## 1 Introduction  
Biochar is a carbon-rich material which is produced through thermal decomposition (pyrolysis) of organic biomass under limited oxygen or no oxygen at relatively low temperature (\(<\)700 \({}^{\circ}\)C) [1]. Biochars are recalcitrant because of their aromatic and crystalline graphing sheet and this explains why they decompose slower than organic matter [2, 3]. Biochars can improve soil and crop productivity because they reside in soils from a few years to thousands of years. As they reside in soils, they are gradually degraded or lost through biotic degradation, abiotic degradation, non-oxidative abiotic degradation, leaching, erosion, and fires. Pyrolytic temperatures produce different honey-comb-like structures because of the original tubular structure of plant cells [4, 5]. Moreover, increasing pyrolytic temperature decreases O-H, C=C, C=O stretching, aromatic C=C, C-H deformation modes of alkenes, C-O-C symmetric stretching of cellulose, and hemicellulose [1]. A conceptual model structure of biochar is presented in Figure 1. The functional groups of biochars are reputed to improve soil structure, soil organic matter, cation exchange capacity (CEC), soil pH, and buffering capacity. Biochars adsorb nutrients through electrostatic interaction, ionic exchange, chemical precipitation, and complexation [1]. The pores of biochars do not only serve as habitat for microorganisms but they also stimulate microbial growth because have significant amount of nutrients for microbes to feed on. Their pores also protect microorganisms from their predators and desiccation [3]. It has been reported that biochars can increase N\({}_{2}\) fixation in soils [6]. Additionally, biochars are noted for improving soil microbial respiration by creating spaces for soil microbes [7]. This increases soil microbial biodiversity and density [7]. Increasing soil fungi improves soil aggregation through increased fungal hyphae. Moreover, microorganisms which survive in biochars are able to produce polysaccharides and these polysaccharides have been associated with good soil aggregation [2]. Although fertilizers play an important role in crop growth and yield production, their usefulness partly depends on how their nutrients are adsorbed and desorbed. Adsorption is the process by which nutrients in their ionic forms, especially the positive charged ions such as K\({}^{+}\), Ca\({}^{++}\), Mg\({}^{++}\), NH\({}_{4}\)\({}^{+}\) and so on get attached to the negative charged surfaces of minerals such as clinoptilolite zeolite and organic amendments [8]. Functional groups such as carboxyl, phenolic, and alcoholic of organic amendments use to adsorb the aforementioned ions followed by their gradual release (desorption) for timely use by plant. Desorption occurs when adsorbed nutrients are released from the surface of soil organic matter and inorganic minerals [9]. Studies on NH\({}_{4}\)\({}^{+}\) adsorption are essential to enable determination of the ability of soils to respond to N fertilization. Adsorption of NH\({}_{4}\)\({}^{+}\) in soils is important for crop uptake besides minimizing groundwater contamination (for example, eutrophication). Adsorbed nutrients are the most important nutrients because they are available for plant uptake compared with absorbed nutrients. Adsorption of nutrients are significantly controlled by soil pH, temperature, flooding, redox potential, and pH buffering capacity [10]. There is limited information on the effects of soil organic amendments on ammonium, phosphorus, and potassium ions adsorption and desorption [10; 11; 12; 13]. The most common isotherms used in adsorption studies are Langmuir, Freundlich, and Temkin. These isotherms were used in this present study. Langmuir isotherm indicates quantitative formation of a monolayer nutrient (adsorbate) on the outmost surface of the adsorbent and no further adsorption. Therefore, Langmuir is only valid for a monolayer surface adsorption mechanism with fixed number of adsorption sites [14; 15].