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Fluid Journal : Fluid Journal 2002-2004
1 Fluid Journal Fall 2003 Balanced P and K Applications Crucial to Optimum Alfalfa Yields W.K. Berg, Drs. B.C. Joern, K.D. Johnson, S.M. Brouder, and J.J. Volenec Summary: Clearly, nutrient imbal- ance (adding P without K) has much graver consequences for alfalfa sur- vival than we had anticipated. From an alfalfa persistence point of view, producers are better off not fertilizing than applying P alone, if they suspect soil K levels are inadequate. In our studies, total annual yield increased with application of P and K, but K ap- plication did not increase first harvest yield. Yield increases were due to greater mass per shoot, whereas stems per unit area were not related to for- age yield. Although P fertilizer in- creased forage yield significantly, it decreased plant populations by pro- ducing fewer but larger individual al- falfa plants. Fertilization with P alone resulted in faster stand thinning than observed in plots provided with both P and K, and those left unfertilized. This rapid stand loss was associated with low concentrations of root reserves including starch, amino acids, and protein in taproots. igh yield and excellent forage quality make alfalfa the forage of choice in many livestock sys- tems, but intensive harvest manage- ment and average winter hardiness can undermine maximum yield, persistence, and ultimately profit. Improved fertil- izer management represents one ap- proach for increasing alfalfa yield and persistence, but our understanding of how alfalfa responds to P and K appli- Tests conducted at Purdue show the grave consequences of adding P without K. cation is rudimentary. Our objective in this study was to determine how alfalfa yield components are altered by P and K application. YIELD Forage yield increased markedly with P and K application (Figure 1). Highest yields were obtained when at least 200 lbs/A of K2O and 100 lbs/A of P2O5 were provided annually. Additions of K without P did not increase yield, whereas the addition of 100 and 150 lbs/ A of P2O5 increased yield of no-K plots over control plots (no K or P). How- ever, these yield increases resulted from greater first-cut (May) yields in 1998 to 2000, and have recently been offset by severe stand losses in the no-K, high-P plots in 2002 (Figure 1). YIELDCOMPONENTS Alfalfa yield is comprised of three yield components: mass/shoot, stems/ area, and plants/area that, when multi- plied together, determine forage yield. An increase in forage yield should be the result of increase(s) in one or more of these yield components. Mass/shoot. Greater alfalfa yield ob- tained with P and K fertilization has pri- marily resulted from increased mass/ shoot (Figure 2). Mass/shoot has con- sistently been associated with P- and K-induced increases in forage yield each growing season, whereas the other yield components have either decreased H Figure 1. Influence of P and K application on cumulative orage yield of alfalfa, 1998-2002. A Dry Matter Yield, tons/A A
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