The research was conducted in North Bank Region of The Gambia which has a population of about 172,835 people (GBOS, 2003) [11] . It lies between Latitude 13˚29'10"N and Longitude 15˚5'48"W. The field work took place in only three districts (Central Baddibou District, Jokadu District, and Upper Niumi District). The Region is characterized by erratic rainfall, poor soil structure, and low soil fertility. The climate is of two different seasons: a wet season (that starts in June and ends in October) and a long dry season (that starts in November and ends in May). The average daily temperature is 30˚C in the dry season and 27˚C in the wet season. Rainfall trends are unpredictable and the annual total is usually less than 800 mm (FAO 2002) [12] . The natural vegetation is mainly savannah woodland and some mangrove trees lining along the banks of River Gambia dissected by an intricate system of water channels. Other areas within the landscape are flood plains and woodlands that are located behind the mangroves.

The locations used for data collection were upland fields in the three Districts. The plots on the fields were used in the study by using a Randomized Complete Block Design (RCBD) with three replications. Soil samples were collected from the replicated plots using a soil probe and screw-type auger. Sukristiyonubowo (2007) [13] method of soil sampling was used to collect soil samples at a depth of 0 - 15 cm and the samples were analysed for pH, Soil Organic Carbon (SOC), Soil Total Nitrogen (TN) and soil moisture content. Soil sampling was done on the 10^th and 12^th October, 2013 in the wet season and 25^th, 26^th and 27^th January, 2014 in the dry season.

Soil total nitrogen was determined using the micro kjeldahl distillation and titration method described by Bremner, J. M., and C. S. Mulvaney, (1982) [14] . Soil organic carbon (SOC) was determined by a modified Walkley-Black procedure as described by Nelson and Sommers (1982) [15] . The gravimetric method of moisture estimation was used where the soil sample was placed in an oven at 105˚C and dried to a constant weight; this took 24-36 hours. Soil pH was determined using the glass electrode method with a soil-to-water ratio of 1:2

SPSS software was used in the data analyses. Significant differences between the select soil contents in the two seasons were determined. The significance level was held at P < 0.05

During the wet season, soil moisture content within the treatment plots fluctuated, which could be attributed to the amount of rainfall and probably the amount of crop biomass at each selected location to conserve moisture in the soil (Figure 1). During the wet season, there was no significant difference (P < 0.05) in soil moisture content between the three locations (i.e. 43% at Njaba Kunda, 41% at Jokadu, and 38% at Niumi Lamin, respectively). In the dry season, on the other hand, although soil moisture content was lower than in the wet season in all three locations (Figure 1), still there was no significant difference in between the soil moisture contents. The decrease in soil moisture content could probably be due to high soil moisture evaporation which generally occurs during a drought period. Under an evapo-transpiration rate, utilization of water by crops declines when 50% of the required soil moisture is lost. For example, the water requirement for groundnut production ranges from 500 mm to 700 mm for the production cycle (FAO, 2013) [16] . This shows that the cropping cycle in the North Bank Region could be highly vulnerable to soil moisture loss as a result of drought, which consequently impacts on

crop production and productivity. Water is the primary factor determining soil nutrient mineralization and uptake by crops. Penning de Vries and Djiteye, (1982) [17] reported that Sahel region of Africa highlighted this deficiency in the soil during production seasons. Therefore, crops grown in the North Bank Region could be vulnerable to about 50% water deficiency in the dry season if crop cultivation is also carried out during the dry season.

The study found no significant difference in soil pH, irrespective of the cropping season, between the three study locations (Figure 2). This is an indication that all the three study locations could probably be of the same soil types that are characterized by low pH. Soil pH has a considerable influence on the availability of nutrients to crops. Low soil pH (i.e. pH between 1 and 4) which is a highly acidic condition tightly holds soil nutrients which are unavailable for plant uptake. Ronen, (2007) [18] reported that most soil nutrients are available for plant uptake in the pH range of 5.5 - 6.5. These low pH values could be the reason for low availability of nutrients for plant uptake.

Soil total nitrogen recorded for Njaba Kunda, Jokadu, and Niumi Lamin for both wet and dry seasons are shown in Figure 3. In either of the growing seasons, soil total nitrogen was less than 1%. This may be attributed to the low crop residue amount that is left on the farms after crop harvest and or low soil moisture which influences nitrogen mineralization for crop uptake as also reported by Wang et al. (1998) [7] .

Although the soil total nitrogen in either cropping season at either of the three study locations was less than 1%, we found a significant difference in total soil nitrogen between Njaba Kunda and Jokadu Darsilami and Njaba Kunda and Niumi Lamin in either of the seasons (Figure 3). This could be probably due to having more crop residues in Jokadu Darsilami and Niumi Lamin, respectively, than in Njaba Kunda that mineralized and released more soil total nitrogen. In Njaba Kunda, the study found no significant difference in soil total nitrogen between the two seasons (Figure 3). It was difficult to explain this phenomenon because the study was expected to have more soil total nitrogen in the wet season than in the dry season because of more mineralization as a result of more soil moisture availability. In Jokadu Darsilami, the study found a significant difference in soil total nitrogen between the wet and dry seasons. There was about twice more soil total nitrogen in the wet season than in the dry season at Jokadu Darsilami. This was probably that in the wet season, there was more mineralization of nitrogen than in the dry season. In Niumi Lamin, there was about one and a half times more soil total nitrogen found in the wet season than in the dry season (Figure 3). Again, just like in Jokadu Darsilami, probably there was more nitrogen mineralization during the wet season than in the dry season in Niumi Lamin (Figure 3). There was no significant difference in soil total nitrogen, irrespective of the season, between Jokadu Darsilami and Niumi Lamin (Figure 3). This could be that in both Jokadu Darsilami and Niumi Lamin there could have been similar amounts of nitrogen mineralization.

Soil organic carbon content among the three study locations is shown in Figure 4. Irrespective of the study location, there was no significant difference between the two seasons in terms of soil organic carbon content (Figure 4). However, there was a significant difference in soil organic carbon content between Njaba Kunda and Jokadu Darsilami and Najaba Kunda and Niumi Lamin (Figure 4), respectively. This could be probably due to more crop residue found in both Jokadu Darsilami and Niumi Lamin than in Njaba Kunda. Irrespective of the season, there was no significant difference in soil organic carbon content between Jokadu Darsilami and Niumi Lamin (Figure 4). Low soil organic carbon content in the three study locations is supported by findings byWilliams and Balling (1994) [19] which states that African soils, and mostly those of West Africa, have low inherent fertility as a result of low crop residue cover of the soils. The same authors went on further to state that soil fertility depletion (mainly N, P, and C) is the primary factor hindering food security in West Africa.