Utilization of Phosphate Rock from Lisina for Direct Application: Release of Plant Nutrients in the Exchange-Fertilizer Mixtures Marija Mihajlovic ́ ,* ,† Nebojs ̌ a Peris ̌ ic ́ , ‡ Lato Pezo, § Mirjana Stojanovic ́ , † Jelena Milojkovic ́ , † Zorica Lopic ̌ ic ́ , † and Marija Petrovic ́ † † Institute for Technology of Nuclear and Other Mineral Raw Materials, 86 Franchet d’Esperey St., 11000 Belgrade, Serbia ‡ Weifa AS, Stuttlidalen 4, Fikkjebakke, 3766 Sannidal, P.O. Box 98, NO-3791 Kragerø, Norway § Institute of General and Physical Chemistry, University of Belgrade, Studentski Trg 12-16, 11000 Belgrade, Serbia ABSTRACT: This study examined the possibility of direct application of phosphate rock (PR) from Lisina deposit, in a mixture with natural clinoptilolite (Cp) and clinoptilolite partially saturated with ammonium ions (NH 4 -Cp). Solution P concentrations of the NH 4 -Cp/PR mixtures were sufficient for plant growth and ranged from 0.36 to 0.82 mg L -1 . The median NH 4 -Cp/PR ratio and the longest proposed mixing time had the most positive impact on the P concentration. Solution Ca concentrations of the NH 4 -Cp/PR mixtures were between 112.5 and 700.5 mg L -1 , indicating that use of the proposed NH 4 -Cp/PR mixtures solves the potential lack of Ca 2+ in the solution, which is typical for substrates of similar composition. Selected artificial neural networks (ANNs) were able to predict experimental variables for a broad range of the process parameters all through assay. Manifold effects of small changes in composition of the mixtures and time on the observed concentrations of nutrients were shown using the sensitivity analysis. KEYWORDS: exchange fertilizer, phosphate rock, Lisina, saturated clinoptilolite, multivariate analysis ■ INTRODUCTION PR is a primal raw material for producing water-soluble phosphorus (WSP), but it can be applied directly only under certain conditions. Direct application of PR (DAPR) has been suggested as an economical alternative to expensive commercial WSP. 1,2 There is also noticeable attention worldwide drawn to the use of natural PR as an eco-friendly fertilizer for organic food production. 3 At the Lisina location, near Bosilegrad in Eastern Serbia, an amount of ∼95 million tons of reactive PR mineral resource, with an average content of 9% P 2 O 5 , has been confirmed. This deposit has been explored for decades. 4 Taking into account that the nearest phosphate ore deposits are located in northern Finland and the North African belt, Lisina is the only major PR deposit in continental Europe of great strategic significance. 5 The agronomic performance of PR correlates well with PR solubility, which is usually estimated with neutral ammonium citrate (NAC), 2% citric acid (CA), or 2% formic acid (FA). 2,6 However, it should be emphasized that highly soluble sedimentary phosphates, which have demonstrated their DAPR efficiency, represent only 1% of the total world’s PR deposits. The remaining 99% are low-soluble igneous and metamorphosed PR (∼10%) and PR of medium and medium/ high solubility (∼89%). 7 The medium and medium/high solubility PR, such as apatite from Lisina, may be suitable for direct application only under specific circumstances. Fertilizer mixtures that combine an ion-exchange agent with PR have the ability to increase the solubility of PR. 8-10 Because of their high cation exchange capacity (CEC), water-retention properties, and rigid crystal structure, zeolites are valuable, widely used, multipurpose soil amendments. 11 In zeoponics, natural zeolites have their ion-exchange sites loaded with selected nutrients such as NH 4 + or K + . 11-13 Charged zeolites interact in the plant rhizosphere, providing gradual release of nutrients through a combination of chemical dissolution and ion-exchange reaction. Cp is the most frequently used zeolite in agricultural practice because of its high absorption level, 14-16 high selectivity for NH 4 + , 17 and some heavy metals and radionuclides. 18,19 Cp saturated with NH 4 + has been reported to increase the solubility of PR and it serves as (i) a sink for dissolving Ca from PR and (ii) a source of NH 4 + . 8,20,21 Batch experiments with Cp-PR blends have been carried out previously, 8,22 and even though the Cp-PR batch equilibration experiments represent static conditions, collected data showed that interactions between the two minerals affect the ion-exchange processes and the dissolution of the PR. It has also been reported that fertilization systems composed of NH 4 -Cp and PR have a potential to supply plant-available N, P, K, and Ca. 22 Barbarick et al. 8 hypothesized that the increase in the zeolite/ PR ratio would lead to an increase in readily available P through the ion-exchange mechanism specified by eq 1, by shifting the equilibrium to the right. − + + ⇄ − + + + + − + − 5(NH ) Cp Ca (PO ) OH 3H O 5Ca Cp 3HPO 5(NH ) 4OH y y 4 5 43 2 4 2 4 (1) Previous studies 8,20-22 prompted the present investigation from the perspective of utilizing a domestic PR mineral source Received: June 10, 2014 Revised: September 17, 2014 Accepted: September 17, 2014 Published: September 17, 2014 Article pubs.acs.org/JAFC © 2014 American Chemical Society 9965 dx.doi.org/10.1021/jf502704j | J. Agric. Food Chem. 2014, 62, 9965-9973 Downloaded via NATL LBRY OF SERBIA on November 28, 2018 at 11:48:07 (UTC). 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