Optimizing irrigation amount and potassium rate to simultaneously improve tuber yield, water productivity and plant potassium accumulation of drip-fertigated potato in northwest China

High crop water productivity (WPc) and fertilizer use efficiency are the key to ensuring the sustainable development of agriculture in water-deficient areas, such as the northwest China. In order to determine the irrigation amount and potassium rate for optimizing potato yield, plant potassium accumulation and WPc of drip-irrigated potato, a two-year field experiment was conducted on the Loess Plateau of northwest China, with three irrigation levels (W1: 60% ETc, W2: 80% ETc and W3: 100% ETc, where ETc is the crop evapotranspiration) and four potassium levels (K0: kg ha−1, K1: 135 kg ha−1, K2: 270 kg ha−1 and K3: 405 kg ha−1). The results showed that leaf area index, chlorophyll content, aboveground dry matter, tuber yield, starch, vitamin C content and WPc generally increased with the increase of irrigation amount, while they first increased and then decreased with the increasing potassium rate. The trends of reducing sugar and potassium use efficiency were contrary to the above trends. The residue of available potassium in the soil increased with the increase of potassium rate, but decreased with the increase of irrigation amount. Irrigation amount and potassium rate had interaction effects on chlorophyll content, dry matter, tuber yield, reducing sugar, plant potassium accumulation and potassium use efficiency. To ensure high-yield, high-quality, water-saving and fertilizer-saving potato production, a water input (irrigation plus effective rainfall) range of 498–520 mm combined with a potassium rate range of 201–393 kg ha−1 was recommended. These results can provide a theoretical basis for the rational application of irrigation water and potassium fertilizer to improve tuber yield, quality and soil environment of drip-irrigated potato in northwest China.