Gravel, Soil Organic Matter, and Texture in Fallowed Alfisols, Entisols and Ultisols: Implications for Root and Tuber Crops

Southwestern Nigeria is replete with soils that have high gravel contents within the subsoils. Most of these soils are being opened up for cultivation as other land uses (roads and housing) are competing for agricultural lands. It is imperative that these soils be properly studied and managed to prevent serious land degradation, which may over the long term militate against food production in most rural communities. Thus, the objectives of these investigations were to assess the soil profile gravel contents, texture, cation exchange capacity (CEC), soil organic matter (SOM), and SOM/silt + clay (or SSC ratio) of these soils; assess which of these pedons will be susceptible to degradation using the SSC ratio; and evaluate the relationship that exists, if any, among the aforementioned variables. A semidetailed survey was conducted on 810 ha of land (7 degrees 40' N and 7 degrees 45'N; 4 degrees 25'E and 4 degrees 30'E) located in the rain forest (RF) agroecological zone (AEZ) of southwestern Nigeria. Eighteen soil types/pedons were identified. Subsequently, profile pits (1.50 m) were dug, and soil samples were collected, bagged, labeled, and air dried. These samples were used to determine the proportion of gravel (>2 mm) in the soil, and the <2-mm fraction was used for the determination of particlesize distribution, organic carbon contents (SOC), and CEC. The SOM was determined by multiplying the SOC by a factor of 1.729. The ratio SOM/(Clay+Silt) (or SSC) was calculated as total SOM (g kg(-1)) divided by clay + silt (g kg(-1)) and multiplied by 100. Results showed that the soil separates (sand, silt, and clay), gravel contents, SOM, and the SSC ratio have high coefficient of variations (CV), which suggests that these soils are highly variable. The gravel contents in the subsoils increased with depth and were generally >= 126 g kg(-1), except in pedons AP (Typic UstiPsamments/Ferric Arenosols), PEA (Typic Kanhaplustalf/Ferric Lixisol), BA (Gleyic Lixisol/Udic Kanhaplustalf), GEA (Typic Kanhaplustalf/Ferric Lixisol), and JGB (Tropaquent/Dystric Fluvisol), which had gravel contents of <= 54g kg(-1). The SSC ratio ranged between 1.33 (IB, Typic Kanhapludalf) and 14.4 (PEA, Typic Kanhaplustalf) and of all these 18 pedons, only 3 out of 18 (or 16.67%) had an SSC ratio of <= 5 and such may be at the risk of being degraded when opened for cultivation. The correlation coefficients among texture, CEC, SSC ratio, SOM/clay, and gravel contents revealed significantly negative relationships.