TY CHARACTERISTICS AND QUALITY Of SOILS UNDER SELECTED fARMING PRACTICES IN SOUTHwESTERN NIGERIA

This study compared soil properties and quality under five different farm practices in a part of the southwest Nigeria. The study indicated that fewer soil properties accounted for more percentage change in total variance at the fallow and mono-cropping plots than at the forest, crop rotation and alley farming systems. It also showed that soils under fallow and mono-cropping systems exhibited the lowest quality values among the farm practices system studied. The study recommends improved soil management approaches in plots under mono-cropping practices, and extensive soil recovery programmes for fallow lands. kEY wORDS: Agricultural practices; Soil physical and chemical attributes, Soil quality index; Plinthic luvisol. CITATION: Eludoyin Adebayo Oluwole, Yetunde Mary Adelere, Olusegun Olufemi Awotoye (2018) Characteristics and quality of soils under selected farming practices in southwestern Nigeria. Geography, Environment, Sustainability, Vol.11, No 3, p. 111-125 DOI-10.24057/2071-9388-2018-11-3-111-125


INTRODUCTION
The physio-chemical properties of soils have been attributed to natural and anthropogenic causes; the natural causes being mostly related to the underlying bedrocks, and the anthropogenic causes to land management practices (Wienhold et al. 2004;Yoo et al. 2006;Verachtert et al. 2009).Many studies on the indices of soil degradation, including nutrient losses, deterioration in soil quality and soil pollution, showed that poor farmland management can pose significant threats to food security in many regions of the world (Malhi et al. 2001;Lavelle et al. 2001;Liebig et al. 2004).
The effects of soil quality on food security can be overwhelming in countries where adequate agricultural technology and environmentally sustainable practices are limited.Recently, the Food and Agriculture Organization of the United Nations (FAO 2010) advocated for advancement of conservation agriculture practices -practices which involves minimal mechanical soil disturbance, minimum tillage and no-tillage systems, direct seeding, mulching, crop rotations and mixed farming.Such advocate is probably because farming activities have become sensitive to the need for environmental sustainability.
Whereas, developed countries, such as the United Kingdom through research-support policies offer guide to farmers on environmental sustainability (Tilman et al. 2001;Withers and Lord 2002;European Environment Agency 2003;Carabias-Martinez et al. 2003;Horsey 2006;Lane et al. 2006;DEFRA 2009;Eludoyin 2013), many farmers in the sub-Saharan Africa countries, including Nigeria, still practice indigenous land management schemes, despite changing climatic factors, competition for land for non-agricultural purposes and many other factors (Scholes et al. 1994, Luoga 2000).
In many sub-Saharan Africa, including Nigeria, practices such as continuous cropping, mono-cropping and mixed cropping are common; relatively new and more organized practice is the alley cropping system.The alley cropping system is an agroforestry practice of hedgerows with annual crops, and was probably introduced to the Nigerian agricultural practices in the 1970s through the International Institute for Tropical Agriculture, Ibadan (Kang et al. 1981).The hedgerows (typically Gliricidia sepium and Leucaena leucocephala) are usually pruned before planting the annual crop, and then periodically to prevent shading of the crops from sunlight (Oyedele et al. 2009).Whereas many studies have shown that many parts of the southwest Nigeria are characterized by fragile soils with poor nutrient status and quality, and that which require care in their management (Lal 2005;Oenema et al. 2006), recent studies have suggested that the assumed poor soil quality may be corrected if information on the practices of farming in the area is improved, as is elsewhere -when compared with agricultural productive heavy clay soils of the Stagni-vertic or Stagni-eutric Cambisols soil groups in the southwest England (Harrod and Hogan 1981) and drained soils (Eludoyin 2013).
The objectives of the present study are (i) to compare the soils under five common farming practice systems (alley cropping, crop rotation, mono-cropping, fallow or shifting cultivation) with that of relatively undisturbed forested sub-region in the study area; and (ii) determine the quality of the soils under selected farm practice systems using the soil quality index (SQI) approach.The SQI approach has been found relevant to soil assessment in tropical areas (Doran and Parkin 1994;Bremer and Ellert 2004;Bastida et al. 2008;Ezeaku 2011;Awotoye et al. 2011;Armenise et al. 2013;Ibrahim et al. 2014;Mukherjee and Lal 2014).The main hypothesis is that soil characteristics and quality vary significantly with farming practices in the area.The study is aimed at contributing to the discussion that land management rather, than natural factors are main causes of the poor soil quality and productivity in the sub-Saharan Africa.

