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Robert L. Mikkelsen

An adequate potassium (K) supply is essential for both organic and conventional crop production. Potassium is involved in many plant physiological reactions, including osmoregulation, protein synthesis, enzyme activation, and photosynthate translocation. The K balance on many farms is negative, where more K is removed in harvested crops than is returned to the soil. Although various organic certification agencies have different regulations governing allowable sources of K, the behavior of soil K is largely governed by its solubility. The slow release of K from soil minerals is generally insufficient to meet the peak nutrient demand of high-yielding crops, but they can contribute to the long-term improvement of soil fertility. There are many excellent K sources allowed for organic crop production, including soluble minerals such as langbeinite, sylvinite, and potassium sulfate. Potassium sources such as wood ash, greensand, and seaweed can also supply K but require special management because of their low nutrient content, their effect on soil pH, low solubility, or bulky nature. The concentration of K in manures and composts is highly variable, but it is generally quite soluble and available for plant uptake. Some rock minerals may supply a portion of the K requirement of plants, but many are too insoluble to be of practical significance.

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Robert L. Mikkelsen

Improving plant nutrient management is important for environmental, economic, and social considerations. The adoption of the “4R” nutrient stewardship framework (right source, right rate, right time, and right place) provides a basis for examination of the underlying scientific principles behind fertilizer use. These 4R concepts are based in global principles related to chemistry, biology, physics, and economics, but the selection of specific practices is adjusted to individual field conditions, relying on local expertise and data. Various stakeholders have input in the selection of nutrient management practices, and their objectives may not always coincide. The development of performance indicators to measure the progress made by adoption of the 4R management techniques needs to be decided by stakeholders.

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Robert L. Mikkelsen and Thomas W. Bruulsema

Tremendous changes have occurred during the past century in the sources and methods for supplying nutrients for horticultural crops. Reliance on animal manure, cover crops, and animal tankage was insufficient to meet the crop nutrient demand for a rapidly expanding population. The Haber-Bosch process for ammonia synthesis (1910s) revolutionized the availability and affordability of nitrogen (N) fertilizer. Discovery of large-scale deposits of rock phosphate in South Carolina (1860s) and Florida (1880s) alleviated widespread nutrient deficiencies. Acidification of rock phosphate and bone material significantly improved phosphorus (P) availability for plants. Discovery of potassium (K)-bearing minerals in New Mexico (1920s) and later in Canada (1960s) now provide a long-term nutrient source. Modern fertilizer technology allows nutrients to be applied in the correct ratio and amount to meet crop needs. Advances in understanding plant nutrition, coupled with slow-release fertilizers, foliar fertilization, soluble nutrients, and the development of soil and tissue testing have all improved the yield and quality of horticultural crops. Future developments will likely focus on fertilization in an increasingly competitive global economy, while requiring sophisticated management to minimize environmental impacts.

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Helen T. Kraus, Robert L. Mikkelsen and Stuart L. Warren

Traditional N mineralization studies have been conducted by soil scientists using soils and temperatures found in field production. As temperature, in part, governs the rate of mineralization, and container substrates reach much higher temperatures than do soils, the effect of these elevated temperatures on mineralization must be considered to begin to understand N mineralization in container substrates during production. The N mineralization patterns of three composts [turkey (Meleagris gallopavo) litter, yard waste, and municipal waste] were determined under three temperature regimes (45, 25, and 45/25 °C). More organic N was mineralized from composted turkey litter (CTL) than from municipal or yard composts, regardless of temperature. The percentage of organic N mineralized from CTL was greater at 45/25 and 45 °C than at 25 °C.

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Helen H. Taylor, Robert L. Mikkelsen and Stuart L. Warren

The N release patterns of composted turkey litter, composted yard waste, and composted municipal waste amended pine bark substrates were measured under simulated diurnal temperature variations [25C, 45C, and 45/25C (14/10 h)] found in container substrates. Temperature regime, compost, and the interaction between temperature and compost affected the NH4 and NO3 availability and the total N released from the composted waste products over the 16-week experiment. Within each temperature regime, the composted turkey litter released greater amounts of NH4 and more total inorganic N than the municipal and yard wastes. The turkey litter yielded the highest NO3 concentrations at 25C, while the municipal waste produced the highest NO3 concentrations at the 45C and the 45/25C temperatures. Temperatures higher than 25C inhibited nitrification in the turkey litter-amended substrates; however, the 45C and the 45/25C treatments resulted in greater total N mineralization than the 25C treatment.

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William E. Little, Jonathan R. Schultheis and Robert L. Mikkelsen

North Carolina is a leading poultry producer in the United States. Thus, much waste by-product also is produced and must be handled in an environmentally responsible way. Using poultry and similar waste products as a fertilizer source for vegetables, such as sweetpotatoes, might serve as a viable use option. Our purpose was to determine the effectiveness of animal wastes and sludges as nutrient sources for sweetpotatoes. The effects of municipal solid waste, composted litter, fresh litter, and synthetic fertilizers were compared for their effects on yield and quality of `Regal' and `Beauregard' sweetpotato varieties. The test was planted as a split-plot randomized complete-block design with each treatment replicated four times. Planting was 3 June, and harvest was 27 Sept. 1994. Yields were similar when fertilized with either organic or synthetic nutrient sources. Root quality was excellent, regardless of fertilizer, because few culls resulted, and there were no differences between treatments. Sweetpotatoes can be successfully grown with various organic nutrient sources without affecting quality or yield and might be marketed as “organically grown” produce. This label may command a higher market price than sweetpotatoes grown traditionally with synthetic nutrient sources.