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Fertilizing: It’s Mainly About Nitrogen

Of all the components of fertilizer, nitrogen is the nutrient that plants need most

Plant fertilizer
Photo: Scott Phillips

I learned my first lesson about fertilizing many years ago as a boy on our family farm. My young mind was having a hard time reconciling the lessons of my elementary school botany studies with the bags of Chilean nitrate fertilizer stacked up to the ceiling of our barn. Impressionable youth that I was, I had come away from the class with the understanding that plants made their own food out of thin air and sunlight through the miraculous process of photosynthesis.

So why, I asked my dad, do we have to feed them all this fertilizer? He thought about it for a few moments and answered: “Well, I suppose those tomatoes might be able to grow on their own, but they’d never make it to market without us. We’re just giving them a little push in the right direction.”

It seemed like more than a little push to me. Whenever I returned home from school, I knew where to find my dad: out on the tractor, side-dressing the crops with fertilizer. I wasn’t sure if the plants required his constant attention or if he just enjoyed the solitude of the job.

I later found out that, like most things we learn when we’re very young, fertilization is a little more complicated than I had been led to believe.

You could spend years studying the science of fertilizing crops. But I try to keep it simple in my garden. I bear in mind the basics of plant nutrition. I’ve learned that one of the most important distinctions among fertilizers is how soluble they are, a concept critical to protecting ground water. And I’ve organized my garden in a way that makes fertilizing easier.

Plants do absorb oxygen, hydrogen, and carbon dioxide from the air. Fueled by sunlight, plants use these elements to manufacture carbohydrates through the process of photosynthesis. But that’s just a part of what they need. In order to make vital proteins and amino acids, they require 13 other elements.

There are the primary nutrients: nitrogen, phosphorus, and potassium. And the secondary nutrients: calcium, magnesium, and sulfur. Then the micronutrients: zinc, iron, manganese, copper, boron, molybdenum, and chlorine. Each plays a vital role in plant growth, and if any one of them is deficient, the plants will suffer.

Nitrogen is the element that gets most of our attention, and rightly so.

Nitrogen is the fuel that makes plants go. It’s used to synthesize amino acids, proteins, chlorophyll, nucleic acids, and enzymes. Plants need more nitrogen than any other element. It’s the nutrient we most often have to apply.

The good news is that nitrogen is in plentiful supply in nature; it comprises 78 percent of the earth’s atmosphere. The bad news is plants cannot extract nitrogen from the air. In fact, whether in the air or in the soil, nitrogen cannot be absorbed by plants in its elemental form. For nitrogen to be absorbed by plant roots, it must be converted, or “fixed,” into nitrates (NO3) or ammonium (NH4) ions.

That transformation occurs naturally in the nitrogen cycle. Some nitrogen is fixed in lightening strikes and delivered via rainfall. But most is converted from organic matter in the soil with the aid of microorganisms, which transform the nitrogen to nitrates.This transformation can be a slow process. But the richer the soil, the higher it is in organic matter and microorganisms, and the faster the nitrogen is made available.

Until about 100 years ago, this natural nitrogen cycle was the only way nitrogen was converted to nitrates. We farmed and gardened under the restrictions of time and nature, and in harmony with the nitrogen cycle—applying manure and wastes, and allowing them to break down over time, thus providing a steady stream of nitrogen. In those days, virtually all nitrogen fertilizers came from natural sources: manure, plant residue, bone and blood meals.

That all began to change in the late 19th century with a breakthrough discovery that nitrogen could be fixed artificially by combining atmospheric nitrogen with hydrogen to form ammonia. That ammonia could then be used to produce nitrates. The result? The nitrogen cycle was speeded up dramatically, and the synthetic fertilizer industry was born.

