Drilled too deep
If deep plowing upsets the soil unduly, it is not surprising. But if crops are simply drilled into the ground point by point with a digging stick, it doesn't sound that dramatic. But some surfaces are too sensitive even for that.
When the Polynesians settled the islands of the Hawaiian archipelago in the 5th century, they already had their main food crops with them - after all, who knows what awaits you in a new homeland. Maybe not with the first pioneers, but definitely before the arrival of the first Europeans, the sweet potato (Ipomoea batatas) found its way there. The new citizens quickly established intensive agriculture in all suitable regions - even in the dry areas of the leeward mountain slopes and up to complex irrigation systems for taro (Colocasia esculenta), an important source of starch.
So the Europeans got the impression of a thriving culture in some places when they first landed at the end of the 18th century. The volcanic subsoil in the drier areas was not that easy to cultivate: very old soils were already infertile because their nutrients had long been washed out over thousands of years. And for young subsoil, the ground cover was still missing, which develops only slowly. In addition, there were limitations due to the precipitation conditions: In the lower elevations, there was not enough rain to practice agriculture. In the high altitudes, however, it was too wet, nutrients were leached out, and the soil was low in oxygen and acidic. In the end, only the middle-age deposits at middle altitudes remained.
Here, the old lava layers are largely covered by a layer of younger volcanic deposits up to one meter thick, which are structured like a sandwich: A 20 to 30 centimeter thick layer of coarser slag - grain size on average four millimeters - is framed above and below by fine layers of ash. When it rains, the upper layer of ash is quickly saturated, but the water slowly drips down through the coarse intermediate layer into the second layer of ash. However, since the slag makes it difficult for roots to go down and capillary water to go up, it acts as a natural barrier for water to return to the surface. The lower ash layer therefore played an important role as a water reservoir.
But the Polynesians' way of farming destroyed this reservoir. What is hard to believe at first glance: the farmers only used a digging stick, which they pushed into the ground, turned and pulled out again. In these holes they planted their taro and sweet potato. Unlike the deep plow ditches of the Europeans, the intervention appears relatively harmless and gentle on the soil surface.
However, the harmless image is deceptive. For the stick pierced the layer of slag and mixed it with the ashes. And since the fields were worked again and again, individual burial holes finally flowed together to form whole systems of holes that ran through the ground. The result: a motley mess of ash and slag that now lacked the natural water barrier.
For the plants, this initially offered the advantage that they could now reach the water reservoir of the lower ash layer much more easily and use a larger supply of nutrients. Because the activity of the digging stick not only mixed the components, it also crushed the slag lumps, which now weathered faster under the more humid conditions and released nutrients.
So everything is fine from the farmers' point of view? no way. Because the increased flow of water not only promoted weathering and the release of nutrients, it also washed them out downwards – with the exception of the poorly soluble phosphorus. And that part that found its way into the plants was withdrawn from the system with the harvest. Over the centuries, the minimal use of the digging stick, which seemed to be compatible with nature, caused a massive impoverishment of the underground, as Anthony Hartshorn from the University of California in Santa Barbara and his colleagues were able to prove.
The researchers analyzed the nutrient levels in such formerly cultivated regions and compared them with unused areas in the vicinity. In the disturbed soil profiles, the concentrations of calcium, potassium and phosphorus were about half to two-thirds lower, magnesium was reduced by a fifth, and the sodium supply was only a tenth of what was probably originally there - estimated based on data from apparently uncultivated soils.
The Polynesians don't seem to have noticed much of this until the arrival of the Europeans: the harvest levels that Hartshorn and his colleagues calculate from the data were on the low end of what produced similar farming systems in Africa or Papua New Guinea. It is possible that the Polynesians increased the yield by mulching their plantings and thus at least reduced the loss of phosphorus from the crops. At least that's what a quote from 1793 suggests, when the botanist and expedition member Archibald Menzies noted that the fields in the dry regions were covered with a thick layer of hay.
The decline of the soil could have started 150 years before the arrival of the Europeans, and the flourishing fields of the dry mountain slopes were probably past their peak. Menzies' observation may not describe long-standing common practice, but rather a first countermeasure in the face of dwindling harvests - due to the lack of written records from the Polynesians, this will never be clarified. How it would have gone on, also not: With the arrival of foreign light-skinned people and their agricultural technology, cultivation on these distant Pacific islands also changed fundamentally.
