Impact of detergents on okra seed germination, plant growth and soil properties

1 Introduction

Environmental pollution remains a critical global challenge, caused not only by industrial expansion and technological processes, but also by everyday domestic and agricultural practices. ¹ Among the wide range of emerging contaminants, synthetic detergents – which are commonly used in households and industrial processes – pose growing threats to terrestrial and aquatic ecosystems. ^{2 , 3} These detergents are chemically complex, and are not only formulated for cleaning and decontamination, but are also used in various applications such as oil spill remediation, pesticide delivery systems and enhancing herbicide efficacy. ⁴ While many modern detergents are marketed as biodegradable, their long-term impact on soil health, microbial communities, and plant development remain inadequately understood. ^{5 , 6 , 7}

The presence of detergent residues in irrigation water is an increasingly recognized as a serious environmental concern, especially in regions where untreated greywater or wastewater is used for agriculture purposes. Some studies have also reported detergent contamination in freshwater bodies around the world. ^{8 , 9 , 10 , 11} Detergents typically contain surfactants (surface active cleaning agents), builders (to remove calcium and magnesium ions from hard water), alkalis (to increase pH and dissolve oily/acidic dirt components), and enzymes. Other additives include anti-redeposition agents, bleaching agents, antimicrobials, softeners, fragrances, optical brighteners, preservatives, foam regulators, hydrotropes, and processing aids. ¹² Many of these ingredients can alter the physicochemical properties of soil, inhibit seed germination, impair plant metabolic activities, and disrupt the ecological balance. ^{3 , 13} In light of our growing reliance on non-traditional water sources for irrigation, it is urgent and necessary to evaluate the environmental and agricultural risks posed by water contaminated with detergents.

Okra (Abelmoschus esculentus L. Moench) is a well-known vegetable and that is cultivated and consumed throughout Africa and many other tropical regions of the world. Vegetables are an important part of the global human diet. They are rich in the nutrients and vitamins necessary for healthy growth and development. ¹⁴ Okra is an important vegetable crop that has high nutritional and medicinal value. Its fruits, in the form of pods, are known to be a rich source of carbohydrates, proteins and vitamins.

Nigeria is the second largest producer of okra in the world after India. According to statistics from the United Nations Food and Agricultural Organisation (FAO), India produced 6.466 million tonnes of okra in 2021, with a productivity rate of 12.177 tonnes per hectare. In contrast, Nigeria produced approximately 1.917 million tonnes with a productivity rate of just 1.281 tonnes per hectare. The total world productivity was 4.367 tonnes per hectare. A careful look at the productivity statistics of okra from the different countries show that, despite the nutritional, medicinal, industrial, and economic value of okra, its optimum yield is far from being reached in many tropical regions ¹⁵ due to factors such as water shortages, water pollution, poor water quality, poor irrigation systems, and climate change.

Shortages of freshwater and salinity in agricultural soils have been identified as major global problems that affect the food supply for an ever-increasing population. Around 70 % of the Earth’s surface is covered by water. However, only 3 % of this is freshwater suitable for agriculture, the remaining 97 % is saline and therefore unsuitable for crop production. Less than 1 % of this freshwater is available as surface water, such as in rivers, springs and lakes. Meanwhile, approximately 99 % of the Earth’s freshwater remains locked up in ice caps and glaciers. Considering these figures alongside the fact that, in addition to agriculture, freshwater is also needed for cooking, drinking and sanitation, it becomes obvious that only a small fraction of the Earth’s water is actually available for crop production. ^{16 , 17}

Considering factors such as population growth, climate change and increased water demand by the industrial sector, water scarcity is predicted to increase. ¹⁸ Another factor contributing to water stress/scarcity is water pollution. Sources of water pollution include agrochemicals (e.g. pesticides and fertilizers), untreated human wastewater, industrial and domestic waste, and contamination of aquifers from various sources.

