Pot studies: Short-term (12 and 24 days) experiments (1–4) were conducted using TSP and/or DAP as granules or as a crushed powder (by a Coffee ‘n’ Spice Grinder, Breville). All experiments were conducted with polyvinyl chloride (PVC) columns (30 cm height, and 15 cm diameter) and arranged separately in a completely randomized design each with three replications in 2010-11. Experiments 1 and 3 were conducted in a naturally-lit glasshouse at Murdoch University, Perth, Western Australia (WA) while Experiments 2 and 4 were in the transparent plastic shelter in Rajshahi, Bangladesh. In Experiment 1 and 3, reddish-brown, slightly acidic sandy clay loam soil (referred to as clay,) from Merredin, Western Australia (WA) (Calcic Red Dermosol, Isbell, 1996) and a subsoil yellow sand were used. In Experiment 2 and 4, a typical HBT sandy clay loam soil (referred to as clay,) (Aeric Haplaquept, Brammer, 1996) was used. The soils used in these experiments were analyzed for the properties. In all experiments, the amount of P fertilizers (TSP or DAP) was calculated on the basis of the recommended P rate (20 kg P or 100 kg TSP or DAP ha-1 ) and row spacing (40 or 60 or 90 cm). A control treatment without addition of TSP or DAP was included in Experiments 1 and 3. Control treatments for Experiments 2 and 4 are described below.
1 PRI, is a single point P sorption measure defined as the ratio: Pads (µg g-1 soil)/Peq (µg P mL-1 ) resulting from equilibrating soil with a 10 µg P mL-1 solution (1 g soil in 20 mL solution) for 16 h at 23 0C, where Pads is the amount of P adsorbed and Peq is the equilibrium P concentration (Ritchie and Weaver, 1993). PRI is usually reported without units, as the ratio of adsorbed P over that remaining in solution.
Fertilizer amount: The amount of fertilizer per seed or per column was calculated and the following considerations: in one ha of land (100 x 100 m), assuming 40 cm row spacing, the number of rows of 100 m length will be 250. The total length of all lines will be 25000 m ha-1. Therefore, the 3.75 cm long strip of fertilized soil per seed will contain 0.151 g of TSP (TSP 100 kg ha-1, 45 seed m-2). Similar considerations were also the basis for calculating DAP density (Experiment 1 only). Experiment 1: Experiment 1 had three TSP or DAP treatments (granule or dust: a control, 40 cm and 60 cm row spacing). Soil (~4 kg) was filled up to about 24 cm from the bottom of the column. The treatment P fertilizer for 40 or 60 cm was mixed with the 2 cm layer of soil. Forty seeds of desi chickpea (cv. Genesis 836) were sown in the middle of the Pfertilized soil layer (2 cm thick). A 2 cm layer of soil (not P fertilized) covered the P-treated layer, i.e., the seed was located 3 cm below the soil surface. At 12 days after sowing (DAS), 6 plants were randomly selected from each column. The selected plants were separated carefully and they were severed at the stem base. The length of each plant’s shoot and root.
Experiment 2: Experiment 2 had TSP granules only as P treatment with the same objective of Experiment 1 plus an extra treatment of TSP concentration at 90 cm row spacing (calculation for 90 cm spacing as for 40 shown in “Fertilizer amount” above). Filling of columns with soil, placement of TSP granules, seed sowing and data collection were similar to that of Experiment 1. The differences were: Experiment 2 was done using the HBT soil and the seed was BARI chola 5 (12 seeds per column). Plants were harvested at 12 DAS. Shoot and root of all plants in a column were retrieved, dried, and weighed as in Experiment 1.