Silicate Rocks and Soil Samples Analytical Methods

60 Chapter 4 DISSOLUTION OF MILLED-SILICATE ROCKS IN THE SOIL

4.1. Introduction

The earlier chapters have shown that milling changed several physical and chemical properties of silicate rocks including increasing the amount of exchangeable base cations, with the effects of dry milling being greater than for wet milling. Milling also increased dissolution of SRFs in dilute mixed acetic-citric acids, and increased the dissolution of amorphous constituents more than for crystalline constituents. However, it remains unclear whether milling will produce a similar increase in dissolution of SRFs in the soil where diverse soil properties and reactions are involved. An important question relating to agricultural practice is to determine those soils for which the application of SRFs will be most advantageous. A dissolution experiment for milled SRFs in soil was, therefore, carried out to investigate this situation. The objectives of this research were to identify 1 the effects of milling on the proportion of major elements dissolved from the SRFs in soil and 2 soil properties that influence the dissolution of ground SRFs. The experiment was carried out under laboratory conditions using 23 soils from south-western Australia.

4.2. Materials and Methods

4.2.1. Silicate Rocks and Soil Samples

Silicate rock fertilizers used in this experiment were dry-milled basalt, dolerite, gneiss, and K-feldspar that had been milled for 0 initial, 10, 60, and 120 min, as used in acid dissolution experiment Chapter 3. The virgin soils under bush vegetation were from 23 soil reference sites in south-western Australia McArthur 1991. The top 15cm excluding the organic matter layer was sampled, air dried and sieved to pass a 2-mm sieve. Selection of the soils was based on the results of previous research by Hughes and Gilkes 1994 who found that these soils were capable of dissolving more than 40 of added rock phosphate. It was postulated that these soils would also dissolve SRFs. Complete information on the sites used 61 for soil sampling and soil profile data are available in McArthur 1991. Soil properties used for investigation of relationships between soil properties and dissolution rate of SRFs are presented in Table 4.1.

4.2.2. Analytical Methods

The methods used for measurement of soil properties presented in Table 4.1 and for soil-SRF mixtures from incubation experiment are as follows. 1. pH H 2 O and pH CaCl 2 and EC at a soil : solution H 2 O and 0.01N CaCl 2 ratio of 1 : 5 were measured with pH- and EC-meters Cyberscan 2000. 2. ∆pH = pH CaCl 2 - pH H 2 O . 3. Organic carbon was determined by wet oxidation with K 2 Cr 2 O 7 Walkley and Black 1934. 4. Cation exchange capacity CEC was determined using 1M ammonium acetate CH 3 COONH 4 pH 7 as an extracting solution Thomas 1982 with 3 extractions. The three filtrates were collected and used for measurement of exchangeable base cations, i.e., Ca and Mg by AAS and K and Na by flame emission. The NH 4 + retained by the soil was displaced using 1N KCl. The excess of KCl in the soil was removed using ethanol, and the remaining K + on the exchange sites of soil was displaced using 1N CaCl 2 , and the filtrate was collected. The concentration of K + in the filtrate was determined by flame emission, and the quantity of K + was considered to represent the value of the cation exchange capacity CEC and is presented in units of centimole charge per kg soil cmol c kg. 5. Effective cation exchange capacity ECEC was calculated as total exchange bases + total exchange acidity. 6. Base saturation BS was calculated as the sum of exchangeable base cations as a of CEC. 7. Total exchange acidity was extracted with 1N KCl and determined by titration using 0.01N NaOH Thomas 1982. 8. Percentages of sand, silt, and clay were determined by the pipette method Gee and Bauder 1986. 62 63 9. Fe, Al, and Si extracted with 0.2M ammonium oxalate COONH 4 2 .H 2 O buffered to pH 3.0 were determined using the method described by Rayment and Higginson 1992. The Fe + Al + Si from this measurement is used as an estimate of the quantity of amorphous constituents in rocks and soils. 10. Measurement of field capacity of the soils used for incubation experiment: 20 g soil 2 mm was transferred into a beaker glass, over saturated with DI water, then transferred onto a filter paper Whatman No 40 on a panel, covered with plastic to minimize evaporation and was allowed to drain overnight. The water content remaining in the soil measured with gravimetric method was defined as the water content at the field capacity of soil. 11. The quantities of CH 3 COONH 4 -extractable basic cations Ca, Mg, K, and Na for soil-SRF mixtures: triplicates of 5g soil-SRF mixture or soil without added SRF were mixed with 25mL 1M CH 3 COONH 4 pH 7 in plastic vials, shaken on an end-over end shaker for 1h. Concentrations of the basic cations in the filtrate were measured by method described in point 4. The values of extractable Ca, Mg, K, and Na for soil-SRF mixture incubated for 0, 2, and 10 months minus those for soil without SRF control are designed as E , E 2M , and E 10M respectively. The amount of elements dissolved during the first 2 months and between 2 and 10 month incubations ∆E 0-2M and ∆E 2-10M respectively were calculated from E 2M – E and E 10M – E 2M .

4.2.3. Incubation Experiment