Fertilizing: It's Mainly About Nitrogen

comments (4) January 17th, 2009

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Click To Enlarge 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 fromnatural 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).

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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