Chapter 4: Results and Discussion4.1 Physiochemical Properties of SoilResults of the soil chemical analysis performed on two selected soils are presented in Table 4.1. Reaction of Wafi soil has indicated that it was a moderately acidic soil (pH 5.15±0.007) while Bug soil was slightly alkaline (pH 7.61±0.0261). Free lime content of the soils was negligible. Finer particle fractions (clay and silt) were greater in Wafi soil compared to Bug soil. Wafi soil belonged to textural class silt loam while Bug was a loam soil.
736601995170Table 4.1 Physiochemical properties of Wafi and Bug soils.
0Table 4.1 Physiochemical properties of Wafi and Bug soils.
10160229743000The amount of P sorbed was higher in Wafi soil than in Bug soil thus Wafi soil had far higher P fixation capacity than Bug soil. Besides this, higher P solubility and mobility is believed to occur in Wafi soil than Bug soil as P solubility and mobility is always high with soil with high sand content (Nascimento et al., 2018). Greater P sorption in Wafi soil could be due to greater clay and silt content compared to those containing small amount of clay (Tening et al.,2013; Chatterjee et al.,2014). Bug soil recorded high N than Wafi soil because greater N content of soil due to inorganic N is known to affect plant P levels as N salts can influence P absorption by altering the root surfaces areas of the crop for P uptake and altering P solubility with its residual acidity or alkalinity (Grunes, 1959; Schwamberger and Sims, 2008).
Soil pH has profound influence on plant growth. In light of the relationship soil pH to crop growth, it was reported that mung bean grows well at the pH range of 6.2 to 7.2 (Varma et al., 2015). Likewise, orthophosphates are readily available for crop uptake at the pH range of 5.0-6.0 (Schachtman et al., 1998) and while maximum solubility and availability of phosphate in alkaline soil occurs at pH 6.5 and declines as the pH increases, forming insoluble calcium phosphate at about pH 8 (Hanes, 1982; Hopkins and Ellsworth, 2005). Taking account of that, there is high possibility of limited P availability, suppressed growth and yield limitation in both soils.
4.2 Chemical Properties of Biochar-10795173482000692151505585Table 4.2 Chemical properties of the biochar derived from kunai grass feedstock
Table 4.2 Chemical properties of the biochar derived from kunai grass feedstock
The biochar produced through slow-pyrolysis at 500’C was an alkaline biochar as indicated in by its pH. This has consolidated relevant reports that biochar generally is alkaline in pH and may increase soil pH, cation exchange capacity, base saturation, exchangeable bases and organic carbon content as well as decreases in Al and Fe saturation in acid soils (Jindo et al., 2014; Sharma et al., 2018).
In light of the availability of soil nutrients like N, P and C, it can release available nutrients it contained directly into the soil from its ash content, depending on feedstock and temperature (Gai et al., 2014; Scott et al., 2014; Ding et al., 2016). It was also divulged that with the amount of P it contained, it can supply a little portion of P which is water soluble for crop uptake (Zhang et al., 2016).
4.3 Effect of Biochar, P Fertilizer and Biochar Co-application with P Fertilizer on Soil pH and P Fractions in Wafi Soil.4.3.1 Soil pHThe effect of addition of biochar, P fertilizer and their co-application on soil pH after day 30 is presented in Figure 4.1. Results indicated that there was significant effect (P;0.05) of treatments. Sole application of biochar and co-application of biochar with P fertilizer resulted in similar pH values 30 days after fertilization. Soil pH for Wafi was 5.15 before addition of different treatments. Application of 64 mg kg-1 P fertilizer raised the soil pH slightly (3.8%).857254507865Figure 4.1 Graph showing pH mean in acidic soil when treatments were added
Figure 4.1 Graph showing pH mean in acidic soil when treatments were added
-5715078740000 Increase in soil pH with addition of 64 mg kg-1 P fertilizer occurred because calcium hydrogen phosphate is the most soluble and mobile fertilizing material can help increase soil pH, P solubility and mobility especially in soil with high sand content (Nascimento et al., 2018).
When 15t ha-1 kunai biochar was added to low pH soil from Wafi, pH significantly increased by 9.5% as addition of biochar to acidic soil has shown to increase soil pH and cation exchange capacity (Martinsen et al., 2015). It could have also increased exchangeable base cations, cation exchange capacity, and base saturation however reduced soil exchangeable acidity, exchangeable Al and exchangeable Fe (Yuan and Xu, 2011; Chintala et al., 2013). It may also the biochar alkalinity that has also shown to be the key factor in controlling the liming effect on acid soils (Yuan and Xu, 2011).
