Leguminous cover crops do not reduce the soil microbial population. In fact, they do the opposite - they aid in increasing the soil microbial population. This is because leguminous cover crops have the ability to fix nitrogen from the air and add it to the soil, providing a source of nutrition for soil microorganisms. Additionally, their roots and leaves also add organic matter to the soil as they decompose, which further supports the growth of soil microorganisms. So, leguminous cover crops are beneficial for soil health and fertility, not detrimental.