Initial soil analysis
The physical and chemical properties of the soil used for the experiment are presented in Table 1. The soil was sandy loam in texture, acidic, and had high bulk density and low total porosity. The 0.96% soil organic carbon (OC), 0.08% total N, 3.85 mg kg− 1 available P, 0.11 cmol kg− 1 exchangeable K, 1.63 cmol kg− 1 exchangeable Ca and 0.32 cmol kg− 1 exchangeable Mg were all very low. Additionally, the cation exchange capacity (CEC) of 2.55 cmol kg− 1 and exchangeable acidity of 0.40 cmol kg− 1 were also very low, indicating poor soil fertility. Therefore, the soil is unlikely to sustain crop yields without the addition of external inputs.
Physical and chemical properties of Tithonia diversifolia used for the experiment
The physical and chemical properties of the Tithonia diversifolia used for the experiment are shown in Table 2. The Biochar exhibited high values of electrical conductivity (EC), nitrogen (N), potassium (K), calcium (Ca) contents, and other micronutrients at the level required for the growth of broccoli. The application of Biochar in the short term is expected to benefit crops and soil. It is alkaline with a pH value of 7.89.
Effects of Tithonia diversifolia Biochar on physical properties of soil
The effects of Tithonia diversifolia Biochar application on soil bulk density, total porosity, and moisture content are presented in Table 3. The application of Biochar resulted in lower bulk density across all treatments compared to the control. Additionally, Biochar application led to increased moisture content and porosity. As the level of Biochar application increased, soil bulk density decreased, while porosity and moisture content showed a corresponding increase compared to the control. Application of tithonia Biochar at 10, 20, 30, and 40 t ha− 1 decreased bulk density by 13%, 20%, 26%, and 33%, respectively, compared to the control. Application of tithonia Biochar at 10, 20, 30, and 40 t ha− 1 increased porosity by 20%, 32%, 41%, and 51%, respectively, compared to the control, while the same treatments increased moisture content by 24%, 44%, 59%, and 75%, respectively, compared to the control.
Effects of Tithonia diversifolia Biochar on soil chemical properties
Application of Tithonia diversifolia influenced soil chemical properties (Table 4). The application of tithonia Biochar at rates of 10, 20, 30, and 140 t ha-1 resulted in increased soil pH, OC, TN, P, K, Ca, and Mg. As the amount of tithonia Biochar increased, there was a corresponding increase in soil pH, OC, TN, P, K, Ca, and Mg. Particularly, applying tithonia Biochar at rates of 10, 20, 30, and 40 t ha-1 led to the higher soil pH, OC, TN, P, K, Ca, and Mg compared to the control. Application of tithonia Biochar at 10 t ha-1 increased soil pH, OC, TN, P, K, Ca, and Mg by 6%, 20%, 29%, 23%, 50%, 16%, and 41%, respectively, compared to the control. Application of tithonia Biochar at 20 t ha-1 increased soil pH, OC, TN, P, K, Ca, and Mg by 12%, 52%, 71%, 73%, 90%, 36%, and 117%, respectively, compared to the control. Application of tithonia Biochar at 30 t ha-1 increased soil pH, OC, TN, P, K, Ca, and Mg by 18%, 83%, 114%, 123%, 120%, 52%, and 159%, respectively, compared to the control. Application of tithonia Biochar at 40 t ha-1 increased soil pH, OC, TN, P, K, Ca, and Mg by 23%, 129%, 171%, 174%, 150%, 74%, and 197%, respectively, compared to the control.
Effects of Tithonia diversifolia Biochar on leaf nutrient concentration of broccoli
Table 5 presents the effects of Tithonia diversifolia Biochar on the leaf nutrient concentrations of broccoli at the flowering stage, four months after sowing. The application of tithonia Biochar significantly influenced the leaf nutrient concentrations of broccoli. Specifically, tithonia Biochar application significantly (p = 0.05) increased the concentrations of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in the leaves, with these concentrations increasing in response to higher rates of tithonia Biochar application compared to the control group. The concentrations of N, P, K, Ca, and Mg in the broccoli leaves increased with tithonia Biochar application rates up to 20 t ha-1. However, beyond this rate, the concentration of N, P, K, Ca, and Mg in the leaves was decreased by the tithonia Biochar application. The application of tithonia Biochar at a rate of 20 t ha-1 resulted in the highest leaf concentrations of N, P, K, Ca, and Mg compared to other treatments. The application of tithonia Biochar at a rate of 20 t ha-1 increased leaf concentrations of N, P, K, Ca, and Mg by 99%, 134%, 103%, 62%, and 88%, respectively, compared to other treatments.
Effects of Tithonia diversifolia Biochar on the growth parameters of broccoli
The effects of Tithonia diversifolia Biochar on the growth parameters of broccoli, including plant height, number of leaves, leaf area, stem girth, and fresh weight of broccoli biomass, observed 90 days after sowing, are presented in Table 6. The application of tithonia Biochar significantly influenced the growth of broccoli. It was noted that the plant height, number of leaves, leaf area, stem girth, and fresh weight of broccoli biomass increased significantly (p = 0.05) with increasing tithonia Biochar application rates, from 0 to 40 t ha− 1. Among the various application rates, the rate of 20 t ha− 1 was found to be the most effective, resulting in the highest plant height, number of leaves, leaf area, stem girth, and fresh weight of broccoli biomass (Table 6). However, application rates above 20 t ha− 1, such as 30 and 40 t ha− 1, tended to decrease these growth parameters, indicating a diminishing return in production. This suggests that while tithonia Biochar can enhance the growth of broccoli up to a certain threshold, excessive application may have adverse effects. The application of tithonia Biochar at rates of 10, 20, 30, and 40 t ha− 1 increased fresh broccoli biomass by 13%, 38%, 26%, and 23%, respectively, compared to the control.