_Article_  
Effects of Amending Phosphatic Fertilizers with  
Clinoptilolite Zeolite on Phosphorus Availability and  
Its Fractionation in an Acid Soil  
**[PERSON] 1,2,*(r)**, [PERSON] 2,3,4 and [PERSON] 4  
1 Faculty of Sustainable Agriculture, University Malaysia Sabah, Locked Bag No 3, 90509 Sandakan, Sabah, Malaysia  
2 Department of Crop Science, Faculty of Agriculture and Food Sciences, Universiti Putra Malaysia, Bintulu Sarawak Campus, 97008 Bintulu, Sarawak, Malaysia; [EMAIL_ADDRESS]  
3 Institute of Tropical Agriculture and Food Security (ITAFoS), Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia  
4 Institute of Tropical Forestry and Forest Product (INTROP), Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia; [EMAIL_ADDRESS]  
* Correspondence: [EMAIL_ADDRESS]  
Received: 22 March 2020; Accepted: 16 April 2020; Published: 1 May 2020  
**Abstract:** Soils of the tropics are highly weathered, acidic, and low in phosphorus (P) because of high contents of Al and Fe. Satisfactory P supply is essential to ensure optimum soil and crop productivity. Thus, there is a need for amending soils with zeolite to improve availability of P in acid soils as this mineral can fix Fe and Al instead of P. This study was undertaken to determine the transformations of P fertilizers in acid soils following application of Clinoptilolite zeolite (CZ) in laboratory (incubation) and pot trials. An acid soil was incubated with a recommended fertilization rate and a reduced amount of the existing recommended fertilization by 25% but substituting this reduction with an equivalent amount of CZ. Triple superphosphate (TSP), Egypt Rock phosphate (ERP), and Christmas Island Rock phosphate (CIRP) were used as P sources. Selected soil chemical properties, inorganic P fractions, available P, and total P of the native soil were determined before and after the laboratory and pot trials. _Zea mays_ L. (test crop) plant dry matter production, P concentration, P uptake, and P use efficiency were also determined using standard procedures. Effects of the treatments with CZ compared to the recommended fertilization on P fixation were similar. In the laboratory study, the treatments with TSP showed lower dominance of Fe-P but more pronounced in Al-P, whereas for the RPs, Ca-P was dominant. In the pot study, Al-P, Ca-P, and Fe-P were rather pronounced in the treatments with TSP, ERP, and CIRP, respectively. There was a decrease in exchangeable Al and soil titratable acidity because of the ability of the CZ to increase soil pH. Although the availability of P was not significant with the inclusion of CZ in the incubation study, dry matter production, P concentration, P uptake, and P use efficiency in the pot trial were comparable with that of the existing/recommended fertilization, suggesting that the CZ is beneficial and could be used to reduce the P fertilizer requirement for _Zea mays_ L. cultivation on acid soils. Regardless of type of P fertilizer, prevalence of the moderately labile P fractions (Al-P, Fe-P, and Ca-P) of the incubation and pot studies acted as slow-release P sources to contribute to long-term P release. Further studies on the potential of CZ to reduce fertilization and its effects on soil and crop productivity are essential. It is also important to determine the economic benefits of including CZ in _Zea mays_ L. cultivation.  
**Keywords:** phosphorus speciation; tropical acid soils; phosphorus fractions; clinoptilolite zeolite; phosphorus fixation