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SOIL AND WATER ANALYSES
Since the subsistence problem of prehistoric Pueblo Bonito was
one of our paramount subjects for inquiry, we dug a number of pits
in order to test the character of soils formerly available for cultivation.
Our first pit was located northwest of the ruin, about halfway
to the north-cliff stairway. Others were dug on the east side, north
of the upcanyon road. Because none of these seemed to offer more
than blown sand and silt they were promptly refilled against possible
injury to wide-ranging Navaho horses and only one, No. 3, left open.
Test Pit 3, 9 feet 3 inches deep and situated on the plain midway
between Pueblo Bonito and the Expedition camp, looked more promising
than the others, and so it was fenced and held for further
study (pl. 7, upper). At the suggestion of C. S. Scofield, soil
chemist and a member of the National Geographic Society's Committee
on Research, earth samples were taken at 10-inch intervals,
bottom to top, and sent for analysis to J. F. Breazeale of the Office
of Western Irrigation Agriculture. Mr. Scofield warned, however,
that the soils represented by those samples would have less agricultural
value than anticipated if, like Chaco well and flood waters previously
analyzed, they were found to contain a high percentage of
sodium in proportion to calcium. (Scofield, 1922, discusses irrigation
and alkali salts.)
Although I have already quoted freely from it, Mr. Breazeale's
report of September 27, 1924 (Judd, 1954, pp. 10-12), on the 11 soil
samples from Pit Number 3 is so pertinent to our present subject
partial repetition seems justified:
All the soils contain a little black alkali, that is, a mixture of sodium carbonate
and sodium bicarbonate, and they all contain approximately the same percentage,
0.144%. . . . In their behavior the soils remind me very much of soils that have
probably originally contained some other alkali, such as common salt . . .
leached out through a long period of time. . . . A long leaching of most good
soils with such water as I have been analyzing for you from Chaco Canyon,
would probably produce just such effects as I see manifested in this set of
soil samples.
As you well know, the first requisite in irrigation agriculture is water
penetration, for unless we can get water into a soil we stand little show of
getting any crop out of it. So I first set about to see if I could make the soils
take water. I rigged up a set of one-inch glass tubes [10 to 12 inches long]
and poured 6 inches of [pulverized] soil into them, and added distilled water to
the top. . . . The water penetrated the soil column very slowly. Soil No. 11,
organic matter, for it percolated faster than the others . . . required about
24 hours to wet the 6-inch column. In the field this, of course, would be much
longer. Nos. 10, 9, 8, and 7 went slower than No. 11. It required about 48 hours
for these columns to become wet. This takes us to 50 inches deep. . . . Below
that level the soils seemed almost impervious, that is, all the samples will
probably require a month each for the water to move downward through the
6-inch layer. I do not think I have ever handled a soil quite so impermeable
to water as are these last six samples.
The Chaco Canyon soil, in all the levels that you sampled, is badly deflocculated
and for the reason that it contains an excess of sodium and a scarcity of
soluble calcium. . . .
I can say without any doubt that these soils . . . will not take water. I do
not believe, even under the most favorable conditions, that such soils can be
successfully cultivated. . . . If all the soils that were available to agriculture in
Chaco Canyon are as bad as these samples, I think you have one reason at
least to explain why the Bonitians left the valley. I do not believe an Indian,
with his primitive methods, could handle any soil like this.1
A year after receiving the foregoing report Pit No. 3 was deepened
at the suggestion of Dr. Bryan and Late Bonitian potsherds
were collected at depths between 10 and 15 feet (Bryan, ibid., p. 58).
These finds established the fact that, unwittingly, I had located this
particular 1922 test within the banks of Bryan's post-Bonito channel.
Our soil samples, therefore, represented a varied alluvium transported
from a distance and not fields cultivated by the Bonitians.
Nevertheless, that transported alluvium originated in the same
place, upcanyon, as had the annual accretions comprising the main
valley fill. And some of those annual accretions, upcanyon, assuredly
were cultivated. But, if all local soils were as impervious to water
as were those from Test Pit 3, and we have no reason to believe
otherwise, farming for a livelihood in Chaco Canyon would have
proven increasingly discouraging and, as Breazeale points out, the
Bonitians would have had ample reason for moving elsewhere.
Each of our 11 samples from No. 3 pit, originally 9 feet 3 inches
deep, was not only deficient in soluble calcium but contained sodium
bicarbonate, or black alkali, in approximately the same amount, 0.144
percent. If as little as 0.144 percent of black alkali can so tighten a
soil that days rather than minutes are required for water to percolate
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