Fertility assessment and phosphorus sorption characteristics of selected crop and grazing land soils in North West Province, South Africa.

Abstract

The assessment of the fertility and phosphorus sorption characteristics of soils in crop and grazing land areas in North West Province of South Africa is crucial for developing appropriate management strategies that promote nutrients replenishment, improve soil physical and chemical and biological conditions, and sustainable use of the soils. The research was initiated due to continuous declining soil fertility, impacting negatively on crop and animal production. Assessing soil physical and chemical indicators and phosphorus sorption characteristics in smallholder crop farm, commercial crop farm and communal grazing lands is crucial to promoting increase productivity and food security. The objectives of this study were to determine selected physical and chemical indicators that affect soil fertility, clay mineralogy and phosphorus sorption characteristics of the selected soils. This was done by analysing soils from selected smallholder crop farms, commercial crop farms and communal grazing land. A total of 180 soil samples randomly collected from 0 - 15 cm, 15 - 30 cm and 30 - 45 cm depths were analysed using routine soil analysis methods. Thirty-six soil samples were used for mineralogical study using XRD and 360 samples for sorption study by batch sorption method. Results of the soil physical and chemical indicators significantly (p ≤ 0.05) varied across location, land use and soil depth. The highest WHC (57.49%), BD (1.41g/cm3), TP (66.06% and silt/clay (1.81) values were recorded from the study. The highest means for pH-H2O, pH-KCl and ∆pH (7.19, 6.14 and 1.14), EC (0.13 dS/m), OC (1.82%), N (1.67 %), P (12.35 cmol/kg), K (415 cmol/kg), Ca (2870 cmol/kg), Mg (493 cmol/kg), Al (1.43 cmol/kg), S (0.84 %), EA (0.87 cmol/kg), CEC (18.31 cmol/kg), Cu (5.94 mg/kg), Fe (60.5 mg/kg), Mn (137.1 mg/kg) and Zn (17.23 mg/kg). Thirteen different mineral phases were identified in the study area: quartz, magnesioferrite, kaolinite, calcite, biotite, dolomite, muscovite, portlandite, nacrite, rutile, albite, brucite and pyrochroite. Quartz (71.2 to 99.4%), kaolinite (0.5 to 16.3%), muscovite (0.7-11.6%), albite (3.2-5.9%), Nacrite (0.1-1.8%), magnesioferrite (0.1-1.6%), portlandite (0.1-0.6%), calcite (0.3-1.2%), biotite (0.1-1.2), dolomite (0.1-1.1%), rutile (0.1-1.2%), brucite (0.5%) and pyrochroite (0.2 %). The sorption maxima of the soils ranged from 70 to 5769 mg P kg-1 and binding energy ranged from -28.8542 to 3.808 mg-1. The coefficients of determination (R2) values of 0.5734 to 1.0 were observed in this study. The slope and intercept ranged from 1.7332 to 18.812 and -51.435 to 111.76. The SPR (P0.2) values of 1.04 to 400 mg/kg were recorded. Phosphorus adsorption capacity decreased in the order of Itsoseng commercial crop farm (LC) > Mathopstad commercial crop farm (RC) > Mathopstad communal grazing land (RG). The study confirmed that LC (20%) clay required more P fertilisation than the other soils. Generally, most of the soil physical and chemical indicators fell within the acceptable limits, except available P and OC were low to medium. In addition, variability of the measured soil physical and chemical indicators helps explain eventual abnormalities of the results of future studies. According to findings in this study, adequate fertilisation and good crop/ soil management must be practiced based on the soil variability in the research area. This would help improve the productivity and fertility of the soils within the research area for sustainable crop production.