As of 2019, data from the World Resources Institute (WRI) reported that 17 countries (home to around 25 % of the global population) were experiencing extremely high levels of water stress. ¹⁹ Twelve out of these 17 countries were located in the Middle East and in North Africa where besides their hot and dry weather conditions, population growth and climate change have exacerbated water stress. The WRI report also showed that water stress is growing globally, as many regions of the world grapple with reduced rainfall, diminishing water supplies, and prolonged droughts, despite the growing demand for freshwater. The current water crisis calls for innovative strategies to increase the availability of freshwater in order to attain the goals of water and food security. ^{20 , 21}

The reuse of greywater for irrigation is a strategy that some consider as to hold great promise for successful water management. ^{22 , 23 , 24 , 25} Greywater includes all wash water (such as waste water from laundry, kitchen sinks, bathtubs, etc.), excluding toilet water. Reusing greywater helps to reduce the pressure on available potable water. In line with the report of Schulze, ²⁶ in many African regions where parts of the country experience less than the required rainfall for rain-fed cropping, greywater irrigation holds much promise, especially for small-scale farming, such as community and household food gardens. Greywater could be particularly useful for irrigating subsistence crops such as vegetables; ²⁷ however, more research is needed into its effects on germination and growth of plants.

Despite of the apparent clarity of greywater, applying it directly to crops without sufficient treatment can pose diverse risks to crop productivity and soil health and sustainability. ²⁸ This is because greywater contains of various chemicals, such as surfactants, oils, metals and salts, which can affect plant growth. Soaps and detergents are major components of greywater. ²⁹ Previous studies have shown that the presence of detergents and soaps in greywater affects crops in different ways, depending on the method of irrigation and the type of crop species. ^{16 , 30}

Detergent solutions have also been used to spray crops and control pests. They are sometimes used with herbicides as surfactants to enhance the herbicide effectiveness by increasing surface contact and leaf penetration and consequently reducing run-off. ^{31 , 32} In a study by Vavrina et al., ³³ detergents at low concentrations controlled white fly infection in tomatoes, but at higher concentrations, a delayed maturation and a significant reduction in yield were also observed. Recently, there has been increased interest in understanding how crops respond to treatment with detergent-contaminated water, as evidenced by related research. ^{5 , 34 , 35 , 36}

It is important to scientifically determine what effects detergents could have on seed germination, crop development and soil characteristics when various crops are exposed to them. The aim of this study was therefore to investigate the effects of detergents on the germination, growth and soil properties of okra, a vegetable commonly cultivated in Africa and other tropical regions of the world.

2 Materials and methods

The experiments were carried out in the screen house and laboratory of the Department of Plant Science and Biotechnology, at Ambrose Alli University in Ekpoma, Edo State, Nigeria. Okra (A. esculentus L. Moench) was selected as it is a commonly cultivated tropical vegetable, that is well known and popular throughout most parts of Africa. For the study, a local, early-flowering okra variety commonly cultivated by farmers in Ekpoma was used.

Two detergent brands commonly used in this region were selected for the study. According to the information label on each brand, the first detergent (D1), contains surfactants, sodium sulphate, sodium carbonate, sodium silicate, sodium aluminosilicate, clay, enzymes, perfume, polycarbonates, optical brighteners and polycarboxylates, while the second (D2) contains builders, surfactants, oxygen-based bleaching agents, polycarboxylates, enzymes, optical brighteners and perfumes, in various proportions. Three different detergent concentrations were prepared from each detergent brand, namely: 1.0 g l⁻¹ of water, 2.5 g l⁻¹ of water and 5.0 g l⁻¹ of water. Water that had not been exposed to detergents (0.0 g l⁻¹) was used as the control treatment.

3 Results

Figures 1 and 2 show the pH and EC values of the irrigation water samples used in the study. As the detergent concentration increased, the pH and EC values of the irrigation water also increased. The effect of different detergent concentrations on germination percentages and radicle length of germinated seedlings is presented in Table 1. The highest germination percentage was obtained in the control treatment (i.e. seeds not exposed to detergent-contaminated water), while the lowest germination percentage occurred in seeds treated with the highest detergent concentration for both brands of detergent. Radicle length was greatest in the control treatments and was significantly reduced in seeds irrigated with detergent at all concentrations.

Figure 1:

pH values of the control (0.0 g l⁻¹) and detergent solutions. Error bars indicate the estimated variability of the data.

Figure 2:

Electrical conductivity values of the control (0.0 g l⁻¹) and detergent solutions. Error bars indicate the estimated variability of the data.

Table 2a shows the shoot height, leaf area and number of leaves of the plants at the end of the experiment. The control pot had the highest shoot height, while plants irrigated with 5.0 g l⁻¹ of D1 and D2 had the lowest shoot height. Leaf area was greatest in the control pot, though this was not significantly different to the leaves of plants irrigated with 1.0 g l⁻¹ of D1 and D2. A significant reduction in the number of leaves per plant was observed in plants irrigated with 5.0 g l⁻¹ and 2.5 g l⁻¹ of both D1 and D2.

