Hawaiian farmers were able to get remarkable results from marginal farm land, but by the time of European contact, some of those lands were depleted of nutrients, according to archaeological and soils research conducted independently on Maui and the Big Island.

And that could have led to significant social upheaval, scientists said.

"Hawaiian settlement in Kahikinui (on Maui) and Kohala (on Hawai'i) had pretty much reached the geographic limits to dryland farming (and) do have some interesting implications for the relationships between people and environment at the time of European contact," said archaeologist Patrick Kirch, of the University of California at Berkeley.

Declining food supplies — particularly when drought caused production collapse on overtaxed soils — could have been a factor in wars of the pre-contact period, he said.

"These conditions may well have triggered periodic warfare and been a fundamental condition underlying the episodes of wars of territorial conquest between Maui and Hawai'i," Kirch said.

Teams of scientists have been studying the Hawaiian agricultural systems of Kahikinui on Maui and Kohala on the Big Island, and are reaching similar conclusions — the farmers were potentially on the brink of trouble.

"We have no evidence that the restrictions of nutrients crossed any sort of critical threshold, but it looks like there was a significant reduction" in soil fertility after as much as 300 years of consistent farming — from about 500 years ago to roughly 200 years ago, said soil scientist Tony Hartshorn, of the University of California at Santa Barbara.

He is the lead author of a paper in the Proceedings of the National Academy of Sciences, "Prehistoric agricultural depletion of soil nutrients in Hawai'i," along with Kirch, Stanford biology professor Peter Vitousek and UC-Santa Barbara soil scientist Oliver Chadwick.

Vitousek, who has been working on ancient dryland agricultural fields in the Kohala mountains, between 'Upolu Point and Kahua Ranch, said he is not comfortable concluding that wars could have resulted from the declining fertility, but said that "pushing the productive capacity of the land as hard as they clearly did ... it made them more vulnerable, and I would be surprised if it didn't play into the the dynamics of the culture in some way."

At Kahikinui, a dry area on the leeward slopes of Haleakala, evidence suggests Hawaiians began growing sweet potato and dryland taro about 500 years ago — generally in the uplands, where there was more moisture than the arid coastal region. Archaeologists have sampled the soils in cultivated areas and compared them with nearby soils that had not been used for agriculture.

The difference was pronounced, Hartshorn said. From untilled to tilled soil, the amount of calcium in the soil dropped by 49 percent, magnesium by 28 percent, sodium by 75 percent, potassium by 37 percent and phosphorus by 32 percent.

The land probably was still capable of producing reasonable yields in good conditions but may have been compromised during challenging times, such as droughts.

"(The research efforts) suggest that in the dryland zones, the Hawaiians had pushed their systems of dryland farming to a remarkable degree. The key unknown factor here is how this interacted with rainfall variation and periodic drought," Kirch said.

"Hawaiian oral traditions frequently refer to the effects of sustained drought. In a bad drought, the high population sustained by the intensive level of production would be hit hard."

Archaeologists believe that Hawaiians, once population pressures used up all the best agricultural lands, moved on to much less productive land like Kahikinui, and with simple tools like digging sticks, or 'o'o.

Archaeologists find the holes made by 'o'o extending deep into the soil under farmlands. Vitousek said that at Kahikinui, the tools were used to pierce a cinder zone under the surface layers of ash, providing crops with access to moister ash layers below.

"That was really brilliant. In areas that we would consider too dry for agriculture, they found a way to provide their crops with access to moisture stored from the winter rains," he said.

Other techniques included extensive mulching. The botanist Archibald Menzies in 1793 reported on the use of a thick layer of hay as mulch on Big Island dryland farm fields. Hartshorn said soil depletion might have been even worse than what is being measured, except for the fertilizing effect of mulch, which provides nutrients to the soil as it decomposes.

"Among other things, our research has shown just how incredibly good the Hawaiian farmers were at making marginal lands productive," Kirch said.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.