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Managing plant nutrientsBy D. Bruce Bosley CSU Extension Agent, cropping systems Colorado Providing a good supply of nutrients is essential for producing adequate and profitable alfalfa yields. Alfalfa continuously depletes soil nutrients. Each ton of alfalfa hay contains about 50 pounds of nitrogen, 10 pounds of phosphorus, 60 pounds of potassium, and 4 pounds of sulfur. Plant deficiencies of other nutrients, while rare, can occur in Colorado fields. Managing the proper levels of plant nutrients begins with assessing the nutrient level present in soils and in plant tissues. Phosphorus and sulfur are the most common causes of alfalfa nutrient deficiencies in the western High Plains region. In addition, agronomists occasionally find that potassium, boron, and molybdenum are deficient in hay fields in the west. Nitrogen deficiencies almost never occur. Poor Rhizobia inoculation, a nitrogen-fixing bacteria found in legume root nodules, is the most likely cause of nitrogen deficient alfalfa. Similarly, iron deficiencies are rare in alfalfa. Both production and stand viability are dependent upon adequate levels of essential plant nutrients. Determine the soil nutrient status for phosphorus and potassium prior to planting. Irrigation water often contains sufficient amounts of sulfur and boron for good alfalfa production. For this and other soil test limitations, conducting tissue tests for sulfur and boron are more reliable in assessing adequate plant levels of all nutrients throughout the life of the alfalfa stand. It is usually a good idea to take multiple soil samples in a large field because of variations in soils and soil history. Likewise, low yielding or especially high areas of a field may require different fertility levels to optimize profitable crop yields. Yearly tissue testing from low, medium, or high yielding areas is a good way to monitor for profitable nutrient management. I had discussed this method using management zones for site-specific (precision) farming in my October news columns. Take soil tests at least two months before planting. This allows time to get the results back and to incorporate the needed fertilizers into the soil. This is especially important for phosphorus, an element that is immobile in soils. Soil test recommendations for phosphorus can call for as much as two hundred pounds of actual phosphorus per acre for new seedings. Apply phosphorus and other needed nutrients to supply two years nutrient needs. When tissue tests call for additional nutrients, a top-dress fertilizer application can supply plant needs for alfalfa without mechanical incorporation. The plant root hairs are able to pick up phosphorus and other non-mobile nutrients near the soil surface, especially on irrigated fields. Soil tests from differing labs vary in their phosphorus extraction methods and the test numbers expressing low, medium, and high levels. For this reason, use the test ranges and recommendations from recommending labs for determining how much fertilizer to apply. If soil tests show low levels of phosphorus, it is best to apply the full amount of this nutrient using broadcast incorporated or subsoil banding. This is not the time to cut fertilizer costs because the reduced phosphorus availability will result in lower yields throughout the life of the alfalfa stand. There is a new method for alfalfa tissue testing that is too complex to describe in this news column. It provides excellent results when done properly. Contact me for a print or electronic copy of the procedure. Please contact me, Bruce Bosley, about this or other cropping systems or natural resource topics at 970-522-3200, ext. 285 in Sterling or 970-542-3540 in Fort Morgan. 11/24/08 Date: 11/19/08 Advertisement
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