Eurasian Journal of Soil Science

Abstract

Tomato (Solanum lycopersicum L.) is a widely cultivated vegetable crop in Kazakhstan, yet its productivity and quality are often limited by soil degradation, nutrient imbalances, and the excessive use of mineral fertilizers. Natural zeolites, particularly clinoptilolite, offer potential as soil amendments due to their high cation exchange capacity, water retention properties, and ability to regulate nutrient availability. This study aimed to evaluate the effects of natural zeolite (2 t/ha), alone and in combination with two mineral fertilizer doses (N₄₅P₄₅K₄₅ and N₉₀P₉₀K₉₀), on tomato yield, fruit quality, soil heavy metal content, and economic profitability under dark chestnut soil conditions in southeastern Kazakhstan. A field experiment was conducted using a randomized complete block design with six treatments and three replications. Results showed that all treatments increased yield compared to the control (21.7 t/ha), with the highest yield (29.1 t/ha) observed under the zeolite + N₉₀P₉₀K₉₀ treatment. Fruit quality improved in terms of dry matter (up to 5.46%) and sugar content (up to 3.80%) with zeolite and fertilizer combinations. Nitrate accumulation in fruits was highest under N₉₀P₉₀K₉₀ alone (78 mg/kg) but decreased significantly when combined with zeolite (64 mg/kg), indicating the mineral’s capacity to reduce nitrate uptake. Heavy metal analysis revealed that zeolite reduced the bioavailability of cadmium and lead in soil, keeping concentrations below permissible limits. Economic evaluation indicated that the zeolite-only treatment provided the highest profitability (171%) due to relatively low input costs and moderate yield gains. Overall, the results demonstrate that zeolite, especially when integrated with moderate fertilizer inputs, enhances tomato productivity, improves fruit safety, and supports sustainable soil management. Its use can be particularly beneficial in resource-limited and environmentally sensitive agricultural systems.

Keywords

Tomato, zeolite, clinoptilolite, mineral fertilizers, nitrate, heavy metals, yield, sustainable agriculture.

References

<p>Ahmed, S., Khan, M., Raza, T., Ahmad, R., Iqbal, J., Eash, N., 2022. Integrated use of bio-organic and chemical fertilizer to enhance yield and nutrients content of tomato. <em>Eurasian Journal of Soil Science</em> 11(2): 126-132.</p>

<p>Anas, M., Liao, F., Verma, K.K., Sarwar, M.A., Mahmood, A., Chen, Z.L., Li, Q., Zeng, X.P., Liu, Y., Li, Y.R., 2020. Fate of nitrogen in agriculture and environment: agronomic, eco-physiological and molecular approaches to improve nitrogen use efficiency. <em>Biological Research</em> 53: 47.</p>

<p>Ay, A., Demirkaya, S., Kızılkaya, R., G&uuml;lser, C., 2022. The effects of two Fe-EDDHA chelated fertilizers on dry matter production and Fe uptake of tomato seedlings and Fe forms of a calcareous soil. <em>Eurasian Journal of Soil Science</em> 11(3): 259-265.</p>

<p>Bertalan-Bal&aacute;zs, B., Rahimi, M., Adelinia, A., Ebrahimi, E., Ojani, M., 2024. Impact assessment of Zeolite, Ca-bentonite and Biochar amendments on Cd bioavailability and fractions in polluted calcareous soils. <em>Environmental Earth Sciences</em> 83: 506.</p>

<p>Bihon, W., Ognakossan, K.E., Tignegre, J.B., Hanson, P., Ndiaye, K., Srinivasan, R., 2022.&nbsp; Evaluation of different tomato (<em>Solanum lycopersicum</em>&nbsp;L.) entries and varieties for performance and adaptation in Mali, West Africa.&nbsp;<em>Horticulturae</em>&nbsp;8(7): 579.</p>

<p>Cataldo, E., Salvi, L., Paoli, F., Fucile, M., Masciandaro, G., Manzi, D., Masini, C.M., Mattii, G.B., 2021. Application of zeolites in agriculture and other potential uses: A review. <em>Agronomy</em> 11(8): 1547.</p>

