The field experiments were conducted at Sunyani (7.3349˚N 2.3113˚W) and Dormaa (7.2671˚N 2.8677˚W) municipalities of the Brong Ahafo Region, on maize and cabbage respectively. The Brong Ahafo region is the food basket of Ghana because a greater amount of the national foodstuffs are produced in the region and supplied to other parts of the country. Also, the Brong Ahafo Region (and selected municipalities for that matter) is known to be a leading producer of maize [30] [31]. The climate of Brong Ahafo Region is classified by Köppen and Geiger as Aw (Tropical Savanna Climate).

The field used for the maize trials was sprayed with sunphosate 41% non-selective herbicide and prepared (using no tillage method) before application of biochar and fertilizer. The no-tillage land preparation technique was used because maize farmers in the selected locations rely mainly on conservational (minimum tillage) methods for land preparation. Soil samples were taken with auger (0 - 30 cm depth) from each experimental unit before and after the experiment was carried out. Soil samples were air-dried, ground, well mixed and passed through a 2 mm sieved and analysed. All macro-nutrients, nitrogen, phosphorus and potassium (15:15:15 - NPK) and biochar amendments were applied in the respective experimental unit plots at different doses using the randomized complete block design (RCBD). Each experimental plot had a size of 10 m by 4 m (40 m²). Treatments covered three different levels of biochar: B0 (0% of recommended dose), B1 (50% of recommended dose) and B2 (100% of recommended dose).The recommended dose of biochar for crop production is 20 ton/ha; therefore, B0 = 0 ton/ha, B1 = 10 ton/ha and B2 = 20 ton/ha. Three different levels of 15:15:15 NPK fertilizer were used: F0 = 0%, F1 = 50% and F2 = 100% of the recommended dose. All the possible combinations of fertilizer and biochar gave rise to nine treatments i.e. B0F0, B0F1, B0F2, B1F0, B1F1, B1F2, B2F0, B2F1 and B2F2. Each treatment combination was replicated three times.

The seedbed of the cabbage field was however prepared using hoes, mattock and rakes which is the common method for cabbage farmers in the selected locality. Soil samples were taken before seedbed preparation and after harvest for analysis. The soil was broken using hoe and mattock, and the biochar incorporated and mixed using the hoes and rakes at a depth of 15 cm. The planting distance of the maize and cabbage was 80 cm inter-row and 40 cm intra-row. Apart from the seedbed preparation, the treatments and experimental design were the same as in the maize field.

Biochar was prepared by carbonizing sawdust in a multi-feed biomass gasifier at a temperature of 350˚C at the University of Energy and Natural Resources, Sunyani, Ghana. The biochar was put through a residence time of 4 hours per batch of production. At the end of each batch preparation, the charred sawdust was quenched with water and left to cool for six (6) hours before packaging. These samples were then packaged and transported to the experimental site.

Moisture content of samples was determined based on mass loss after twenty-four hours at 105˚C. Volatile matter was determined by heating dried samples in the oven to 950˚C and recording the loss of the mass. Ash content of biochars was determined by heating the same samples to 750˚C in a muffle furnace for four (4) hours. Proximate analysis of the biochar was done using the ABNT NBR 8112 methods [32].

Soil samples were collected at a depth of 0 - 30 cm after crop harvesting at random from each treatment plot. The samples from each treatment plot were composited, placed in tagged plastic bags and dried at room temperature. These samples were taken to the soils laboratory at the Soil Research Institute of Ghana’s Council for Scientific and Industrial Research in Kumasi for physio-chemical analysis. The soil samples were passed through the 2 mm sieve to obtain the soil fractions for the determination of soil textures. Total organic carbon (TOC), soil organic matter (SOM) and total organic nitrogen (TON) contents were determined by the dry combustion method using an N-C analyzer (Sumigraph NC-90A) as described by [34]. Exchangeable cations [calcium (Ca), magnesium (Mg), potassium (K), sodium (Na)] were first extracted with ammonium acetate (1.0 M NH₄OAc) and the contents were then determined by inductively coupled plasma-atomic emission spectroscopy. Exchangeable acidity was determined by first extracting with potassium chloride (KCl) and then titrating the extract with sodium hydroxide as described by McLean [35]. Cation exchange capacity (CEC) was calculated as the sum of exchangeable cations (Ca, Mg, K, Na) and exchangeable acidity.

