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Major Nutrients
Minor Nutrients
Micronutrients
Let's first cover some basic
concepts. Fertilizing is not feeding. Fertilizing is more
like taking vitamin supplements. What plants 'eat' is sunlight and
carbon dioxide and water - the raw ingredients of photosynthesis - which
are used to make starches and sugars, which are what plants burn as food. What we add as fertilizer
are elements such as nitrogen, phosphorus, potassium, iron, sulfur,
magnesium etc. These are comparable to vitamin and mineral
supplements, not bread.
No fertilizer can ever replace inadequate
sunlight. Each species of plant has an inborn requirement for a
certain amount of direct sunlight (sunlight falling directly on the
leaves, not just an absence of night). Planting a plant in too
much shade will starve it, and adding extra fertilizer will never make
up for that, since fertilizer is not food. Roses require full-day
sun, especially in our cloudy climate.
Plants absorb nutrients by the roots, from the soil water.
Nutrients must be dissolved in water, and in the correct chemical form,
to be absorbed by plant roots. Soil pH, water content, mineral
content, and the
proportions of clay and organic matter compared to sand and gravel will
determine what chemical form nutrients will be in, and whether or not
they will be in solution and available to plant roots. Most
nutrients are most available at pHs ranging from 5.5 to 7.5.
Magnesium and molybdenum require the higher pHs, and iron, copper, zinc,
manganese and cobalt require the lower pHs. Another basic
concept is the differences between organic fertilizers, and non-organic.
Non-organic fertilizers may be called chemical, or synthetic, or
conventional. On this website we will use the term synthetic.
Organic fertilizers come from once-living tissues, either plant or
animal, or from other naturally-occurring sources such as greensand or
rock phosphate. Synthetic fertilizers are manufactured in chemical
plants, although they may use natural sources for feedstock.
Proponents of synthetic versus organic fertilizers will say that plants
don't care where nitrogen comes from. This is true, to an extent.
It's equally true that human bodies don't care where sugar comes from.
But which is healthier for you to eat: an apple, where the sugar
is in the form of fructose, or a spoonful of table sugar, which is pure
sucrose. What your body burns is sucrose, but eating table sugar is not
the same as eating an apple! The sucrose you get from eating an apple is released
slowly, preventing a sugar-rush (and the development of adult-onset
diabetes), and is accompanied by many other necessary things such as
fiber, minerals and vitamins, and tiny traces of proteins.
Similarly, applying nitrogen in the form of ammonium or nitrate creates
a 'nitrogen-rush', overwhelming the ability of the plants to absorb the
nitrogen, and of the soil to store it for later use; it also deprives
the soil of other necessary compounds contained in organic sources like humic and fulvic acids,
chelating compounds, amino acids and
micronutrients. It can
also damage the soil by poisoning soil microbes through the release of
ammonia gas. Synthetic fertilizers may be high in the
macronutrients such as nitrogen, but most don't contain any nutrients
beyond the big 3 - nitrogen, phosphorus and potassium. Minor
nutrient and micronutrient deficiencies are more common in gardens
fertilized solely with synthetic fertilizers.
There are situations where synthetic fertilizers will be best, and
those where organic will be best. Most gardens will benefit from
the use of both types of fertilizers over the course of the year.
Information on soil-nutrient cycles on this website comes from The
Nature and Properties of Soils, 13th Edition; Brady, NC and Weil, RR.
2002, Prentice Hall, publishers. Information on fertilizers comes
from WSU-Extension.
The major nutrients are nitrogen (N), phosphorus (P) and potassium
(K). These are the nutrients that plants need the most of.
When you buy fertilizer, the three numbers on the label refer to the
amounts of these three major nutrients, in the order given. An
8-4-4 fertilizer is 8% nitrogen, 4% phosphate (P2O5) and 4% potash
(K2O).
These are the only three nutrients required to be on the fertilizer
label. Nitrogen
Nitrogen is by far the most limiting nutrient in our soils, which are
leached of nutrients by the rain. If you do nothing else for your
roses, give them nitrogen. Nitrogen
occurs in many forms: N2 (atmospheric nitrogen), NO3
(nitrate) and NH4 (ammonium) in particular. Plants
absorb nitrate and ammonium from the soil through their roots, but since
these forms of nitrogen are rather water-soluble, they are lost from the
soil pretty quickly if not absorbed. Synthetic
fertilizers contain ammonium and nitrate nitrogen. Nitrogen can
also be applied in organic form, such as compost or blood meal.
