Lewis Lawyer (Note. Lewis Lawyer wrote this article for the Fall 1993 issue of the Almanac)
Almanac, Spring 2002 Vol.30, No.2: 6-9
[Adapted and slightly modified for the web]
By and large, plants can survive in our gardens without any additional fertilization. Even in the Pygmy Forest of Mendocino County, California, a place so near sterile that it draws national attention, plants do survive.
Mere survival, however, is only one of the hopes we have for our plants. With our increasing population, we universally apply fertilizers to increase food crop production. In the same spirit, we fertilize our garden flowers to make them larger and more lush.
Pacifica iris probably require less attention than most flowers, but still we feel a need to toss them an occasional handful of fertilizer. We often get questions about this, and a few years ago, I wrote an article for the Region 14, American Iris Society Bulletin, with some facts about fertilizers to make it easier for each of us to decide when, how, and how much.
To begin with, soil is one of the most complicated things on earth! Physically, it varies in particle size from chunks of rock and sand to microscopically small platelets of clay. These particles, along with bits of organic matter, are intermixed in combinations ranging from scree to almost pure clay, adobe, or gumbo. The chance of getting any two shovelfuls alike is less than winning the California Lottery.
Chemically, soil is so complex that even after centuries of study, no one yet fully understands it. Biologically, it includes countless organisms, all interdependent and intercompetitive in such a teeming mass that understanding our most complicated factories and assembly lines is easy by comparison.
In this stew, the roots of our plants forage for food. On the whole they do very well, but since we often have the urge to make them do even better, here are a few facts that may help.
Before the early 19th century, most growers believed in the "humus theory of plant nutrition". All plant food was thought to be derived from brown organic humus in the soil. De Saussure's proposal that green plants get most of their solid matter from carbon dioxide in the air, not from substances in the soil, was not easily accepted. Who could believe that no plant can ingest anything as large as even the smallest organic moledule in the humus?
But in fact, plant dry matter is roughly 45 percent each of carbon and oxygen, and 5 percent hydrogen derived from the air. Nitrogen content varies from less than one percent in woody tissue to as high as 10 percent in some soft tissues. All remaining elements, like phosphorus and potassium, which are left behind as ash when plants are burned, make up only 1 to 5 percent of the total.
So why are we so preoccupied with fertilizer applications, if they are responsible for only 3 - 15 percent (at most) of the of a plant's weight? The answer is that without the essential elements derived from the soil, there would be no plants.
More than 40 elements have been found in plant tissues. Eleven are believed essential to all plants, and three others to some plants. Nitrogen, sulfur, phosphorus, chlorine, potassium, calcium, magnesium, and iron are, in the order listed, the soil-supplied elements most abundant in plant tissue.
Four items need to be considered in any discussion of fertilizer practices:
1. Physical soil properties. Wild Pacifica iris generally grow in loose or gravely soils. While we are stuck with our garden's soil type, we can usually amend it to more closely fit the requirements of a particular plant we want to grow there.
Gravely scree and sandy loam is much more permiable than clay or other heavy soil. Water penetrates easier, but disappears rapidly. Soluble fertilizers are more quickly available to plant roots, while over doses more readily burns the plants. At the same time, soluble fertilizers soon leach out, so you need to apply fertilizers more often, but in reduced amounts.
2. Root type. Most annuals are shallow rooted, while perennials tend to be deep rooted. (There are many exceptions: tap-root annuals tend to feed deeply, and many perennials can be shallow rooted.) Pacifica iris have some deep anchoring roots, but I believe most of their feeding is done through the white roots that occupy the top 6 to 8 inches of soil.
3. Other soil organisms. Most plant roots or their root hairs exude materials that attract or stimulate the growth of certain minute organisms in the microscopically small area around the root known as the rhizosphere. This colony of organisms helps the root hairs, or may even be essential to their absorption of nutrients from the surrounding soil. There is little we can do about this phenomenon, but it is important that we know about it because it can become crucial following soil fumigation and the resultant change in the balance of the soil organisms.
4. Chemical composition. Most soils support plant life without any human intrusion. This may be humiliating, but it is a fact to consider when you are mulling over what you want to accomplish when you apply your manures. Only you can be the judge of this as it applies to your particular soil.
Nitrogen: Nitrogen (N) is the element most often supplied to plants through the soil. It can be applied in any of three forms: nitrate, ammoniacal, or organic. In commercial operations the primary consideration is cost, and you often see tractor-drawn equipment injecting pure gaseous ammonia into the soil. In orchards it is often supplied as ammonium sulfate. Plants don't care what form you feed them. Unless you know from experience that one specific form works best, cost per unit is a good starting point on which to base your choice.
Nitrate: The nitrate form of nitrogen can be obtained in several formulations including ammonium nitrate and calcium nitrate. Applied in this form, it is washed into the root zone with water and becomes available immediately to the plant root.
Ammoniacal: Ammoniacal nitrogen is available in many forms including ammonia gas, ammonium sulfate, and urea. Some plant species are known to be able to absorb the large ammonium molecule directly through their roots, but how prevalent this ability is remains controversial.
Controversy aside, we do know that all forms of ammoniacal nitrogen are converted to ammonium ion almost immediately after contact with the soil, and that in that form they bind tightly to the soil and can not be leached to the root zone.