STUDY AREA
The study area, plots of farmlets in of Leventis Farms are in the southwestern Nigeria (7 o 40'N -7 o 48' N, 4 o 10'E -4 o 45' E) (Fig. 1), and underlain by the Iwo soil association (Smyth and Montgomery 1962) or Plinthic luvisols of FAO/UNESCO (1974)'s classification.The World Reference Base for Soil Resources of FAO (2006) classified the Plinthic luvisols as Fe-rich, humic, and capable of changing irreversibly to a layer with hard nodules, a hardpan or irregular aggregates on exposure to repeated wetting and drying.The Plinthic luvisols soil covers about 60 million ha, globally, and is associated with the rainforest area where it supports food and tree crops (Ekanade 1991;FAO 2006).The study area is underlain by the basement complex geology, rich in gneisses and schists.The schist belt in Ilesa area is a black-arc basin where there is a subduction of an ocean slab into the mantle while the gneisses have resulted from part of the rocks that have metamorphosed into banded gneisses from which granite gneisses were derived (Oyinloye 1998).Both schists and gneisses, being metamorphic-having been subjected to varying degrees of pressure and temperature-are associated with slow weathered and fragile soils.Annual rainfall in southwest Nigeria averages 2500 mm with over 90% of occurring between April and October; average monthly temperatures vary between 22.5°C and 31.2°Cwhile average relative humidity is about 76.1 %, annually.
The general relief of the area is plain, varying between 360 m and 400 m above the mean sea level.
The Leventis Farms was created in 1988, and the Farms with the 'control' site had been under the indigenous farming system which was characterized by a mix of fallow or shifting cultivation, mixed cropping and crop rotation based on the perception of individual farmers.Since 1988, when the farmlands were cleared for cultivation, the 'control' site was left fallow and allowed to regrow as forest (secondary regrowth).In other words, until the use of the farmlands as experimental sites, the area was under similar farming (at individual farmer's discretion, however) practices.

Data collection
First, plots under different farming practice systems (alley cropping, mono-cropping, crop rotation and a fallow that has been part of a shifting cultivation practice) were identified in the entire experimental fields in the study area.Also, part of the Farms that is relatively undisturbed and treated as an agro-forestry system (since 1988) was identified for selection as 'control' .Identified plots are as follows: a. Alley plot: about 4 years old and comprised of hedgerows intercropped with Zea mays.The hedgerows were 2 m apart, and the Zea mays was planted in a single row between them.
b. Mono-cropping plot has been under cultivated for 10 years, also with Zea mays A 45 by 45 m spread-out portion was randomly carved out at the centre of each plot, and further delineated into three (15 by 15 m subplots).Each of the subplots was further divided into 3 by 5 grids for soil sampling, such that soil samples (a triplicatecomposite sample for physiochemical analysis and another with a 5.5 by 10 cm cylindrical corer) were obtained from each grid.Subsequently, 45 composite soil samples each were obtained with the aid of soil auger, each at 0 -15 cm, 15 -30 cm soil layers in each of the five plots and another 45 samples using the cylindrical corer for determination of soil bulk density.The soil samples were collected in labelled black polyethylene plastic bags, and taken for laboratory analysis at the Soil Laboratory, Department of Soil and Land Resources and Management, Obafemi Awolowo University, Ile-Ife, Nigeria.