Protein, both animal and vegetable, is an organic form of nitrogen.
Organic forms of nitrogen are much more stable in the soil than the
nitrate and ammonium forms, but they're not available for plant
absorption until they're converted to nitrate and ammonium by soil
microbes.
Good sources of organic nitrogen are animal remains, such as fish
fertilizers or blood meal, and legumes, such as clover, vetch, or alfalfa.
Animal protein contains a lot of nitrogen, and legumes fix atmospheric
nitrogen, so can be high in nitrogen also. Alders are
nitrogen-fixers too, so if you have access to alder leaves, use them in
compost and mulch! Alders actually fix more nitrogen than peas and
beans. Chicken manure also contains a good amount of
nitrogen, since birds eliminate urine and solid waste mixed together -
it's the urine that has the nitrogen. Roses love chicken manure.
Mammals separate the two wastes, so animal manures are not good sources
of nitrogen, except in composted horse or rabbit bedding, where the
animal has been urinating on the bedding. Common, inexpensive lawn
fertilizers use ammonium sulfate, ammonium nitrate, or urea to supply a
lot of nitrogen. These fertilizers tend to make soil more acid,
and our soils are plenty acid as they are. Avoid these
fertilizers, especially in the rose garden. Nitrogen is stored in the
soil as organic matter, or as ammonium adsorbed onto clay and humus
particles. Typically our soils are very low in clay and humus, so there's little storage capacity for nitrogen.
Adding organic matter in the form of compost and manures will increase
both the storage capacity and the nitrogen itself, as well as
stimulating the soil microbes necessary to convert organic N to nitrate
and ammonium. Phosphorus
Phosphorus (P) is the second of the major nutrients listed on
fertilizer labels, and like nitrogen is a basic building block of all
life, animal and vegetable. But where nitrogen is stored in soil
in organic forms and is relatively readily transformed into
plant-available forms, phosphorus is stored in unavailable, insoluble
mineral forms. Plant-available phosphorus occurs as phosphate, PO4.
However, within hours of applying phosphate fertilizer, the phosphate
molecules bind to minerals and clay particles in the soil to form
unavailable, insoluble phosphorus compounds. These compounds tend
to become more tightly bound, and more insoluble, as time progresses,
unlike nitrogen compounds which can be broken down by microbes over time. Organic forms of phosphorus can be broken down over
time, but once the phosphorus is released from the organic form, it is
also quickly bound up in mineral and clay particles. There's a
rather short window of opportunity for plants to absorb dissolved
phosphorus before it's bound up by the soil and lost to the plants, or
lost through leaching and erosion. So, soluble phosphate must be
continually added to the soil, either through the breakdown of organic
matter, or repeated applications of phosphate. The tricky part is
that excess phosphate is easily eroded away into streams, where it's a
serious pollutant and potentially leads to death of aquatic life such as salmon
and trout. Phosphate availability peaks at soil pH of about 6.5.
Little phosphate will be available below about pH 5.5, or above pH 7.
Phosphates are also more available to plants that have mycorrhizal fungi
associated with their roots. The mycorrhizae extend the effective
root zone of the plant by as much as ten times, reaching out to
phosphate ions before they are locked up by the soil. Potentially available phosphorus is stored in the soil in
the form of organic matter, adsorbed onto clay particles, and in iron
oxides. Microbial breakdown of organic matter can provide a slow
but relatively steady stream of available phosphate to plants.
Some phosphate can be made available from that stored on clay and in
oxides, but not much. Organic matter also itself binds to the same
compounds that bind up phosphorus, reducing the phosphorus-binding
capacity of the soil, and increasing available phosphorus as a result.
Manures, fish fertilizers, and bone meal are good sources of organic
phosphorus. Rock phosphate is a natural form of phosphate, but is
relatively unavailable. Triple super phosphate is a synthetic
phosphate form, and has a high percentage of available phosphate
compared to rock phosphate, high enough to cause pollution problems if
used regularly.
