Conventional fertilizers and the risks associated with them

In order to enrich the soil with nutrients to increase the efficiency of plant production, multi-component mineral fertilizers are mainly used. According to statistics, the consumption of this type of fertilizer is increasing every year. Comparing the data, it can be noticed that in the last 20 years the consumption of fertilizers in Poland per hectare has almost doubled, from 86 kg/ha in 2000 to 142 kg/ha in 2018¹. The comparison of the consumption data for Poland and other European Union countries reveals the fact that Poland is the leader of countries with the highest consumption of phosphorus and nitrogen fertilizers (Fig. 1). The statistical data thus refutes the repeated myth about the quality of Polish food as organic, better compared to food produced in Western Europe, where the advantage would be a balanced fertilizer management or its significant limitation.

This situation is very worrying because it should be noted that the soil is not always fertilized properly. Furthermore, the excess of fertilizers used can lead to an increase in nitrogen and phosphorus content in the soil, which, when washed away by rainwater, end up in water reservoirs including water eutrophication. 

Fig. 1 Consumption of phosphorus and nitrogen fertilizers in tonnes for individual EU countries 2017³

Processes causing soil depletion and degradation

Human activity largely determines the condition and quality of soil and can also affect the processes occurring in water reservoirs, which are unfavourable for the environment. Incorrectly cultivated plants aim only at obtaining high yields, which are generated by intensive and unbalanced soil fertilization, and leads to a number of unfavourable phenomena. Their effects can be a serious problem for both the environment and plant producers.

The concept of devastated and degraded land refers to land that has lost its value in use as a result of various natural phenomena or resulting from human activity. Such phenomena include erosion, desertification, weathering, sterilization and acidification of soils.

Erosion is one of the main processes leading to soil degradation. It consists in the destruction of the soil surface as a result of the impact of physical forces on it related to rainfall, flowing water, temperature changes, wind, ice and sun rays. Water erosion, which flushes and leaches nutrients from the soil, thereby losing fertile farmland, is a special type of soil erosion. In addition, it contributes to the deterioration of the soil structure and the disappearance of the levels of its differentiation⁴. 

Water erosion affects not only soil, but also pollutes surface waters. Rainwater leaches the soil of nutrients, which also include fertilizers used in cultivation. Water runoff from agricultural areas causes a large number of biogenic elements, mainly nitrogen and phosphorus, to enter the surface waters. The excess of these elements leads to an increase in water fertility, known as eutrophication. The described processes are presented in Fig. 2.

The phenomenon of eutrophication is extremely unfavourable from the ecological point of view (the relations and balance of the biosphere are disturbed). The increasing content of nitrogen and phosphorus in the water causes the appearance of a large amount of algae. The phenomenon is based on the principle of positive feedback, because in the process of formed algae dying off, subsequent portions of phosphorus and nitrogen get into the water. The phenomenon of eutrophication causes unfavourable changes in both the flora and fauna of water reservoirs. The reduced oxygen content in the water causes the fish to die off, while the reduced water transparency caused by algae bloom leads to the disappearance of aquatic plants⁵.  In 2018, the Chief Inspectorate for Environmental Protection carried out research assessing the general condition of surface waters in Poland. According to the available data, out of 1,472 tested water reservoirs, only 9 of them were in good condition¹. 

Fig. 2 Different mechanisms related to the flow of fertilizers and pesticides⁶.

Soil acidification is another process causing soil degradation. Acidification involves leaching of alkaline cations from the soil. These processes are intensified mainly in autumn and winter and are particularly favoured by rainfall and low temperatures. Such a phenomenon significantly reduces plant production, because the acidic environment makes it difficult for plants to absorb phosphorus, which is responsible for their balanced growth and development. Among the European Union countries, Poland has the highest acidification of agricultural land. According to the data of the National Chemical and Agricultural Station, the share of very acidic and acidic soils in the total area of agricultural land is as high as 58%¹. Unfortunately, human activity does not improve this condition. Additional fertilization of crops, and in particular an inadequate ratio of physiologically acid and calcium fertilizers, or the removal of alkaline cations along with the yield, further aggravates the occurrence of this disturbing phenomenon. 

