Surfactants are amphiphilic molecules that serve as wetting agents and possess both hydrophobic and hydrophilic moieties and are able to decrease the surface tension of water and increase the dissolution capacity of non-hydrophilic organic compounds (Daneshnia et al., 2015).
Decreasing the surface tension surfactants decline the contact angle between soil particles and soil water, facilitating infiltration (Arriaga et. al, 2009).
Application of surfactant to soils that perform water distribution problems has been shown to be beneficial as infiltration responded at a quicker rate into hydrophobic soils while surfactant solution concentration was increasing (Feng et. al, 2002, Cooley et. al, 2009) and water drop penetration time reduced (Feng et. al, 2002).
On the other hand, Cooley et. al, (2009) and Sullivan et. al, (2009) exhibited that surfactant had either no effect or unpropitious effects on infiltration in case of soils that are not notably hydrophobic.
According to Alvarez et. al, (2016) and Urrestarazu et. al, (2007), surfactants are able to have positive impacts in water homogeneity in soil and therefore in the moisture distribution of the root-zone. So as a result surfactants may increase water use efficiency since irrigation needs would be reduced.
This claim is also supported by Chaichi et. al, (2016), who mention that the properties of surfactants can change the flow dynamics of water, so they affect positively the hydrological conditions of soils, improving the root zone- water holding capacity, the uniformity of soil moisture distributions and as a consequence crop yield. Furthermore, where soil wettability is less than optimal, surfactant use in conjunction with appropriate irrigation treatments is able to improve the hydrological performance of soil and consequently to ameliorate irrigation efficiency and water preservation.
Jafarian et. al, (2016), indicate that apart from reduction of water consumption ,another benefit of limited irrigation needs, which are promoted by the application of surfactants, is the increment of the areas that can be cultivated
Surfactants may also be used to enhance the nutrients’ uptake and as a result the efficiency of fertilisers and to optimize the water relations of water repellent soils (Lehrsch, 2013, Guillen et. al, 2005). Baratella et. al, (2016) and Arriaga et. al, (2009), agree that surfactants can be used as a possible agricultural technique to reduce nitrate leaching losses from potatoes and enhance their nitrogen utilization.
It is essential to mention that anionic and ionic surfactants can be toxic for the plants. Non-ionic surfactants do not have a charge in solution and are chemically less active and as a consequence less toxic for the plants comparing to ionic and anionic (Reinikainen et. al, 1997). For that reason non –ionic surfactants are used most common in agriculture and horticulture, as in the recommended concentration are harmless for the plants (Nemati et. al, 2017, Baratella et. al, 2016).
The economic view of surfactants has been reported by Jafarian et. al, (2016) and Daneshnia et. al, (2016), who support that any extra cost from the application of surfactants is counterbalanced by limited irrigation and increased yield that will be produced, which will have a higher profit as a result.
Water use efficiency
There are studies that prove the positive effects of surfactants on water use optimization.
A number of researches have shown that surfactants have positive effects on turfgrass, as they reduce hydrophobicity in root-zone and soil water repellency (Alvarez et. al, 2016). As a result surfactants are able to improve moisture distribution and infiltration rate in turfgrass (Dekker et. al, 2004, Karnok et. al, 2001).
It is also commonly agreed that surfactants improve turf quality (Sciavon et. al, 2014) and decline often problems such as localized dry spots (Karnok et. al, 2001, York et. al 1993).
Even if is recorded that surfactants improve water homogeneity in soil, this is not necessary correlated with increment of volumetric water content (Alvarez et. al, 2016, Sciavon et. al, 2014).
On the other hand other authors support that they found significant differences in volumetric water content between treated (with surfactant) and untreated (without surfactant) experimental trials (Oostindie et. al, 2008, Barton et. al 2011).
Recently it is also reported that the application of surfactants and different irrigation techniques affected positively the water use efficiency of basil and berseem clover. The affection was appeared in clover leaf/stem ratio but also in other parameters of both plants such as plant height, seed yield and total forage dry matter (Daneshnia et al., 2015).
Surfactants have also enhanced plant growth and reduced water repellency in case of tomato. Chaichi et. al, (2017), reported that the application of wetting agents in combination with non-saline water had beneficial effects on plant height, leaf number plant, stem dry weight, leaf dry weight, root dry weight, shoot dry weight compared to control experimental trials.
Also, Chaichi et. al, (2015) studied the impacts of surfactants on yield and irrigation use efficiency under limited irrigation treatments, in corn. Significant differences were found between shoot dry matter and leaf dry matter of controlled and treated trials. It is also mentioned that the highest shoot dry matter was recorded at full irrigation by application of surfactant. Furthermore, they were recorded positive effects of surfactants in ear yield, total dry matter and grain yield between treated and untreated trials. Irrigation use efficiency both based on grain yield and total dry matter has shown higher rates in treated trials rather than controlled.
In case of potatoes crop yield and water content were estimated and compared to untreated trials (Cooley et. al, 2009, Oostindie et. al, 2012). According to the results of both authors surfactants are able to minimize the dry zones of treated plots since moisture distribution will be improved. Furthermore, an increased trend on tuber quality and yield performance was noted in both cases of treated plots.
