The middle of the Pacific seems a strange place to learn about soils. But histories of tiny Pacific islands provide perfect examples of how soil management practices can sustain or doom a society.
Both Mangaia (Cook Islands) and Tikopia (Solomon Islands) were covered with thick forests and uninhabited until small groups of Polynesians arrived some 2500 to 3000 years ago. Forests were cleared and cultivation expanded as the populations grew to 5000 at its peak.
On Mangaia, taro cultivation on volcanic slopes led to loss of topsoil. With a shrinking resource base, violence erupted and by the time Captain Cook arrived in 1777 fewer than 1000 inhabitants remained.
On Tikopia, a specially adapted form of agroforestry was developed that sustained soil fertility and crop yields. Local legend has it that pigs were eliminated from the island because they were seen to be causing too much damage to the plant crops. The agroforestry practices are highly sophisticated and remain largely in place today following a 1000 year history of relative stability. This was coupled with a zero population growth ideology that was strictly enforced.
These are just two examples of David Montgomery’s book dirt: The Erosion of Civilizations. His decision not to capitalise the first word of the book’s title is perhaps a not-so-subtle indictment of soil’s seemingly unappreciated status as the root of all things. It is also an excellent primer for 2015, the United Nations International Year of Soils.
Montgomery goes on to describe the gradual erosion of soils in the Roman Empire, leading to increasing dependence on imported grain from North Africa. He notes that a lost shipment of grain from the fertile fields along the Nile would translate into months of hunger or even starvation for some in Rome.
How well are we retaining our soils in New Zealand? For techniques on retaining soils check out the Menu of practices to improve water quality: cropping land at www.farmmenus.org.nz. Cultivation techniques, bunds, sediment traps, grazing management and cut outs on tracks and races are among a number of practical things we can do to slow down the loss of sediment from our farms.
For a good introduction to the soil loss through degradation, urbanisation, land grabbing and over-exploitation visit bit.ly/itsnotjustdirt. It’s a timely reminder of the value of dirt and the very long time it takes to develop.