Small particles of organic material are called

Organic materials found in soil such as dead plants, animal fesis, bacteria, and fungi cause decomposition in the soil or a decaying breakdown of organic materials which is needed for proper soil health. Decomposition releases small particles of microorganisms or detritus which other living organisms feed on and release energy as well as recycles inorganic materials back into the food chain. These small particles are so small that the naked eye cannot see. The process of photosynthesis is very important for the food chain. Healthy soil helps plants grow, decomposition feeds the plants, the sun provides energy which releases carbon dioxide so the plants can grow, and the plants release oxegen into the atmosphere which living organisms need to survive.

~potters6/te801/biology/biounits/respiration.htm