I’ve been studying for my RHS course, and recently read more about ‘green manure’. Green manure is when certain plants are grown in an area, with the intention that they will be dug into the soil at some stage to improve the nutrient content and structure of the soil. This has the advantage that no artificial fertilisers need to be applied, but more importantly will improve the structure of the soil, and genereally improves the health of the soil. Organic material is broken down slowly by the microorganisms in the soil, so it feeds a whole living system that contributes to the growth of plants.
Improving soil structure also allows it to hold water and other nutrients more effectively, so it’s something every gardener should do. Many of the plants used in green manure happen to be useful for bees when their flowers bloom. These include: Borage, Chicory and Phacelia (pictured right). The seeds of these are all easily available in garden centres, and simple to sow and grow. You could also consider using members of the 'pea' family of plants, which includes all peas and beans, as well as clover, lupin and vetches. These plants all have special nodules in the roots which contain bacteria that take nitrogen from the air and convert it to forms that plants can use. If it wasn't for nitrifying bacteria like this, there would be almost no natural nitrogen available for plants. Nitrogen is one of the most important plant nutrients, so adding more to the soil this way can improve growth.
The advice for most green manures is to dig them into the soil before the flower and set seed - otherwise they'll keep growing in subsequent years, even if you don't want them there. But this is clearly of no benefit to bees - they need the flowers! I leave mine to flower, and don't mind if they set seed, as I prefer to have a wilder appearance to my garden anyway.
You can also use the leaves of many of these plants to create your own liquid fertiliser. Just cut the leaves of borage or phacelia, and leave them in a bucket of water or watering can for at least a month. In this time they will break down, releasing their nutrients into the water. This can be quite strong, so dilute it with ten parts water before applying.