World Reference Base For Soil Resources Research Articles

Characterization and classification of rice-growing soils on Imo clay shale residua in Eastern Nigeria and current suitability for rice production: I. Omasi-Agu and Omor agrarian communities

The soils on Imo clay shale residua at Omasi-Agu and Omor areas in Anambra State, Nigeria were sampled in order to characterize classify and evaluate their current suitability for rice production following standard scientific procedures. The soils generally belong to loamy textural classes or finer, especially in the subsoil layers. The morphological and physical properties of soils showed that the soils were poorly drained favouring high water retentivity due to low hydraulic conductivity and slow permeability. All these soil conditions support good rice performance. Though the soils at Omasi-Agu contained high levels of exchangeable bases, exchangeable acidity was equally high leading to low base saturation of < 35% but at Omor the base saturation was > 35%. Thus, the soils were, respectively, classified as Typic Kandiaquults (Soil Taxonomy) correlated with Gleyic Cambisols of the World Reference Base for Soil Resources (WRBSR) and Typic Kandiaqualfs (Soil Taxonomy) correlated with Gleyic Cambisols (WRBSR). Even though the soils of Omor appeared to be generally more fertile than Omasi-Agu soils, both have similar constraints to rice production, that is, insufficiency of nutrients (f) and possibility of erosion by flooding (e) in limited areas (< 25%). Therefore, the current suitability of the soils for rice production is suitability class 2 with fertility constraints (S2fe). For sustainable rice production, supplementary addition of nutrients especially P2O5 from triple superphosphate and some minor levelling and bunding are required to control erosion due to flooding in the limited areas.

Characteristics of Rendzina soils in Serbia and their WRB classification

According to the Serbian official soil classification system, Rendzina is a soil type with an A-AC-C-R profile, developed on parent rock containing more than 20% of calcareous material (except soils with an A-R profile on hard pure limestone or dolomite). Previous investigations have shown that 29 Rendzina soil profiles from Serbia belong to the reference soil groups (RSGs) of Leptosols, Regosols and Phaeozems according to the World Reference Base for Soil Resources (WRB 2015). The present study addresses the correlations among three WRB RSGs in terms of soil texture, mean weight diameter (MWD), total N content, and humus fractional composition using Principal Component Analysis (PCA). The objective is to better understand the mutual relationship between the classification soil units used in Serbia and the international WRB system. The results show that PCA cannot unequivocally distinguish between these three RSGs. Leptosols and Regosols are highly incoherent groups while the group of Phaeozems is highly coherent, leading to the conclusion that the physical and chemical properties of the soil profiles of Phaoeozems are specific. It is obvious that soil depth and color, which are the overriding factors in the differentiation of Rendzina soils into three WRB RSGs, had no significant effect on these properties. The results further show that soil properties such as texture, MWD, humus fractional composition, etc. cannot be used to correlate Rendzina soils from Serbia with WRB. Instead, careful correlation of individual soil profiles is needed based on quantitative soil data analysis as required by WRB.