As for chlorophyll content, there was no significant difference in the total chlorophyll of okra leaves, except for those in the plants treated with the highest detergent concentrations (Table 2b). The fresh and dry fruit weights decreased with increasing detergent concentration (Table 2b). Table 3 reports the number of weeks to 50 % flowering as affected by the different treatments.

The effect of the detergent on the physical and chemical parameters of the soil is shown in Figures 3–7. Soils treated with detergent solutions had an increased pH and electrical conductivity (Figures 3 and 4). Additionally, the cation exchange capacity (CEC) of the soil decreased after the experiment in the control pots, whereas the CEC of the soil exposed to detergents increased, as illustrated in Figure 5. The phosphorus value decreased in soil treated with water only (the control treatment), but increased in soil treated with detergent solutions (Figure 6). The carbon content of the soil decreased in all soil treatments (Figure 7), while total nitrogen values were higher in soils treated with detergents than in the control treatment (Figure 7).

Figure 3:

Soil pH before experiment (BE), compared with soil pH after experiment in the different treatments ((0.0, 1.0, 2.5, and 5.0) g l⁻¹). Error bars indicate the estimated variability of the data values.

Figure 4:

Electrical conductivity (EC) of soil before experiment (BE) compared with EC after the experiment in the different soil treatments ((0.0, 1.0, 2.5, and 5.0) g l⁻¹). Error bars indicate the estimated variability of the data values.

Figure 5:

Soil cation exchange capacity (CEC) before experiment (BE) compared with soil CEC after the experiment in the various soil treatments ((0.0, 1.0, 2.5, and 5.0) g l⁻¹). Error bars indicate the estimated variability of the data values.

Figure 6:

Phosphorus content of soil before experiment (BE) and after the experiment at various soil treatment concentrations ((0.0, 1.0, 2.5, and 5.0) g l⁻¹). Error bars indicate the estimated variability of the data values.

Figure 7:

Carbon and nitrogen composition of soil before experiment (BE) and after experiment at various soil treatment concentrations ((0.0, 1.0, 2.5, and 5.0) g l⁻¹). Error bars indicate the estimated variability of the data values.

4 Discussion

As shown in Figures 1 and 2, the pH and EC values of the irrigation water samples were higher in the presence of detergent solutions. Similar observations regarding the impact of detergents on the pH and EC levels of irrigation water have been reported by Saeed et al. ¹⁶ and Sawadogo et al. ²⁹ The high pH values confirm the presence of alkalis in the detergents. Alkalis are substances with a high pH and are usually an important component of detergents. According to Smulders, ¹² alkalis increase the pH of wash water, thereby facilitating the breakdown of oily and acidic dirt components. When agricultural soils are exposed to liquids with a high pH, such as those found in detergent-contaminated water, the liquids act as dispersing agents. This causes the soil particles to separate, resulting in a decline in soil structure over time ⁴³ They are therefore not recommended for irrigating crops.

The electrical conductivity of a solution indicates its salinity and total dissolved solids. ^{44 , 45 , 46} Higher EC values indicate higher salinity. The EC results of the irrigation solutions indicate the presence of salts in the detergent, which caused the salinity of the solutions to increase.

The presence of detergents in irrigation solutions was found to significantly reduce germination percentages (see Table 1). This reduction in germination was due to the high salinity and pH of the irrigation in water; pH values below 3 or above 8 have been reported to inhibit germination. ⁴⁷ Physiological activities leading to seed germination are impeded and inhibited when the pH of the available water is too low or too high. ⁴⁸ Ultimately, this negatively affects germination, either delaying or hindering it, or leading to reduced vigour in the germinated seedling due to alkalinity stress. Reduced germination could also result from induced oxidative stress, which brings about lipid peroxidation and increases the permeability of cell membranes to toxic ions. ⁴⁹

The radicle length of germinated seedlings was found to be significantly reduced by detergent solutions. This reduction was due to the high salinity of the detergent solutions, as indicated by their EC values. Heidari ⁴⁹ reported that salinity stress delays and depresses seed germination. It has also been reported ⁴⁹ that a high dose of detergent around a germinated seed reduces water uptake due to the high osmotic potential generated. This leads to reduced metabolic activity and the production of oxygen free radicals, which can damage the cell membranes. ⁵⁰ In summary, the reduction in seed germination caused by detergent can be explained by the high salinity of the irrigation water and the resulting high osmotic water potential.