Another process causing soil degradation involves desertification. It mainly affects dry and semi-arid land and is associated with climate changes related to the increase in average temperature and the intensity of droughts. In addition, incorrect human activity is another cause of desertification. This is mainly due to poor irrigation techniques and overgrazing. Deforestation is also one of the causes of desertification, as it removes vegetation that has kept the soil moisture at an appropriate level and stores water itself. Desertification not only affects the condition of the soil but can also affect the climate. The degraded land areas are devoid of plants, which prevents the absorption of greenhouse gases. This can contribute to the warming of the climate, as a result of which an increased intensification of storm phenomena and fires will be observed in a given area⁷. 

Methods of preventing devastation and degradation of soil

In order to counteract the processes of devastation and degradation of soils, a number of activities can be applied. They aim not only at preventing the increasing deepening of degraded soils, but also at increasing the efficiency of plant production.

• Management of plant residues and winter crops
• Contoured cultivation
• Precision farming
• Limited ploughing
• Introducing new plant species
• Agriculture automation
• Soil deacidification 

Contraindications to the use of conventional mineral fertilizers

Currently, plant cultivation poses huge challenges for producers, in which the most important thing is the efficiency of the process and the resulting profits from plant production. Various methods of fertilizing arable land are used to increase the amount of the crop. According to the Institute of Soil Science and Plant Cultivation, in order to increase the nitrogen content, which is conducive to the proper development and growth of plants, mainly mineral fertilizers are used. They constitute 56.9% of the sources of nutrients (nitrogen and phosphorus) in the soil of agricultural crops (Fig. 3). Such data are worrying because mineral fertilizers can contribute to a number of unfavourable environmental and climate phenomena. 

Fig. 3 The structure of sources of nitrogen supply to arable soils.

In addition, it is worth noting that the use of mineral fertilizers is associated with using a large amount of raw materials and minerals which are the source of key elements. One of such minerals is phosphate rock. Phosphorus is obtained from phosphate rock. However, it should be noted that the phosphate rock comes from a non-renewable source. In addition, it is imported to Europe because there are no natural deposits of it in this part of the world. The use of an excessive amount of conventional fertilizers, in addition to contributing to soil degradation or water eutrophication, can also deplete non-renewable resources of phosphorites³.

The use of conventional fertilization methods consumes a large amount of this compound. Moreover, the efficiency of such a fertilization process is low. Conventional fertilizers can be washed away from the soil after the first rainfall of increased intensity, moving to its deeper layers.  For plants, such a source of phosphorus becomes unavailable, and as a result, their yields can become much lower.

Scientific data and statistics on the use of synthetic mineral fertilizers, correlated with the rate of soil degradation and water eutrophication, force the Polish legislator to develop tools to effectively stop the negative effects of excessive soil fertilization. The legislator amended the Act on Fertilizers and Fertilization and the Act of the State Plant Health and Seed Inspection Service of June 22, 2020. Such amendment prohibits the use of granulated urea, which is a source of nitrogen for plants. On the other hand, the act excluded mineral fertilizers containing a biodegradable coating⁸. It seems that this is a good direction for changes in the field of fertilizers and their application, it also forces research on looking for new solutions for fertilizing industrial crops. Amended laws seem to be one of the most effective tools for forcing plant producers to change their habits when it comes to preparing land for cultivation and taking shared responsibility for the condition of the natural environment (including at least slowing down or stopping soil degradation processes).

A new alternative for agriculture, CRF 

CRF (controlled release fertilizers) are defined as fertilizers that contain nutrients that are available in a form that the plant cannot immediately absorb. They are typically coated or encapsulated with materials that control the rate, mechanism, and timing of nutrient release. It is the ability to control the release of substances that is the main factor distinguishing CRF from RF (slow release fertilizer)⁹.

