Fertilizing: It's Mainly About Nitrogen

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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 3⁄4 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. Over-application 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.

  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.

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.

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 1⁄2 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.

by Warren Schultz
June 1999
from issue #21

posted in: fertilizer, manure

Comments (4)

DerekSpencer writes: Love it
Posted: 4:27 am on July 6th
marquesssmith writes: Thanks for sharing. It helps allot
Posted: 7:41 am on May 23rd
AliceFulter writes: Thanx for sharing this informative article
Posted: 6:00 am on January 26th
Mark_in_Indiana writes: Thank you for this well written article!
Posted: 11:40 am on January 18th
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