Co-application of 15tha-1 kunai biochar with 64 mg kg-1 P fertilizer raised the soil pH remarkably by 9.9% as application of chemical fertilizers with organic amendments has shown to ameliorate phosphorus fixation in acid soils. Biochar had the ability to increase soil pH, reduce exchangeable acidity, exchangeable aluminium, and exchangeable iron, thereby increasing available P for crop uptake (Huck et al., 2016). All in one, sole addition of 15t ha-1 kunai biochar or P fertilizer at 64 mg kg -1 P or their co-application with 64 mg kg-1 P fertilizer have significantly raised pH in Wafi soil.
4.3.2 Dynamics of P fractions in Wafi Soil in Response to Treatments
Addition of various treatments to the acidic soil had no significant effect on the amount of soluble P, Al-P and Fe-P on the day 30, however affected significantly the distribution of P in calcium bound P fraction (Figure 2.4). Application of 64mg kg-1 P fertilizer into the acidic soil increased soluble P, inorganic aluminium and calcium bound phosphorus by 8.7%, 19.8% and 48.9% respectively while decreased iron bound phosphorus by 46% as the soil pH increased by 3.8%.
The 8.7% increase in soluble P when 64 mg kg-1 P fertilizer alone was added was due to the fact that calcium hydrogen phosphate based fertilizers, as the most soluble and mobile fertilizers can help increase soil pH, P solubility and mobility especially in acidic soil with high sand content (Nascimento et al., 2018). Opala (2016) also reported from his study that application of P fertilizer can increase soluble P thus crop dry matter without liming effect. It was also disclosed that addition of P fertilizer to soil can considerably increase the concentration of P in the soil solution and the readily available pool for crop uptake, however, higher P applications or concentrations, can encourage more P to precipitate out of solution through reactions with charged molecules and soil particles (Hargreaves, 2015).
19050-229870Figure 4.2 Distribution of P fractions in Wafi soil at day 30.
0Figure 4.2 Distribution of P fractions in Wafi soil at day 30.
In light of that, the slight increase in Al-P and significant increase in Ca-P noted with addition of 64 mg kg-1 P fertilizer in reference to the quantity of Al-P indicated fixation of 0.03 mg kg-1 P fertilizer applied on the soil colloidal surface. Application of P fertilizer such as monoammonium phosphate, single superphosphate or triple superphosphate enables addition of phosphoric acid into the soil to react with H+ ions causing soil acidity and thus fixation of phosphate ions on the aluminol clay surface (Hansel et al., 2014; Schmitt et al., 2017). This could cause a series of reactions resulting in the diffusion of P from the fertilizer granules into the soil solution, sorption of P into soil particles, and, with time, P precipitation (Hedley and McLaughlin, 2005; Schmitt et al., 2017). In other words, the orthophosphate from fertilizer reacted with free Al3+ ions in soil solution causing additional adsorption of P thus increased Al-P.
On the same token, inorganic iron bound phosphorus, however, decreased with addition of 64 mg kg-1 P fertilizer though iron has a very high affinity to fix phosphates. The reduced amount of inorganic iron bound phosphorus may occur as a result of reduced concentration, crystallinity, soil surface area, configuration and concentration of hydroxyl groups on the surface of iron oxides (Fink et al., 2016).
Taking account of the aforementioned, it was also disclosed after 90-days cropping period there was significant difference amongst means of iron bound P and insignificant difference amongst means of soluble P, inorganic aluminium and calcium bound P (Figure 4.3).
Soluble P and Fe-P decreased by 3.41 mg kg-1 and 0.11 mg kg-1 respectively while there was an increase in Al-P and Ca-P by 0.35 mg kg-1 and 1.11 mg kg-1. The decrease in soluble P and increase in Al-P and Ca-P indicated the capacity of the acidic soil to fix soluble P making it unavailable for crop uptake in the long run coupled to the plant uptake of available P (Hedley and McLaughlin, 2005; Hansel et al., 2014; Schmitt et al., 2017). All in one, this study clearly showed that much of the added P is either absorbed by the mung bean plants or converted to non-extractable residual P.
Sole biochar treatment on Wafi soil on day 30 increased soluble P, Al-P and Ca-P by 1.87%, 79.2% and 13.5% respectively while Fe-P decreased by 52.6% in reference to the control soil (Figure 4.2). A concurrent increase in soil pH was also recorded with this soil amendment.
