The vast majority of questions I get regarding organic fertilizer fall into two groups. The largest number run along the lines of "Why should I use organic fertilizer?". Running a respectable second is "What exactly is an organic fertilizer?". I've intended to write about this for some time, and a recent letter has prompted me to finally do it. I am a proponent of organic fertilizers, but I'm going to try steering away from the hype and towards the facts.
There is often a lot of confusion about organic fertilizers. Some of what I'll discuss will be slightly technical, but it's not really that difficult of a subject. Part of the problem stems from the fact that "organic" is used in several different ways, yet people tend to lump them all together.
At the most basic level, organic fertilizer simply means a fertilizer derived from organic matter, such as cotton, bone, kelp, or manure. Chemical fertilizers are often processed from things like petroleum, or some of the gasses that are found in places where petroleum exists. Sometimes things get a bit confusing, because people who use organic fertilizers also use mineral rocks (such as Jersey Greensand or Dolomite limestone) and call them "organic fertilizer", when actually they are not organic in this strict sense of the word.
People also use organic to refer to methods that are perceived as being more earth-friendly. Additionally, the term organic is used when referring to fertilization methods that require some sort of soil action to make the nutrients available (which is how mineral fertilizers fit into the picture).
Chemical fertilizers, when mixed with water, generally are in a form a plant can use immediately. Nitrogen is present as nitrate salts or urea, which dissolve in water into the nitrate ion (among other things). Phosphorus is incorporated as phosphate, which provides the phosphate ion. Potassium is generally in the form of potash, which ... well off-hand I forget what the form of potassium is that the plants actually take up (I'm sure someone will write and remind me!). Of course there are also somewhere around 20 micronutrients, but I'm not going to go into all of them! Chemicals are more easily over-applied since they are essentially 100% available right away. Also chemicals are much more easily removed from the root zone since they are water soluble, and some people water WAY too much... but that's another story! Additionally, as I brought up before, chemical fertilizers generally are derived from non-renewable resources.
Organics are intrinsically different. They are not in a form the plants can use right away. Organics require the presence of soil microlife (various bacteria and fungi) to break them down, and convert them to the chemical form that is "available" for the plants. From a strictly "how to feed your plants" point of view, organics have several advantages: 1) It's much harder to over-feed your plants, since they will only have available what is broken down by the soil microlife. 2) You can't wash an entire "feeding" away. You can certainly wash away what's been broken down at a given point in time, but it won't take long for the soil life to make more available. 3) Most importantly, you get a much more even feeding over a long term, which basically follows the same logic I used in #2. Just how long term the action is depends to a large degree on your soil temperature. Here in the Maritime Northwest, where soils just don't get that hot, I can generally feed a plant once (at sowing or transplanting time) and that takes care of it.
One thing many people don't realize is that chemical fertilizers can and do feed many of the soil microbes as well. However, I think there is some decent evidence that some of the common source chemicals (ammonium sulfate, for instance) are harmful to some of the soil's macrolife, such as earthworms. However I'm not an authority on that aspect of things; I believe it, but if you're skeptical it'd be better to do your own reading and make your own decisions.
An additional "earth-friendly" benefit of organics is that many of them are just byproducts of some other already-existing process. Bone meal and blood meal are made from slaughterhouse leftovers. Seed meals are taken from what's left after cotton is harvested or certain oil seeds are processed. Since I prefer the products I use to be as low-impact on the earth as is practical, I don't like one common organic amendment, rock phosphate, because it is mined. Bone meal is a better source of phosphorus from this point of view.
Occasionally you'll hear an additional argument for organic fertilizers. Usually it'll come from permaculturists or others found on the fringe of the organic movement. It runs like this: The big advantage to organic fertilizers is that their breakdown results in positive ions (cations), while chemicals overwhelmingly break down into negative ions (anions). Since clay and humus hold cations better than anions, organics are superior because they don't leach as readily. Well, I don't buy this for several reasons. First, the only time this even would apply is in the case of overwatering, which (hopefully!) is a pretty limited circumstance to begin with. Second, the form that plants take in most nutrients is as anions. Plants (or their root-dwelling fungi) take in nutrients using a mechanism called an ion pump (which I'm not going to explain!), and that's just how they work. Because of this, chemical fertilizers produce negative ions by design, while this argument presupposes that it's a weakness. Finally, since the nutrients in cationic form have to be converted to some sort of anion before they can be absorbed, they can be leached just as easily as the chemical that started out as an anion.
So there it is. I've tried to show why organic fertilizers are worth considering, even if you aren't an organic gardener. I hope that in future articles we perhaps can discuss other benefits to the organic method.
All contents © Travis Saling
This page was last updated July 9, 2011