This article is part of a series written for HTGSupply by plant biologist Dr. E.R. Myers.
This is the fourth of a multi-part series on plant growth and liming factors. This month I will discuss nutrients and plant growth. Next month light and plant growth.
The most important thing when considering plant growth is understanding limiting factors. Plant growth is determined by limiting factors. In biology this is often called Liebig’s law of the minimum. Basically, whatever element that is first to be limited (not meeting the demand of the plant) will determine plant growth. Perhaps the best way of demonstrating Liebig’s law is with an imaginary barrel. One barrel is made up of several pieces of wood called staves. Each stave in this barrel represents a factor for growth (one stave for light, water, CO2, nitrogen, phosphorus, iron etc.) If one stave is shorter than the rest, you can never fill the barrel above the shortest stave no matter how tall any other staves are. The short stave is the limiting factor. The moral of the story is, if your plants do not receive enough of any one (1) factor they won’t grow at the maximum rate no matter how much of any other factor you give your plants. To be a better grower you must find your limiting factor. Increasing the lowest stave on a barrel, your limiting factor, will improve growth without having to adjust or do anything else. Be mindful that it is also true that you can add the most expensive nutrient product to your plants, and if it does not include your limiting factor it won’t improve your growth.
There are several major factors that can limit plant growth, in order of importance they are, light, water, temperature, carbon dioxide and nutrients. Light is the energy source for photosynthesis, and photosynthesis provides the energy for all the plants physiology. If you don’t give your plants enough light it does not matter how much fertilizer, CO2 or any other factor you give your plants, they won’t grow bigger. If you are reading this, I assume you are one of the smart people who have a high quality light source from www.HTGsupply.com
Plant Nutrition 7/08
This information is broken down into two parts, first I go over basic nutrient information such as
what nutrients are needed for plant growth from seedling to flowering and in what amounts etc.
Then, for those interested there are more details about nutrient uses in plant physiology. This
knowledge will help with understanding nutrient deficiency symptoms and possible cures, of
which I have included a few.
Basic nutrient information: Plants like all organisms need nutrients to survive. Some are
needed in large quantities called macronutrients. Carbon (C), hydrogen (H), and oxygen (O),
which come from CO2 (carbon dioxide) and H2O (water) are macronutrients obtained by the
processes called photosynthesis, which is driven by the absorption of light energy (see Light
8/08). Other macronutrients such as nitrogen (N), phosphorus (P), and potassium (K) are in the
soil or growth medium. The nutrients NPK are (or should be) listed on every fertilizer. They are
listed in the same order (NPK) and as a percentage. For example, if you buy a fertilizer that has
the numbers –10-15-12– it has 10%N 15%P and 12%K. Nitrogen is usually the most limiting
nutrient to plants. Its presence promotes vegetative growth, and increased leaf growth of plants.
When plants are growing vegetatively (not flowering) you should use a fertilizer that has N as
the highest number, like 10-4-5 would be suitable for vegetative growth. The actual numbers are
not as important as the ratio, the first number (N) should be the highest 3-1-3 would be just as
acceptable. You do not want to add too much of any nutrient. Too much nitrogen can cause
plants to grow lanky* and can prevent or cause poor flowering. Two weeks before flowering cut
back the nitrogen and switch to a high phosphorus fertilizer. I grow in soil usually, so I flush the
plants with pure water to leach nitrogen out two weeks before I start flowering. I water the plants
using pure water and let a substantial amount flow through the soil coming out the bottoms of the
container thus “flushing” excess nutrients from the soil. Phosphorus is involved with flowering
and budding. Increased phosphorus promotes bigger flowers and fruits. Phosphorus also
promotes root growth. Seedlings should be given a diluted all purpose phosphorus fertilizer like
10-15-10 and flowering plants given a different fertilizer low in N such as 1-5-4. K (potassium)
is all around good, and I think helps with flowering in many plants. In my opinion, you should
add K in all stages, seedling, vegetative, and flowering. Liquid seaweed, such as Maxigrow is an
excellent natural source of K (potassium).
