Application of natural zeolites and biological preparations for increasing soil fertility

A characteristic feature of the 21st century has been rapid population growth, urbanization, intensive mining, and active anthropogenic human activity. All this testifies to a narrowing of living space, including the land fund, which creates the need for an urgent solution to the food problem.

It should be noted that attempts to increase crop yield by applying increased application rates of mineral fertilizers proved unjustified and led to such negative consequences as soil degradation, deterioration of product quality, and environmental pollution.

In this situation, the application of zeolites to the soil, as well as the use of bacterial preparations and plant growth regulators, become important.

Advantages and properties of natural zeolites

Zeolites improve soil properties by performing the following functions:

• adsorb ammonium and potassium ions;
• retain soil moisture;
• prevent plant root diseases;
• serve as a source of nutrients.

Zeolites belong to the group of alkaline and alkaline-earth aluminosilicates, predominantly of calcium and sodium, and less often of potassium and barium. There are about 40 species of them, among which the following are distinguished:

• amicite, barrerite, bikitaite, brewsterite, harmotome, garronite, heulandite, gmelinite;
• gonnardite, dachiardite, phillipsite, cowlesite, clinoptilolite, laumontite, levyne, mazzite;
• mesolite, merlinoite, mordenite, natrolite, offretite, paulingite, silicalite, scolecite;
• stellerite, stilbite, thomsonite, ferrierite, phillipsite, faujasite, chabazite, edingtonite;
• epistilbite, erionite, yugawaralite.

Geological origin and occurrence conditions

Zeolites are found in sedimentary, volcanic, and metamorphic rocks in many regions of the globe. They are formed at moderate temperatures and pressures and have a limited stability field, which is why they are rarely found in rocks older than the Mesozoic.

Zeolites are most widespread in sedimentary rocks (mordenite, clinoptilolite, phillipsite, chabazite, erionite), where their extent and thickness can reach several kilometers. Zeolites are found in many lithic and feldspathic sandstones in association with clastic rocks of Paleozoic and Miocene age.

Zeolites with well-developed crystalline aggregates are found in the cavities and voids of igneous volcanic rocks, mainly in basalts. Apparently, they are the result of late deposition from fluids that penetrated the basalts after their crystallization.

Metamorphic zeolites are formed mainly from highly reactive volcanic glasses, ferromagnetic minerals, and diagenetic zeolites that may be present in primary sedimentary rocks. Zeolites are also detected in marine muds, which indicates their formation in an aquatic environment in the presence of appropriate conditions for synthesis from solutions. This is not the only way zeolites originate, and perhaps not the main one.

Heulandite (clinoptilolite) is also found in muds, but it is of terrigenous* origin and is washed from the land surface into water bodies. In this case, it is concentrated in the finest fraction (<2 μm) in the muds. Zeolites have also been discovered in freshwater and salt lakes and lagoons. The formation of zeolites as a result of pore solution interaction is also possible.

Currently, about 1,000 deposits of various zeolite varieties are known in more than 40 countries of the world. Volcanogenic-sedimentary zeolite deposits of industrial importance were first developed in the USA and Japan in the mid-50s. Rich deposits of these minerals have been discovered in Africa, France, Bulgaria, Hungary, and other countries. In the territory of the Russian Federation and neighboring countries, potential zeolite resources amount to several billion tons; proven reserves total more than 1 billion 804 million tons. They are mainly located in Transcarpathia (Sokirnitsy, Vodichi, Lipcha), the Kemerovo region (Pegas river),

Terrigenous – (from Lat. terra 'earth' and Gr. genos 'born') – clastic components of sedimentary rocks

the Primorsky and Khabarovsk territories (Chuguevskoye, Seredochnoye, and Ugolnoye deposits), on Sakhalin (Lyutogskoye), Kamchatka (Yagodninskoye), Tuva, Yakutia, and Buryatia. In the North Caucasus, zeolite-bearing rocks, which are raw materials for multi-purpose use, are also widespread. The most studied, promising, and accessible are the Paleogene zeolite-bearing formations, which can be traced from west to east (over 300 km) along the northern foothills of the Caucasus. Large deposits of zeolites are found near the city of Khadyzhensk and in the Krymsk district of the Krasnodar Territory.

An idea of the chemical composition of natural zeolites is provided by the data in Table 156 (Yakovlev E.N., Nagorokava L.M., Zavoyskaya O.A. et al., 1990).

Table 156 – Content of major oxides in zeolites, % Type SiO2 Al2О3 Fe2O3 CaO K2O Na2O of zeolite ores

 Clinoptilolite 68.8 12.0 2.80 2.10 0.95 1.59 3.4 Mordenite 71.8 12.7 1.47 1.43 0.34 1.88 5.8 Heulandite 70.1 10.6 1.70 3.30 0.80 4.40 6.0

The main criterion used as a basis for the classification of zeolites is structural characteristics related to crystal morphology. According to this classification, zeolites are divided into three main groups: 1) three-dimensional framework structures such as phillipsite and chabazite; 2) fibrous structures formed by chains of tetrahedra weakly linked to each other in the longitudinal direction, such as natrolite; 3) lamellar structures formed by tetrahedra strongly linked in one plane and weakly in the perpendicular direction, such as heulandite and clinoptilolite. There is another classification of zeolites based on framework topology. In this case, each framework is depicted as separate structural units, for example, rings of tetrahedra. Further, these constructions are grouped to form easily identifiable polyhedra or structure "blocks". This classification is of interest in terms of the industrial use of zeolites.