The amount of biochar and fertilizer required for each plot was determined based on the biochar application rate and plot size. The required quantity was weighed using a beam balance and incorporated into /mixed with soil using tools such as hoes, mattocks and rakes. The fertilizer was applied using the side dressing method.

Statistical analysis was done by subjecting the data to analysis of variance (ANOVA) using the Statistical Package for Social Scientists (SPSS). The normality of the data was checked using the explore function of the SPSS and TOC, TON, SOM, pH and Yield of Maize were all normally distributed. The ANOVA was used to determine if there is significant difference (at p < 0.05) between the means of the various factors. The Tukey HSD posthoc test was used to isolate the means with significant differences at confidence level of 95%.

The largest proportion (55.7%) of the sawdust biochar was in the coarser size regime between 500 µm and 2500 µm. The smallest size fractions between 63 and 125 µm was 12.9%. The sawdust biochar had about 41.4% of its particles between 125 µm and 250 µm. The bulk density of the biochar was found to be 0.25 mg/m³. Proximate analysis of the biochar showed 25.6% moisture content, 25.3% ashes, 53.8% volatiles and 20.9% fixed carbon. Generally, biochar was light and therefore as soil amendment, will have to be incorporated and mixed very well with the soil to avoid losses through water and wind erosion. The size range indicates that there is no need for further grinding of the sawdust biochar before incorporation. Surface application will not be ideal as they will float on water should the soil be irrigated and/or even rainwater.

The statistical analysis of the soil data before and after harvesting showed that application of biochar significantly (as shown in Table 1 and Table 2) increased total organic carbon (TOC), Soil organic matter (SOM) and soil pH. Application of biochar increased TOC from 2.25% for biochar rate of 0 ton/ha or control, to 3.32% for biochar application rate 20 ton/ha (see Table 1). SOM also increased correspondingly from 3.88% (control) to 5.72% (for biochar application rate 20 ton/ha). The increase in the TOC and SOM apparently helped to address the major soil nutrient deficiencies at the trial site, which is consistent with results found by past research works [36] [37]. TOC is an indicator of SOM quality, which aerates the soil and helps retain water and nutrients. SOM also provides substrate for soil microbial biomass, which in turn make nutrients much more readily available to plants. Studies have shown that a sandy soil amended by coconut shell biochars increased soil organic carbon (SOC) and persisted after the second crop harvest, while TOC in manure treated soil was not significantly different from the fertilizer-only treatment after the first and second maize harvest [38]. Recent studies have reported that TOC was significantly increased due to different biochar applications [39] [40] [41]. Soil TOC in this study was averagely above the recommended lower limits of 1.8%, whilst the average total

TOC—total organic carbon; TON—total organic nitrogen; TEB—total exchangeable cation; CEC—cation exchangeable capacity; Base Sat—base saturation.

organic nitrogen (TON) content, although increased, were still below levels considered sufficient (0.25% for N), which may be due to high N uptake by plants, and high contents of above and below-ground plant residues. This research discovered that the soil of the study area (humid tropical Ghana) is sufficient in TOC but deficient in TON. The increase in pH has been isolated in the post hoc test (Table 1(c)) which shows that the rise in pH was mainly due to application of biochar not NPK. Previous studies by Vašák, et al. [42] revealed that application of NPK to soil reduces pH which goes to affirm the findings of this paper. The increase in pH with increase in dosage of biochar and fertilizer implies that application of biochar to agricultural soil in Ghana can help ameliorate the soil acidity problem identified by Dovana & Cassy [3]; Smith-Asante, [2]. In a similar study, Mensah and Frimpong [27] found that application of biochar with compost increased the soil pH. The increase in the pH could be attributed to the increase in available soil P due the presence of P in biochar [43] [44].

Soil pH was significantly (Table 1 and Table 2) different among soil samples of different treatments. Highest soil pH (7.30) was found in the experimental unit having B2F0 treatment while the lowest was found in B0F2 (6.55) (Table 1). The pH increased due to the application of biochar implies that the acidic soil in Ghana can be amended using biochar.