This breakthrough changed the way we looked at fertilizer. Unlike in natural fertilizers, the nitrogen in these synthetics was available to plants almost as soon as it hit the ground. We could practically watch the plants green-up and grow before our eyes. But there was, and is, a downside to these fast-acting, water soluble synthetics. They are also very mobile in the soil. They can rapidly wash out of the reach of plant roots and into groundwater. So they must be used carefully and applied frequently. If you apply too much at one time, the excess nitrates can leach into groundwater and pose a health hazard; too little and plants suffer.

Phosphorus and potassium round out the big three nutrients

Phosphorus is second only to nitrogen in the amount required by plants. It is a vital element early in the season, as it stimulates early shoot growth and root formation. When phosphorus levels are low, plants grow slowly and may have poor fruit or seed development. Phosphorus is especially important in cool weather. That’s why most starter fertilizer contains high amounts of it.

The problem with phosphorus is the opposite of that with nitrogen. Soils generally contain a good supply of it, but it is not readily available to plants. Phosphorus is extremely immobile in the soil. It does not travel in the soil solution, and plant roots must be in contact with phosphate ions to absorb them.

All phosphate fertilizers originate from phosphate rock, generally in the form of francolite. But in its natural form, it takes forever to become available in the soil. However, in 1842 it was found that treating phosphate rock with sulfuric acid would greatly speed the release of phosphorus. The result was superphosphate.

Superphosphate (0-20-0) is produced by reacting finely ground phosphate rock with sulfuric acid. Concentrated, or triple superphosphate, containing as much as 45 percent phosphate, is formed if phosphoric acid is used.

Finely ground phosphate rock (0-30-0) is still used as a natural source of phosphorus, as are colloidal phosphate (0-20-0) and bone meal (0-12-0). They all release their nutrients very slowly. No matter what type of phosphate fertilizer you use, the key is location, location, location. Make sure to work the fertilizers into the root zone of the soil. Add the required amount of phosphorus in fall or early spring. Don’t bother to side-dress during the year. If the soil is cold, use a liquid starter fertilizer containing ammonium phosphate. The nitrogen in the formula seems to make the phosphorus more readily available.

Potassium, the third primary nutrient, also encourages root growth and helps plants resist disease. It helps increase the size of vegetables and improves cold hardiness. Signs of potassium deficiency include weak plants, slow growth, small or shriveled fruit, and leaf burning at the tips and margins. As with  phosphorus, only about 1 percent of the soil potassium is available to plants.

Potassium fertilizer comes in several forms. Potassium chloride (0-0-60), also known as muriate of potash, is the most common. Derived from sylvanite ore, it is available to plants almost immediately. However, potassium chloride is rather acidifying, and some crops, notably beans, potatoes, and tomatoes, have a low tolerance to chlorides.

Potassium nitrate (13-0-45) is produced when potassium chloride reacts with nitric acid. Its advantage is that it does not acidify the soil and does provide nitrogen as well as potassium. However, it leaches from the soil rapidly. Sulfate of potash magnesia (0-0-21), sold as Sul-po-mag or K-mag, is derived from the mineral langbeinite. It is in a form that is available to plants rapidly.

Potassium sulfate (0-0-50) another mined product, provides sulfur as well as potassium. Other common sources of potassium include greensand, from the mineral glauconite (0-0-6), wood ashes (0-0-10), and granite dust (0-0-7).

Small amounts of other elements aid plant growth

The secondary nutrients, calcium, magnesium, and sulfur, are not required in great quantities by plants and are often present in the soil in adequate amounts. Also, some nitrogen and phosphorus fertilizers contain small amounts.

Calcium must be present in plants for the construction of new cells, where it strengthens the walls and membranes. The soil usually has sufficient quantities, except in alkaline or very dry conditions. Calcium deficiencies show up as tip burn on young leaves, or abnormally green leaves. Limestone is a good source of calcium, as are calcium nitrate and superphosphate fertilizers.

Magnesium is an essential element in the process of photosynthesis. It may be deficient in sandy soils and it will show in yellowing of leaves. Dolomitic limestone is a good source of magnesium. You can also provide magnesium with magnesium sulfate, epsom salts, and sulfate of potash magnesia, Sul-po-mag.