The term "zeolites" has been known in scientific literature since the mid-19th century. For instance, J. Way wrote in 1850 that the absorption of cations by the soil is carried out by humus and zeolites. By the latter, the author understood a component of the soil soluble in hydrochloric acid. He explained the nature of absorption through a chemical reaction between the solid phase of the soil and the salt solution. Somewhat later, Van Bemmelen (1876) explained cation absorption by the presence in the soil of semi-decomposed organic remains, humus, iron hydroxides, colloidal silicic acid, and amorphous zeolite-like silicates. In turn, K.K. Gedroits (1935) believed that the main carriers of the absorption capacity of soils are organic, organomineral, and mineral colloids. He called the latter the "zeolitic part" of the soil. From the above, it is not entirely clear what was originally meant by the term "zeolites": individual minerals or a part of the inorganic components of soils. Clarity was first brought to this issue by K.D. Glinka (1906), who called zeolites individual minerals of a specific chemical composition. By their chemical nature, they are crystalline aluminosilicates with a framework structure.

Zeolites consist of an aluminosilicate framework containing voids and channels in which alkali and alkaline-earth metal cations and water molecules are located. With careful heating, "zeolitic water" can be completely removed without disrupting the mineral structure. This is the fundamental difference between zeolites and minerals containing constitutional water, the release of which is possible only upon the destruction of the entire structure. When heated, zeolites swell. The ease with which water escapes from zeolites even with slight heating determined the name of this mineral: from Greek "boiling stone". Cations and water bound to the framework can be partially or completely replaced or removed during the process of ion exchange and dehydration. After the removal of water, the mineral becomes like a microporous "sponge" with a pore volume of up to 50%. This creates a high cation exchange capacity, which predetermines the special value of this mineral for ensuring long-term plant nutrition. In various zeolites, the cation exchange capacity reaches 1–5 mmol-eq/g of mass. A second very important property of zeolites is their selectivity toward cations. Ion exchange occurs with a pronounced selectivity for large cations such as NH4+, K+, Ca2+, Ba2+, and Mg2+. Their constant selectivity for Na+ is also noted. Pesticide residues, alcohols, hydrogen sulfide, and heavy metal ions are adsorbed on zeolites. The latter is especially important for the agricultural reclamation of lands along highways with heavy traffic, where the content of copper, zinc, and lead significantly exceeds the maximum permissible concentrations.

In general, after saturation with NH4+, zeolites can be considered as a long-acting complex fertilizer, including, in addition to nitrogen, K+, Ca2+, Mg2+, and many microelements of the zeolite itself. The deposition of the ammonium ion in the channels prevents its nitrification and reduces nitrogen losses from fertilizers and the soil. The role of zeolite as a prolongator of mineral and organic fertilizers is also great. By absorbing chemical elements introduced into the soil, it protects them from leaching and then gradually returns them to the soil solution.

The molecular-sieve and catalytic properties of zeolites determine their wide potential for increasing soil fertility and the productivity of greenhouse substrates, as well as for improving the environmental condition. The use of zeolite-containing substrates in greenhouse management is particularly effective. From an agronomic and production point of view, zeolite substrates are characterized by good physical properties, long service life, lack of weeds, sterility, and an aesthetically pleasing appearance. Greenhouse plants develop a powerful root system, and faster development and fruiting are observed. Moreover, such substrates can be used in areas unsuitable for farming. The application of zeolites in irrigated agriculture, in particular in rice cultivation, is very effective. The use of zeolites for rice allows nitrogen fertilizer to be applied in a single dose and its rate to be reduced by 25–30% without reducing the yield.

The effectiveness of zeolites is largely determined by the method and rate of application. In protected ground, zeolite substrates are prepared from zeolite and greenhouse soil in a ratio of 1:1 by volume. For rice, they are applied in an amount equal to the physical mass of the nitrogen fertilizer before sowing, simultaneously with mineral fertilizers (Table 157; Sheudzhen A.Kh., 2005).

Table 157 – Effectiveness of zeolite application for rice on meadow-chernozem soils of the Kuban Height Length Empty- Grain Mass of Yield of plants, of panicle, filled yield, 1000 grain, cm cm spikelets, % grains, g c/ha

 N120P80K60 75,7 13,5 16,0 30,0 48,2 N120P80K60+Ц50 % 76,9 14,0 15,9 30,2 49,9 N120P80K60+Ц100 % 80,4 14,6 14,8 31,0 53,9 N120P80K60+Ц150 % 74,8 13,3 16,6 29,9 50,1 N90P80K60+Ц100 % 79,9 14,3 15,5 31,1 53,2

Higher application rates of zeolites are used for rain-fed crops. In this case, they range from 0.5–5.0 to 20–30 t/ha depending on soil and climatic conditions and the biological characteristics of the crops.