<p>Choo, L.N.L.K., Ahmed, O.H., Talib, S.A.A., Ghani, M.Z.A., Sekot, S., 2020. Clinoptilolite zeolite on tropical peat soils nutrient, growth, fruit quality, and yield of&nbsp;<em>Carica papaya</em>&nbsp;L. cv. Sekaki.&nbsp;<em>Agronomy</em>&nbsp;10(9): 1320.</p>

<p>Damian, F., Damian, G., Lăcătușu, R., Postolache, C., Iepure, G., Jelea, M., Năsui, D., 2013. The heavy metals immobilization in polluted soils from romania by the natural zeolites use. <em>Carpathian Journal of Earth and Environmental Sciences</em> 8(4): 231&ndash;250.</p>

<p>Filcheva, E.G., Tsadilas, C.D., 2002. Influence of clinoptilolite and compost on soil properties.&nbsp;<em>Communications in Soil Science and Plant Analysis</em>&nbsp;33 (3-4): 595&ndash;607.</p>

<p>GOST 13192-73: Wines, wine materials and cognacs. Method for determination of sugar. Available at [Access date : 18.11.2024]: https://gostperevod.com/gost-13192-73.html</p>

<p>GOST 28561-90: Fruit and vegetable products. Methods for determination of total solids or moisture. Available at [Access date : 18.11.2024]: https://gostperevod.com/gost-28561-90.html</p>

<p>GOST 29270-95: Fruits, vegetables and derived products. Determination of nitrate content by potentiometric method. Available at [Access date : 18.11.2024]: https://gostperevod.com/gost-34570-2019.html</p>

<p>GOST 30178-96. Raw material and food-stuffs. Atomic absorption spectrometric method of determination of toxic elements. Available at [Access date : 18.11.2024]: https://gostperevod.com/gost-30178-96.html</p>

<p>Grandillo, S.; Zamir, D.; Tanksley, S.D., 1999. Genetic improvement of processing tomatoes: A 20 years perspective.&nbsp;<em>Euphytica </em>110: 85&ndash;97.</p>

<p>HelgiLibrary, 2024. Tomato Production in Kazakhstan. Available at [Access date : 18.11.2024]: &nbsp;https://www.helgilibrary.com/indicators/tomato-production/kazakhstan/</p>

<p>Ippolito, J.A., Tarkalson, D.D., Lehrsch, G.A., 2011. Zeolite soil application method affects inorganic nitrogen, moisture, and corn growth. <em>Soil Science</em> 176(3):136-142. &nbsp;</p>

<p>Jarosz, R., Szerement, J., Gondek, K., Mierzwa-Hersztek, M., 2022. The use of zeolites as an addition to fertilisers &ndash; A review. <em>Catena</em> 213: 106125.</p>

<p>Javaid, A., Munir, N., Abideen, Z., Siddiqui, Z.S., Yong, J.W.H., 2024. The role of natural and synthetic zeolites as soil amendments for mitigating the negative impacts of abiotic stresses to improve agricultural resilience. <em>Plant Stress</em> 14: 100627.</p>

<p>Kavvadias, V., Ioannou, Z., Vavoulidou, E., &amp; Paschalidis, C., 2023. Short term effects of chemical fertilizer, compost and zeolite on yield of lettuce, nutrient composition and soil properties. <em>Agriculture</em> 13(5): 1022.</p>

<p>Khamkure, S., Reyes-Rosas, A., Gonz&aacute;lez, A.Z., Samaniego-Moreno, L., Gamero-Melo, P., 2025. A sustainable approach to fertilizer management: Microwave-assisted synthesis of slow-release ammonium-natural clinoptilolite. <em>Sustainable Chemistry and Pharmacy</em> 45: 102004.</p>

<p>Maffia, A., Marra, F., Canino, F., Oliva, M., Mallamaci, C., Celano, G., Muscolo, A., 2023. Comparative study of fertilizers in tomato-grown soils: Soil quality, sustainability, and carbon/water footprints. <em>Soil Systems</em> 7(4): 109.</p>

<p>Malekian, R., Abedi-Koupai, J., Eslamian, S.S., 2011. Influences of clinoptilolite and surfactantmodified clinoptilolite zeolite on nitrate leaching and plant growth. <em>Journal of Hazardous Materials</em> 185: 970&ndash;976.</p>

<p>Maxotova, A., Nurbayeva, E., Aitbayev, Т., Nurgaliyeva, B., 2021. Yield and yield components of five tomato varieties (Solanum lycopersicum) as influenced by chemical NPK fertilizer applications under chestnut soil conditions. <em>Eurasian Journal of Soil Science</em> 10(4): 327 - 331.</p>

