| Summary: | Potassium (K) fertilizer plays a crucial role in the formation of the biological and economic yield of cotton (<i>Gossypium hirsutum</i> L.). Here we investigated the effects of the amount of K on biomass accumulation and cotton fiber quality with lowered N amounts (210 kg ha<sup>−1</sup>) under late sowing, high density and fertilization once at 2 weeks after squaring. A 2-year field experiment was performed with three K fertilizer amounts (168 kg ha<sup>−1</sup> (K<sub>1</sub>), 210 kg ha<sup>−1</sup> (K<sub>2</sub>), and 252 kg ha<sup>−1</sup> (K<sub>3</sub>)) using a randomized complete block design in 2016 and 2017. The results showed correspondingly, K<sub>3</sub> accumulated cotton plant biomass of 7913.0 kg ha<sup>−1</sup>, next to K<sub>2</sub> (7384.9 kg ha<sup>−1</sup>) but followed by K<sub>1</sub> (6985.1 kg ha<sup>−1</sup>) averaged across two growing seasons. Higher K amounts (K<sub>2</sub>, K<sub>3</sub>) increased biomass primarily due to a higher accumulation rate (32.68%−74.02% higher than K<sub>1</sub>) during the fast accumulation period (FAP). Cotton fiber length, micronaire, and fiber strength in K<sub>2</sub> were as well as K<sub>3</sub> and significantly better than K<sub>1</sub>. These results suggest that K fertilizer of 210 kg ha<sup>−1</sup> should be optimal to obtain a promising benefit both in cotton biomass and fiber quality and profit for the new cotton planting model in the Yangtze River Valley, China and similar climate regions.
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