• Organic substances: this group includes substances of natural origin and synthetic substances, which in turn are divided into biodegradable compounds (based on urea aldehyde condensation products) and compounds undergoing chemical decomposition, including composts or organic-mineral fertilizers.
• Low solubility inorganic substances: include, among others, metallic ammonium phosphates (MgNH4PO4) and partially acidified phosphate rocks.
• Water-soluble inorganic substances, limited by physical barriers, are formed by coating cores or granules with sulphur or polymers, as well as by incorporating nutrients into matrices that limit their dissolution¹⁰.

Fig. 4 Classification of controlled-release fertilizers

An alternative method of fertilization is the use of fertilizers with a controlled release of micronutrients. These types of fertilizers slowly release nutrients into the soil. For this reason, plants would  have continuous access to micronutrients, ensuring their sustainable growth and development. In addition, slow and controlled-release fertilizers would be able to prevent the leaching of nutrients from the soil, which could occur during heavy rainfall.

An additional advantage of controlled-release fertilizers involves the possibility of using them not only for fertilizing plant crops growing in the soil, but also as a source of valuable elements for plants grown in water, e.g. rice. The additional coating allows the salts contained in the fertilizer to enter the water at a much slower pace, and the plants are not exposed to the effects of a sudden increase in their concentration.

In order to compare the activities available on the mineral fertilizers market and the new concept of fertilizers which are with a controlled action, CRF’s were designed based on a matrix made of a biodegradable polymer, additionally enriched with fillers. This is the purpose of CRF’s, in which a salt mixture was used - a source of biogenic elements, and a biodegradable polymer was used as a biodegradable matrix and natural fillers, i.e. a fibre that was a good channel for transporting water inside the fertilizer granule and a carbon filler with a large surface area, trapping crystallized mineral salts containing biogenic elements in its pores. These salts are enclosed in the granule on several levels, and the granule available on the surface is released immediately, as is the case with mineral fertilizers. The next level includes salts clamped directly in the biodegradable polymer matrix. They are released after some time after its use. The rate of release of these salts depends on the degradation rate of the biodegradable polymer, i.e. on the microcracks that penetrate into the granule, allowing water access, and thus rinsing salt to the soil. The presence of hygroscopic natural fibre facilitates the transport of water to the inside of the fertilizer granule, causing its additional degradation from the inside. The process of washing out the salt from the carbon macro and micropores is the last level of mineral salt release. The rate of this release depends on the diameter of these pores and the availability of moisture in the decomposition process. 

Thus, by using a matrix combined with fillers, one of which has a developed surface, it is possible to gradually release the salts trapped in them, and the complexity of their release mechanisms allows the gradual leaching of nutrients into the soil. The biodegradable polymer shell itself ultimately decomposes into water and CO2 (carbon dioxide), and thus into naturally occurring products found in the environment. The ratio of the individual components of the fertilizer is shown in the pie chart (Fig. 5).

Fig. 5 Diagram of CRF structure and the ratio of individual fertilizer components 

The research on controlled-release fertilizers is ongoing at the University of Science and Technology in Kraków, at the Faculty of Materials Science and Ceramics at the Department of Biomaterials and Composites. It was possible to optimize the production process of such fertilizers, as well as to test and evaluate the properties of such fertilizers over time by comparing them to analogous conventional mineral fertilizers (CRF - UST and the research on mineral fertilizers had the same content of biogenic elements).  The research on the operation of the designed CRF was carried out on the basis of various crops, including honey phacelia, oats or millet, which were carried out in laboratory conditions. The sample that is part of the cultivation is shown in the diagram in Fig. 6. Rainfall was simulated on the crops and the concentration of salt washed out was determined on the basis of the water that flowed through the soil and was then collected in a sealed container.

 Fig. 6 Scheme of the sample and cultivation for testing the degree of release of biogenic elements from CRF.

