SU0013451 - Laserfiche WebLink

Home Browse Search

out that there is no linear relationship between the values and that a value of five may have several hundred times more phosphate content than a sample with a value of two (Woods 1975:23). Carbonates testing is an integral part of overall soil analysis. As middens develop, relatively high levels of calcium, nitrogen, carbon and phosphates accumulate in the soil. Carbonates result from interaction of calcium, carbon dioxide and phosphates. Age estimates for archaeological sites have been done using the depth of leached carbonates as a chronological control (Johnston 1975). One problem is that carbonates will not last long in acidic soil. Therefore, its use as a chronological indicator would be limited to sites with neutral or alkaline soils. The carbonates testing was made using a 2.5 g soil sample on a clean glass sheet. Three drops of muriatic acid were added to the sample and the observed reaction was subjectively recorded: None=O; Weak=l; Medium=2; Strong=3. The pH reading is an important component of soil analysis. In acid soils (pH values below 7.0) the phosphate is bound with iron and aluminum compounds. With alkaline soils (pH above 7.0), phosphate will usually be bound with calcium. These phosphate compounds will remain in the soil where deposited and generally are not subject to leaching. Of course, if the soil itself is disturbed, so is the phosphate. Studies have been done on the relationship of the soil pH to prehistoric sites. In general, the evidence shows that midden sites tend to have higher pH values than the surrounding soils. Although the initial occupation of the site would lower the pH because of humic and fulvic acids formed in the midden soil, after site abandonment the soil pH will increase as a result of complex chemical processes. The pH of each soil sample was determined using a Coleman Metrion IV pH meter. A volume of 25 ml of distilled water was added to a 20 g air-dried soil sample which had been screened through a #28 Tyler screen. The mixture was stirred every fifteen minutes for one hour prior to taking the pH reading. RESULTS Phosphate Testing Sixty-five samples were positive for phosphates and fifteen were negative. The highest PO, values, a 6.00 reading, were from depressions at site The four samples from depressions in off-site contexts in the vicinity of . were minimal (one was negative). Five of the fifteen negative readings came from boundary loci at site, The mean value of the sixty-five positive samples was 4.45 with a standard deviation of 1.22 and a variance of 1.49. A3-2 '1