A substrate that is of nearly uniform size and used for the inert hydroponic medium. Each aggregate is made up of soil particles of different sizes held together by both the attraction of soil particles and the binding of organic matter between soil particles.

Soil organic matter holds aggregates together, making them stable and structural. At the same time, aggregates protect the organic matter from decomposition. Aggregate structures also provided large and small pores which have structural functions. Large soil pores allow water to quickly infiltrate the soil. Smaller soil pores can store plant available water in times of limited rainfall.