Putting grasses, leaves, pods, and dung in the vagina can cause infection and irritation.
Washing out the vagina (douching) with herbs or powders does not prevent pregnancy. Sperm move very fast and some will reach the inside of the womb before they can be washed out.
Urinating after sex does not prevent pregnancy. (But it can help to prevent infections of the urine system.)
Sources
Burns, A. A., Niemann, S., Lovich, R., Maxwell, J., & Shapiro, K. (2014). Where women have no doctor: A health guide for women. Hesperian Foundation.