AGRO-GEOPHYSICAL ASSESSMENTS OF SPATIAL VARIABILITY IN AGRICULTURAL SOIL PROPERTIES IN IBADAN, SOUTHWESTERN NIGERIA

Abstract

Agricultural soil nutrients variability assessment for sustainable crop production has usually been through soil geochemical/chemical analyses which are laborious and expensive, thus necessitating the need for faster and cheaper alternatives. The application of geophysical methods to resolve this has gained acceptability globally. However, there is paucity of data from Nigeria on the application of geophysical investigation for soil properties variability determination. Therefore, this investigation was designed to use geophysical methods to assess the physical properties that can substitute for geochemical analysis of agricultural soil nutrients in Ibadan, southwestern Nigeria. The investigation was at the research farms of the Cocoa Research Institute of Nigeria, Ibadan. The Apparent Electrical Conductivity (ECa) and Volumetric Water Content (VWC) of the soils were determined using resistivity earth-meter and VG-meter-200 moisture-meter. The 912 (cocoa farm) and 700 (kola field) points were classified into zones of Low ECa (LECa), Moderate ECa (MECa) and High ECa, (HECa) on which other investigations were based. Thermal Conductivity (TC), Volumetric Heat Capacity (VHC) and Thermal Diffusivity (TD) at 90 (Cocoa farm) and 67 (Kola field) points were determined by KD2PRO analyser. The ECa, VWC, TC, VHC and TD were assessed in both wet and dry seasons. Falling and constant head permeability tests were conducted on duplicated ten cored soil samples per farmland for water infiltration assessment. Soil textural classes were established in the cocoa (54-sample) and kola (42-sample) farms using Bouyoucos method. Soil (20-sample/farmland) were analysed for pH, Electrical Conductivity (EC), organic carbon, total nitrogen, available phosphorus, acidity, Na, Mg, K, Ca and Cation Exchangeable Capacity (CEC) using standard soil science procedures. Soil mineralogy (6-sample/farmland) was determined using X-ray diffractometer. All investigations were limited to the root zone (0.3 m). The soils EC a, VWC, TC, VHC and TD were 10-545 µS/cm; 2-69%; 0.700-2.715 W/mk; 0.760-4.578 mJ/m3k and 0.351-1.994 mm2/s, respectively. The soils were categorised into LECa (1-49 µS/cm), MECa (50-99 µS/cm), and HECa (>100 µS/cm). The HECa had high TC (1.668-2.148 W/mk), high VHC (2.604-2.721mJ/m3k), and low TD (0.622-0.835vii mm2/s), while LECa had inverse distribution, indicating that heat energy retained in soils aided mobility of ions. Soils’ permeability ranged from 6.2x10-6-3.97x10-3 cm/sec across the field. Infiltration rate was low (HECa), moderate (MECa) and rapid (LECa) accounting for the moisture variation. Texturally, the soils were sandy loam (HECa/MECa/LECa), loamy sand (MECa/LECa) and sandy clayey loam (HECa). The soils’ pH, EC, organic carbon, total nitrogen, available phosphorus, acidity, Na, Mg, K, Ca, CEC ranged from 6.1-7.6; 30-180 µS/cm; 0.270-1.667%; 0.03-0.17%; 3.50-12.71 mg/kg; 0.32-1.20 cmol/kg; 0.15-0.42 cmol/kg; 0.25-2.84 cmol/kg; 0.13-1.33 cmol/kg; 0.46-5.84 cmol/kg and 1.92- 10.33 cmol/kg, respectively. The saturation of basic cations in HECa (81.38-87.73%), MECa (73.24-81.82%) and LECa (71.80-77.87%) indicate that HECa had more nutrients than others. Kaolinite (4.7-41.2%), microcline (6.8-24.6%) and quartz (14.3-67.2%) were the main minerals in the soils. The HECa had low quartz (22.5-41.3%) and microcline (9.85-15.05%), but high kaolinite (31.1-37.6%). Soil physical properties from geophysical methods were effective in evaluating the spatial agricultural soil nutrient variability. This method can therefore be adopted for cost effective agro-soil evaluation.