All these techniques are different but somewhat interrelated.
Mulching is the use of organic materials (plastic mulch is expensive and non-biodegradable) to cover the soil, especially around plants to keep down evaporation and water loss, besides adding valuable nutrients to the soil as they decompose. Mulching is a regular process and does require some labour and plenty of organic material, but has excellent effects, including encouraging the growth of soil fauna such as earthworms, preventing soil erosion to some extent and weed control.
Green manuring is an age-old practice prevalent since ancient times. A crop like dhaincha (Sesbania aculeata), sunnhemp or horsebean is sown (usually) just before the monsoons. A mix is also possible. Just around flowering (30-45 days after sowing), the crop is cut down and mixed into the soil after which the season’s main crop is sown. Green manuring is beneficial in two ways – firstly it fixes nitrogen, and secondly the addition of biomass (around five to ten tons/acre) greatly helps in improving the soil texture and water holding capacity. Green leaf manuring can also be carried out if sufficient leguminous tree leaves are available.
Cover cropping is normally carried out also with nitrogen-fixing crops that grow fast and require little or no inputs like water or additional manuring. While cover crops can yield some returns, they are mostly used for covering the soil in the fallow months, adding nitrogen to the soil, suppressing weeds, preventing soil erosion and later used as biomass or fodder. Velvet bean is an example, and it finds use as a fodder crop and biomass generator. Another useful cover crop is Dolichos lablab which is a source of fodder and food.