<p>Milo&scaron;ević, T., &amp; Milo&scaron;ević, N., 2017. Influence of mineral fertilizer, farmyard manure, natural zeolite, and their mixture on fruit quality and leaf micronutrient levels of apple trees. <em>Communications in Soil Science and Plant Analysis</em> 48(5): 539&ndash;548.</p>

<p>Mohammed, E., Tewodros, M., Lulseged, T.,&nbsp;Feyera, L., Wuletawu, A., Amsalu, T., 2024.&nbsp;Agronomic and socio-economic drivers of fertilizer use and crop productivity in smallholder wheat production systems in Ethiopia.&nbsp;<em>Discover Environment </em>2: 130.</p>

<p>Mondal, M., Biswas, B., Garai, S., Sarkar, S., Banerjee, H., Brahmachari, K., Bandyopadhyay, P. K., Maitra, S., Brestic, M., Skalicky, M., Ondrisik, P., Hossain, A., 2021. Zeolites enhance soil health, crop productivity and environmental safety. <em>Agronomy,</em>11(3): 448.</p>

<p>Nakhli, S.A.A., Delkash, M., Bakhshayesh, B.E., Kazemian, H., 2017. Application of zeolites for sustainable agriculture: A review on water and nutrient retention. <em>Water, Air, &amp; Soil Pollution</em> 228: 464.</p>

<p>Omer, E., Szlatenyi, D., Csenki, S., Alrwashdeh, J., Czako, I., L&aacute;ng, V., 2024. Farming practice variability and its implications for soil health in agriculture: A review. <em>Agriculture</em> 14(12): 2114.</p>

<p>Rahmani, R., Khalesro, S., Heidari, G., Mokhatssi-Bidgoli, A., 2023. Vermicompost and zeolite improve yield, nutrient uptake, essential and fixed oil production, and composition of Nigella sativa L. <em>Frontiers in Sustainable Food Systems</em> 7: 1214691.</p>

<p>Sadenova, M.A., Abdulina, S.A., Tungatarova, S.A., 2016. The use of natural Kazakhstan zeolites for the development of gas purification catalysts. <em>Clean Technologies and Environmental Policy</em> 18: 449&ndash;459. &nbsp;</p>

<p>Saparov, A., 2014. Soil Resources of the Republic of Kazakhstan: Current Status, Problems and Solutions. In:&nbsp; Novel measurement and assessment tools for monitoring and management of land and water resources in agricultural landscapes of Central Asia. Mueller, L., Saparov, A., Lischeid, G., (Eds.). Environmental Science and Engineering. Springer International Publishing. Switzerland. pp.61-73.</p>

<p>Shmelev, S.E., Salnikov, V., Turulina, G., Polyakova, S., Tazhibayeva, T., Schnitzler, T., Shmeleva, I.A., 2021. Climate change and food security: The impact of some key variables on wheat yield in Kazakhstan.&nbsp;<em>Sustainability&nbsp;</em>13: 8583.</p>

<p>Sultanbayeva, G., Agatayeva, A. Kaiynbayeva, R., Kozhabekova, N. Chernyakova, R., Jussipbekov, U. 2022. Study of the sorption properties of natural zeolite in relation to Indium(III) and Gallium(III) cations on the model systems. <em>Crystals</em> 12(9): 1220. &nbsp;</p>

<p>Tastanbekova, G., Tokbergenova, Z., Sharipova, D., Jantassova, A., Surimbayeva, K., Azhimetova, G., Zhylkibayev, A., 2024. Enhanced tomato (Solanum lycopersicum L.) yield and soil biological properties through integrated use of soil, compost, and foliar fertilization under greenhouse conditions. <em>Eurasian Journal of Soil Science</em> 13(4): 366 - 375.</p>

<p>Vassilina, T., Nasiyev, B., Rvaidarova, G., Shibikeyeva, A., Seitkali, N., Salykova, A., Yertayeva, Z., 2023. The effects of clinoptilolite type of zeolite and synthesised zeolite-enriched fertilizer on yield parameters of Cucumber (Cucumis sativus) plant and some chemical properties in dark chestnut soil. <em>Eurasian Journal of Soil Science</em> 12(3): 277-281.</p>

