Just so I’m not operating under false pretenses, let’s get one thing straight: I don’t garden. I don’t seed, I don’t weed, I don’t plant, I don’t compost, and I don’t spread manure (this column excepted). My one connection with the plant world is mowing the grass, and I wouldn’t do that if I had my druthers.
But many people do do all these things. Some of even claim to enjoy it. I don’t pretend to understand this obsession with green growing things (and the multi-legged things that eat green growing things), but live and let live, that’s my motto.
One topic that seems to come up whenever two or more gardeners are gathered together is fertilizers. Granules, pellets or liquids, organic or synthetic–the arguments are endless and (for fertilizer companies) endlessly profitable. But just what is a fertilizer?
A fertilizer is anything which provides plants with nutrients not available, or in short supply, in the soil. Plants, unlike humans, don’t need to ingest complex compounds like carbohydrates and amino acids–they can synthesize everything they need. But that synthesis requires raw materials.
The three primary nutrients required by plants are nitrogen, phosphorus and potassium, also known by their chemical symbols, N, P and K. Nitrogen and phosphorus are literally building blocks, found in DNA, cell walls and other plant structures. Potassium carries messages from cell to cell, triggering the production of proteins and regulating enzymes and the movement of water. Without it, you get tasteless tomatoes and pitiful potatoes.
Plants also require smaller amounts of calcium, magnesium and sulfur, the “secondary nutrients,” and trace amounts of boron, chlorine, cobalt, copper, iron, manganese, molybdenum and zinc, the “micronutrients.”
Most soils contain plenty of the secondary and trace elements: what’s sometimes missing (especially if the soil has already been extensively farmed or gardened over the years) is one of the Big Three.
People have recognized the need for fertilizer since the beginning of agriculture. The Greeks and the Romans wrote extensively about the benefits of various kinds of manure on various kinds of soils and crops, and also touted composting. Other fertilizers still in use today are also mentioned in ancient writings, from green manure (legumes and other plants plowed back into the soil) to bones and ashes.
Synthetic fertilizers–fertilizers produced through the chemical manipulation of raw materials, as opposed to naturally occurring substances–came along in the mid-1800s, and their production and use have boomed in the 20th century. (Part of our own economy has boomed with them. The potassium salts–“potash”–mined in Saskatchewan are the primary source of potassium for fertilizer manufacturers across North America. The terms “potash” and “potassium” both refer to one of the earliest sources of potassium, the wood ashes boiled in a kettle as part of the soap-making process.)
Although organic farmers decry the use of manufactured fertilizers, the fact is the the plant couldn’t care less whether its nitrogen, phosphorus and potassium come from manure or DOW Chemicals: elements are elements.
Fertilizers are either “simple” or “complete.” A simple fertilizer contains only one plant nutrient. Ammonium sulfate, for example, supplies nitrogen, but no phosphorus or potassium. A “complete” fertilizer supplies some of all three major nutrients. However, the proportions vary from fertilizer to fertilizer, and are indicated on the label in the order N-P-K. For example, a 10-10-5 fertilizer would contain 10 percent nitrogen, 10 percent phosphorus and five percent potassium. (Among organic fertilizers, manure and guano contain nitrogen; bone meal contains small quantities of nitrogen and larger quantities of phosphorus; wood ash contains lots of potassium.)
Nitrogen is the most important component, because nitrogen is most easily leached from the soil and therefore has to be replenished regularly. Phosphorus and potassium linger much longer. Therefore, the fertilizer with the highest concentration of nitrogen for the lowest amount of money is usually the best bet.
Sometimes, however, you may have a soil that is in specific need of phosphorus or potassium. The best way to find out is to have your soil tested. (Soil tests usually don’t include nitrogen anyway, because the amount present in the soil varies greatly over time. For a lawn, the rule of thumb is one pound of nitrogen per 1,000 square feet of lawn.)
Manufactured fertilizers are sold as dry granules, solid pellets or water-soluble liquids. Granular products are usually the least expensive and most commonly used. Solid pellets are composed of compressed particles. They usually cost more, but they release their nutrients more slowly and therefore last longer. Finally, liquid fertilizers provide nutrients quickly, but they cost more and have to be applied more often because they’re not incorporated into the soil.
Organic fertilizers usually aren’t as rich in nutrients as manufactured ones, are less water-soluble and release their nutrients more slowly.
Fertilizers aren’t the answer to all gardening problems. They can’t substitute for building up a soil with organic material, for example–and a soil with abundant organic material won’t need much fertilizer anyway. They don’t do a thing about insects or disease or bad growing conditions. And they have their own set of problems.
The flow of nutrients into streams and lakes, especially shallow lakes surrounded by large human populations (Wascana Lake, for instance) promotes an abnormally rich growth of algae and water plants. This is called eutrophication. Although both nitrogen and phosphorus are part of this problem, fertilizers mainly contribute nitrogen: fertilizer phosphorus is fixed in the soil where it’s applied. Phosphorus pollution usually comes from detergents and industrial wastes.
Too much fertilizer can be as bad as too few nutrients, producing watery potatoes or forage grasses that cause nutritional problems in cattle. Some fertilizers contain, in addition to the three main plant nutrients, some of the secondary nutrients and micronutrients, which can be toxic in large amounts. As well, though the nitrogen element of the fertilizer leaches away, potassium and phosphorus can sometimes build up in the form of mineral salts, lowering soil quality.
When it comes to fertilizer, the real trick is applying just the right amount at just the right time: and that, as any gardener will tell you, is as much an art as it is a science.
