Rhizosphere soils of some fruit and spices plants from Agricultural Research Station, Khulna and Regional Horticultural Agricultural Research Station, Patuakhali during 2015-2016 and 2016-2017, and Regional Horticultural Agricultural Research Station, Chapainawabgonj were collected during 2016-2017 for the assessment of arbuscular mycorrhizal association. Rhizosphere soils with thin root were collected from the plants.
Assessment of spore population density:
Assessment of spore population was done by following the Wet Sieving and Decanting Method (Gerdemann and Nicolson, 1963). Soil samples from the rhizosphere of the respective plant species were mixed thoroughly by breaking up any large lumps. Any large unwanted particles such as stone, roots, twigs etc. were removed when done this process. Then 100 g of mixed soil was kept in a bucket (8 litres) and filled three quarters with tap water. The soil with water was agitated by stirring vigorously by hand and washed into the bucket and left to settle for one minute. The suspension was sieved by following the wet sieving and decanting method (Gerdemann and Nicolson, 1963). Two sieves (400 mm and 100 mm mesh) were used throughout the experiment. The supernatant was poured through a 100 mm sieve into the second bucked (10 litres) to avoid the loss of useful materials. After allowing the suspension to settle for one minute, the supernatant was decanted into the 400 mm sieve. This time water was discarded and the material was back washed from the sieve into a beaker (250 mL) with a small quantity of water. The solution with spores was distributed in 4 equal size test tubes evenly and balanced up the tubes with water for equal weight. The tubes were plugged properly and then centrifuged for 4 minutes at 3,000 rpm. The supernatant was poured in test tubes and the test tubes were filled with sucrose solution and stirred vigorously with the round-ended spatula to re-suspend the precipitate. The test tubes were balanced properly to equal weight and they were plugged. Then the plugged test tubes were centrifuged for 15 seconds at 3,000 rpm. After centrifuge, the sucrose supernatant was poured through a 400 mm sieve and rapidly washed with water to remove the sucrose from AM spores by back washing the materials from the sieve into watch glass for observation.
Counting of AM spores:
All the AM spores were isolated from the extract with the help of a fine forcep into a watch glass with small quantity of water. The extract, with AM spores, was observed under stereomicroscope and the number of spores was counted. Spore numbers from the three replicates per samples were averaged and the result was expressed as number per 100 g of dry soil basis.
Assessment of root colonization infection:
The percentage of AM infection was estimated by root slide technique (Read et al., 1976). One hundred root segments were examined for each sample. The stained root pieces were mounted in acidic glycerol on slides and the cover slip was placed and slightly pressed. The roots were observed under microscope. A root segment was considered as positively infected, if it showed mycelium, vesicles and arbuscules or any other combination of these structural characteristics of AM infection. The presence or absence of infection in the root pieces was recorded and the percent infection was calculated as follows: % root colonization = {( Number of AM positive segments) / (Total number of segments scored)} x 100.
Methods of chemical analysis:
Soil pH was measured by a combined glass calomel electrode (Jackson, 1958). Organic carbon was determined by wet oxidation method (Walkley and Black). Total N was determined by modified Kjeldahl method. Calcium, K and Mg were determined by NH4OAc extraction method. Copper, Fe, Mn and Zn were determined by DTPA extraction followed by AAS reading. Boron was determined by CaCl2 extraction method. Phosphorus was determined by Modified Olsen method (Neutral + Calcareous soils). Sulphur was determined by CaH4(PO4)2.H2O extraction followed by turbidimetric turbidity method with BaCl2.