<p>Vassilina,&nbsp;T.,&nbsp;Nasiyev,&nbsp;B.,&nbsp;Shibikeyeva,&nbsp;A.,&nbsp;Seitkali,&nbsp;N.,&nbsp;Kossanov,&nbsp;S.,&nbsp;2024. The role of zeolite and mineral fertilizers in enhancing Table Beet (Beta vulgaris L.) productivity in dark chestnut soils of Southeast Kazakhstan. <em>Eurasian Journal of Soil Science</em> 13(4): 312 - 319.</p>

<p>Wea, T., Unonis, M., Khairuddin, M., Shaharuddin, S., Chai, N., 2018. Mechanism in using commercial high efficient zeolite-base greenfeed slow release fertilizers. <em>Journal of Agricultural Chemistry and Environment</em> 7: 1-9.</p>

Abstract

Tomato (Solanum lycopersicum L.) is a widely cultivated vegetable crop in Kazakhstan, yet its productivity and quality are often limited by soil degradation, nutrient imbalances, and the excessive use of mineral fertilizers. Natural zeolites, particularly clinoptilolite, offer potential as soil amendments due to their high cation exchange capacity, water retention properties, and ability to regulate nutrient availability. This study aimed to evaluate the effects of natural zeolite (2 t/ha), alone and in combination with two mineral fertilizer doses (N₄₅P₄₅K₄₅ and N₉₀P₉₀K₉₀), on tomato yield, fruit quality, soil heavy metal content, and economic profitability under dark chestnut soil conditions in southeastern Kazakhstan. A field experiment was conducted using a randomized complete block design with six treatments and three replications. Results showed that all treatments increased yield compared to the control (21.7 t/ha), with the highest yield (29.1 t/ha) observed under the zeolite + N₉₀P₉₀K₉₀ treatment. Fruit quality improved in terms of dry matter (up to 5.46%) and sugar content (up to 3.80%) with zeolite and fertilizer combinations. Nitrate accumulation in fruits was highest under N₉₀P₉₀K₉₀ alone (78 mg/kg) but decreased significantly when combined with zeolite (64 mg/kg), indicating the mineral’s capacity to reduce nitrate uptake. Heavy metal analysis revealed that zeolite reduced the bioavailability of cadmium and lead in soil, keeping concentrations below permissible limits. Economic evaluation indicated that the zeolite-only treatment provided the highest profitability (171%) due to relatively low input costs and moderate yield gains. Overall, the results demonstrate that zeolite, especially when integrated with moderate fertilizer inputs, enhances tomato productivity, improves fruit safety, and supports sustainable soil management. Its use can be particularly beneficial in resource-limited and environmentally sensitive agricultural systems.

Keywords: Tomato, zeolite, clinoptilolite, mineral fertilizers, nitrate, heavy metals, yield, sustainable agriculture.

References

Ahmed, S., Khan, M., Raza, T., Ahmad, R., Iqbal, J., Eash, N., 2022. Integrated use of bio-organic and chemical fertilizer to enhance yield and nutrients content of tomato. Eurasian Journal of Soil Science 11(2): 126-132.

Anas, M., Liao, F., Verma, K.K., Sarwar, M.A., Mahmood, A., Chen, Z.L., Li, Q., Zeng, X.P., Liu, Y., Li, Y.R., 2020. Fate of nitrogen in agriculture and environment: agronomic, eco-physiological and molecular approaches to improve nitrogen use efficiency. Biological Research 53: 47.

Ay, A., Demirkaya, S., Kızılkaya, R., Gülser, C., 2022. The effects of two Fe-EDDHA chelated fertilizers on dry matter production and Fe uptake of tomato seedlings and Fe forms of a calcareous soil. Eurasian Journal of Soil Science 11(3): 259-265.

Bertalan-Balázs, B., Rahimi, M., Adelinia, A., Ebrahimi, E., Ojani, M., 2024. Impact assessment of Zeolite, Ca-bentonite and Biochar amendments on Cd bioavailability and fractions in polluted calcareous soils. Environmental Earth Sciences 83: 506.

Bihon, W., Ognakossan, K.E., Tignegre, J.B., Hanson, P., Ndiaye, K., Srinivasan, R., 2022.  Evaluation of different tomato (Solanum lycopersicum L.) entries and varieties for performance and adaptation in Mali, West Africa. Horticulturae 8(7): 579.

