Soybean response to potassium fertilization was evaluated over a twelve-year period in a field experiment on a Haplohumox soil, in Campos Novos, State of Santa Catarina, Brazil. Treatments consisted of a combination of four levels of K2O applied in the first year (0, 80, 160 and 320 kg ha¹), and three levels of K2O (0, 40 and 80 kg ha-1) applied annually from the fifth year. The critical level of K in the soil and in the leaves was determined based on the K concentration responsible for 90% of the soybean yield. Potassium in the soil was highly correlated with yield and K concentration in the leaves, and may be a suitable procedure to predict the availability of K to the plants. The critical level of K in the soil was 63 mg dm-3 of K, and in the leaves at flowering it was 14 g kg-1. Normal leaves, with no visual symptoms, had K concentrations from 12 to 19 g kg¹. A decrease in the soil K supplying capacity increased Mg concentration and decreased K/Mg ratio in the leaves. Four classes of K content in the soil and in the leaf tissue were defined: very low, when K concentration was below 38 mg dm-3 in the soil and below 9g kg¹ in the leaves; low, when between 39 and 49 mg dm³ in the soil, and between 9 and 12g kg-1 in the leaves; medium, when between 50 and 63 mg dm-3 in the soil and between 12 and 14 g kg-1 in the leaves; high, when higher than 63 mg dm³ in the soil, and higher than 14 g kg-1 in the leaves.
Glycine max (L.) Merrill; potassium availability; K in leaf; deficiency symptoms