Laboratory analysis
The composite soil samples were first were divided into two subsets; one set for determination of particle-size distribution by Hydrometeric method in a Bouyoucos cylinder (as described by Gee and Bauder 1986), and the other air-dried, sieved (2 mm) and analysed for pH, soil organic matter (SOM), total N, organic carbon (OC), available P (AP), exchangeable Ca 2+ , Mg 2+ , K + and Na + , cation exchange capacity (CEC), NO 3-and NH 4 , exchangeable acidity (EA) and SO 4 2- . The pH was determined as pH in H 2 O and pH in KCl with a standardized pH meter (Page et al. 1986), SOM by the loss on ignition method (Motsara and Roy 2008) and OC by Walkley-Black method (Walkley and Black 1934).The total N was determined after digestion with Technicon AAII (Technicon Instrument Corporation 1971) while concentrations of AP were determined at 640 nm wavelength of an Atomic Absorption Spectrophotometer (AAS) (Bray and Kurtz 1945).The exchangeable Ca 2+ , Mg 2+ , K + and Na + were determined with AAS at their respective wavelength values while CEC was determined with Flame Photometer (Uehara and Gillman 1981).Concentrations of NO 3-N and NH 4-N were determined by weighing and centrifuging using Technicon AAII (Technicon Instrument Corporation 1971) whereas the EA was extracted with 1M KCl as described by Thomas (1982).Lastly, SO 4 2-was determined by weighing, filtration and centrifuging (Daji 1970).
In addition, bulk density was determined as described by the International Organisation for Standardisation, ISO 11272 (ISO 1998) as the ratio of oven-dried (after 24 hours at 105°C) soil samples to volume of the core (after the volume of materials greater than 2 mm and other organics that were removed had been subtracted) (1) Where Ms = Mass of oven-dried soil Vt = Total volume of the cylindrical core Wt = Volume of solids greater than 2mm plus that of litter or vegetation taken in by the corer.The Wt was determined by water displacement method, such that volume of materials larger than 2 mm and removed organics were estimated from the volume of water displaced, in a water cylinder Porosity was determined in terms of the percentage of the ratio of the bulk density and particle density (Brady and Weil 2008).i. 'Minimum data sets' (MDS) were determined for each farm site.The MDS are variables which exhibits significant (95%) between-plots difference and possessed eigenvalue greater than 1.0 after rotated under the Varimax normalization procedure for PCA.The MDS were considered to have ranged in the 10% of the highest component loading, and be nonredundant (i.e.exhibits high correlation with other variables) (Armenise et al. 2013).

Data
ii.The MDS were then transformed into unit scores ranging from 0 to 1 (1 represents the optimum level for the index), accounting for their contribution to soil functions, and scored as either 'more is better' (when increasing values suggest high quality) or 'less is better' (when increase in variable values is associated with declining quality) (Wymore 1993).
iii.Subsequently 'less is better' was used to assess the physical properties particle size (percentage sand, bulk density, acidity and porosity) whereas nutrients (including AP, total N, organic matter content and carbon) were assessed with the 'more is better' index.iv.Scores were then used to generate the SQI values for each plot using equation 2 W = the weighting factor S i = MDS score VE = Variable Explained (for variables with at least 1.0 Eigenvalue) Furthermore, mean stocks of selected nutrient variables were described as recommended by Kiely et al (2010) (equation 3) N = concentration of selected variable d = soil depth at which samples were taken.

