Effects of Nitrogen, Phosphorus, and Potassium Rates and Fertilizer Sources on Yield and Leaf Nutrient Status of Short-day Onions

Abstract

This is a compilation of several studies that were performed to address specific grower concerns or questions about onion fertilization, to assess onion fertility, to make adjustments in soil test recommendations, and to test specific fertilizers for clients covering the 1999–2000 to 2004–2005 seasons. The synthesis of these studies was to evaluate levels of nitrogen (N), phosphorus (P), and potassium (K) fertilizers and their effect on yield, graded yield, and leaf tissue nutrient status in short-day onions over 6 years. In addition, various fertilizers were evaluated for their effect on these parameters. There was a significant increasing quadratic effect on yield from increasing N fertilizer from 0 to 336 kg·ha−1 with an R2 of 0.926. Maximum calculated yield was at 263 kg·ha−1 N fertilizer; however, the yield at this rate did not differ, based on a Fisher's least significant difference (P ≤ 0.05), from our current recommendations of 140 to 168 kg·ha−1 N. Jumbo (7.6 cm or greater) yield performed in a similar fashion. Phosphorus fertilizer rates from 0 to 147 kg·ha−1 had no effect on total yield, but did affect jumbo yields, which decreased linearly with an R2 of 0.322. Evaluations of P fertilizer in the 2001–2002 and 2002–2003 seasons only, when the exact same P fertilizer rates were used, showed a decreasing quadratic effect for jumbo yields with the lowest jumbo yields at 83 kg·ha−1 P fertilizer and jumbo yields increasing with 115 and 147 kg·ha−1 P fertilizer rates. Potassium fertilizer rates from 0 to 177 kg·ha−1 had a quadratic affect on total yield, with the highest yield of 52,361 kg·ha−1 with 84 kg·ha−1 K fertilizer rate. As would be expected, N and P fertilizer rates affected leaf tissue N and P levels, respectively. In addition, N fertilizer rates affected leaf tissue calcium (Ca) and sulfur levels. Potassium fertilizer rates had a significant linear effect on leaf tissue K 3 of 6 years. In addition, K fertilizer rates had a significant effect on leaf tissue P levels. Several fertilizers, including Ca(NO3)2 and NH4NO3, along with complete fertilizers and liquid fertilizers, were used as part of a complete fertilizer program and showed no differences for total yield or jumbo yield 4 of 5 years of evaluation when applied to supply the same amount of N fertilizer. Based on the results of this study, soil test P and K recommendations for onions in Georgia have been cut 25% to 50% across the range of soil test levels.