Alright.. this is what I have in mind... then you can judge if I'm totally crazy: As in any other experiment we wish to only hold 1 variable, while other thinkable variables are held constant. In addtion to that we need to ref. soil and call that 0. Now some theory: We know from medicals they can have synergetic effect if mixed. Same can go for nutrients, we therefor test each soil mixed and alone and see the effect. We call the nutrients A, B and C, (continue with more if you like) Now this is our setups: 0, A, B, C, AB, AC, BC, ABC. In this way we can make compare to a ref soil (0) and also check for synergetic effects... this is a bit considered a multivariable experiment, but we also have A, B and C as ref. against the mixed soils.