Saturday 8 December 2012

Forest Garden Notes - 2 : Nutrient Budget Calculations

In my last post I considered the water requirements for the fully cropping fruit and nut trees for the sort of food forest I am proposing for my plot. Whilst natural forests do not need feeding, by definition, forest gardens, whilst modelled on young forests, require extra nutrients to replace those harvested since we are usually seeking greater yields from them.
Top terrace

Martin Crawford (1) in chapter six of his excellent book details the nutrient demand for heavy cropping, moderate cropping and undemanding trees and shrubs. He goes on to detail the nitrogen supply from various sources and the annual requirement for heavy croppers, moderate croppers and vegetables. He also details similar data for the supply and annual demand for potassium. For all this data the supply and demand figures were quoted per square metre.

Middle terrace

Since my ambition is that these requirements for my plot should be met sustainably I decided I needed to do some early thinking and a few quick calculations to get a feel for the levels of both nitrogen and potassium I would need for the sort of food forest I was envisaging.

The area on the plot that most readily suggests a good starting point for an initial food forest garden consists of a series of three terraced areas that already contain a few apple, pomegranate, and fig trees together with a couple of dead or dying old walnut trees. In total this is an area of approximately 750 square metres.


From the data contained in Martin Crawford's book:

The size of semi dwarf fully cropping fruit trees is assumed to be 4 metres high by 4 metres wide i.e. an area of 12.56 sq.m per tree and such trees require 8 grams per sq.m of annual nitrogen to maintain fertility.

Canopy nitrogen fixing trees and nitrogen fixing shrubs, both in full sun, will provide 10 grams of nitrogen per sq.m.

Using first level algebra:

Let X = the number of semi dwarf fully cropping trees in proposed 750 sq.m plot then:

12.56 x X x 8 grams of nitrogen is required for the proposed forest garden.

Similarly the maximum nitrogen that can be sustainably provided by interplanting in a single layer with nitrogen fixing canopy trees or shrubs in full sun is  (750 - 12.56X) x 10 grams.

Since my aim is for a self sustaining system demand and supply need to be balanced. Therefore, for this simple one layer sustainable model demand must equal supply. ie.

12.56 x X x 8 = (750 - 12.56X) x 10

Therefore X = 33.17 say, 33 fully cropping semi dwarf fruit trees for the proposed 750 sq.m plot.

This is somewhat lower than the 40 or so that the natural spring water supply (see previous post) can support.
Lower terrace

However, immediately adjoining the the proposed 750 sq.m three terrace area is an area around the currant water storage tank of about 60 sq.m that I will plant up solidly with nitrogen fixing trees, shrubs and ground cover. In addition outside the fenced area (necessary due to marauding flocks of mountain sheep and goats!) on the northern side of the proposed food forest plot I am planning to plant a major nitrogen fixing windbreak with drought resistant over canopy, canopy and shrub layers. This will cover an area of approximately 100 - 120 sq.m. These additional plantings will easily raise the number of semi dwarf fully cropping fruit trees possible on this proposed plot to around 40 (in line with the estimated water supply requirements) since, by careful planning and adopting a 3 layer (canopy, small trees/shrubs and ground cover) nitrogen fixing approach to planting it should be possible to increase the nitrogen yield per sq.m significantly.

In addition further up the steep area of the plot itself, in what is zone 4/5 land in permaculture terms, I will also be planting, over time, drought resistant nitrogen fixing and pioneer trees to further increase nitrogen levels should this be necessary.


Again using data from Martin Crawford's book heavy cropping fruit trees require 10 grams of potassium per sq.m per year. This equates to 3.768 kg per year for 30 trees and 5.024 kg for 40 trees. On the supply side he quotes Comfrey as supplying 10 grams per cut  and wood ash as supplying 80 grams per kilogram. Using solely Comfrey would therefore require 380 cuts and 502 cuts for 30 and 40 trees respectively. Similarly using wood ash solely (since will have wood fired cooking range and heating stoves in planned house) would require 47 kg and 63 kg per year. In reality the requirement will be met by a combination of the two sources.


The above consideration of the nutrient requirements and the water requirements, detailed in my previous post, has given me a set of basic data that I can incorporate into the more detailed forest garden plan I will be putting together over the winter months. It has also brought a real sense of what will be possible and enabled me to get a very real picture, in my mind's eye, of how my initial food forest garden will look.

It also means that I have already changed my original plan to incorporate an annual vegetable area into the proposed forest garden plot in favour of more fruiting trees and supporting nitrogen fixers. I may, however, incorporate a series of intensive vegetable beds as a transitional measure until the trees and shrubs become established.  

I appreciate that such a simplistic logical approach may be at odds with the more intuitive approach normally favoured by experienced permaculture designers but it has helped me , and may help others, to start to build and balance the complex systems that are required for a successful design.


(1) Martin Crawford - "Creating a Forest Garden".