Holmgren by principles & pathways

Author:principles & pathways [principles & pathways]
Language: ita
Format: epub
Published: 1601-01-01T00:00:00+00:00

we have spring bulbs that are also active well before the trees are in leaf. This system yields

less fruit than a traditional orchard of the same size but, as well as providing other yields,

it is more efficient in growing organic matter, using natural rainfall, recycling nutrients and

preventing leaching.

In milder coastal areas of Victoria a wide range of subtropical evergreen species, such as

avocado, citrus, macadamia and sapote, can be grown. This astounds visitors from the

northern hemisphere, where at similar latitudes winter cold precludes such species. In

these areas high-density evergreen food forests become a design option.

Fertility-loving and infertility-loving plants

The adaptation of plants to fertile (high-energy) and infertile (low-energy) soils also

illustrates subtle but fundamental patterns important for design. It is often assumed that

plants that are found naturally growing on infertile soils prefer these conditions. In fact, it

is their relative efficiency and therefore competitive advantage on poor soils that accounts

for their natural occurrence. Most plants grow better the higher the fertility, but only plants

evolved (or bred) to high-fertility soils are reasonably efficient in these situations.

Although our aim may be to increase productivity, matching plants and animals to suit

current conditions is also an important strategy. The hardy nature of local indigenous and

other Australian native plant species makes them ideal for providing shelter and other

functions in unirrigated and unfertilised soils. When we plant a new garden, the soil fertility

may support only a limited range of vegetables, but over time we can introduce more

demanding ones. The use of pioneer plants that grow in and improve poor soils is a strategy

which is considered further in Principle 12: Creatively Use and Respond to Change.

Plant and animal biomass as indicators of fertility

The relative amounts and types of plant and animal biomass in natural and managed

ecosystems can be used as a general indicator of mineral fertility. Although the Permaculture (and now mainstream scientific) view is that substantial perennial plant biomass is an important characteristic of sustainable agriculture, there are understandable reasons

why humans have tended to reduce woody and perennial biomass in managed systems.

Ecosystems with large amounts of woody plant biomass often predominate on infertile,

leached soils in high-rainfall areas, whereas the archetypical human habitats tended to be

drier regions with more grassy vegetation. These regions have vegetation with a higher

mineral content which supported large numbers of animals, and heavy seed-producing

grasses (parents of modem grains).

North American landscapes

The ecological and soil mineral balances in the short-grass prairie, tall-grass prairie and

eastern deciduous forests of the American mid-west were closely studied by William

Albrecht.21 He showed that the balance of minerals in the lower biomass and climatically

drier prairies favoured large herbivores, food crops, and thus people; while the deciduous

forests to the east with higher rainfall had more leached soils, less ideal for crops and

naturally supporting fewer large herbivores. These deciduous forests with abundant seed

crops of acorns, beechnuts and hickories that feed squirrels and turkeys are replaced on

lower-fertility soils by conifer forests. The conifer forests have abundant woody biomass

but few nutrients, large seeds or animals.

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