The soil samples were taken from designated fields in the period 1977-1982. A single composite surface (0-15 cm) sample was taken from each field. The composite consisted of 30-50 cores (depending on the size of the field) which were hand mixed in a bucket. A 1,000 gram sample of the mix was then placed in a polyethylene bag and taken to the laboratory. The sample was air-dried, ground with a wooden roller, and screened to pass a 2-mm sieve. The screened soil was stored in scaled cardboard containers until analyzed. The 239 samples were analyzed for the following:
| Analysis | Number of Samples |
|---|---|
| Total Cd, Cu, Ni, Pb, Zn | 239 |
| Total Cr | 186 |
| DTPA Cd, Cu, Ni, Pb, Zn | 59 |
| Total K | 53 |
| Total P | 26 |
| Total Kjeldahl Nitrogen | 25 |
| Bray PI Available Phosphorus | 23 |
| pH | 23 |
| pH: 1: 1 in water | |
Bray PI Available P: Knudsen (4). Absorbance measured at 730 nm instead of 660 nm. Detection limit 1.0 µg P/g soil.
Total Kjeldahl Nitrogen: Bremner (3). Digestion with concentrated H2SO4 and catalyst (K2SO4/CuS04/Se) on Labconco microkjeldahl digestion apparatus. Neutralize with 10N NaOH and steamdistill into boric acid. Titrate with O.O1N HCl Detection limit 100 µg N/g soil.
Total Metals: A 2-g sample of soil is placed in a 100 ml pyrex glass tube in a machined aluminum block on a hot plate in a perchloric acid hood. Five ml of concentrated perchloric acid are added, the tube covered with a small glass funnel for refluxing, and the sample digested for 75 minutes at 200° C. After cooling, the sample is filtered with No. 1 filter paper, brought to 50 ml in a precalibrated test tube, mixed, and metals analyzed by atomic absorption spectroscopy. The six metals (Cd, Cu, Cr, Ni, Ph, Zn) were all analyzed on a Varian Model 375 spectrophotometer with an air-acetylene flame and background correction. Detection limits for the metals were: Cd-0.25; Ni-1.25; Pb-4.0; Cu-2.0; Cr-2.0; Zn-3.0 µg/g soil.
Total Phosphorus: An aliquot of the filtered perchloric acid digest for total metals which was diluted to 50 ml is further diluted 50-100 fold as needed and P is analyzed as ascorbic acid reduced phosphomolybdate at 730 nm. Detection limit 25 µg P/g soil.
Total Potassium: An aliquot of the filtered perchloric acid digest for total metals which was diluted to 50 ml is further diluted 25 fold and K is determined by flame emission on the Varian Model 375 spectrophotometer. Detection limit 200 µg K/g soil.
DTPA Extractable Metals: Soils were extracted with 0.005M DTPA (diethylene triaminepentaacetic. acid), O.1M triethanolamine and 0.01M CaC12 at pH 7.3 according to Lindsay and Norvell (5). The extracted metals were analyzed as for total metals. Detection limits were: Cd---0.02; Ni--0.10; Pb-0.32; Cu--0.161; Cr-0.16; Zn--0.24 µg/g soil.
The data were analyzed statistically with Statistical Analysis System (SAS) computer programs at The Ohio State University Computer Center.