Although in case cotton where surfactant had been applied in a wettable soil no significant differences were recorded on yield or soil water content (Sullivan et. al, 2009).
Possible effects of surfactants have been also investigated in studies based only on soil. In the most of them soil types that have been examined are sandy, loam, silt loam, silt clay loam, sandy loam, sandy clay loam and peat (Reinikainen et. al, 1997, Karagunduz et. al, 2001, Lehrsch et. al, 2011, Chaichi et. al, 2016).
Surfactants seem to reduce water repellency as it was concluded by Water Drop Penetration Time tests that were taken place (Lehrsch, 2013, Lehrsch et. al, 2011). Also, they are in agreement with Reinikainen et. al, (1997), who claim that surfactants apart from effects on reduction of soil water repellency, increase even slightly the water content of soil.
Nutrient use efficiency
A few studies have been made regarding the effects of surfactants on nutrients’ uptake of soil and the potential improvement in the efficiency of fertilisation.
Evidences from Baratella et. al, (2016), show that when surfactant is applied at an appropriate dose to fertilized soil, is able to increase root length and mass of lettuce and consequently to improve efficiency of nutrients’ use. As a result the hypothesis that surfactant might effects on the root-soil mingle by facilitating water uptake in root system and correspondingly the nutrient absorption by lettuce, may be supported.
Moreover, surfactants have been successfully used in tomatoes where they declined salinity stress by helping the plants to retain their nutrient balance especially reducing sodium (Na) immersions, as in high contents disrupt the nutrient stability and therefore ion toxicity caused (Chaichi et.al, 2015).
On the other hand, Guillén et. al, (2005), support that assimilation of cations and anions in case of tomatoes had no differences by the application surfactant in both soils that had been tested.
Cooley et. al, (2009) also determined that in some cases decreasing NO3- leaching below the potato plant root-zone can be achieved by surfactant use. That study is in accordance with Arriaga et. al, (2009), who claim that there are some potentials for non-ionic surfactants to decline nitrogen (N) losses in potato yields.
Soil water repellency
Soil water repellency is a global agricultural challenge as it has been reported in a large number of countries along the world such as Australia, New Zealand, Colombia, Puerto Rico, United Kingdom and Ireland (Sullivan et. al, 2009, Snyder et. al, 2004).
Soil water repellency is considered to be a seasonal phenomenon; lowest when the soil is wet, and most intense during long dry periods and often occurs at a critical water content, which can vary depending upon soil type (Barton et. al, 2011).
This phenomenon has been noticed in soils such as sand, sandy loam, loam, clay, sandy peat and clayey peat (Oostindie et. al, 2012).
Water repellency in soils is caused by organic hydrophobic compounds surrounded the surfaces of soil particles. Some of the compounds are excrete by roots and some others are produced by fungi and microbes (Cisar et. al, 1999, Kostka et. al, 1999, Alvarez et. al, 2016). In some other cases hydrophobic coatings are caused by highly humidity conditions. Main problems that can be caused in soils by hydrophobicity are decrease of water retention, percolation and infiltration.
Infiltration rate depends on organic matter content, soil texture, soil moisture content, and compaction (Chaichi et. al, 2016), which all are effected by soil water repellency.
Furthermore hydrophobic soil circumstances are able to lead to the performance of preferential flow paths in soil, increased run-off and declined moisture distribution (Alvarez et. al, 2016). As a consequence, obtainable water for plants, nutrient availability and erosion processes are negatively affected (Sullivan et. al, 2009).
Water repellency may affect negative water and solute movement and has been proved to defy nonuniform preferential and water paths in many soils including sandy soils (Dekker et. al, 2004, Aamlid et. al, 2009; Soldat et. al 2010).
Hydrophobicity in soils is often correlated with sand based root-zones. Some research has shown that hydrophobicity is due to organic coatings that entwine soil particles (Karnok et. al, 2001, Dekker et. al, 2004). Snyder et. al, (2004) mentions that a potential factor of water repellency is “an oily or waxy layer on the soil particles”. Also, Nemmati et. al, (2017), agrees mentioning that hydrophobic organic materials such as waxes, rosins and different organic acids develop “non-polar coatings” on the surface of soil particles. These coatings are not necessarily thick nor cover the whole area of soil particles. (Oostinfie et. al, 2012).
Soil water repellency is considered to be a seasonal phenomenon; lowest when the soil is wet, and most intense during long dry periods and often occurs at a critical water content, which can vary depending upon soil type (Barton et. al, 2011). Nemati et. al, (2017) specify that in case of peat, when soil moisture is below 40% (by weight), these organic components are able to develop hydrophobic properties which do not allow soil to be wetted properly (Nemati et. al, 2017).
Furthermore, sometimes particulate organic matters such as bits of stems and leaves may be mixed in with mineral soil particles and cause significant repellent problems (Oostinfie et. al, 2012).
Humic and fulvic acids are both involved as the organic coating substance that are responsible for soil hydrophobicity, as both of them become hydrophobic after elongated dry conditions (Karnok et. al, 2001, Snyder et. al, 2004).