Soil characterisation
Table 1 shows the mean values and variations of selected variables at both 0 -15 cm and 15 -30 cm soil strata at the different farm plots.The soils are generally characterised by low acidity (pH value of 6 -8), CEC (1.0 -10 cmolc kg -1 ), organic matter (1.1 -5.1 g kg -1 ), and by particle sizes varying from medium to coarse-texture sandy-clay-loam or loamy-sand soil types when plotted on the FAO soil texture chart (Jeffery et al. 2010).The soil bulk density was in the range of 1.1 -1.6 g cm -3 .Also, the 15 -30 cm soil layers was generally more acidic than the 0 -15 cm layer, and pH values were generally higher at the forest plot than either the plots used for mono-cropping, rotation or fallow.Except for the percentage sand, silt and clay, exchangeable cations and other nutrients were more concentrated in the 0-15 cm layer than at 15 -30cm layer, except for the plot for mono-cropping (for cation exchangeable capacity) and pH.Fig. 2 shows that nitrogen stock at the 0 -30 cm soil stratum was more at the fallow and forest plots while organic carbon stock was more at forest plot despite that the plot for alley cropping exhibited richer organic matter contents than the other cultivated soil plots.Available phosphorus was dominant among the selected chemical variables, although the results of the regression analysis done to establish significant prediction of one nutrient by the other only showed that the organic matter content is a significant predictor of the organic carbon and total N in the area (R 2 = 0.43 and 0.63, respectively, R 2 = 0.39 in the relationship between organic matter and available P) (Table 2).Furthermore, Fig. 3 shows that fewer soil properties (out of the selected) also accounted for more fractions of the total variance at the fallow and mono-cropping plots (75% and 59%, respectively) that at the forest, crop rotation and alley cropping plots, suggesting lesser diversity in the nutrient inputs in the latter plots (fallow and mono-cropping) than the former plots.Fig. 3. Distribution of the eigenvalues of variables that explained the highest fraction of the total variance across selected plots.Total variance explained is provided in parenthesis for each plot
The parts of the soils in the present study area is in the group of coarse-texture sandy-clayloam or loamy-sand, and the results of the study showed that about 63%, 47% and 39% of the total nitrogen, organic carbon content and available P, respectively, in the soils were accounted for by the distribution of the organic matter.These results suggest that the soils in the area would benefit from sufficient use of organic matter.Soils with low organic matter contents often exhibit low carbon nitrogen and available P (van Breemen et al. 1983;Blackwell et al. 2010;Awotoye et al. 2011;Marschner 2012).Also, the stock (product of bulk density and concentration per soil depth) of the investigated variables (especially organic carbon, total nitrogen, nitrate and available P) in the soil indicated more deposits of available P and organic matter, and this suggests that the organic matter content contains more available P than organic carbon and total Nitrogen.
In terms of the soil quality, soils under the different farm practices exhibited different level of quality; the lowest rated being Fallow system.Existing studies (e.g.Mendoza-Vega and Messing 2005) have indicated that fallowing practice may have lost its sustainable relevance in the face of increasing urbanization and population-induced pressure on land resources, and as shorter (than the previously known 5-10 years) period of fallow now cause fallow land not to regain full restoration of its fertility.In addition, the second lowly rated practice, mono-cropping has been discouraged in many ecosystems because it is found to be characterized by lesser quantity of soil microbial biomass and soil enzymes than the more effective practices of alley cropping and crop-rotation (Acosta-Martinez et al. 2004;Tian et al. 2005;Liang et al. 2004).Mono-cropping is an important cropping practice in West Africa, and in the present study area, it was a 10-year period of the continuous maize cultivation.
In general.forest, alley cropping, and crop rotation practices ranked 1 -3, in the five practices examined in this study, suggesting that the soils under such practices exhibited quality better than those under fallow and mono-cropping practices.The results of the SQI obtained in this study is in line with the reports from other parts of Nigeria (e.g.Awotoye et al. 2011;Ezeaku. 2011;Ibrahim et al. 2014) that show the soils to be fragile and very reactive to management methods.
Main limitations of this study is the inability of the researchers to access long-term farm records, which studies have argued to be useful for assessing management roles (e.g.Minae et al. 2008;Rothamsted Research 2009;Eludoyin 2013;Eludoyin et al. 2017).The problem of poor historical data is not peculiar to the study area as Minae et al. (2008) argued that farm data systems in many sub-Saharan African countries comprise fragmented and disjointed multi-source that may greatly be subjective.In addition, the procedure for the SQI will be further investigated, especially since it involves some subjective ranking (e.g.categorization into 'less or more is better') (Bremer and Ellert 2004).

CONCLUSION AND RECOMMENDATIONS
The study compared soil properties and quality under five different farm practices in a part of the southwest Nigeria.The results showed disparity among the soils under the farm practices, and based on available records.The study showed that dominant cultivation practice system exacerbated the condition of the already fragile soils in the region.The results of the SQI also indicated that fallow and mono-cropping possessed the lowest quality values among the farm practices system studied.The main hypothesis that soil characteristics and quality vary significantly with farming practices in the area is accepted for this study.The study recommends improved soil management approach for the area, and extensive soil recovery programmes for the already depleted soils, because poor soil may herald difficulty for sustainable food production, given the unsound agricultural technology in many parts of the sub-Saharan Africa.
were analyzed in the IBM SPSS 12 for descriptive statistics, variance, Factor and Principal Component Analysis.Principal Component Analysis (PCA) was used to reduce the factors of the parameters, and to deduce those which significantly (p ≤ 0.05) contributed to spatial variations in each fig. 2. Mean stock of selected soil variables under the different farm practices

Fig. 4 .
fig. 4. Soil quality index (SQI) scores and quality rank of selected farm practices