Agronomic response of phosphorus and calcium rates for processing and table stock potatoes in northeast Florida

Phosphorus (P) and calcium (Ca) are required nutrients for potato plant growth and yield. Notably, even in soils with high P levels, tuber yield has responded to P fertilization. Additionally, the application of calcium sulfate (CaSO ₄ ) to improve tuber quality has become a common practice, despite the absence of guidelines considering soil Ca levels as a critical threshold for such applications. The objective of this research was to assess potato plant growth, yield and tuber quality in response to P fertilizer (0, 45, 90, 135, 180, 225 kg ha ^-1 of P ₂ O ₅ ) and Ca fertilizer (0, 560 kg ha ^-1 of CaSO ₄ ) rates arranged in a factorial design. A three-year trial was conducted at with processing cultivar Atlantic at Site 1. The trial was repeated in a commercial potato farm (Site 2) with an additional CaSO ₄ rate of 1120 kg ha ^-1 with table stock cultivars ‘Fabula’ in 2016 and ‘Red La Soda’ in 2017. In Site 1, there was a quadratic response for total yield as function of increased P fertilizer rates. However, total yield and marketable yield increased quadratically but leveled off above 90 kg ha ^-1 of P ₂ O ₅ . In Site 2, there was no effect of fertilizers rates on total and marketable tuber yield, internal and external quality for any table stock cultivar. Total yield averaged 33 Mg ha ^-1 (2016) and 33.9 Mg ha ^-1 (2017) while marketable yield averaged 19.3 Mg ha ^-1 and 23.6 Mg ha ^-1 in 2016 and 2017, respectively. Tuber specific gravity averaged 1.042 in 2016 and decreased linearly with the increasing the P fertilizer rates in 2017, ranging between 1.061 to 1.059. Potato yield responses to P fertilizer were limited and site-specific regardless of the M3 soil testing results indicating poor reliability of the M3 extractant with bioavailable P. Calcium sulfate provided no measurable benefit to soil Ca, P availability, plant uptake, yield, or quality, indicating routine gypsum use is unwarranted in high-Ca soils and that P applications should be conservative and guided by soil tests.