Does your garden need phosphorus? How much? Hard to say. Use
a soil test to see how much phosphorus the soil already has. If
it's low, adding organic matter will both add phosphate and keep it from
being tied up in the soil. When planting new roses, add a little
bit of phosphate fertilizer with the backfill along the sides of the
hole (not at the bottom). Placing phosphates in the vicinity of
the plant's roots will help the plant reach the phosphate before it's
lost to leaching or adsorption. Avoiding practices that discourage
mycorrhizal fungi helps too, such as fungicide sprays that drip onto the
ground, frequent soil tilling, and use of super triple phosphate.
Potassium
Potassium is the third element listed on fertilizer labels.
Potassium is not exactly a building block. It is used in a variety
of critical chemical reactions, but is not used to build tissue.
Still, plants need about as much potassium as they do nitrogen, and much
more than they do phosphorus. Unlike phosphorus and nitrogen, most
soils have plenty of potassium, which is continually made available to
plants through natural mineral decomposition, if only in small amounts.
Fertile soils that don't have their plant material harvested and
removed, or that are regularly composted, may not need additional
potassium from fertilizers.
Most potassium in soil is part of mineral complexes, particularly clays.
Physical and chemical decomposition (not microbial) releases potassium
from its parent minerals. Potassium in the simple ionic form K+ is
what plants can absorb. K+ floats around in soil water, or adsorbs
onto clay particles and soil organic matter. Sandy soils low in
clays and organic matter tend to not have much potassium available for
plants. Potassium is available to plants at
a wide range of soil pHs.
Potassium in fertilizers is listed as K2O, or potash.
In our sandy soils, especially when growing roses, some potassium will
probably have to be added as fertilizer. Seaweed or kelp meal,
fish meal, and wood ashes are all good organic sources of potassium.
Wood ashes in fact can add too much, creating toxic conditions.
WSU recommends one cup of wood ashes per rose bush every two years - not
every year, and no more than one cup at a time. Chicken manure and
alfalfa are also good organic sources of potassium. Greensand is a
very good source of potassium. Synthetic sources of potassium
include potassium chloride, potassium sulfate, and potassium magnesium
sulfate (sul-po-mag). You will probably want to apply potassium as
part of a balanced fertilizer, however, as it is unlikely your garden
will need potassium only, and not nitrogen also.
The minor nutrients are sulfur (S), calcium (Ca) and magnesium (Mg). They're
called 'minor' nutrients only because they're needed in smaller amounts
than the major nutrients, not because they're any less necessary for
plant health. All three tend to be low in western Washington
soils.
Sulfur
Although not required in as large amounts as the major nutrients, sulfur
is often deficient enough to limit plant growth. Sulfur, like
nitrogen, is used in building proteins and amino acids. Unlike
nitrogen, sulfur is not typically included in all-purpose synthetic
fertilizers, or the amounts are too low for sulfur-deficient soils.
Before the Clean Air Act, sulfur in the air from burning coal and
smelting metals would settle on the ground and add sulfur, so
fertilizers didn't need to add very much (never mind the damage the
sulfur caused to our lungs). Now with cleaner air, we need to add
more sulfur to our gardens.
Plant-available sulfur occurs as sulfate, SO4. Like
nitrogen, most sulfur in the soil is stored as various organic compounds
that must be decomposed to the sulfate form by soil microbes.
Sulfate can be easily leached from the soil. Clays and organic
matter can hold sulfate in the soil, and release it slowly for plant
uptake. Like potassium, sulfate is available at a wide range of
soil pHs.
Good sources of sulfur are alfalfa meal, seaweed and kelp meal, fish
meal, gypsum, Epsom salt, potassium sulfate, iron sulfate, and
sul-po-mag. Any animal flesh-based fertilizer should supply some
sulfur. Sulfur-based fungicides may add sulfur to the soil as
well; typically, only a small percentage of garden sprays stay on the
foliage, and quite a bit ends up on the soil. Do not add elemental
sulfur to your garden. Elemental sulfur can make your soil
extremely acidic, potentially poisonously so, and can leach lethal
acidity into any nearby streams or ponds.
