## 4 Discussion
### Dry Weight, Fresh Cob Weight, and Nitrogen Use Efficiency  
The framework of the CZ which comprises tiny pores enables ammonium to be held in the cation exchange sites of this mineral [40]. It thus works in a similar manner to slow-release fertilizers where ammonium ions are released slowly to be taken up by the maize. This is further supported by the findings of [PERSON] et al. [27] who also reported that the combined application of urea and CZ led to an increase in crop dry weight. This was attributed to CZ serving as a control release fertilizer. The rice straw and paddy husk composts also contributed to the increase in the dry weight of the maize shoots. As reported by [41], the mineralization and timely availability of N from compost application during the cropping season can contribute to an increase in dry matter production of cereals. The results of [PERSON] et al. [42] showed that co-application of CZ and N fertilizer minimized N leaching but increased N utilization of crops grown on sandy soil.  
The findings in a previous study support the ability of clinoptilolite zeolite to adsorb ammonium [12]. The CEC and higher adsorption by CZ caused significant sorption of ammonium released from the urea, rice straw compost, and paddy husk compost. This was due to CZ's properties namely, aluminosilicate framework, high cation exchange capacity, ion adsorption, and preferential selectivity for ammonium [43]. In addition, the high organic matter in the plots with rice straw compost or paddy husk compost resulted in higher plant N content and uptake for T3-T6 compared with the recommended chemical fertilizer application (T1). This was possibly because organic N in the organic matter of the rice straw and paddy husk composts were converted to inorganic forms (ammonium and nitrate), through mineralization, for maize uptake. [PERSON]'s [44] findings showed that the co-application of composts and inorganic fertilizers led to an 8% increase in wheat yield than was achieved with the use of inorganic fertilizers alone. [PERSON] et al. [45] also reported that steady and slow mineralization of N in compost amended soils resulted in sustained N supply to crops. The lower plant N content and uptake for T1 are related to poor retention of ammonium and nitrate and this was the reverse for T2-T6 (Figure 1). The combined use of compost and inorganic fertilizers resulted in a build-up of N in the soil with improved NUE [46]. [PERSON] and [PERSON] [47] demonstrated that CZ could be used to temporarily fix ammonium but it can also be used to gradually release NH4\({}^{+}\) similar to the way in which slow-release fertilizers release N. The retention of N in soil and the associated enhancement of N uptake in the plants (Figure 1, Figure 8, and Figure 9) explain why the ear weight was significantly higher (Figure 11) for the different treatment combinations consisting of chemical fertilizers, CZ, rice straw compost or paddy husk compost as compared with the recommended chemical fertilizer application alone. The results of [PERSON] et al. [48] showed significantly higher NUE for the combined application of inorganic and organic fertilizers than with the application of inorganic fertilizers alone.