Cation exchange capacity (CEC) increased with increase in the application rates. Highest soil CEC of 23.66 cmol/kg was observed in B1F2 and the lowest was in B0F0 of 13.48 cmol/kg. Soil amendment with biochar significantly improved the CEC, indicating that soil retention of non-acidic cations increased. CEC is an important parameter in retaining inorganic nutrients such as K⁺ and NH 4 + in soil [45], and biochar has been associated with the enhancement in CEC of some biochar-amended soils [14] [46], thereby increasing the availability and retention of plant nutrients in soil and potentially increasing nutrient use efficiency.

Soil texture at the selected site varied from silt loam to sandy loam (Table 3) with visible contamination from construction materials both vertically in each profile and among the plots. The average soil texture was loamy sand (65.3% sand, 25.4% silt, 9.3% clay) in the profile. Soil texture is the most stable soil physical characteristic that has influence on a number of other soil properties including structure, soil moisture availability, erodibility, root penetration and soil fertility [47]. The coarse texture soil exhibited at the site controls the variability of nutrient storage capacity, limit the water holding capacity and increases permeability. Variability in soil texture was attributed to the complex nature of parent material as well as the past activities that have been carried out at the site (land use). The land area has been under cultivation for more than ten years with some history of construction activities. The high soil pH in control plots (Table 1) may therefore be as a result of previous liming.

The biochar amendments significantly (p ≤ 0.05) affected the yield of maize as shown in Table 4. The yield parameters such as cob length, grain dry weight, and stover dry weight of maize were significantly affected by the increase in dosage of fertilizer but showed marginal changes with biochar. The yield of dry grain generally ranged from 12.31 ton/ha for B0F0 (0 ton/ha of biochar, 0% of recommended dose of NPK) treatment to 13.13 ton/ha for B2F0 (20 ton/ha, 0% recommended dose of NPK) (Table 4, Table 5 and Figure 1) representing 6.66%. These variations were probably due to variations of residual fertilizer in the different sections of test site. These results are consistent with a study conducted by Major et al. [24], which looked at maize yield and nutrition during 4 years after biochar application to a Colombian savannaoxisol. They observed that biochar application had no significant effect on maize yield in the first year after biochar application. However, in the subsequent years maize yield increased with increasing biochar application rate. The increased SOC influences many soil characteristics including colour, nutrient holding capacity (cation and

Means for groups in homogeneous subsets are displayed. Uses Harmonic Mean Sample Size = 3.000.

L—length; FW—fresh weight; DW—dry weight. All values are the average of 6 measures, except for 100 kernels FW, which are the average of 4 measures.

anion exchange capacity), nutrient turnover and stability, which in turn influence water relations, aeration and workability [48]. But SOC can significantly influence these soil characteristics after some form of decomposition and increase in microbial activities, both of which take several weeks to build up. This study therefore argues that the first season is used by microorganism to condition soil (with biochar as catalyst) for the observed yield increase in the subsequent years after application.

The biochar amendments however had significant (p ≤ 0.05) effect on the yield of cabbage as shown in Table 6 and Figure 2. The yield of cabbage was increased by 7.57% from 1.98 kg for B0F0 (0 ton/ha of biochar and 0% of recommended dose of NPK) to 2.13 kg for B2F0 (20 ton/ha of biochar and 0% of recommended dose of NPK). The observations in this study is consistent with a

study conducted by Tariku et al. [49] where different crops responded differently to the application of biochar. Carter et al. (2013), revealed that cabbage yield was maximum in the first crop season and lowest in the third crop cycle/season. Their finding was the reverse of finding of Major et al., [24] cited earlier in this study. The variance in the yield responses to application of biochar by maize and cabbage could be attributed to variance in optimum growth condition of the two crops. According to Wunderground [50] and FAO [51], the maximum growing conditions for cabbage is pH of 5.3 - 8.0. However, for maize production (increase in grain yield) the most important factor is the ready availability of Nitrogen. The application of biochar was able to create the optimum growth condition of cabbage in the first season but needs more (at least a season more) time to establish that for maize [52].