The increase in soluble P, Al-P and Ca-P with 15 t ha-1 alkaline biochar application was in conformity with a report that 15 t ha-1 alkaline biochar produced at 500 0C has shown promising result to increase soil P availability in acidic soil (Dumea et al., 2017). Other soil studies also reported that addition of alkaline biochar increased soil pH in acidic soil thereby availing more P for crop uptake (Huck et al., 2014; Zhang et al., 2016; Dumeb et al., 2017; Ding et al., 2016). This is due to the ability of the alkaline biochar to alter soil chemical properties by increasing the soil pH and reducing the exchangeable acidity, exchangeable aluminium, and exchangeable iron thereby increasing inorganic phosphorus fractions like soluble inorganic P, aluminium bound inorganic P, iron bound inorganic P, calcium bound P and significantly available P (Huck et al,. 2014).
Besides this, biochar can also alter P availability through sorption of chelating organic molecules like phenolic acids, amino acids and complex proteins or carbohydrates by directly adsorbing cations such as Al3+, Fe3+ and Ca2+ resulting in delayed P adsorption or precipitation in soil (Joseph et al., 2010; Xu et al., 2014). The soluble P (0.766 mg kg-1) contained in the biochar also may have the effect in increasing available P in the soil (Zhang et al., 2016). The decrease in Fe-P however could be due to the reduced concentration of P in solution, crystallinity, soil surface area, configuration and concentration of hydroxyl groups on the surface of iron oxides (Fink et al., 2016).
Taking account of the aforestated, it was also seen that after 90-days cropping period, amount of soluble P, Al-P and Fe-P decreased remarkably by 4.45 mg kg-1, 0.37 mg kg-1 and 0.55 mg kg-1 respectively while Ca-P increased by 2.87 mg kg-1 (Table 4.3). The decrease in soluble P, Al-P and Fe-P could be due to the short period of liming effect of biochar. Benefits of reduced P fixation does not last longer in acidic soil thus decreases with extensiveness of time (Huck et al.,2014; Cornelissen et al., 2018). Alkaline biochar can increase soluble P and inorganic aluminium, iron and calcium bound P in the short-run however, in the long run, its effect faded due to the leaching of the biochar-associated alkalinity (Chintala et al., 2013; Huck et al,. 2014; Dume et al., 2017;Cornelissen et al., 2018). Also certain amounts of these P pools served for crop use.
Co-application of 15 t ha-1 alkaline biochar with 64 mg kg-1 P fertilizer on acidic soil increased soluble P, Al-P and Ca-P by 7.7%, 76.5% and 58.7% respectively while reducing Fe-P content by 3.3%. A concurrent increase in soil pH was also marked with this soil amendment. An increase in soil pH, soluble P, inorganic aluminum and calcium bound P occurred because soil amended with chemical fertilizer and organic materials can ameliorate P fixation in acid soils to improve crop production because organic amendment has a larger residual effect than chemical fertilizers only (Huck et al., 2016). The amount of soluble P contained in the biochar also may have the effect in increasing available P in the soil (Zhang et al., 2016). In light of that, it can also delay P adsorption by fixing Al3+, Fe3+ and Ca2+ and had the potential to keep the inorganic phosphorus in a bioavailable labile P pool for a longer period compared with application of P fertilizer without organic amendments (Zhu et al., 2018).
-1911356269355Figure 4.3 Distribution of P fractions in Wafi soil at day 90.
Figure 4.3 Distribution of P fractions in Wafi soil at day 90.
-138430142113000Several researchers co-applied different level of biochar with varied level phosphate fertilizer on acidic soils and discovered that the combination showed a positive synergistic effect in reducing phosphate fixation concurrently enhancing P availability (Abolfazli et al., 2012; Huck et al., 2014). This is because when biochar is applied with fertilizers, it works as a stimulator and increases the efficiency of mineral fertilizer application (Blackwell et al., 2010).
After 90-days cropping period, soluble P and Ca-P decreased remarkably by 4.63 mg kg-1 and 0.19 mg kg-1 respectively while Al-P and Fe-P increased by 1.57 mg kg-1 and 1.00 mg kg-1. The reduction in soluble P was attributed to the fading effect of liming and residual effect of the biochar enabling adsorption of more P on aluminol and ferrol soil colloidal surfaces.
4.3.3 Effect of Biochar, P Fertilizer and Biochar Co-application with P Fertilizer on Crop Yield in Wafi Soil838206089650Figure 4.4 Crop yield in Wafi soil under different treatments.