Sulfur is necessary for protein synthesis. Much of it is absorbed through the air and from the soil. When sulfur is deficient, plants are small and spindly, and the youngest leaves are light green to yellow. To supplement, apply Sul-po-mag, gypsum, or superphosphate.

An even smaller set of dietary elements also influences plant development. We call them micronutrients, and plants need only traces of them. For example, just ¾ ounce of Borax, the laundry detergent, provides all the boron necessary for 100 square feet of garden.

Zinc, manganese, and copper contribute to the formation of enzymes and hormones in plants. Iron and chlorine are necessary for the formation of chlorophyll. Boron regulates the metabolism of carbohydrates in plants. Molybdenum helps convert nitrates to amino acids. Most of these micronutrients are available in chelated forms, formulas that dissolve easily, making them readily available. Properly fed soil with well-adjusted pH should require no added micronutrients.

Though it’s fine to add the three primary nutrients to your garden soil as a matter of course, the secondary and mi­cro­nutrients should not be applied unless indicated by a soil test. Overapplication may cause more harm than good by contributing to a mineral imbalance in the soil.

Organic or synthetic?

As a teenager in the 1960s, I reacted against my father’s stacks of chemical fertilizers with their acrid, nose-twitching odor and planted an organic vegetable garden in a corner of the farm. I soon learned what all organic gardeners come to understand: that organic fertilizer is bulky, occasionally inconvenient, sometimes sloppy, and often smelly.

But it works as long as you don’t expect instant results. If you’re patient and have time to build up the soil, organic fertilizers pay dividends over the long run. If you work into the soil about one bushel of manure per 100 square feet of garden early in the year, every year, you will be providing virtually all the nutrition most plants need. The residual organic matter means that the plants never starve, and you won’t overfeed or underfeed.

However, we often don’t have the luxury of time. Or after years of building the soil in our garden, we pull up stakes and move and must start all over again. Or the pepper plants lag just when the compost bin runs out, and you can’t lay your hands on some mellow, aged manure.

It was during one of those times, after I had just started a garden in soil as sandy as the beach, that I began to wonder: What’s the harm in spritzing those plants with a little bit of Miracle-Gro? I would never consider using just a touch of synthetic pesticide, but I confess, I couldn’t think of a compelling reason not to use a little bit of synthetic fertilizer.

So now, my fertilizer program, like many things in my life, is perhaps less pure and a little more utilitarian. I do occasionally supplement organic fertilizer with a synthetic pick-me-up. To me, the important distinction is not whether a fertilizer is organic or synthetic, but whether its nitrogen is water insoluble or water soluble. I believe water-insoluble nitrogen is superior, because it is released gradually for steady feeding. Whereas water-soluble fertilizers are here today and gone tomorrow. Applying them is like the old joke about voting in Chicago: You have to do it early and do it often. Not only do you have to reapply regularly, there is also a danger of harmful nitrates leaching into the groundwater.

Nitrates in drinking water at levels greater than the federal standard of 10 parts per million can cause a potentially fatal condition in infants commonly known as “blue-baby” syndrome, also called methemoglobinemia. Babies can develop blue-baby syndrome after drinking water contaminated with nitrate levels greater than 10 parts per million for as little as one week, according to the Environmental Work Group, an activist organization based in Washington, D.C. The group estimated that between 1986 and 1995 more than 2 million people, including approximately 15,000 infants, drank water from systems that had nitrates in excess of 10 parts per million. The survey dealt mainly with farms.

Some of the newer synthetics mimic the slow-release quality of organics. Some, such as sulfur-coated urea, come in a shell that breaks down to release the nutrients over time. Others, like isobutylene urea (IBDU) or methylene urea contain nitrogen forms that are less water soluble, relying on temperature and microorganisms to release the nitrogen over time. They eliminate the need to constantly reapply fertilizer, but they offer none of the soil-building qualities of organics.

