On behalf of Foss Leach:
I am surprised at the comments made by your correspondent because there was nothing primitive about these ancient gardeners at all; on the contrary. I have great admiration for the way that pre-European Maori so quickly adapted their horticultural practices to the harsh New Zealand conditions when arriving here from the Pacific with tropical root crops. There is abundant archaeological evidence of the diligent and enterprising research which these people
carried out in order to find ways of successfully growing and storing these root crops in the face of great difficulties with this unfamiliar and generally unsuitable climate. Their experimental research on subterranean storage pits is an excellent example of this. So too, the way that they deliberately modified poorly drained soils by the addition of coarse sand and fine gravel. This practice had a number of beneficial effects on garden soil for tropical root crops, not all of which may have been anticipated.
Yes of course all archaeologists are familiar with the literature your correspondent cites; but I must point out that there has been a great deal of authoritative research on ancient gardens since these two papers were published so long ago (Rigg and Bruce 1923, and McNab 1969).
For example, Mike Burtenshaw of the Open Polytechnic of New Zealand is near the end of a ten-year research project examining changes in soil fertility in experimental garden plots growing traditional kumara cultigens. As far as the use of charcoal as a fertiliser is concerned, I can only suggest that my earlier comments on this matter should more carefully be read again.
There is no archaeological evidence, or for that matter any other form of evidence, that pre-European Maori deliberately added charcoal to garden soils in an effort to improve fertility.
Charcoal occurs in garden soils in dense localised patches where people were burning rubbish (this is not fertilising activity) just as we still do today. Charcoal also occurs as finely divided particles more evenly distributed in soils both inside and outside gardens due to forest fires nearby (again this is not a fertilising activity).
On the subject of `other forms of evidence', early Europeans in New Zealand were in an excellent position to observe and record at first hand the gardening activities and knowledge of Maori about such issues as the agencies of fertility. One of most astute of these observers was William Colenso, who had this to say: "One striking peculiarity, however, should not be omitted - in which, too, I think, they differed from all agricultural races - their national non-usage of all and every kind of manure ... They also never watered their plants, not even in times of great drought, with their plantations close to a river, when by doing so they might have saved their crops" (Colenso 1880: 11). Surely this should dispell any doubt on this subject?
Finally, on the question of "would adding charcoal to soil make it more fertile?" I asked a senior soil scientist at the Hill Laboratories in Hamilton this question and got the clear answer: "Charcoal No, but ash perhaps". And just to show that even experts don't always know everything, I refer the interested reader to some recent scientific research by Lehmann et al. (2003) which suggests that the addition of charcoal to soil may indeed do exactly what it is purported not to be able to do. The reasons for this are complex, not well understood, and the subject of ongoing research.
Colenso, W. 1880. On the vegetable foods of the ancient New Zealanders before Cook's visit. Transactions and Proceedings of the New Zealand Institute 13: 3-89.
Lehmann, J., Silva, J.P. de, Steiner, C., Nehls, T, Zech, W. and Glaser, B. 2003. Nutrient availability and leaching in an archaeological Anthrosol and a Ferralsol of the central Amazon basin: fertilizer, manure and charcoal amendments. Plant and Soil 249: 343-357.
McNab, J.W. 1969. Sweet potatoes and Maori terraces in the Wellington area. Journal of the Polynesian Society 78(1): 83- 111.
Rigg, T. and Bruce, J.A. 1923. The Maori gravel soil of Waimea west, Nelson, New Zealand. Journal of the Polynesian Society 32: 85-93.