Several other nutrients are needed in small quantities and are termed micronutrients. Some
micronutrients are sulfur (S), magnesium (Mg), calcium (Ca), iron (Fe), manganese (Mn), cobalt
(Co), zinc (Zn), molybdenum (Mo), boron (B), copper (Cu), and chlorine (Cl). Micronutrients
are important and come from the soil or growth medium. Most high quality potting soils will
contain enough micronutrients for your plants. Hydroponic growers will need to make sure
micronutrients are added to the reservoir. Small quantities or not, any micronutrient can be a
limiting factor, so you should consider them once you have met the NPK needs. A good fertilizer
should list micronutrients as part of their contents. Compost (see Compost 6/08) would also be a
good source for possibly all micronutrients for soil gardens. Other sources of organic micronutrients are kelp/seaweed, worm castings and even ocean water. Organic hydroponics growers could try adding 1-3% ocean water to your reservoir to add micronutrients if you think they are lacking. Please note, however, I have not tested this myself.
*Lanky – plants that have long internode lengths, are weak stemmed and have a tendency to fall
Nutrients are needed to carry out photosynthesis reactions, the art in growing is when you give
the plants enough so that photosynthesis takes place as fast as possible, but not so much that you
create a toxic growth medium and stress or kill the plant.
Plant nutrients can be broken down into four groups, each group is needed in fewer quantities:
1) C,O,H. These nutrients are obtained in the form of CO2 and H20 via photosynthesis which
begins with light energy. These nutrients are the basis of all organic molecules and all that is
needed to make carbohydrates (sugars). From 40- 60% of a plants weight can be carbon. These
nutrients are used for various purposes in plants like structural support in the form of cellulose
and lignin and as energy in the form of sugars and starches. When any of these nutrients are
limiting, reduced energy (growth) will result. You make water easy to obtain for indoor plants,
but remember too much of any thing is bad. Indoor growers could, without proper ventilation
and air movement, have reduced growth due to CO2 being limited. A CO2 generator or a bottled
CO2 dispenser is a good source of CO2, look for future article on CO2 and ventilation (09/08)
2) N,P,S. These are the nutrients most likely to limit productivity in plants. Nitrogen is used
in the chlorophyll molecule, which absorbs light to start off photosynthesis. Nitrogen migrates
from old growth to new, in general. This means that nitrogen deficiencies are seen in older leaves
turning yellow (lacking/losing chlorophyll — no N, no chlorophyll). Most plants respond quickly
to nitrogen fertilization showing obvious green new growth. So, when you think of nitrogen
think green and leafy. By the way, adding N fertilizers to your lawn is one way to get the
greenest lawn on the block. Urine is a good source of readily available organic nitrogen, one cup
(bladder full) per gallon. (0.5N-0.003P – 0.003K).
Phosphorus is used by most plants during flowering and fruiting. Indeed, many manufactures sell
fertilizers high in phosphorus specifically for blooming plants. However, it is also important for
root growth in early plants and when taking cuttings. Be sure to use a diluted high phosphorus
fertilizer with trace minerals when starting seeds and if you take cuttings. Good growing begins
the second the seed germinates or the scissors cut the stem to make a cutting. Phosphorus
deficiencies in broadleaf plants usually result in smaller darker green leaves with reddish stems.
Sulfur is not needed in large quantities by most plants, but can be limiting in natural ecosystems.
For most plants 90% of sulfur is used in just 3 amino acids methionine, cystine and cysteine.
These amino acids are used in many different enzymes and proteins in plants. Most high quality
potting soil and hydroponic nutrient mixes contain enough sulfur. If I have it in the house, I will
add Epsom salt (1 teaspoon per gallon of water) to seedlings around the third watering, as I begin
to fertilize. Epsom salt supplies sulfur and Mg (magnesium) and is easy to get (try your local
drug store) and is relatively inexpensive.
3) Ca++,K+,Mg++ The nutrient K is responsible for stomata behavior (getting CO2 into plants)
and photosynthesis processes, where as Ca (calcium) is needed for proper cell membrane
function and Mg (magnesium) is part of the chlorophyll molecule. K and Mg are also important
parts of many plant enzymes. I have noticed plants grown in CocoCoir (coconut husk growing
medium) tend to need additional Mg and Ca and products such as Botanicare’s Cal-Mag+ are
excellent sources of both Mg and Ca. Find it here.
