To produce leafy growth plants need more nitrogenous compounds. Through photosynthesis they break these down to extract the nitrogen required to create leaf cells. This means they need a compound that is rich in nitrogen. Usually we use urea or ammonium sulphate based fertilisers to increase the nitrogen available for plants. Chicken manure is high in nitrogen but is too strong to add directly to the plants. Instead it’s a great additive to compost. The bacteria will break it down to make it even more readily accessible to plants.
It’s important to realise that plants require nutrients in minute quantities. A little goes a long way. We currently have a farming attitude of saturating the soil with an excess of fertiliser for a bumper yield. Over time the level of nutrients rises to the point where the water run off is toxic to aquatic life. The best method is to rely on natural organic fertilisers. Usually they are slower to release their nutrients, often lasting several growing seasons and the run off water is less toxic to aquatic life. An added benefit is that they usually break down to release other nutrients in lesser quantities, unlike artificial chemical fertilisers that leave a residue that builds up with each application.
Another way of increasing nitrogen in the soil is to grow plants that “fix” nitrogen in the soil. These are plants that have nodules on their roots with special bacteria that trap nitrogen in a form readily usable by plants. The most common plants used are all the members of the pea family and lupins. Sometimes driving around in the country, you will see fields of bright blue or yellow lupins. A farmer is replenishing the nitrogen content of the soil naturally. It’s a lot cheaper than applying artificial fertilisers and it can be done over the less productive time of the year.
As a home gardener, there are a different varieties of lupins that make a bright show of flowers as well as step up the nitrogen in your soil. Simply plant them and when they have flowered for a while (but before they have all died off and developed seed) dig the entire plant into the soil. In a month they will have decomposed into a nitrogen rich humus.