How do manure and soil-improving crops add organic matter to vegetable soil?

In vegetable farming, both manure and soil-improving crops are essential for maintaining soil organic matter, which is vital for soil fertility and structure.1

Manure

Farm manure consists of animal excreta mixed with bedding materials like straw. When incorporated into the topsoil, it adds organic substances and humus, which improve the soil's physical properties, such as water storage and aeration. Because manure contains nutrients in unmineralized forms, soil microbes must break down this organic matter to make the nutrients bioavailable to plants.23

Soil-Improving Crops

Soil-improving crops, which include green-manure crops and cover crops, are grown specifically to prepare the land for future vegetable production.14
* Mechanism: These crops are turned under (plowed) into the soil while still green. As the plant material decomposes, it returns nutrients and organic matter to the soil.1
* Types: Both legumes and nonlegumes are effective, though legumes are particularly valued for their ability to fix atmospheric nitrogen.51
* Decomposition: The rate at which these crops add organic matter depends on the plant's maturity, soil temperature, and moisture levels. Succulent, immature plants decompose more rapidly than mature, dry material.1