Potassium (K) gives strong sturdy stems. With deficient K, plants tend to be long stemmed and
thinner. Yellow spots (Necrotic patches) can develop on the larger leaves and leaf tips with
severe K deficiencies. Potassium is important in seed development in many plants. If you are
breeding plants for seed, give them fertilizer high in K! In my personal opinion, high K is good
in all forms of plant growth from seedling to flowering. Fire wood ash (NOT commercial
CHARCOAL for BBQ which is toxic) is water soluble and high in K (0-1-6), but is also very
alkaline. It could be used once per growing season as a good source of organic K. You could also
try burning pine, Douglas fir, or other conifers, which are acidic and therefore will have less
alkaline (basic) ash. Indeed, the easiest way to introduce K to your garden is by using a liquefied
or powdered Seaweed product such as MaxiGro.
Magnesium is the central molecule in chlorophyll so its deficiencies are similar to N, they appear
in the lower leaves as yellowing, with green veins. For plants grown in soil, adding one teaspoon
Epsom salts to a gallon of water will provide magnesium to your plants. Soil growers can also
add dolomite lime to add Mg (and Ca too). Even without signs of deficiency adding Epsom salt
or dolomite lime in small quantities is generally good for all plants growing vegetatively. Adding
either to your lawn is also another easy way to have the greenest lawn on the block.
4) Fe, Mn, Co, Mo, Z, Ni, Cu and B. Members of this group of nutrients are needed in small
amounts but are used in numerous physiological processes and can be a limiting factor. All are
metals (except B) so solubility i.e. availability to plants depends on organic compounds called
chelates. Solubility increases with chelation (Chele is Greek for crab claw) in chelation metal
ions are surrounded (grabbed) by a large organic molecule (chelate) forming a chelate complex.
The complex can enter plants/plant cells. Chelates such as humic and fulvic acid are secreted by
some plant roots, mycorrhizae and some bacteria. This is why products promote mycorrhizae and
bacteria, to help plants take up nutrients even when the pH is not in the optimal range. Be
warned, not all mycorrhizae or even bacteria will work well with all plants. Many fertilizers
produced by Humboldt Nutrients contain chelated nutrients and/or mycorrhizae, humic acid,
fulvic acid and beneficial bacteria. I use many of their products regularly. Find it here.
Keep in mind, sometimes simply adjusting the pH can remove a limiting factor. Moreover, the
nutrients Fe, Mn, Cu, Zn, Mo, and Ni are all insoluble to plants (can’t get into the plant) at
certain pH ranges. This means that there abundance is NOT a good measure of availability to
plants. In other words, just because it is in your growth medium does not mean it can be used by
plants. This is the basic reason pH is so important, it determines the plants ability to uptake
nutrients it needs out of the growth medium via its roots. (A pH tester is a must for hydroponic
growers and a good idea for people growing in soil indoors – HTGSupply has several types
available from easy to use pH test papers and kits to more exact electronic meters.
Many nutrient deficiency symptoms are similar: leaf curling over or under, leaf tips turning
brown or yellow, either the veins or the tissue between the veins turning yellow (not whole leaf
like flowering/senescence). If you see any of these symptoms first apply a general fertilizer high
in NPK (10-10-10) if that does not help, you may have a micronutrient deficiency and should
look for a fertilizer high in micronutrients like compost etc. HOWEVER! Keep in mind that over
fertilization has many of the same symptoms. So, if you have given your plants 4 times the
recommended dosage of your standard petrochemical fertilizer, and the leaves are turning under,
and/or your have necrosis (brown spots on the leaves) you probably DON’T have a deficiency. If
you think you over fertilized your plants, you can sometimes save them if you flush them with
water to dilute the fertilizer. Hydroponics growers can change the water and flush the grow
medium. However, by the time the signs of over fertilization occur, it is usually too late to save
the plant or have it come back and grow to its full potential. Too much fertilizer makes the soil
salty so that plants can’t take up water and die of thirst in a sea of excess nutrients. The salt in the
soil/nutrient solution actually draws the water from your plants! You need to flush the growth
medium to remove/dilute the fertilizer salts. Too much of any thing is bad. The optimal amount
of everything for plants is good growing!