Experimental location Pot experiments were conducted in the net house of the Soil Science Division, Bangladesh Agricultural Research Institute (BARI), Gazipur with 20 top soils (samples collected at 0–15 cm soil depth) during January to March and April to June 2017. The crop used was maize (cv. BARI maize9). The geographical position (GPS) of the experimental field was 23 590 38.43300 N Latitude and 90? 240 57.75600 E Longitude.
Collection of soil samples The 20 soils in bulk from plow layer (0–15cm) were collected from five Agro–Ecological Zones (AEZs) in Bangladesh, namely Tista Meander Floodplain (AEZ–3), Karatoya–Bangali Floodplain (AEZ–4), High Ganges River Floodplain (AEZ–11), Low Ganges River Floodplain (AEZ–12), and Madhupur Tract (AEZ–28). Encompassed here were six districts Rangpur, Nilphamari, Pabna, Bogra, Rajshahi, and Gazipur. Each of the 20 soils was collected from three different locations of the same soil series for pot trials. The sampling sites had a wide range of land types. The collected soil samples were air-dried, finely ground, and finally passed through a small 2 mm sieve separately for laboratory analysis. Soil pH was determined by a combined glass calomel electrode (Jackson 1962). Organic matter was determined by the wet oxidation method (Walkley and Black 1934). Potassium (K), calcium (Ca), and magnesium (Mg) were determined by the ammonium acetate (NH4OAC) extractable method. Phosphorus was determined employing the Bray and Kurtz method (Hanlon and Johnson 1984).
Zinc extraction At first 16.67 g diethylene triamine pentaacetic acid (DTPA) was weighed. Then 1 L of distilled water was added and dissolved under constant stirring. Eventually, 149.2 g triethanol amine (TEA) and 14.7 g CaCl2.2H2O were weighed separately and 1 L of distilled water was added to dissolve. After that the DTPA solution was poured under constant stirring conditions into the TEA–CaCl2 mixture. When the DTPA was dissolved completely, the solution was diluted to 9 L. Then the pH was adjusted to 7.3 with HCl and the volume was made up to 10 L with distilled water. Thus, the DTPA solution was prepared. We then calibrated the atomic absorption spectrophotometer by providing it with a hollow cathode Zn tube, with a standard solution containing 0.5 mg Zn mL1. The standard was prepared by pipetting 0.5 mL of a stock solution containing 1000 mg Zn mL1 into a 1 L volumetric flask and diluting to volume with distilled water (Lindsay and Norvell 1978).
Pot preparation and experimental setup There were two Zn treatments viz. 0 and 5 mg kg-1 applied on soils in pots. Each of the treatments was replicated thrice in a completely randomized design (CRD) to give a total of 6 (2 zinc rates 3 replication) pots for each soil. Thus, the total number of plastic pots used in this study was 120 (6 pots 20 soils). Pot dimension was 26 cm 31 cm (height diameter). During the first sowing, 15 kg of each soil was weighed into a series of 6 pots and five maize seeds were planted in each pot. A basal application was made with 150 mg kg1 N from urea, 60 mg kg1 P from triple super phosphate, 100 mg kg1 K from muriate of potash, 40 mg kg-1 S from gypsum, 10 mg kg-1 Mg from magnesium sulfate, and boron 2 mg kg1 from boric acid in each pot to support normal plant growth. Zinc sulfate heptahydrate served as the source of Zn which was applied with other fertilizers. Soils of all pots were kept moist with distilled water being added periodically. Finally, two maize plants were grown per pot. In all pots, 150 mg kg1 N as urea was applied at a vegetative stage after four weeks of planting.
Data collection The plants were cut at tasseling stage at day 62, then washed with distilled water and dried in an oven at 65 C for 48 hr for recording DM yield. Dried plant samples from each pot were powdered separately by a stainless still grinder. After that, the powdered plant samples were digested in a mixture of 10:4:1 of HNO3:HCLO4:H2SO4 on a hot plate and filtered by Whatman no. 42 to estimate Zn by atomic absorption spectrophotometer. Post-harvest soils were used as an initial soil for the next sowing. Fertilizers were given according to the soil test values.
CL determination The CL of soil Zn was determined by graphical method (Cate and Nelson 1965). According to the procedure (Cate and Nelson 1965), a scatter diagram of the relative yields (Bray’s percent DM yield) as Y-axis versus soil test values as X-axis was plotted. Bray’s percent DM yield was determined via the following Eq. (1).
Bray’s percent DM yield = DM yield without Zn treatment / DM yield with Zn treatment x 100