Figure 4.4 Crop yield in Wafi soil under different treatments.
0130619500Results presented in Figure 4.4 indicated that amount of orthophosphate taken up by the crop in Wafi soil without any amendment was 1.02 mg kg-1. Addition of 64 mg kg-1 P fertilizers increased plant P by 3.8% and thus yield by 80.6%.
The yield increased because of the increased soluble P in the soil solution (11.8%) and the ability of the crop to take 1.6% of it thereby improving its agronomic efficiency by 2.3%.
-793757213600Figure 4.5 Quantity of plant total P in Wafi soil under different treatment.
Figure 4.5 Quantity of plant total P in Wafi soil under different treatment.
-74930244030500The significant effect of P fertilizer on crop yield (Figure 4.4) occurred as addition of P fertilizer had the potential to elevate soil pH and soluble P because of having additives in its formulation (Nascimento et al., 2018). Its effect, however, may not last for longer cropping period without organic amendment or other forms of P activators because organic amendment has a larger residual effect than chemical fertilizers (Huck et al., 2016; Zhu et al., 2018). This may also be due to direct reaction of P fertilizer with the soil ions or mobilized soil metal ions resulting in enhanced mobilization of soil metal ions, precipitation and formation of amorphous Fe-P and Al-P (Shen et al., 2011).
Disclosure from the study also indicated that the crop was unable to fully use the applied P fertilizer to produce economic yield as there is low availability of P due to slow diffusion and high fixation (Shen et al., 2011). Inability of the crop to fully use P may also occur because of root-induced acidification, and or reduction in growth of primary roots due to P deficiency causing inefficient use of P fertilizer (Marschner, 1995; López-Bucio et al., 2003; Desnos, 2008). All in one, application of P205 from 60-90 kg ha–1 P fertilizer can still achieve favorable yield with mung bean (Khan et al., 1999).
Besides that, the slight increase (78.9%) in pod yield with addition of 15 t ha-1 biochar (Figure 4.4) occurred as a result of increased soluble P in soil solution and total plant P by 2% and 17.4% respectively. The amount of soluble P was 6.7% lesser than amount of soluble P with application of 64 mg kg-1 P fertilizer, however, it was able to supply more total plant P than the P fertilizer because biochar had the ability to alter the chemical characteristics of the soil by increasing the soil cation exchange capacity, pH; aluminum and iron bound inorganic P (Yuan and Xu, 2011; Chintala et al., 2013; Huck et al,. 2014). Increasing the aforestated reduced exchangeable Al, Fe and Ca thereby increasing soluble P for crop uptake (Huck et al., 2014; Melese et al., 2015). The favorable yield increase may also be attributed to the potential of biochar to delay P adsorption by fixing Al3+, Fe3+ and Ca2+ instead of P, keep the inorganic phosphorus in a bioavailable labile phosphorus pool for a longer period and alleviate nutrient stress and increase plant growth (Zhu et al., 2018; Panditb et al., 2018). Likewise, the amount of soluble P contained in the biochar may also have the effect in increasing available P in the soil thus crop yield (Zhang et al., 2016). All in all, sole application of biochar has shown to enhance P availability for crop uptake however it was unable to improve the efficiency of the crop to convert the plant available P into economic yield.
Co-application of 15 t ha-1 biochar with 64 mg kg-1 P fertilizer in Wafi soil has increased crop yield significantly by 84.7% with a concurrent increase in plant P (15.9%). The crop was also able to use 1.7% of the 64 mg kg-1 P fertilizer applied and consequently improved its agronomic efficiency in terms of yield productivity by 3.2%. The yield output was much higher than the PUE and agronomic efficiency of the crop in comparison to the yield attributes recorded with sole addition of 64 mg kg-1 P fertilizer.
The significant effect of the soil amendment on the pod of mung was due to the combining effect of biochar and P fertilizer on the soil properties. The significant increase in soluble P (7.7%) occurred as a result of a significant increase soil pH (9.4%) and insignificant elevation of Al-P and Fe-P by 76% and 3.4% respectively. This has occurred because the co-application of 15 t ha-1 with 64 mg kg-1 P fertilizer had the ability to overcome nutrient stress, slow diffusion of available P, P fixation, root induced acidification, root growth reduction and other problems associated with the bioavailability and acquisition of P based on the chemical and biological processes in the rhizosphere (Marschner, 1995; López-Bucio et al., 2003; Desnos, 2008; Shen et al., 2011).