Employment of zeolitites in open field experiments: characterization and monitoring of the cation exchange mechanisms in relation to the crop cycle progress

The project “ZEOLIFE - Water pollution reduction and water saving using a natural zeolitite cycle” (LIFE+10 ENV/IT/000321) has been conceived to test an innovative integrated zeolitite application having the aim to experience the employment of natural and ammonium exchanged zeolitite (NH4-zeolitite hereafter) in a two-years open field test (www.zeolife.it for more details). This summary describes some of the results relating to the characterization of zeolitite samples collected in the experimental field during different stages of the crop cycle (i.e., after the application of synthetic fertilizers but before the development of the plants, and at end of cultivation cycle after harvest). In fact, during the first crop year, the experimental field was subdivided in six plots of about one hectare each: one plot was amended with 7Kg/m2 of NH4- zeolitite prepared by the prototype, two plots with 5 and 15 Kg/m2 of natural zeolitite, and three plots were not amended and used as control. The NH4-zeolitite plot was amended with 50% chemical fertilizers comparing to the quantity used in the control plot, while 5 and 15Kg/m2 natural zeolitite plots with 70% chemical fertilizer. All plots were sown with grain sorghum. Fertilization was done with urea and diammonium phosphate using 5.561 Kmoli/ha and 0.1945 Kmoli/ha (relative to the control plot), respectively, according to the production disciplinary. Since the conversion of urea to ammonium occurs according to the following reactions: (NH2)2CO+H2O→NH3(gas)+H2NCOOH <-> 2NH3(gas)+H2O→2NH4(+)+2OH(-) it follows that for each mole of urea can be formed, as a maximum, two moles of NH4 (diammonium phosphate has the same conversion ratio). Therefore, in the control plots may be formed, as a maximum, 11.557 Kmoli/ha of NH4 (5.578, 8.090 Kmoli/ha in the plots in which the reduction of synthetic fertilizers compared to control were 50 and 30%, respectively). The laboratory tests indicated that the maximum operational C.E.C. of the selected zeolitite for ammonium is 0.389 moles/Kg; it is likely to assume an “in-situ (on field) C.E.C.” not higher 50% of the operational C.E.C. It follows that in the two plots amended with natural zeolitite, all the ammonium that may be formed from synthetic fertilizers can potentially be absorbed by zeolitite itself. On the other hand, this should not occur in the plot amended with NH4- zeolitite that should be completely ammonium saturated. Experimental measurements (chemical analysis of major elements, exchangeable ammonium and total nitrogen) on soil-zeolitite samples collected in the experimental field, show that there is a net change in the chemistry of zeolitite after the cultivation cycle. In particular, at the end of the first one: 1) in NH4-zeolitite there is almost no residue of exchangeable ammonium (0.248mmol/Kg, equivalent to 17.3 moles/ha referred to the parcel amended with 7Kg/m2); 2) in the natural zeolitite (i.e., parcels amended with 5 and 15Kg/m2 of natural zeolitite) remains a modest amount of exchangeable ammonium (0.638 and 0.428 mmol/kg, equal to 31.9 and 64.1 moles/ha); 3) in the zeolitites of all plots the content of Ca and Mg increases, whereas Na and K decreases, both with respect to NH4- and natural zeolitite at the beginning of the experiment.