For further analysis of the main factors affecting this increasing trend of soil
available K, we separated the soil samples into two categories based
on crops planted: grain crops and cash crops. The soil K values for
both grain crops and cash crops increased with the time going from
1990 to 2012. For grain crops, soil available K content increased only
slightly with a slope of 0.553 based on the linear model (Fig. 2B),
however, for cash crops, the value increased dramatically over
the period with a slope of 4.919 for the trend line (Fig. 2C).