I have 10 gram of soil sample in my media and would like to know what is the right number of each type of plastics to be added in. My plastics are poly ethylene, poly polypropylene and other two. At the moment I used 8 pieces from each, which makes it 32 all together. but I was advised to add it 32 from each so when I sacrifice the plastic for analysis each month I do not decrease the concentration. Does anyone know how to maintain the concentration and how it should be calculated. My media volume was 175 ml and soil sample was 10 gram in water = 75 = total volume 250 does V/W ratio is needed?

thanks for your help