Calcium
Calcium (Ca) is one of the most abundant nutrients in the soil, but can
become deficient in plants due to either too-low pH, or inadequate soil
moisture. Calcium is most available to plants at pHs ranging from
6-9, so it is possible for garden soil in our area to be too acidic for
good calcium availability. Summer drought can also make calcium
unavailable to plants, as calcium is picked up and transported passively
through transpiration of soil water, not by active translocation or ion
diffusion. Calcium weathers from the minerals the soil is
made from, and then attaches to clay or humus particles, from which it
is picked up by plants. Our soils are deficient in the clay and
humus particles needed to keep calcium available, so it leaches away
(leaching of calcium is also what leads to our soil being so acid).
Adding lime both raises the soil pH and adds calcium.
Compost and manures are also good sources of calcium, and add the
organic matter necessary to keep calcium from leaching away.
Magnesium
Magnesium (Mg) is a central element of chlorophyll, the molecules
responsible for photosynthesis. Magnesium is not as abundant in
the soil as calcium is, but it is not needed in as large amounts either.
Like calcium, plants get most of their magnesium from that held on clay
and organic particles. Magnesium is most available in alkaline
soils rather than acidic ones, with only minor availability at soil pH
of 6. Dolomitic lime is a good source of magnesium, along with
compost and manures. Magnesium can be deficient in western
Washington soils, so we recommend liming with dolomitic lime rather than
plain lime.
Micronutrients, basically, are trace minerals. The main
micronutrients are iron (Fe), manganese (Mn), zinc (Zn), copper (Cu),
molybdenum (Mo), boron (B), cobalt (Co), chlorine (Cl), and nickel (Ni).
They are used in a variety of enzymes and metabolic processes, such as
photosynthesis and oxidation/reduction reactions. Most
micronutrients are most available in slightly acidic soils. Most
are not particularly deficient in western Washington soils. Iron
deficiency can be a problem with roses at higher pHs. While the
required amounts of micronutrients are small, so are the toxic levels.
You can poison your plants by over-applying micronutrients, so follow
label directions when applying synthetic micronutrient fertilizers, or
when liming.
Micronutrients come from the minerals that make up the soil, and are
released as the parent minerals break down through weathering and
chemical decomposition. Ionic micronutrients react with other
minerals to become unavailable to plants. Organic matter can hold
micronutrients in forms that are relatively available to plants.
Microbial decomposition of the organic matter releases the
micronutrients. Soil pH has a large effect on availability of
micronutrients. Iron and zinc deficiencies are common in neutral
or alkaline soils, and can occur in soils that have had too much lime
added. Dolomite in particular can cause zinc deficiencies if too
much is applied, through antagonism between zinc and magnesium, so check
your pH every year or two before liming! Iron and zinc
deficiencies can also be caused by over-application of phosphate
fertilizers.
You may have seen fertilizers labeled as containing "chelated" iron.
Chelates are complexes of certain micronutrients (the positively charged
ones like iron and zinc) with organic molecules, to form compounds that
can hold the micronutrients in a form that is available to plants.
Chelated micronutrients are much slower to form unavailable compounds,
or to leach away. Chelating compounds occur naturally in organic
material in the soil, or can be synthesized to make chelated
fertilizers. Plant roots are able to take the micronutrient
directly from the chelate, and release the chelating compound back to
the soil.
Compost and manures are excellent sources of most micronutrients.
Gardens that are routinely composted or manured may not need any other
source of micronutrients. Seaweed and kelp provide micronutrients.
Something called "spoiled legume hay" is also listed as an excellent
source of micronutrients - probably something akin to alfalfa tea,
alfalfa being a legume hay, and the tea-making process involving
fermentation and decomposition. Sewage sludge, like Tagro, is a
very good source of micronutrients, but can also contain unwanted heavy
metals, so use in moderation (and only on ornamentals, never edibles).
Sawdust, bark, and wood chips, while not normally recommended as soil
amendments due to very low nitrogen content, can provide micronutrients.
These wood wastes decompose very slowly and are best used as mulches
rather than soil amendments.
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