In the last two decades significant progress has been made in understanding the factors and processes affecting the K requirement of crops. Simultaneously analytical techniques to study the behaviour of K in soil have been developed, on the understanding that soil controls the supply of nutrients to crop plants by adsorbing and sometimes fixing nutrients. As in solution culture, the concentration of K in the soil solution governs the uptake of K by plants. The soil matrix serves as a reservoir from which the soil solution is replenished. Monitoring K status of the soil solution under fixed crop sequences can provide a useful basis for improving fertilizer recommendations. Kconcentrations in the soil solution have, therefore, been measured on a soil in which a number of crops have been grown in fixed annual sequences in field experiments.