Plant growth studies revealed that detergents reduced shoot height. This is because the detergents constituted a source of abiotic stress for the plants. As a result of this stress, the decrease in plant height may have been an adaptive response to prevent shoot transpiration ⁵¹ reduction in cell size and internode length, and the accumulation of abscisic acid. ⁵² Other studies have reported a reduction in plant height due to environmental stresses. ⁵³

Also, a high detergent concentration led to a reduction in leaf area and the number of leaves per plant, as well as a decrease in chlorophyll content. A decrease in leaf area and number of leaves per plant could also mean a decrease in plant productivity, since okra leaves are the main photosynthetic organs. The reduction in the number of leaves per plant and leaf area can be attributed to water stress. Leaf area expansion depends on leaf turgor, temperature and assimilating supplies for growth. Researchers have reported that water deficits reduce the number of leaves per plant and the size and longevity of individual leaves by decreasing the water potential of the soil. ^{54 , 55} Based on the work of Rucker et al., ⁵⁵ suppression of leaf expansion as a result of reduced photosynthesis is responsible for drought-induced reductions in leaf area. A decrease in chlorophyll content also indicates a reduction in photosynthesis when plants are irrigated with detergent-contaminated solutions. Significant reductions in chlorophyll content in plants exposed to detergents have also been reported by Uzma et al. ⁵⁶ and Branislav et al. ¹ Branislav et al., ¹ reported a 12 % decrease in chlorophyll concentration in bean leaves irrigated with detergent-contaminated water, and showed that photosynthetic activity in the leaves decreased by around 45 %. The significant reduction in chlorophyll content at the highest detergent concentrations used may also be due to the inhibitory effects of toxic chemicals on chlorophyll synthesis in exposed plants, as reported by Singh et al. ⁵⁷ It should also be noted that chlorophyll synthesis is expected to decrease when the nutrients responsible for aiding the process are unavailable to plants in optimal quantities. Ehilen et al. ⁵⁸ reported that detergents lead to nutrient deficiency/toxicity, disturb the nutrient status of the soil. This disturbance to the nutrient status of the soil may also have contributed to the decrease in chlorophyll content of the leaves.

The study also revealed that the effect of stress caused by detergents was more pronounced in crops during the early growth stage than in later weeks. Over time, the effect of stress on the crops decreased as the plants became more adapted to the stress factors. This proves that a plant’s tolerance to stress depends on its age and developmental stage. This agrees with earlier reports stating that salt stress responses in crop plants throughout their growth cycle depend on several interacting variables, including the cultural environment, the developmental stage of the plant, the salt concentration, and the duration of the stress over time. ^{28 , 29 , 36} Munns ⁴⁶ further stated that, when plants are exposed to salinity, there is a rapid and temporary drop in the growth rate, followed by a gradual recovery to a new, reduced growth rate. These temporary effects are attributed to rapid or transient changes in plant water relations. The phenomenon observed in the current study can also be explained by the fact that, as plants continue to grow under stressful conditions, they develop more adaptive strategies to cope with the stress, thereby increasing their tolerance to the stressful conditions.

It was also noted that all the plants survived the treatments and produced flowers and fruits, although better results were obtained in the control treatments. The results showing the number of weeks to 50 % flowering indicate that 5.0 g l⁻¹ of detergent delayed flowering in both brands of detergent used. Similar findings were reported by Vavrina et al. ³³ regarding the effect of detergents on tomatoes. The delay in flowering can be attributed to salinity stress, water stress, and nutrient imbalances in the soil, as these factors are well known to affect flowering in plants. ^{58 , 59}

Additionally, the fresh and dry weights of the fruits were significantly reduced in the plants that were treated with detergents (see Table 2b). The reduction in fruit weight shows that, although the plants may survive the effects of stress, abiotic stresses (such as water and salinity stress from detergent-contaminated water) ultimately affect the productivity of crops. ^{33 , 53 , 56}