The results of the simulation research on precipitation and concentrations of salts washed out by water are presented in the bar graph in Fig. 7. It can be seen that the concentration of salts washed out in samples with commercial fertilizer is at a much higher level. A significant difference in their level can be noted on the 26th day of the research, for which the estimated amount of rainwater was approx. 62 l/m2 of soil surface (while according to the Institute of Metrology and Water Management, the average amount of precipitation in Poland, for example for the month of May, 2019 was approx. 85l/m2) [own research of the UST]. 

Fig. 7 Results of salt concentrations for rainfall simulation in selected plant samples for individual days of the research. 

After the rainfall simulation was completed, the plants were harvested and examined. Based on the research, it was found that plants on which no fertilizers were used were clearly more fragile and brittle. In addition, their colour was less intensive, which can prove nutrient deficiencies. The plants grown on the fertilized soil substrate were in much better condition. However, those for which CRF was applied showed a greater stem thickness and a more intense and distinct colour, which can also be seen in Fig. 8. In addition, the ability to accumulate water of these plants was greater compared to those grown on other substrates (CRF-UST fertilizers accumulated and retained water in the soil - the effect of natural filler) [own research of the UST].

Fig. 8 Images from an optical microscope showing plant fragments from the left: without fertilizer, with CRF and with commercial fertilizer.

  CRF’s perform very well when it comes to their use for fertilizing plants grown in water. This is a great chance, especially since the possibility of fertilizing this type of plant has so far been very limited and burdensome due to the occurrence of a sudden increase in the concentration of mineral salts in water. In order to examine the rate of salt release, tests of CRF-UST fertilizers in water were carried out. The obtained results allowed to conclude that both in the initial phase and after a longer period of time, CRF-UST fertilizers gradually release the salts retained in them. CRF-UST fertilizers proportionally released their salts into water, which means that at the same time intervals the concentration increased by the same value. These fertilizers would make it possible to precisely fertilize plants growing in water, e.g. rice. It would also involve much less interference with the natural environment, both in the fields and in their vicinity. Fig. 9 shows a photo of the solubility of CRF in water.

Fig. 9 Examination of salt release from CRF-UST commercial fertilizers.

Aquatic plants can also need to be supplied with biogenic elements, in this application mineral fertilizers dissolve immediately, leading to eutrophication and salting of water. For this reason, it is impossible to use them in such a case. The presence of a slowing barrier (several release mechanisms simultaneously) in CRF’s ensures that CRF’s applied in the aquatic environment (e.g. for fertilizing aquatic plants) decomposes in a controlled manner, which cannot be achieved with conventional synthetic fertilizers, as they dissolve immediately.

Challenges facing the agricultural industry.

Soil degradation and devastation are irreversible processes (soil formation depends on many complex factors, and the whole process sometimes takes thousands of years). Human activity leads to overexploitation of soil resources, the pace of which is many times faster than the natural processes of its regeneration. It is important to thoroughly investigate the effects of human activity through the industrial cultivation of plants on the quality and speed of damage to the natural environment (mainly soil and water pollution) in order to react quickly and efficiently and eliminate the negative effects of the food industry.

The agricultural sector is facing enormous demands. Between 2019 and 2100 the population will increase from 7.7 billion to 10.9 billion people¹¹. It is estimated that food production should be increased by as much as 70% in order to fully cover the demand for food products of a much larger population¹². People's eating habits are also changing. The average amount of calories consumed by an average person in the world is growing¹³.

 This alarming data is the basis for taking measures in reducing the harmful effects on the environment that accompany the use of fertilizers. The achievements of science and technology, including precision farming, new plant species or agriculture automation are an effective tool for the implementation of such production. It is also crucial to develop methods of supplying plants with minerals in such a way as to minimize negative environmental effects. The use of modern controlled-release fertilizers, which are produced on the basis of biodegradable substances, do not remain in the environment, or cause its degradation, make it possible. In addition, they can contribute to reducing the amount of fertilizer used and the frequency of fertilization, while increasing the efficiency of plant cultivation. 

Authors: Katarzyna Suchorowiec, Piotr Szatkowski - University of Mining and Metallurgy in Krakow, Faculty of Materials Science and Ceramics, Department of Biomaterials and Composites

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