Response of high density apple orchards on coarse-textured soil to form of potassium applied by fertigation

This study tested the effects of fertigated potassium sources on orchard cation status. A randomized, complete block experimental design was maintained from 2000 to 2003, in a high density 'Jonagold'/M.9 apple (Malus x domestica Borkh.) orchard planted in 1993 on a loamy sand. Seven K-fertigation treatments included annual application of either no K (control), 15 g K/tree as either potassium chloride or potassium magnesium sulphate (KMag) or 30 g K/tree as potassium chloride, KMag, potassium sulphate or potassium thiosulphate, applied daily during 6 wk midsummer to six replicate, four-tree plots. Fertigated K-forms did not affect yield, but increased soil K after 3 yr to 30-cm depth beneath the drip emitters. This increased leaf and fruit K concentrations. Fruit K/Ca ratio was also increased by K-fertigation. A high incidence of bitter pit at harvest was unaffected by fertigating K, but rather was associated with low harvest fruit Ca concentration and large fruit size. KMag increased soil Mg availability, but leaf and fruit Mg concentrations were slightly affected, indicating the difficulties of improving apple Mg status when co-applying K. Leaf and fruit Ca concentrations were minimally affected by treatments. Soil Ca declined slightly after 3 yr of K fertigation.