   Plant Mineral Nutrition - December 4, 2002 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Last week, we covered essential plant nutrients: what they were, where they came from, and a little about how they get into a plant. We did not cover absolute amounts of each nutrient needed for normal plant growth and development. This topic is complicated because plant species differ in their nutrition needs, ability to extract scarce nutrients, and adaptations that allow them survive where other plants cannot. Native plants succeed because of these factors. Crop plants are usually not as well adapted and rely on humans to create an environment where they can succeed. In an effort to simplify this topic, we will focus on crop plants. Chemist, Justus von Liebig, is largely credited with pioneering plant nutrition theories. His published works in plant chemistry date from the 1840's. Liebig's Law of the Minimum states that yield is proportional to the amount of the most limiting nutrient, whichever nutrient it may be. From this, it may be inferred that if the deficient nutrient is supplied, yields may be improved to the point that some other nutrient is needed in greater quantity than the soil can provide, and the Law of the Minimum would apply in turn to that nutrient. Liebig's model is often illustrated with a barrel having varying stave lengths. The capacity of the barrel is limited by the length of the shortest stave, and can only be increased by lengthening that stave. When that stave is lengthened, another one becomes the limiting factor. The concept of the law of the minimum has been modified as additional elements have proved to be essential in plant nutrition. It has been extended to include other factors such as moisture, temperature, insect control, light, plant population and genetic capacities of plant varieties. The model is simple and it still holds water (so to speak). In most systems, nitrogen is the nutrient most limiting to plant growth. Once the need for nitrogen is met, then, very often phosphorus becomes limiting. From here, the next limiting nutrient often depends on the crop being grown. Potassium is often sufficient in our native soils due to the parent material mineral composition. Zinc or iron may limit certain woody plants and fruit or nut crops. Nutrient deficiency symptoms may become evident when observing plants. Slow or stunted growth, abnormalities of leaf/fruit color, shape, or development are common indicators of major deficiencies. However, deficiency symptoms can be subtle or confounded by multiple factors. Fertilizer additions can rectify most nutrient deficiency problems, but apply them sparingly and according to label directions. Excess nutrients can leach out of the root zone and become harmful to the environment. Just as nutrients can be deficient, on overabundance can lead to nutrient toxicities. If you suspect a plant nutrient deficiency and are unsure where to start, then do a soil test. This can be done by a commercial lab or a simple test kit available from your local garden center or garden supply catalog. Commercial labs use standardized analytical methods, can test for more nutrients, and produce higher quality, repeatable results. Do-it-yourself kits are less expensive, are limited in their capacity, and produce more general, qualitative results. Soil testing will be the topic of next week's column. Got bark beetles? The University of Arizona Cooperative Extension, Yavapai County is offering a free forest health seminar at the Pushmataha Center (Keep Sedona Beautiful), 360 Brewer Rd on Wednesday December 18, 2002 from 9:00 AM to noon. I hope to see you there! The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/. |