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3. GENERAL METHODS
In this C hapter only general aspects of m aterials and m ethods will be described. M aterials and m ethods specific to particular Chapters will be described therein.
3.1. Soil selection
The soil selection for the prelim inary experim ents is described in Section 4.2; acid soils were collected from sites 1, 2, 3, 4, 5, 6, 7 and 8 of the Cotter catchm ent area (Figure 4.1 presented on P 78) as identified by Talsma (1983). From these, three sites were selected which will be described in detail in Section 4.2. Later an acid soil even higher in exchangeable A1 and low er in exchangeable Ca than the previous soils w as selected (Section 4.3). For this purpose, 14 soil sam ples from 6 new sites in the Cotter catchm ent area and 7 samples from three sites from com partm ent 152 of the U riarra forest w ere preview ed. One site from com partm ent 152 of the U riarra forest was finally selected (Section 4.3).
3.2. Soil collection and processing
Soils w ere collected from carefully exam ined sites and d e p th s in polythene bags. If the field m oisture content was high (much m ore than about 20%) then the soil was air dried in the shade to about 20% m oisture content before being sieved. Soil sam ples used for site selection were passed th ro u g h a 2 mm sieve, w hile soil collected for p lan t grow th experim ents was sieved through 5 mm sieves. Following sieving, soils w ere m ixed th o ro u g h ly either in a sm all cem ent m ixer w hen bulk collection was m ade or by hand for smaller quantities. After sieving and mixing, soils for chemical analysis were stored in a cold room until the com pletion of analyses. All analyses on soil were carried out at about
20% moisture conditions and then results expressed on an oven dried basis after oven drying a sub sample taken at the time of analysis. Soils used in plant growth experiments were stored in open heavy duty polythene bags in a cool, dry place.
3.3. Soil analysis
3.3.1. General
pH
Soil pH was measured both in water (pHw) and in 0.01 M CaCl2 solution (pHs) with a glass electrode pH meter (Anax) after 30 minutes of shaking. The sample was made up at a soil : water or salt solution ratio of 1 : 5.
Electrical conductivity
Electrical conductivity (EC) (mS/cm) was measured using a conductivity meter (Radiometer, CDM 3). For soils, the measurement was made on the same extract as for pHw (i.e., at a soil : water ratio of 1 : 5). For soil solutions, EC was measured on a 1 : 1 extract.
Particle size analysis
Particle size distribution was determined by the hydrometer method (Bouyoucos, 1928).
Organic matter
Soil organic matter was determined by a chromic acid digestion method which is based on spontaneous heating by dilution of H2SO4 (Walkley and Black, 1934).
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Field capacity
Field capacity was determ ined by packing soil above gravels in a large cylinder. The soil surface was flooded by adding w ater and then allowed to drain freely for 48 hours while its surface was kept covered to check evaporation. The m oisture content was determ ined as the field capacity (Wilde et al 1979). M oisture content at 0.1 bar was also determ ined, but only for com p artm en t 152 soil using a pressure plate. The m oisture content thus obtained was low er (27.7%) w hen com pared to the field capacity determ ined (46.6% ) for the same soil.
3.3.2. Extraction m ethods
Extractable cations
For the initial soil selection, exchangeable cations w ere extracted by shaking the soil w ith IM KC1. Three extractants w ere com pared for determ ining exchangeable Ca, Al, Mn, Mg, K and N a for com partm ent 152 soils. These were: IM KCl, 1M N H4C1 and 0.1M BaC^. Values for Ca, Al, Mn, Mg and K w ere not significantly different betw een those obtained w ith IM KC1 and 1M N H4C1 although 0.1M BaCl2 extracted significantly less Al and Mn than the other two. The am ount of N a extracted was significantly different for each extractant (Figure 3.1, for display values of cations w ere m ultiplied by factors). Since 1M N F ^C l extract has the ad v an tag e of allow ing K m easurem ent from the sam e extract it was decided to use it for soil analysis for the rem aining study. Recently
Shum an and D uncan (1990) also reported that IM KC1 and IM N H4CI
com pared favourably in extracting Al from 248 soil sam ples. Slightly m ore Al was extracted by IM N H4CI which is also true in the present case. Finally soil sam ples w ere shaken for 1 hour with 1M NIT4CI at a soil to solution ratio of 1 to 20 and then the suspension w as filtered
through Whatman No 42 filter paper. Different cations were estimated from the filtrate by methods described in the Section 3.3.3.
EZ 1MKC1 0 1M NH4C1 □ 0.1 M BaC12