When shopping for fertilizers, read the label carefully

The label will list the percentages of water-soluble and water-insoluble nitrogen. The bag, of course, will show the amount of other nutrients in percentages. A 100-pound bag of 10-10-10 fertilizer has 10 pounds of each of the nutrients, with stabilizers making up the rest. If you need 20 pounds each of nitrogen, phosphorus, and potassium, you would need two bags of the fertilizer.

You need to keep in mind the actual amount of the ingredients, not only to get the biggest bang for your buck, but also to determine how much to apply to different crops.

Choosing your nitrogen source
Not all sources of nitrogen are created equal. The synthetic sources of nitrogen carry a high percentage of the fertilizer and offer a quick boost to plants. But they do nothing to build the soil and may leach into groundwater. The organic sources contain less nitrogen, but last longer and contribute to a healthy soil matrix.
Fertilizer % Nitrogen Tendency
to leach
Period of
in soil *
Urea 46 high 2 weeks
Sulfur-coated urea 38 moderate 6 months
Urea formaldehyde 38 moderate 3 months
Ammonium nitrate 33 high 1 month
Isobutylene urea (IBDU) 31 low 9 months
Methylene uree 28-41 moderate 6 months
Ammonium sulfate 21 high 1 month
Nitrate of soda ** 16 high 3 months
Calcium nitrate 15 high 3 months
Potassium nitrate 13 high 3 months
Bat guano 11 low 3 months
Blood meal 10 low 1 year ***
Fish meal 10 moderate 3 months
Cottonseed meal **** 6-8 low 1 year ***
Alfalfa meal 5 low 1 year ***
Cow manure (dry) 2-3 low 1 year ***
Poultry manure 2 low 6 months ***
Seaweed (dry) 2 low 9 months ***
Horse manure (fresh) 1 moderate 1 year ***
* Assumes idea soil conditions of neutral pH, moderate moisture, and warm temperature
** Though a natural product, not necessarily certified as organic
*** Available 2 weeks after application
**** May contain pesticide residues

Organizing the garden around feeding plants

Different plants have very different fertilizer requirements. Potatoes, for example, require about four times as much nitrogen and potash and twice as much phosphorus as beans. A 100-square-foot patch of potatoes needs about ½ pound each of actual nitrogen, phosphorus, and potassium per year for good growth. That’s about 5 pounds of a 10-10-10 fertilizer.

Root crops and leafy vegetables, such as lettuce, cabbage, and spinach, need about 1/3 pound of actual nitrogen, 1/4 pound of phosphorus, and 1/3 to 1/2 pound of potash per 100 square feet. Fruit crops, such as tomatoes, cantaloupes, and peppers, need 1/4 pound of actual nitrogen and phosphorus and 1/3 pound of potash per 100 square feet. While legumes, such as beans and peas, require only 1/10 pound of nitrogen, phosphorus, and potash for the same amount of space.

Trying to meet the diverse needs of a whole garden full of crops could make your head spin. But I have an easy way to keep the meal plans straight. Some people plan their kitchen gardens for aesthetics, some for succession and rotations, and some for ease of harvest. I take all of those elements into account, but plan my garden primarily according to the feeding needs—basically the nitrogen requirements—of the plants.

Potatoes, the heaviest feeders of all, get their own bed. I group the medium-feeding fruiting crops—tomatoes, peppers, melons, cucumbers—in a bed. Root crops get a bed, and so do the greens and legumes. That way, I can apply the same amount of fertilizer to a single bed, and know that every plant in it is getting the optimum amount of nutrition.

Over the years, I’ve learned that a fertilizer doesn’t have to be natural, but using it has to feel natural to you. That is, it must be in a form you feel comfortable with, one you will use faithfully. Because you need to feed. Choose the finest, tastiest, and best-looking varieties you can find—it doesn’t matter if they’re heirlooms or hybrids—and feed the plants properly. They will reward you with a harvest that’s everything you expected.

—This article originally appeared in Kitchen Gardener #21 (June 1999).

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