Cataldo, E., Salvi, L., Paoli, F., Fucile, M., Masciandaro, G., Manzi, D., Masini, C.M., Mattii, G.B., 2021. Application of zeolites in agriculture and other potential uses: A review. Agronomy 11(8): 1547.

Choo, L.N.L.K., Ahmed, O.H., Talib, S.A.A., Ghani, M.Z.A., Sekot, S., 2020. Clinoptilolite zeolite on tropical peat soils nutrient, growth, fruit quality, and yield of Carica papaya L. cv. Sekaki. Agronomy 10(9): 1320.

Damian, F., Damian, G., Lăcătușu, R., Postolache, C., Iepure, G., Jelea, M., Năsui, D., 2013. The heavy metals immobilization in polluted soils from romania by the natural zeolites use. Carpathian Journal of Earth and Environmental Sciences 8(4): 231–250.

Filcheva, E.G., Tsadilas, C.D., 2002. Influence of clinoptilolite and compost on soil properties. Communications in Soil Science and Plant Analysis 33 (3-4): 595–607.

GOST 13192-73: Wines, wine materials and cognacs. Method for determination of sugar. Available at [Access date : 18.11.2024]: https://gostperevod.com/gost-13192-73.html

GOST 28561-90: Fruit and vegetable products. Methods for determination of total solids or moisture. Available at [Access date : 18.11.2024]: https://gostperevod.com/gost-28561-90.html

GOST 29270-95: Fruits, vegetables and derived products. Determination of nitrate content by potentiometric method. Available at [Access date : 18.11.2024]: https://gostperevod.com/gost-34570-2019.html

GOST 30178-96. Raw material and food-stuffs. Atomic absorption spectrometric method of determination of toxic elements. Available at [Access date : 18.11.2024]: https://gostperevod.com/gost-30178-96.html

Grandillo, S.; Zamir, D.; Tanksley, S.D., 1999. Genetic improvement of processing tomatoes: A 20 years perspective. Euphytica 110: 85–97.

HelgiLibrary, 2024. Tomato Production in Kazakhstan. Available at [Access date : 18.11.2024]:  https://www.helgilibrary.com/indicators/tomato-production/kazakhstan/

Ippolito, J.A., Tarkalson, D.D., Lehrsch, G.A., 2011. Zeolite soil application method affects inorganic nitrogen, moisture, and corn growth. Soil Science 176(3):136-142.  

Jarosz, R., Szerement, J., Gondek, K., Mierzwa-Hersztek, M., 2022. The use of zeolites as an addition to fertilisers – A review. Catena 213: 106125.

Javaid, A., Munir, N., Abideen, Z., Siddiqui, Z.S., Yong, J.W.H., 2024. The role of natural and synthetic zeolites as soil amendments for mitigating the negative impacts of abiotic stresses to improve agricultural resilience. Plant Stress 14: 100627.

Kavvadias, V., Ioannou, Z., Vavoulidou, E., & Paschalidis, C., 2023. Short term effects of chemical fertilizer, compost and zeolite on yield of lettuce, nutrient composition and soil properties. Agriculture 13(5): 1022.

Khamkure, S., Reyes-Rosas, A., González, A.Z., Samaniego-Moreno, L., Gamero-Melo, P., 2025. A sustainable approach to fertilizer management: Microwave-assisted synthesis of slow-release ammonium-natural clinoptilolite. Sustainable Chemistry and Pharmacy 45: 102004.

Maffia, A., Marra, F., Canino, F., Oliva, M., Mallamaci, C., Celano, G., Muscolo, A., 2023. Comparative study of fertilizers in tomato-grown soils: Soil quality, sustainability, and carbon/water footprints. Soil Systems 7(4): 109.

Malekian, R., Abedi-Koupai, J., Eslamian, S.S., 2011. Influences of clinoptilolite and surfactantmodified clinoptilolite zeolite on nitrate leaching and plant growth. Journal of Hazardous Materials 185: 970–976.

Maxotova, A., Nurbayeva, E., Aitbayev, Т., Nurgaliyeva, B., 2021. Yield and yield components of five tomato varieties (Solanum lycopersicum) as influenced by chemical NPK fertilizer applications under chestnut soil conditions. Eurasian Journal of Soil Science 10(4): 327 - 331.

