An introduction to elements essential for plant growth
Like people and all other organisms, plants need many different nutrients to grow. Nutrients needed
in large amounts are sometimes called “macronutrients,” while nutrients needed in smaller amounts
are called “micronutrients.” The following table lists macro and micronutrients needed by plants, the
percentage of an average plant that is made up of each nutrient, and the relative number of atoms of
each nutrient needed for healthy growth. Note that even the macronutrients account for only a small
percentage of the weight of a dried plant – almost all the rest of its weight is made of the element
carbon (C). If you were to analyze a living plant (that is, one that had not been dried out), you would
find that most of its weight was composed of water.
It is worth noting that many of these nutrients are not actually used by plants in the pure, elemental
form shown in the table above. Nitrogen (N), for example, is most abundant on our world in the form
of a gas, which consists of two N atoms bonded to each other make the molecule dinitrogen, or N2.
This gas makes up 78 percent of the atmosphere, but plants can’t use it. They rely instead on sev-eral other forms of nitrogen, including nitrate (N03
-
), ammonium (NH4
+
), and nitrite (N02
-
). These
molecules are created by a number of biological and nonbiological processes that occur in the atmosphere, in the soil, and in the roots of special plants called legumes. Humans can (and do) also
create large volumes of the plant-available forms of nitrogen using industrial processes. These
processes require large amounts of fossil fuels, and the nitrogen fertilizers that result from them are
not used in organic food production. Nitrogen-containing fertilizers used in organic production are
made instead from animal wastes (manure), animal parts (bones, blood, feathers, etc.), or nutrientdense plant products (ground-up seeds or fruits).
Like nitrogen, other plant macro and micronutrients exist in forms that are available to plants and
forms that are not available to plants. They can be made into fertilizers using methods that are considered either acceptable or unacceptable for organic production.
Why use organic fertilizers ?
It’s fair to ask why organic farmers, gardeners, and consumers should care about how plants get
their nutrients. After all, nitrogen is nitrogen, yes? Well, yes and no. To a plant, a molecule of ammonium (NH4
+
) is indeed a molecule of molecule of ammonium, no matter where it comes from. But that molecule of ammonium can get to the plant in ways that have either good or not-so-good consequences elsewhere. One concern about production of conventional (non-organic) fertilizers has to
do with reliance on fossil fuels (coal, oil, and natural gas). As mentioned above, making plantavailable forms of nitrogen in conventional industrial ways requires large amounts of fossil fuels.
Fossil fuels are increasingly scarce, which is one problem, but scientists who study global warming
feel that we need to cut use of fossil fuels even before we run out of them because the carbon dioxide and other gases that result from burning fossil fuels are changing global climate. Organic fertilizers can be produced with much less fossil fuel input.
Another reason to use organic fertilizers instead of conventional, fossil fuel-based fertilizers has to
do with how they release nutrients into the soil. When farmers apply conventional fertilizers to their
fields, the nitrogen they contain is all immediately available to plants. Unfortunately, plants can’t use
it all (or even most of it) right away, and the unused portion tends to leave the field, either evaporating or getting washed away. When nitrogen gets into the air and into ground and surface water, it
disrupts various natural systems. Organic fertilizers come in large particles that break up slowly and
release their nitrogen over a much longer period of time. While this can sometimes mean that plants
don’t get as much nitrogen all at once as they might like, it does mean that at least some nitrogen is
available to support plant growth over a long time period, and less nitrogen is free to evaporate or
wash away. The same is true for other macro and micronutrients.
Buying organic fertilizers ,
Broadly speaking, there are two types of organic fertilizers – those that provide macronutrients, and
those that provide micronutrients. Because organic fertilizers are made from complex living things,
even the macronutrient fertilizers contain some micronutrients. However, the two are sometimes
sold separately.
Like conventional fertilizers, organic macronutrient fertilizers are usually labeled prominently with
three numbers. You might see a bag of composted turkey manure, for example, labeled 4-6-4 or 5-
2-4. The three numbers represent available percentages of nitrogen (N), phosphorus (P), and potassium (K), in that order (N-P-K). If you put 100 units (kg, lb, whatever) of a manure-based 4-6-4
fertilizer on your crops, 4 units out of 100 would be nitrogen in a form available for plant growth, 6
units would be phosphorus in a form available for plant growth, and 4 units would be potassium in a
form available for plant growth. The remaining 86 units would be various forms of carbon, together
with small amounts of other nutrients.
