Ammonium reduces growth, fruit yield and fruit quality of watermelon

Watermelon (Citrullus lunatus Thunb.) plants cv ‘Sugar Baby’ were grown hydroponically in a greenhouse under NO3:NH4 ratios of 3:1, 1:1, and 1:3. Plants receiving the high ammonium treatment expressed symptoms of NH4‐toxicity and declined rapidly after bloom. Reducing NO3:NH4 from 3:1 to 1:1 significantly reduced growth, water use, fruit yield, flesh soluble solids and uptake of NO3, NH4, K, Ca and Mg.