It is only after the soil organisms convert the ammonium to nitrate that it moves freely through the soil and become usable to the root.
Organic: If you want to pay a little more and get a little less, you can use any of several organic fertilizers. One possible advantage of organic fertilizers is that, because of their more complex structure, it takes a longer time for the soil organisms to break them down to a usable form; thus they may be more slowly available and longer lasting in the soil. In the organic form, however, they are completely useless to plants.
Feedlot manures when used in large quantities are useful as a mulch or soil amendment. But they are a poor source of nutrients, having only about one percent each of N, P, and K. The greatest danger is that feedlot manures sometimes have a high salt content and can be the source of serious weed pests such as nut grass and bindweed.
Sulfur: Sulfur is the second most abundant soil-derived nutrient found in plants, and is seldom deficient in garden soils. Under normal conditions we can rely on the sulfates in most garden fertilizers as an adequate source.
Pacifica iris, like some other plants, prefer slightly acidic soils, and sulfur is sometimes used for this purpose. Such use should be carefully monitored, but in any case it is not strictly a nutritional consideration.
Phosphates: Two important characteristics in the behavior of phosphorous fertilizers help us plan for its application in our gardens. First, phosphorus (P) is taken up by the plant in the form of phosphate. Phosphate is very active chemically, and readily forms compounds with any available chemical. Phosphates quickly bond to the soil and do not move from the point of application. The second quality is that only a minute amount of the phosphate dissolves in the soil water at any one time. The part that does not dissolve cannot be washed from the soil, so a single application can last for months or years.
Since phosphates do not wash through the soil, plant roots have to grow to where the phosphate is, and even then they find only a very dilute solution of the nutrient. This suggests two things: plants need a well developed root system, and gardens require a strategic placement of phosphorus. The best way to accomplish the latter is to apply phosphate fertilizer before planting and spade or till it into the anticipated root zone.
Phosphate-containing fertilizers broadcast on the soil around living plants are not entirely wasted. Minute amounts do leach slowly downward, but on the whole, the phosphate simply waits where applied until it gets tilled into the root zone for the next planting.
Phosphorous fertilizer is commonly available in one of three forms: rock phosphate, superphosphate and bone meal. Rock phosphate is almost insoluble, and only slightly available to plants. Superphosphate is rock phosphate that has been treated with acid to make it soluble. It can be purchased as either single superphosphate or as treble superphosphate. The former can be used almost without any restriction, but treble superphosphate has to be carefully measured to avoid burning the plants. Bone meal is the organic form of phosphate made from ground bones. Chemically it is almost like rock phosphate and is almost insoluble.
For the record, just before planting Pacifica iris, I broadcast a visible amount of single superphosphate over the entire bed and spade it in. For some Pacifica plantings and for plantings of other individual plants like tall bearded iris or chrysanthemums, I trowel in about two heaping tablespoons of single superphosphate fertilizer at each plant site.
Chlorine: Chlorine is so abundant in soils in the form of chloride, that we worry more about its excess than its deficiency. There is no need to run around your garden with a saltshaker.
Potassium: Potassium (K) is the fifth most abundant soil-derived element found in plants, and third on the list of elements - N, P, and K - contained in complete fertilizers. Potassium is soluble in water, but because it bonds to clay particles and humus in the soil, it is about midway between nitrogen and phosphorous in its availability and persistence.
Most western soils have adequate potassium, and a response to its application is rarely spectacular if even measurable. There are exceptions, however, and heavily-cropped vegetable farm soils are often deficient. You will have to judge its importance in your particular circumstances.
All potassium fertilizers are mined from the ground, either as potassium sulfate or potassium chloride. They can be bought in these forms, but are usually purchased as part of a complete fertilizer.
All the rest of the soil-derived elements found in plants are usually lumped together under the term minor or trace elements. By and large we need not be concerned with any of them unless a "deficiency symptom" shows up on our plants.
The symptoms of minor element deficiencies usually show as chlorosis or a yellowing of the leaves. They can be corrected by the addition of specialty fertilizers including chelates, or by foliar sprays especially formulated for this purpose. Many books and articles describe these symptoms, but the best solution is to take a representative part of the plant to someone knowledgeable for diagnosis. Persons capable of answering such questions are usually available in County Agricultural Offices, universities, or some of the better nurseries.
The most commonly deficient minor element is iron. Despite its possible presence in large amounts, iron can get tied up in an insoluble form, especially in alkaline or poorly drained soils. Zinc deficiency is fairly common in some orchards, especially citrus groves.
There are also a few areas where excess minor element symptoms appear, like excess boron around Hollister and Woodland in California, and excess cadmium in the foothills near Salinas.
In most areas where they can be grown, the Pacifica iris will grow without using any fertilizer. However, if you desire spectacular blue ribbon show stalks, you will probably benefit by feeding your plants. And since we all like to see our plants performing to the best of their ability, adding a good nutrient supply will certainly help.
The only disadvantage that I can think of is that they will increase faster, require more space, and need transplanting more often. Pacifica iris are not the easiest plants to transplant, so starving them a little bit will alleviate this problem.
Each garden's soil is different, and what applies to one gardener's needs may not apply to another. This article has given you enough basic information to help you decide what is best for you. There is ample literature on the subject to fill you in on what I have omitted.