I originally wrote this as a post on the garden blog (7th Nov 2010), but thought it might be worth adding to the notes section.
I thought I had planned my garden quite carefully, but having been on the Introduction to Forest Gardening course at ART recently I realised that I had not worked out a nitrogen budget. If the garden is to be truly sustainable it is important that it doesn't rely on imported nutrients. I've also been thinking about adding nitrogen fixing plants into the design, but want to know how many I need.
The principles for the budget are simple:
1. estimate the nitrogen requirements for the plants in your garden
2. estimate the nitrogen inputs
3. see if the inputs meet the requirements and if not increase the sustainable inputs through including more nitrogen fixers or other sources.
I used Martin Crawford's Creating a Forest Garden as a guide. It has figures for annual nitrogen requirements for different classes of plants, in order of decreasing requirements: annual vegetables, heavy cropping (apples, pears etc.), moderate cropping (e.g. currants) and undemanding (e.g. hawthorn) that don't need additional feeding. I calculated that when the garden is mature I will have about:
- Annual vegetables: 8m2
- Heavy cropping plants: trees 92m2, shrubs 12m2, total 104m2
- Moderate cropping plants: trees 26m2, shrubs 15m2, total 41m2
- Nitrogen fixing plants: Not many at the moment. I have crimson clover sown through much of the lawn. I have plants which accumulate nitrogen (comfrey and nettles), but as I understand it these don't add nitrogen to the system only concentrate it from the soil and subsoil and can be cut and applied as a mulch or to compost. Based on the number of comfrey plants and how often I cut them this accounts for only 0.006kg per year.
- Compost: I've got a couple of compost heaps. Now that most of the main planting has been done in the garden I use this for top dressing to feed plants and to improve the thin soil. I estimated that I empty the kitchen compost bin every three days. It's raw contents normally weight about 1.7kg. Creating a Forest Gardensuggests compost has about 0.5g/kg. I found a reference somewhere on the web that suggested that raw compost reduces in mass by about 10-30% when it is composted. I assumed mine reduces in mass by 20%, giving about 165kg of compost and 0.825kg of nitrogen each year. That is getting close to the requirement value.
- Human urine: This is something that makes some people uncomfortable, but it is a fantastic source of nitrogen in a form that is readily available to plants. I first read about it's use as a compost accelerator and learnt from experience that it works. I've used it to turn a pile of chipped Leylandii into compost in just a few months. A quick search of the web gives lots of information on its potential use in gardening and agriculture and of course it saves water used in flushing it away. There is even a great little book that makes the case as well as recent scientific studies. It can even help to keep cats and other animals away. So doing the sums - I estimate two pees a week from myself and my male offspring (counted as half an adult), for six months of the year. The total is 0.576kg per year during the growing season.