Now days rubber ash is used for zinc deficient soil.Rubber ash contain 98.8percent Zinc and also there is no chance of Cd accumulation. My paper on rubber ash is recently accepted .When it come on line you can see that article.It will help you alot

I also used sand culture to study rice morphology. Here I used river sand and they were washed with running water. Yoshida nutrient medium was used for the study. There are few concerns when using Yoshida medium. You can refer "Laboratory manual for physiological studies of rice" from IRRI web site to see these conditions.