Why when i added a fertiliser to my mung beans did it stunt their growth?

I am doing a biology experiment and i added 10 mung beans to each beaker filled with a different concentration of 1:1:1 N:K:P fertilier, yet as the conc. increased the rate of growth decrease. Im utterly confused as i thought fertilier would increase the rate of growth. I cannot find anything on the net to help me, can anyone explain why this occurred, or are all my results faulty?