Photo: Courtesy of Robert Gardner
|Irrigated sweet potato fields in the Grand Valley|
The Dani subsistence system is base on three separate components:
Sweet potatoes supply 90% of the caloric intake in the Dani diet. Pigs are of course the main protien source.
irrigation system is based on digging ditches to tap the many
small streams that run through the valley bottom and mounding up the land
between these channels to form raised rectangular fields. The ditches serve
both to water the crops in the dry season and drain them during the rains.
Heider does not discuss how the flow is actually controlled to adjust for
fluctuations or ensure that all the users receive adequate amounts, but
presumably the Dani have established some system of water regulation.
The irrigation ditches are in themselves a twofold resource for the farming system. They supply not only water but also fertilizer in the form of washed off top soil that becomes mixed with rotting plant wastes deposited after the harvest. Of course farmers must undertake the arduous work of scooping the muck from the ditch bottoms and spreading it on the fields to replace the soil that is constantly eroded. Fertility is enhanced by the scoopings from the ditches and the activities of the pigs who are placed on the fields to root for the smaller tubers after the harvest.
Sweet potatoes are planted on the irrigated plots by digging up the soil with sharpened wooden poles and are set in regular rows as a monocrop, i.e., they are not intermixed with the other food plants, which the Dani reserve for their swidden fields. After six months of growth, the crops are gradually harvested as they are needed by the household. The Dani stagger their planting so that they can depend upon crops maturing throughout the year without any need to store them, a possible reason for their development of a constant water supply.
After harvesting the Dani may either replant or fallow their fields. Some fallowing is present in many intensive systems, but unlike extensive cultivation forms it is never in excess of the cultivation period so that the land is in effect permanently cropped. Heider observes that: "the fertilization system is so effective that with care the fallow system is not even necessary".
Heider does not give a measure of the actual yields obtained in the Dani system and indicates that his attempts to do so were frustrated. However, he was generally impressed by its high apparent productivity. The sweet potato crop does support a population density of over 400 per square mile, well above the standard limit for extensive cultivation, and Dani subsistence techniques could probably bear a much higher concentration of people since many plots are not in use. Rice based irrigation systems in east Asia can maintain rural densities of over 1,000 per square mile.
In all of its aspects, the Dani system of sweet potato production represents the standard features of pre-industrial intensive agriculture. A further consideration of its development raises an obvious question of why this apparently isolated people with an otherwise simple material culture have created such a sophisticated production technology. Unfortunately, Heider does not consider this issue.