitrogen and the release is considerably slower going into
several weeks. The nitrogen derived from the ammonium sulfate,
however, is identical to the nitrogen derived from the compost.
Fertilizers are nutritional products used to improve the
quantity or quality of plant growth. Soil amendments are
products used to improve the properties of the soil. It is
important that the distinctions be kept in mind.
There have been laws on the books that have prevented the
labeling of packaged manures and composts as
"fertilizers" because of the low mineral content of
these organic products. Specifically, there have been, and
perhaps continue to be, laws in different locations that have
required packaged manures and composts could only be sold as
soil amendments and not as fertilizers. This, however, was
mostly the result of marketing.
Manures and composts are both fertilizers and soil amendments.
Composts and manures are in a class by themselves as first
rate soil amendments and as nutrients for the plants and the
soil organisms. See
Section B.02.* on Soil Properties and Soil
Some of the synthetic fertilizers incorporate resins and/or
other water insoluble materials to bind the mineral nutrients
into the structure of the fertilizer particle or bead. The
intent is to extend the release rate of the mineral content
over a longer period of time. In essence, to slow down the
release of the nutrients.
All of the fertilizer salts are toxic to germinating seeds,
plant roots, and the soil organisms at high levels. Slowing
the release of the nutrients is a way around the build up of
toxic concentrations. This sub-section is about the release
rates of the various synthetic and organic products. Some of
the organic products break down and release their nutrients
faster than others. It is also about the relative toxicity of
the essential plant nutrients.
Most of the mineral nutrients in the soil are in the form of
mineral salts dissolved ina sort of soil soup. The roots feed
on the soil soup and assimilate the mineral nutrients into the
plant by osmosis. There are a couple of exceptions.
The primary exception is nitrogen. Nitrogen is the one
essential macro nutrient that is not a mineral. It must be in
the nitrate form before it can be taken up by the roots.
Nitrogen applied to the soil in the form of a protein from
organic sources, ammonia or ammonium salts from organic or
synthetic sources must first be decomposed or oxidized to the
nitrite form and then decomposed or oxidized further to the
nitrate form. This oxidation process is the work of special
nitrobacteria resident in the soil. Legumes have nodules on
their roots which contain "nitrogen fixing" bacteria
that can take nitrogen directly from the air present in the
soil to short-cut the normal nitrogen cycle.
The second major exception is phosphsorus which is normally
present in the soil in the form of relatively insoluble
phosphate salts. Very little of the phosphate salt is released
into the soil soup. Instead, trace amounts of the phosphate
salt are released to the microscopic film of moisture
surrounding the nutrient particles and the root hairs are left
with the responsibsility of seeking out these phosphate salts.
This pushes the root hairs to seek the phosphorus in new
Foliar feeding offers some advantages in some specialized
situations but it also offers problems or the threat of
problems in general use. There is a fine line between the
amount of nutrient needed for adequate feeding and the amount
that is toxic to the foliar surface. This is particularly true
with the micro nutrients. Foliar feeding is also not very cost
or product efficient as a large amount of the nutrients simply
drip off in the course of application or are washed off by the
rain or watering.
In some cases, foliar feeding is a good way of dealing with
specialized problems. For, example, foliar feeding gives a way
around root slow down in cold soil or problems with nutrients
being tied up in the soil as a result of soil alkalinity. This
sub-section is about the application of foliar feeding and the
ways the plant assimilates nutrients from foliar
Mycorrhizae are symbiotic soil fungi, present in most soils,
that attach themseselves directly onto the roots of most
plants. They help the host plants absorb more water and
nutreints while the host plants provide food for the fungi.
Because the surface area of the hyphae, the feeding structures
of the mycorrhizae, may be several hundred times the surface
area of the roots, the mycorrhizae can feed on a larger soil
mass and they can also feed more thoroughly. Mycorrhizae have
the potential to be a central nutritional source for most
The mycorrhizae are also relatively fragile and can be damaged
or destroyed by a number of soil conditions, cultural
practices, high levels of macro or micro nutrients, and many
of the synthetic fungicides. This sub-section is about the way
the mycorrhizae work, their application and the various ways
of protecting these beneficial organisms.
Plants show a variety of symptoms when fed with too much or
too little of the essential mineral nutrienets. The symptoms
of too much or too little caused by any one of the mineral
nutrient sometimes overlap with the symptoms of too much or
too little caused by another mineral nutrient. The symptoms
might also overlap with a variety of other conditional
Interpreting symptoms of plant distress is mostly an issue of
knowing the possibilities. Correcting nutritional problems is
sometimes straight forward and involves no more than
supplemental feeding. Sometimes, however, it can be complex.
Alkaline soil or water, for example, can cause a variety of
nutritional problems. Getting around the alkalinity may take a
bit of doing. This sub-section is about the identification and
correction of nutritional problems including problems of