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Robert L. Mikkelsen

framework to guide nutrient management decisions, it is universally recognized that it is in our best interests to use the right source of nutrients, in the right rate, at the right time, and in the right place—termed the four rights or 4R. A systematic

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Bielinski M. Santos

Selecting the “right” nutrient rate for fertilization programs is one of the most important decisions growers face. On one hand, increasing fertilizer prices and environmental concerns have increased the awareness of accurately managing fertilization programs, thus reducing fertilizer amounts during cropping seasons. By contrast, many growers fear not obtaining the desired crop performance and economic returns, especially when fertilization is assumed as “inexpensive insurance” to improve yields, thus leading to overfertilization. The objective of this paper was to provide general principles for selecting and monitoring the right nutrient rate within the framework of the “4R” nutrient management concept (right rate, right source, right placement, and right timing) to protect environmental quality while maintaining productivity. Some methodologies to determine, apply, and adjust fertilization rates during the growing season were discussed, including in-season monitoring procedures, such as petiole sap testing, plant diagnostic analysis, leaf color evaluation, and plant growth index.

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Neil S. Mattson and Marc W. van Iersel

The 4R nutrient stewardship framework presents four concepts to consider when applying fertilizers in a responsible matter; the “right source” of nutrients should be applied at the “right rate” during the “right time” and supplied to the “right place” to ensure their uptake. In this article, we provide ideas to consider when attempting to provide nutrients at the right time. When nutrients are applied at a time when they are not required by the plant, the result can be economic and environmental losses. Oversupply relative to plant demand can result in losses of applied nutrients because of leaching or volatilization. Undersupply relative to demand, especially in the case of phloem-immobile nutrients, may limit plant growth and yield. Several factors interact to affect plant nutrient demand such as growth stage, life history (annual vs. perennial), environmental conditions, and plant health. Techniques such as soil and tissue testing, isotopic labeling, and spectral reflectance have been used with varying degrees of success and expense to measure plant nutrient demand and guide fertilizer decisions. Besides knowledge of plant nutrient demand, efficient nutrient supply also depends on systems that allow precise spatial and temporal delivery of nutrients. Future improvements to the timing of nutrient delivery will depend on improvement in knowledge of plant nutrient demands. For example, targeted gene expression chips show promise for use in rapidly assessing plant status for a broad suite of nutrients. Future developments that allow more precise nutrient delivery or more robust agroecosystems that scavenge available nutrients before they are lost to the environment will also help producers use nutrients more efficiently.

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Robert Conway Hochmuth, Marina Burani-Arouca, and Charles Edward Barrett

attain the 206 kg·ha −1 N rate by using the BMPs associated with the 4-Rs Principles: right rate, right place, right source, and right timing ( FDACS, 2015 ) and when weather related nutrient leaching is not a major factor. In situations where N would be

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Mark Gaskell and Tim Hartz

-grown ornamentals Hort. Rev. 38 253 297 Mikkelsen, R. Schwab, G. Randall, G. 2009 The four fertilizer rights: Selecting the right source Crops Soils 42 3 28 31 Mitchell, J. Gaskell, M. Smith, R. Fouche, C. Koike, S. 2000 Soil management and soil quality for organic

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David R. Bryla

The 4R nutrient stewardship concept was introduced by Bruulsema et al. (2009) to define the right source, rate, time, and place to apply fertilizers to produce not only the most economical outcome in any given crop but also to provide desirable