Potassium (K) is the third major nutrient after N and P, required by plants for buildup of biomass. It exists in soil in different forms and these forms are in quasi equilibrium with each other. Now a day, K is recognized as an important limiting factor in crop production.

In the absence of adequate K fertilization, significant depletion of soil K reserve takes place, effect of which is substantial yield loss and higher economic risk of farmers. In the year 2020, the deficit of K in Indian agriculture is projected to be around 10 million tonnes/annum while the estimates for N and P balances are positive (Srinivasarao et al., 2001). Such a deficit will create serious nutrient imbalances with major implications on factor productivity and environment. It is worthwhile to note that even the most progressive and productive states like Punjab and Haryana, have most skewed N:P₂O₅:K₂O ratio. The focus has been on N followed by P and very little use of K resulting in a huge imbalance.

One hundred twenty one samples were collected on grid basis with coordinates through GPS and thematic soil maps were prepared using TNTmips (2010) GIS software.

Wide variation in water soluble K (3.5 ppm to 67 ppm), exchangeable K (12 ppm to 274 ppm), available K (17 ppm to 330 ppm), non-exchangeable K (65 ppm to 2101 ppm), nitric acid soluble K (126 ppm to 2431 ppm) and total K (469 ppm to 22471.20 ppm) were recorded in soils of North Bihar and are present in following order viz. total K > nitric acid soluble K > non-exchangeable K > available K > exchangeable K > water soluble K.

\Spatial distribution of pools of potassium generated on the basis of interpolation of point data provides spatial information for potassium management in North Bihar. Soil map generated by combination of exchangeable and non-exchangeable K of soil into nine classes among which, Class I, Class II, Class IV and Class VII cumulatively cover 45.23% area comprising in Motihari, Bettiah, Jaynagar, Sitamarhi, Darbhanga, Kisanganj, Purnia and Araria districts of North Bihar, where K application must be done to realize full yield potential of different cropaping systems.