Why does soil bulk density and organic matter per layer of soil positively correlate?

Soil bulk density and organic matter per layer of soil positively correlate due to several reasons:

1. Organic Matter Content: Organic matter contributes to the overall volume of soil but has a much lower density than mineral particles. Therefore, soils with a high organic matter content tend to have lower bulk densities.

2. Soil Structure: Organic matter contributes to the formation of soil aggregates, which can reduce bulk density. This is because aggregates create pore spaces in the soil, reducing the mass per unit volume.

3. Water Holding Capacity: Organic matter can retain water, which adds to the soil's volume but not its mass, reducing the bulk density.

4. Decomposition: As organic matter decomposes, it can lead to a decrease in volume, thus increasing the bulk density.

5. Soil Compaction: Organic matter can help prevent soil compaction, which tends to increase bulk density. This is because organic matter can create a more porous soil structure, reducing compaction.

So, as the organic matter in a soil layer increases, the bulk density tends to decrease, showing a positive correlation between the two.