The concept of Q/I relationship was developed by

Beckett 1964 has developed the concept of the Q/I relationship.
K is potassium ions here in the soil solution and I is the Intensity factor while Q is the Quantity factor.
As per this concept, K+ in the soil solution to the changes in exchangeable K+ in the soil thus, a develop a Quantity - Intensity (Q/I) relationship for predicting the K availability to plants.