Most common fertilizers contain nitrogen, phosphorus, and potassium as described above, but if you look at the table of necessary plant nutrients, you’ll notice that plants also need significant amounts of calcium, magnesium, and sulfur. You might think these nutrients would be added to every fertilizer as well, but they aren’t. In a nutshell, the explanation of this is that calcium, magnesium, and sulfur are more readily available in soils than are nitrogen, phosphorus, and potassium. This is partly a function of the fact that they are more stable in soil than the others. When a plant dies, much of the nitrogen it contains leaves the soil fairly quickly by one route or another, but most of the calcium it contains stays in the soil to be taken up later by other plants. You can certainly buy organic fertilizers that contain calcium, magnesium, and sulfur, and many gardeners would do well in particular to apply calcium to their soils (you can’t have too much of it, and it is critical for crops like tomatoes, cucumbers, and melons), but most of the time you will buy fertilizers based on their N-P-K ratings. Nitrogen particularly encourages growth of leaves, while phosphorus and potassium particularly encourage root growth.
So. Which organic fertilizer to buy, and how much to buy ?
Beginning organic gardeners would do well to start off the season with one 18-22 kg (40-50 lb) bag of general, all-purpose organic fertilizer.
What you buy will depend on what products are available in garden stores in your area. N-P-K ratings vary widely, but in general you should look for something fairly balanced (N = P = K), like the 4-
6-4 fertilizer mentioned above. If you see fertilizers with numbers that add up to more than 15, or
where one of the numbers is more than 8, they are probably NOT organic and should be avoided
(there are organic exceptions to this rule – fertilizers made from bat guano can be 0-13-0).
If you are a farmer or know a farmer who has livestock (cows, pigs, chickens, turkeys, etc.), you may be able to use the manure of your animals as fertilizer, but you’ll need to be careful – fresh, raw manure (especially from chickens and turkeys) is too concentrated to apply directly to crops. It must either be composted somehow (see compost) or mixed well with soil a week or two before you plant your crops. The nutrient content of animal manure varies a lot.
When you buy fertilizer, you may also wish to buy soil amendments, which are not fertilizers but improve your soil’s ability to make use of any fertilizer you do buy.
Applying organic fertilizers ,
If you have purchased an organic fertilizer that is fairly balanced (see above), you can apply it in one
or more of three ways,
1) When preparing a bed for planting, scatter fertilizer over the surface of a bed before you dig into
the soil. The fertilizer will end up incorporated randomly in the soil, and plant roots will find it and
use it. For some plants (lettuce and other salad greens, radishes, and carrots, to name a few) this is
enough fertilizer to ensure a good crop. How much fertilizer you scatter around depends on how
strong it is – read the package for suggested application rates. If you cannot find a suggested rate
for bed preparation in a vegetable garden, try applying it at rate of about 2.3 kg per 10 square meters (5 lb per 100 square feet). A garden bed 1 m wide and just over 9 m long (40 in wide and 30 ft
long) would take that amount of fertilizer. If you only fertilize one way, this would be the way to do it.
2) Some plants, especially those that are transplanted and that produce large fruits (tomatoes, eggplants, and melons, for example) benefit from more fertilizer than the scattering method above provides. While it’s still good to fertilize the planting bed as described, you should also add some fertilizer to each planting hole before the plant goes in, working it into the soil so it’s not all in one place. This provides a more concentrated source of nutrients for these plants to grow with. The crop pages for these crops discuss how much fertilizer to use; also see transplanting for pictures of what this might look like.
3) Some plants (onions, broccoli, cauliflower, cabbages,
and kale, to name a few) also benefit from scattered fertilizer, but are too small when they are young to make use
of fertilizer applied as described in method 2, above. Instead, wait until these plants are about halfway toward
maturity, then fertilize them again at the same rate you
used in preparing the bed. You can’t scatter the fertilizer.
