A mature forest garden is known to create maximum output for minimum effort. Forest gardens also referred to as food forests and know under the umbrella term agroforestry are popular in permaculture for this and other reasons.
A forest garden can accomplish a good yield with little effort after establishment. This is because most plants in a forest are perennial and appear year after year. A natural forest also consists of multiple layers of plants that can exist in the same area with little interference. The natural forests of the world are grown on a succession of decaying organic matter from past plants built up over many years. This rich forest soil can be established faster from a depleted soil by choosing nitrogen fixing and nutrient accumulating species of plants.
Outlined below are examples of plants that improve the soil at each level of the forest garden maximizing fertility for itself other plants to grow. Techniques like ‘chop and drop’ mulches, coppicing and pollarding from these plants in particular can release the nutrients they have extracted over time from the earth or air.
Even if you don’t have room for any trees, the soil can still be improved by using examples from the lower layers. Even something as small as a window box could contain Herbaceous, ground cover, vertical and as well as root layers. Growing more than one type of plant at different layers can aid plants nearby as well as provide food even if one crop totally fails.
If you want to get an idea of the forest garden concept in action Robert Hart Forest Gardening with Robert Hart (Complete)
Black Locust: Robinia pseudoacacia
Large tree can be coppiced or pollarded for fuel or left on the forest floor to break down. Roots fix nitrogen and the flower and seed can be made edible.
Honey Locust Gleditsia triacanthos
Another large tree reported in most permaculture publications to improve soils via nitrogen fixing. The tree also produces excellent timber and edible leguminous pods.
Dwarf Tree Layer
Siberian pea tree: Caragana arborescens
A relatively small tree fixes nitrogen that produces an edible seed. Suited to temperate areas with hot summers and colder winter.
Twisted Baby™ Dwarf Black Locust
A dwarfing version of Robinia pseudoacacia bred for ornamental purposes. This variety features contorted growth despite being much smaller it will still grow to around 15 feet/4.57m tall
Any Elaeagnus plant will fix nitrogen. However why not choose the two below as they noted in particular for their edible fruits.
Elaeagnus multiflora: -Goumi – Regarded as the best fruiting variety of Elaeagnus.
Elaeagnus angustifolia – Easier to acquire this species in many areas.
Lupins are mostly herbaceous perennial plants commonly seen in gardens. They also fix nitrogen and with caution can be eaten.
However annual versions Lupinus mutabilis and Lupinus angustifolius have cultivars that have been bred with a low alkaloid content making them a more suitable crop for eating.
Certain Bean Species
Broad/Fava beans: Vicia faba
Dwarf beans: Phaseolus lunatus
Certain beans make for nitrogen fixing herbaceous layer plants. They are also easier to get hold of than some of the other plants mentioned from this list. Some maybe bush or dwarf varieties of runner beans.
Ground Cover Layer
Red clover: Trifolium pratense
White clover: Trifolium repens
Both clovers above make excellent nitrogen fixing ground covers and make excellent bee forage among other uses as a food and medicinal source. White clover it more tolerant of being cut back on a regular basis than red clover which is a much taller plant overall.
Runner beans: Phaseolus coccineus
Garden pea: Pisum sativum
These two plants surely need no introduction both common nitrogen fixers used in agriculture. Growing them up trees as shown by Robert Hart instead of the typical cane can provide an unconventional way of growing a conventional crop. The tree gets a nitrogen boost and is a lot less likely to snap from the weight of an extremely productive beanstalk!
An excellent mineral accumulator. With Comfrey’s long deep taproot it brings up nutrients from deep in the soil. Comfrey makes for an excellent liquid fertiliser, mulch and is used in traditional medicine. Interestingly comfrey is popular with slugs and snails so using it as a mulch can help prevent them eating your other plants.
Dandelion: Taraxacum officinale
A surprisingly versatile plant usually just referred to as a common weed! However with its large deep taproot this nutrient accumulating plant should be treated with more respect. Every part of the dandelion is edible or useful, however older plants tend to develop too much of a strong bitter flavour to be palatable for most people. Leaves can be used for salads and the roots are usually prepared as a tea or coffee substitute. In terms of weight the root contains more calcium than milk. The plant’s flower is also an important source of food for bees in early spring.
Ground Nut: Apios americana
Ground nut is a climbing plant that produces a tasty edible tuber. The plant also fixes nitrogen in the soil.
Bonus Layer – Pond
Robert Hart doesn’t include this layer in his version of the forest garden. However ponds do have a place in many gardens and can be put to good use building soil.
Azolla: Azolla filiculoides
This highly productive plant it is worth mentioning. It grows so fast across ponds you may wish to use it with some caution. However pulled out of the pond it can be used as an excellent source of nitrogen for composting etc.
This small selection of plants should help you on your way to improving soil at all different levels of the forest garden. There are of course many more plants out there waiting to be applied in a similar manner. Also as I can’t take into account where you are in the world you might apply this information please use it with some care and discretion. Lupins and azolla have been known to crowd out longer established species in various countries. However the attributes that make them so problematic in these countries when used with care in the right environment also make them incredibly productive and beneficial.