Critical Leaf K Concentration Based on the Maximum RDM

The critical leaf K concentration was constructed based on the critical data points at which RDM was neither limited nor enhanced by K. A critical data point was determined by each sampling data that include K-limiting treatments and non-K-limiting treatments. With increasing K application, both the storage root weight and leaf K concentration of sweet potato increased significantly (P < 0.05). With the continuous increase of K application, there were no significant changes in the storage root weight (P < 0.05), but the leaf K concentration increases continuously with increasing K application (P < 0.05). Therefore, the K treatments for each sampling were divided into two groups: (i) K-limiting treatment was defined as a treatment in which additional K supply led to a significant response in the storage root weight and leaf K concentration and (ii) non-K-limiting treatment was defined as a treatment in which additional K supply did not lead to a significant increase in the storage root weight but resulted in a significant increase in the leaf K concentration. The RDM values in different K treatments were compared using ANOVA (SPSS Statistics 19.0 software, IBM Corp., Armonk, NY, USA) at a probability level of 5%.

The critical leaf K concentration based on the maximum RDM is defined as the minimum leaf K concentration required for maximal root growth. A critical leaf K concentration model for sweet potato roots was constructed according to the method of the critical N concentration model (Justes et al., 1994). First, the RDM and the leaf K concentrations were used to identify whether root growth was restricted by K nutrition. A regression line was used to fit data from the K-limiting treatments. A vertical line was calculated with the data from the non-K-limiting treatments as the average of the maximum RDM. The critical leaf K concentration point was the intersection point of the oblique and vertical lines. The following equation describes the critical leaf K concentration curve (K_leaf) based on the maximum RDM:

where a₁ and b₁ are the empirical coefficients.