Which of the following is indicative of the compactness of the soil?

Bulk density is a key measure of the compactness of soil. It is defined as the mass of soil per unit volume, which includes both the soil particles and the pore spaces. High bulk density indicates that the soil particles are closely packed together, leading to less pore space and potentially reduced air and water movement. This compaction can affect root growth and soil organism activity.

In contrast, porosity measures the volume of pore space found within the soil, which is inversely related to bulk density. While porosity gives information about the potential void spaces within the soil, it does not directly quantify how compact the soil is. Soil pH, which measures the acidity or alkalinity of the soil, and soil carbon content, which relates to organic matter, do not provide information about soil compaction. Therefore, bulk density is the most direct indicator of soil compactness, making it the correct choice in this context.