Which property of the soil is primarily determined by the Standard Proctor test?

The Standard Proctor test is specifically designed to determine the optimum moisture content and maximum dry density of soil. This test evaluates how the moisture content in the soil affects its compaction, which is a critical aspect in construction and geotechnical engineering. By compacting the soil at various moisture levels, the test identifies the specific moisture content at which the soil can achieve its highest density, referred to as the optimum moisture content.

Understanding the optimum moisture content is crucial for ensuring effective soil compaction in the field, thereby enhancing the soil's load-bearing capabilities and stability. When designing foundations, roads, or other structures, engineers must account for this optimal level to prevent differential settling and maintain structural integrity. Therefore, the primary property determined by the Standard Proctor test is indeed the optimum moisture content of the soil.