However, it is necessary to state here that the results of this study show that, compared with experiments carried out on other vegetables such as African spinach (Amaranthus hybridus) and lettuce (Lactuca sativa), okra appears to exhibit a greater tolerance to water and salinity stress. This is consistent with the findings of Ehilen et al. ⁵⁸ and Sawadogo et al. ²⁹ Ehilen et al. ⁵⁸ reported that A. hybridus subjected to solutions contaminated with 5.0 g l⁻¹ of laundry detergent experienced very slow growth and did not survive beyond six weeks after planting. Sawadogo et al. ²⁹ also reported from his investigation that okra was found to be more tolerant of salinity stress than lettuce. This confirms the statement of Saeed et al. ¹⁶ and the results of other investigations ^{5 , 29 , 36} that the tolerance of plants to stress is species-dependent. The better tolerance of okra to salinity stress imposed by detergents may be attributed to the relative thickness of its stem and to other morphological and anatomical characteristics of its stem tissues. ⁶⁰

The life and healthy growth of plants depends greatly on the properties of the soil in which they are grown. An unfavourable change in soil properties could greatly reduce crop productivity. Soil pH increased where detergent concentration was higher (see Figure 3). This increase in soil pH was due to the alkalis present in the detergents. Neina ⁶¹ described soil pH as a ‘master soil variable’ that impacts various physical, chemical, and biological soil processes and properties influencing plant growth and biomass yields. The optimal pH range for okra growth is between 6.5 and 7.5. Higher pH values beyond this range lead to decreased biological activity and could also cause dissolved organic matter to leach out of the soil. ^{43 , 61} It has been proven that soil pH affects the availability of nutrients to crops. For example, Oluwatoyinbo et al. ⁶² showed that the mineral element phosphorus is readily available to plants at pH ranges of 5.6–6.7. An increase in pH brought about by detergents is therefore detrimental to the soil and crop production.

EC values were found to increase with an increase in detergent concentration. This demonstrates that detergent solutions result in higher salinity in soils, thereby threatening plant growth. Pinto et al. ²⁴ reported high soil EC and pH after greywater application to silver beet plants. Similar results were also reported by Saeed et al. ¹⁶ and Sawadogo et al. ²⁹

The CEC value, which is the sum of the cations (K⁺, Na⁺, Ca²⁺ and Mg²⁺), was found to have increased in soils irrigated with detergent solutions. Higher CEC values in soil indicate the presence of more organic matter. These results are consistent with the work of Sawadogo et al. ²⁹ who reported an increase in soil organic matter and nutrients when irrigated with detergent solutions. Other nutrients, such as nitrogen and phosphorus, were also found to be present at higher levels in soils irrigated with detergent solutions. However, this increase in nutrient content would have been advantageous if other factors affecting nutrient availability and uptake by plants, such as pH, had not been altered to unfavourable levels, as stated earlier. A pH value of 9.3 was recorded in D1 soils and 9.8 in D2 soils, both of which were treated with 5.0 g l⁻¹ of detergent. Such high levels would, for example, limit the uptake of essential elements such as phosphorus by plants. ^{29 , 43 , 62} It must be noted that normal growth is only possible when the concentration of nutrients in the soil is balanced. When one nutrient becomes excessive, it is often at the expense of another nutrient. If some nutrients become too high, they can have an adverse effect on plant growth by affecting the absorption of other nutrients. For example, a higher level of phosphorus in the soil has been found to reduce the absorption of two essential mineral nutrients (zinc and manganese), thus limiting plant growth. ⁶³

5 Conclusion and recommendation

The results of this study have shown that detergents, especially at higher concentrations, have a negative effect on the germination and growth of okra plants and on the soil in which they grow. Seed germination percentages and radicle length were significantly reduced in detergent treatments due to the high pH of the irrigation solution, the low osmotic water potential, and the high salinity. Plant growth and productivity were also found to be reduced in okra treated with a detergent solution due to abiotic stresses (such as water and salinity stress) caused by the irrigation solution. Most importantly, detergent solutions had an adverse effect on the soil, which is the bedrock of crop production. Soil treated with detergent solutions had high pH and EC values, which are not favourable for crop production. Although certain nutrients, such as phosphorus, were present in higher concentrations in soil treated with detergents, this did not result in an overall positive effect on crop productivity due to other soil properties being adversely affected.

To ensure the safety of crop production and soil sustainability, it is crucial that irrigation solutions are free from detergent contamination and that their physical and chemical properties do not exceed the safety limits. To ascertain their safety for consumption, it is also recommended that investigations be carried out into the nutritional status of crops treated with detergent-contaminated solutions. Furthermore, research into developing more environmentally friendly detergents that retain their cleansing and pesticidal properties is recommended.