Please say why you want to prevent decomposition.  Most protectants will have other effects so you need to choose with a mind to your study

The simplist way is to lower the temperature and moisture in storage. Good luck!

Ping

I actually would like to add it because I want to improve the engineering properties of

the soil, like shear strength. it  totally differs from agricultural view

simply I do want it to decompose over time

thank you very much 

thank you Dr.Vladimir

You can reduce the rapidity of decomposition by keeping the mixture dry or by adding chemicals such as creosote.  However, the latter is carcinogenic, and this and other more soluble and miscible chemicals will alter the engineering properties such as strength and freezing point.

Sruart.