## 4 Discussion
### Adsorption Isotherms of Nitrogen, Phosphorus, and Potassium  
The fact that N adsorption followed only Freundlich isotherm (Tables 11 and 12) suggests that N adsorption in all of the treatments occurred on the heterogeneous adsorbent surfaces of the samples [10]. Moreover, 1/n \(>\) 1 in all of the samples suggests that the N adsorption is an unfavorable adsorption reaction [25]. In terms of P adsorption isotherms, soil alone showed the highest bonding energy  
Figure 2: Linear regression between the added mmol H\({}^{+}\) kg\({}^{-1}\) sample and the pH of suspension show coefficients of determination (R\({}^{2}\)) \(>\) 0.73 and ** significant at P = 0.01.  
constant (K\({}_{\rm L}\)) (Table 16) because P fixation by exchangeable Al\({}^{3+}\) and Fe\({}^{2+}\). This reaction transforms for example, orthophosphates or soluble P into insoluble P [32]. Moreover, highly weathered tropical soils tend adsorb P ions because of their anion exchange capacity [33]. On the other hand, B1, B2, and B3 showed lower K\({}_{\rm L}\) compared with soil alone because of the high CEC of the CLB (Table 2). This suggests that the negative charges of the CLB might have repelled phosphate ions [34].  
Maximum adsorption capacity (q\({}_{\rm m}\)) is the maximum mass adsorbed at saturation conditions per mass unit of adsorbent in monolayer manners where, high q\({}_{\rm m}\) requires less P saturation maximum mass adsorbed at saturation conditions per mass unit of adsorbent [35]. Thus, the higher maximum adsorption capacities (q\({}_{\rm m}\)) of B1, B2, and B3 compared with soil alone suggest that B1, B2, and B3 required less P to saturate the adsorbent because of the highly negative charged exchange sites of the chicken litter biochar repelled P from being adsorbed. Furthermore, the relatively higher P of the chicken litter biochar (Table 2) reduced P requirement from other sources to saturate the adsorbent. Langmuir P adsorption isotherm on soil alone, B1, B2, and B3 indicates that P was adsorbed by formation of a monolayer on the outermost surface of the adsorbent [12; 13]. Phosphorus adsorption was consistent with Freundlich isotherm because of lower \(\chi^{2}\) value compared with Temkin isotherm. This finding suggests that P adsorption occurred on the heterogeneous adsorbent surface of CLB and this is inconsistent with Langmuir monolayer formation assumption [10]. Maximum buffering capacity (MBC) is defined as the ability of an adsorbent to replenish ions in for example, soil solution [35]. This explains why soil alone showed the highest release P into the soil solution compared with B1, B2, and B3.  
In terms of K, the lower Langmuir bonding energy constants (K\({}_{\rm L}\)) of B1, B2, and B3 (Table 17) was due to the initial K content of the chicken litter biochar (Table 2) which might have saturated the adsorption sites with K. The higher maximum adsorption capacity (q\({}_{\rm m}\)) of B3 compared with CLB suggests that the soil with B3 required less K to saturate the adsorbent [35] because of lower exchange sites to adsorb K (Table 19). The soils with B3 showed higher maximum buffering capacity (MBC) compared with CLB because soils with low CEC can easily release K into soil solution (Table 19). According to [PERSON] and [PERSON] [25], 1/n \(>\) 1 of B1 and B2 indicate unfavorable K adsorption.