   Plant Mineral Nutrition, Part 2 - December 9, 1998 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County This article is a continuation of last week's article on the 17 known essential plant nutrients. In that article, function and deficiency symptoms of several plant macronutrients (hydrogen, carbon, oxygen, nitrogen, potassium and calcium) were discussed in order of their relative abundance in plant tissue. To reduce reader suspense, let's continue with three more macronutrients and the eight micronutrients. Magnesium is taken up by plants in the ionic form (Mg++). It is extremely important because it becomes the central atom of the chlorophyll molecule. Chlorophyll is the green pigment in plants that absorbs light energy during photosynthesis. When magnesium is deficient, older leaves are chlorotic (pale yellow) but the veins remain green. Magnesium is also needed to activate enzymes needed for photosynthesis, respiration, and DNA synthesis. Luckily, magnesium is hardly ever a limiting factor in our soils. If it were, an addition of Epsom salts (magnesium sulfate) would easily correct the situation. Phosphorus is almost always a limiting nutrient in Arizona soils. Plants take it up as one of many forms of phosphate depending on soil pH. Signs of phosphorus deficiency can be stunted growth, dark green color or purplish leaf color, and oldest leaves turning dark brown before prematurely dying. Phosphorus is highly mobile within a plant and can be redistributed from old to new leaves. It is used during cellular energy transfer (remember ATP from biology class), in metabolic processes, DNA, and membranes. Many soils can quickly make added phosphorus unavailable. Therefore, it is best to concentrate phosphorus fertilizers in the rooting zone rather than broadcasting them on the soil surface. Sulfur is taken up as sulfate ions which is usually abundant in native soils. Deficiency symptoms are total chlorosis (including veins). Plants need it to produce certain amino acids within the plant with will be incorporated into proteins. Sulfur is also needed to produce some vitamins and an intermediate compound (coenzyme A) used in respiration and fatty acid metabolism. Soil sulfur is commonly used to amend native soils not to add sulfur, but to lower soil pH. So far, we've only discussed macronutrients which are not nearly as elusive as the micronutrients. Micronutrients, by definition, exist only in quantities between 0.01 and 0.00001%. This makes it much more difficult to unlock their secrets. Chlorine is absorbed by plants in its ionic form: chloride (Cl-). It is needed to stimulate break down of the water molecules used during photosynthesis (see hydrogen in last week's article). In addition, it is important to roots, cell division in leaves, and osmotic regulation. There is abundant chloride in the atmosphere, so it is never a limiting factor. Iron is also absorbed in its ionic forms, but maintaining plant available supplies in native soils of the Verde Valley can be extremely difficult. This is because it is highly immobile within plants and ionic iron is relatively unavailable in soils with a pH above five (most soils in Arizona). Iron is needed by plants for energy transfers during photosynthesis and respiration. Iron deficiency symptoms are interveinal chlorosis (between veins) of new leaves. Those of you growing Photinia know all about this. To treat this common problem, a foliar application of chelated iron is the best immediate treatment. Chelated iron can also be incorporated into soil and is manufactured by chemically bonding an iron ion to a large organic molecule that maintains its solubility in our basic soils. Boron is absorbed by plants as boric acid. Boron deficiencies are rare in our area. Deficiency symptoms are varied, but include failure of root tips to elongate, inhibited DNA synthesis, and stunted shoot tips. Boron's role in these processes is still unclear. Manganese, zinc, and copper absorbed by plants in their ionic forms. I have grouped them together because, in many ways, they are similar to iron and only one of these elements is commonly deficient in our area. Zinc is commonly deficient in fruit and nut trees: especially apples and pecans. Symptoms are stunted, chlorotic branch tips which have been named "little leaf" disease. To correct the problem, spray the foliage with a solution of zinc sulfate. Nickel is a somewhat recent addition to the list of essential nutrients. It has been demonstrated as essential to many crops and is part of the enzyme that breaks down urea. Molybdenum is our final essential nutrient. Virtually nothing is known about the forms in which plants absorb it and what happens to it once it is absorbed. Plants require it in extremely small amounts for changing nitrate ions to nitrite ions in addition to two other small but critical plant processes. This brings us to the end of a long, technical topic. If you couldn't tell, I'm excited and fascinated by plant nutrition and, if you don't mind me saying so, all gardeners should be. The jury is still out on whether or not sodium and selenium should be added to the list. In closing, I'd like to add that I find it refreshing when I learn there are some things scientists still don't know. I guess it makes them seem more like the rest of us. The University of Arizona Cooperative Extension has publications and information on gardening and plant science. If you have other questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |