Evaluation of the effect of zeolite particle size in the soil properties and nitrogen volatilization

Six zeolite fractions were evaluated for particle size: < 0.25 mm, 0.25 a 0.50 mm, 0.50 a 1.00 mm, 1.00 a 2.00 mm, 2.00 to 3.00 mm, and 3.00 to 5.00 mm in the properties of an inceptisol and in the volatilization of ammoniacal nitrogen. Fifteen per cent of each size fraction was applied to the soil, which was sieved (2 mm sieve). The dose of zeolite was determined based on previous incubation evaluations without zeolite and with doses of 7.5, 15, and 30 %. The recommended dose of 120 kg hm-2 of urea was also used, which contained 46 % N.

The pH was evaluated using a Sartorius pH-meter (PP20); the content of exchangeable bases and the cationic exchange capacity (CEC) were evaluated using an atomic absorption spectrophotometer (GBC 902, Australia), and the Effective Cationic Exchange Capacity (ECEC) was determined as the sum of exchangeable bases in the soil, measured 45 d after planting. The loss of N by volatilization was determined using the methodology described by ^{Lara et al. (1997)}; a semiopen-static collector was utilized, with two polyurethane foam discs soaked in a H2SO4 0.5 N solution and glycerine 3 %. The lower piece of foam, located 20 cm from the surface, captures ammonium from the treatments, and the upper piece, at 35 cm, adsorbs the compound that enters from the atmosphere.

Evaluation of the dose of zeolite in the dry matter of Rhodes grass

Zeolite (0.00, 1.88, 4.74, 6.88, and 9.38 g kg-1 soil) was evaluated in allitic, grayish, brown with carbonate and and humic siliceous soils. Allitic soils are characterized by their ABC type profile, intense alteration of primary minerals, and exchangeable aluminum saturations of over 50 %. Brown soils are characterized by the presence of siliceous horizons, which present mineral clay-like compositions of a koalinite type. Humic siliceous soils present AC type profiles, they seldom present horizon B and are characterized by the predominance of silica accumulation processes that are reflected in the clay-like composition of horizon A and abrupt limits between horizons. The response variable was the dry mass per experimental unit, determined after keeping the tisues for 24 h in a drying oven (Memmert, UF 750 plus, Alemania) at 60 °C.

Evaluation of zeolite in the soil properties and dry matter of Rhodes grass

In grayish brown, brown with carbonates and humic siliceous soils, zeolite proportions of 4.73, 6.87, and 9.37 g kg^-1 were evaluated, according to the results of previous evaluations. From each type of soil we included one control without zeolite. After 45 d, pH was determined in potassium chloride (1:1), organic matter (Walkley - Black method), exchangeable bases content (quantification by absorption and atomic emission spectrophotometer), CEC and ECEC (extraction with ammonium acetate at pH 7.0 and sum of exchangeable bases, respectively). Dry matter was also determined.

Evaluation of the effect of zeolite in the nutrition of Rhodes grass in three soils

Six zeolite doses (0.00, 1.88, 4.74, 6.88, and 9.38 g kg^-1 soil) in brown soils (grayish, with and without carbonates) were evaluated for: foliar N, determined using the Kjeldahl technique and volumetric quantification; P, determined in a flame spectrophotometer (Jenway, PFP7, United Kingdom); foliar K, Ca and Mg contents, determined by wet digestion and absorption and atomic emission spectrophotometry.

Chemical analyses were performed according to the NRAG 879.88 standard (Cuban Soil Institute, 1996) of the Soil Institute, and the foliar analyses, according to the Ramal NRAG/ CNTN-O5 standard (Instituto de Suelos de Cuba, 2010).

The statistical analysis of the data consisted in the verification of its normal distribution using the Shapiro Wilk test, and the homogeneity of variances, with Levene’s test. Bifactorial analysis of variance and Tukey test were carried out to establish significant differences between treatments. IBM SPSS Statistics 20.0 was used for the analyses.

Effect of zeolite particle size on soil properties and on nitrogen volatilization

The pH range was between 4.35 and 5.38, with significant differences between treatments (p≤0.05) (Table 2). The pH of the treatment with the largest particle size did not present significant differences with the control, yet it was more acidic than in other treatments. A higher pH than 5.30 was observed in the different particle sizes, which reduced the acidity for plant growth. ^{Li et al. (2009)} report that the pH in soil increased with the zeolite doses. This increase in pH may be due to the increase in CEC by zeolite, which also increases the capacity of retention of exchangeable bases. Also, when reducing the particle size, the specific surface of the mineral increases, favoring ionic exchange, and the natural cations of zeolite, such as Ca²⁺ and Na⁺, can be passed on to the soil solution (^{La Iglesia, 1989}).

The exchangeable Ca contents were located in middle leves, where the largest fraction gave the lowest content of Ca²⁺, without significant differences with the control. The highest content was observed in particles with 0.5 to 1.00 mm, yet with no differences with the other sizes. The Ca levels increased due to the contribution of zeolite and its high affinity towards this element (^{Machiels et al., 2006}). The concentrations of Mg+2 were low and medium, according to ^{Fernández et al. (2006)}; the lowest concentrations were for the control and particle sizes higher than 0.25 mm, with the exception of the sizes between 0.50 and 1.00 mm and <0.25 mm, and with significant differences with the control. Zeolites also have an affinity to Mg, they increase their retention and availability in the soil (^{Obregón et al., 2016}).

The lowest exchangeable Na values came from particle sizes 1.00 to 3.00 mm, and sizes lower than 1.00 mm had the highest value, which affects the physical conditions of the soil; this element promotes the spreading of clays. Particles smaller than 1.00 mm withheld the highest amounts of exchangeable K, and an inverse proportion was observed between particle size and capacity of retention of exchangeable bases. The best results were provided by particle sizes 0.25 to 3.00 mm, with a clear influence in fractions sized 1.00 to 2.00 mm; the monovalent cations showed a tendency to increase with the lower particle sizes, although the values of K⁺ remained similar to the control, because zeolite provides less of this element than of Ca. Also, in the selective adsorption chain (lyotropic series) of the minerals, the one with bivalent cations is more favorable that the one with monovalent cations when their concentration is greater in the medium, due essentially to its greater ionic radius, which produces a change in selectivity (^{Machiels et al., 2006}). ECEC was of approximately 10 cmol(+) kg^-1, with no significant differences between particle sizes, and was higher (12 %) than in the control. The highest CEC was observed with particle sizes of less than 1.00 mm, and in greater particle sizes, it was similar to the control. This difference is due to the greater-sized particles having a long-term effect, since the ionic exchange process takes places slowly.

Applying zeolite with particles larger than 1.00 mm, significantly reduced losses by ammonial N volatilization into the atmosphere (Table 3). Zeolite exerts a greater effect on the soil in the presence of nitrogenated fertilizer when the particles in both materials are in close contact and have a certain similarity in particle size (^{Malekian et al., 2011}). The control presented the greatest loss of NH3 and the greater particle size in zeolite presented the lowest loss of N into the atmosphere. Applying zeolite with particle sizes between 3.00 and 5.00 mm reduced N loss into the atmosphere by up to 57 % in comparison with the control, probably because, as the specific surface increases, the cationic exchange sites in the structure increase, capable of retaining the NH₄ ion, which delays the biological transformations of the N and favors the assymilation by the plant (^{Obregón et al., 2016}). According to ^{Espécie et al. (2015)}, the use of Cuban zeolite (40 %) combined with urea, in the form of pellets, reduced the volatilization of ammonia from aqueous solutions by 33 %. It is possible that the zeolite also contributed to reducing the volatilization of other chemical forms of N, such as nitrous oxide (N2O), by delaying the nitrification and denitrification processes in the soil. This effect was observed by ^{Zaman and Nguyen (2010)}, when they reduced N2O emission by 11 % when applied with cow urine in fields with white clover.

Effect of the doses of zeolite in the dry matter of Rhodes grass

With the exception of the allitic soil, we observed that as zeolite doses became larger, so was the dry matter content in each pot (Table 4). These results are due to the significant increase of CEC by zeolite, which helps retain more nutrients and reduce acidity, favoring the availability andsolubility of essential elements such as P, which participates in the radical growth and benefits the absorption of water and nutrients (^{Li et al., 2009}).

The soil-zeolite-nitrogen dynamics are variable, depending on the physical and chemical characteristics of the soil, the dose applied, crop management, and the time of the year in which the experiments are carried out (^{Kolyagin and Karasev, 1999}). This confirms the convenience of periodic evaluations on the soils where they are to be used, since the constant mobility of N in the soil affects plants.

In allitic soil there were significant differences between treatments; a tendency to increase yields between 22 % and 26 % was observed in the presence of the mineral. In the grayish brown soils, the highest zeolite dose (9.38 g kg^-1) showed a tendency towards the higheest content of dry matter (12 % and 27 % with respect to the control), with no significant differences, with4.74 and 6.88 g kg-1 of zeolite. The highest zeolite doses in the brown soils with carbonates produced the most dry matter in comparison with the orher doses; in humic siliceous soil, the highest dose showed a significant increase of 22 % with regard to the control. All soil types increased the percentage of dry matter due to the effect of the zeolite, although humic siliceous soil gave the best results, due probably to the predominance of smectite clays, whcih delay internal drainage and could contribute, along with zeolite, to delaying the washing of bases (^{Fookes, 1997}).

Effect of zeolite in soil properties and Rhodes grass dry matter

The zeolite increased the pH, the exchangeable base contents, and CEC in all three soils (Table 5). These results are similar to those reported by ^{Florez Macías et al.(2007)}, who point out the increment of pH due to the exchange between NH₄ ⁺ and cations such as Ca²⁺ and Na⁺, with the soil solution and the release of OH^-. In the grayish brown soil, the Mg²⁺ increase was the greatest (61 %), and for Ca²⁺ and Na₄ ⁺, this was found in brown soils with carbonates (16 and 71 %). The humic siliceous soil presented the highest increase in K+ (30 %), due to its higher zeolite and smectite clay retention capacity, avoiding their lixiviation. Zeolite increases CEC, which improves the acidity conditions of the soil and promotes the availability of nutrients for plants. Na concentrations increased due to the zeolitic mineral used, and therefore periodic evaluations would be necessary when using zeolite, since Na is a dispersal agent, and harmful for soil structure. With the predominantly calcic zeolite, in an aqueous medium, NH₄ ⁺ replaces the Na₄ ⁺ located in the change complez and the content of available Na₄ ⁺ increases. Due to this, it would be convenient to carry out selective extractions in the deposits located above underground waters in order to know their content of Na₄ ⁺.

Effect of doses of zeolite in Rhodes nutrition in three soils

Zeolite increased with the foliar contents of N, P, K, and Ca (Table 6). As zeolite doses increased in the grisáceos soils, so did the levels of de N, K, and Ca; in contrast, Mg increased only with the highest doses; P increased in all treatments, possibly because zeolite reduces soil acidity, due to the equivalent adsorption of Ca2+ and Mg2+ by the H+ of the respective solution; this helps increase P availability in the soil (^{Obregón et al., 2016}). To this end, ^{Rodríguez et al., (2012)} point out that the zeolites loaded with NaH₄ ⁺ increased the solubilization of the P. Similar results were observed in brown soils, without carbonates, and humic siliceous soils, with the difference that in the former, zeolite increased Mg content, and in the latter, Ca levels remained similar and Mg levels fell in comparison to the control. These results are similar to those of ^{Soca et al. (2004)} and ^{Millán et al. (2008)}, who resport with significant increases in foliar contents and yields of plants when using zeolite.

Zeolite increased the foliar indices in comparison with the control, due to its effect in the development of the radicle and the presence of nutrients, since it reduced its loss by fixation, volatilization, and lixiviation (^{Obregón et al., 2016}). Zeolite is used to reduce the amount of N-NO3and residual NH₄ ⁺ in the soil soultion, the rate of nitrification (^{Florez-Macías et al., 2007}), or to increase the N content in plat tissue (^{Millán et al., 2008}).