Milošević, T., & Milošević, N., 2017. Influence of mineral fertilizer, farmyard manure, natural zeolite, and their mixture on fruit quality and leaf micronutrient levels of apple trees. Communications in Soil Science and Plant Analysis 48(5): 539–548.

Mohammed, E., Tewodros, M., Lulseged, T., Feyera, L., Wuletawu, A., Amsalu, T., 2024. Agronomic and socio-economic drivers of fertilizer use and crop productivity in smallholder wheat production systems in Ethiopia. Discover Environment 2: 130.

Mondal, M., Biswas, B., Garai, S., Sarkar, S., Banerjee, H., Brahmachari, K., Bandyopadhyay, P. K., Maitra, S., Brestic, M., Skalicky, M., Ondrisik, P., Hossain, A., 2021. Zeolites enhance soil health, crop productivity and environmental safety. Agronomy,11(3): 448.

Nakhli, S.A.A., Delkash, M., Bakhshayesh, B.E., Kazemian, H., 2017. Application of zeolites for sustainable agriculture: A review on water and nutrient retention. Water, Air, & Soil Pollution 228: 464.

Omer, E., Szlatenyi, D., Csenki, S., Alrwashdeh, J., Czako, I., Láng, V., 2024. Farming practice variability and its implications for soil health in agriculture: A review. Agriculture 14(12): 2114.

Rahmani, R., Khalesro, S., Heidari, G., Mokhatssi-Bidgoli, A., 2023. Vermicompost and zeolite improve yield, nutrient uptake, essential and fixed oil production, and composition of Nigella sativa L. Frontiers in Sustainable Food Systems 7: 1214691.

Sadenova, M.A., Abdulina, S.A., Tungatarova, S.A., 2016. The use of natural Kazakhstan zeolites for the development of gas purification catalysts. Clean Technologies and Environmental Policy 18: 449–459.  

Saparov, A., 2014. Soil Resources of the Republic of Kazakhstan: Current Status, Problems and Solutions. In:  Novel measurement and assessment tools for monitoring and management of land and water resources in agricultural landscapes of Central Asia. Mueller, L., Saparov, A., Lischeid, G., (Eds.). Environmental Science and Engineering. Springer International Publishing. Switzerland. pp.61-73.

Shmelev, S.E., Salnikov, V., Turulina, G., Polyakova, S., Tazhibayeva, T., Schnitzler, T., Shmeleva, I.A., 2021. Climate change and food security: The impact of some key variables on wheat yield in Kazakhstan. Sustainability 13: 8583.

Sultanbayeva, G., Agatayeva, A. Kaiynbayeva, R., Kozhabekova, N. Chernyakova, R., Jussipbekov, U. 2022. Study of the sorption properties of natural zeolite in relation to Indium(III) and Gallium(III) cations on the model systems. Crystals 12(9): 1220.  

Tastanbekova, G., Tokbergenova, Z., Sharipova, D., Jantassova, A., Surimbayeva, K., Azhimetova, G., Zhylkibayev, A., 2024. Enhanced tomato (Solanum lycopersicum L.) yield and soil biological properties through integrated use of soil, compost, and foliar fertilization under greenhouse conditions. Eurasian Journal of Soil Science 13(4): 366 - 375.

Vassilina, T., Nasiyev, B., Rvaidarova, G., Shibikeyeva, A., Seitkali, N., Salykova, A., Yertayeva, Z., 2023. The effects of clinoptilolite type of zeolite and synthesised zeolite-enriched fertilizer on yield parameters of Cucumber (Cucumis sativus) plant and some chemical properties in dark chestnut soil. Eurasian Journal of Soil Science 12(3): 277-281.

Vassilina, T., Nasiyev, B., Shibikeyeva, A., Seitkali, N., Kossanov, S., 2024. The role of zeolite and mineral fertilizers in enhancing Table Beet (Beta vulgaris L.) productivity in dark chestnut soils of Southeast Kazakhstan. Eurasian Journal of Soil Science 13(4): 312 - 319.

Wea, T., Unonis, M., Khairuddin, M., Shaharuddin, S., Chai, N., 2018. Mechanism in using commercial high efficient zeolite-base greenfeed slow release fertilizers. Journal of Agricultural Chemistry and Environment 7: 1-9.