Acta agriculturae Slovenica, 117/3, 1–12, Ljubljana 2021 doi:10.14720/aas.2021.117.3.1456 Original research article / izvirni znanstveni članek Investigation about wetting ability (surface tension) of water used for preparation of pesticide solutions Donyo GANCHEV 1, 2 Received January 27, 2020; accepted May 17, 2021. Delo je prispelo 27. januarja 2020, sprejeto 17. maja 2021 1 Agricultural University – Plovdiv, Bulgaria, Faculty of Plant Protection and Agroecology, Department of Chemistry and Phytopharmacy 2 Corresponding author, e-mail: donyo@abv.bg Investigation about wetting ability (surface tension) of water used for preparation of pesticide solutions Abstract: The investigation about surface tension of water used for preparation of pesticide solutions reveals it is quite diverse and changeable without any logical correlation towards location, time, and type of water source. Moreover, spraying with solutions with lower surface tension give big- ger flow rates due to the lower resistance of fluid to the noz- zles. The conducted trials show that plant surfaces with more rough texture require to be sprayed with pesticide solutions with lower surface tension. The wax content of the surfaces has no significant impact on surface tension requirement. Key words: surface tension; pesticides; plant protection products; sprayers; wetting ability Preučevanje omočitvene sposobnosti (površinske napetosti) vode za pripravo raztopin pesticidov Izvleček: Raziskave glede površinske napetosti vode up- orabljene za pripravo raztopin pesticidov so odkrile, da je ta zelo raznolika in, da se spreminja brez logične povezave glede na lokacijo, čas in vir vode. Škropljenje z raztopino z manjšo površinsko napetostjo daje večje pretoke zaradi manjšega upora tekočine v šobah. Izvedeni poskusi kažejo, da površine rastlin z bolj grobo teksturo zahtevajo škropljenje z raztopino pesticidov z manjšo površinsko napetostjo. Vsebnost voska na površini nima značilnega vpliva glede zahtev o površinski napetosti raztopine pesticidov. Ključne besede: površinska napetost; pesticidi; priprav- ki za zaščito rastlin; razpršilci; omočitvena sposobnost Acta agriculturae Slovenica, 117/3 – 2021 2 D. GANCHEV 1 INTRODUCTION Using of pesticide solution with low surface ten- sion (good wetting ability) is crucial for achievement of satisfactory level of pesticide effectiveness (Crease & Thacker, 1991; Ellis et al., 2001,). If the surface ten- sion is too high (wetting ability – too low), pesticide solutions will be at the form of drops on the sprayed surfaces (they will not cover all surface and can be eas- ily dropped down) on one hand and they will not pen- etrate fully in the rough surfaces from the other. (Ellis et al., 2001). In both cases, the effectiveness of pesticides can be dramatically lowered, especially when pesticide solutions are sprayed in relatively low temperatures (which increase surface tension of the water) and on plant parts with rough surface textures (which require solutions with lower surface tension for full cover in order pesticide solution fully to penetrate into of the surfaces plant tissues - cuticle). The spraying of plants with pesticide solutions over the point of run-off can cause significant risk for contamination of soil and wa- ters (Bergström, 1990). Traditionally it is expected that common tap wa- ter or water from rivers or lakes used for preparing of pesticide solutions have a constant surface tension only slightly depending from temperature (Gittens, 1969; Grisso et al., 1988). Different surface tension means dif- ferent flow rates and respectively different sprayed dose rates (Matthews, 2008). The main aim of present investigation was to reveal that the surface tension of the water is highly diverse and changeable and it depends on numerous factors and cannot be predicted in any way unlike common acceptance (Claussen, 1967,; Pallas & Pethica, 1983; Kalová & Mareš, 2015;). Also, the common acceptance that plant surfaces with high content of wax like cab- bage leaves require to be sprayed with pesticide solu- tion with lower surface tension (better wetting ability) also was wrong – the solutions with lower surface ten- sion is required for surfaces with more rough textures, not with higher wax content (Bartell & Zuidema, 1936; Hess and Foy, 2000; Wagner et al., 2003). The spraying of solutions with different surface tension can signifi- cantly change the flow rates of the sprayers and must be taken into account during treatments with plant pro- tection products (Semiao et.al., 1996; Miller and Ellis, 2000; De Schampheleire et al., 2009) 2 MATERIALS AND METHODS Water samples from different sources in Bul- garia were taken during 2018 year. The sources were: - tap water from different towns and villages (from dif- ferent districts of given town / village); - river and dam lake water from sources situated nearby different towns and villages in Bulgaria; - well water which although rarely, also is used some- times for making pesticide solutions (especially during summer drought) from sources situated in different towns and villages; The 3 samples from one source (location) were taken four times in different time different seasons – spring (in May), winter (in February) , summer (in Au- gust) and autumn ((in October). The surface tension of water samples was established by tensiometer K6 Du Noüy Ring (Macy, 1935), produced by Krüss Scientific, at 24 °C checked with digital thermometer and average results was presented (used) for each taken sample. Samples (leaves, barks, fruits) from different plants (mainly agricultural, but also decorative and forest plants) in different BBCH growth stages were taken and were sprayed with solutions made from distilled water and organosillicone surfactant Silwet L 77, pro- duced by Momentive Performance Materials with dif- ferent surface tension at 24 °C (Stevens et al., 1988). The ability of solutions to form even film, to spread easily and uniformly over the treated surfaces (wetting abil- ity) was examined. A liquid with high wetting ability forms a thin, continuous film when it spreads over the surface which allow compete cover of the treated sur - face with pesticide solutions for maximal effectiveness (maximum cover, exposure and retention) from one side and minimal pollution of the environment (mini- mal amounts of pesticides fall on soils and waters from treated plants) – from the other (Prado et al., 2016; Zhu et al.,2019 ). Lower surface tension means better wet- ting ability (Bonn et al., 2009). Effective wetting ability requires the surface tension of the adhesive (Y adhesive ) to Figure 1: Wetting surface requirement Acta agriculturae Slovenica, 117/3 – 2021 3 Investigation about wetting ability (surface tension) of water used for preparation of pesticide solutions be less than or equal to that of the substrate (Y substrate ) (Yuan & Lee, 2013) (Figure. 1) The flow rate with solution with different surface tension was examined by hand compression Knapsack Sprayer Matabi super green 12 , produced by Goizper. The size of droplets was visually determined by water sensitive papers, produced by Syngenta by method described by Turner and Huntington (1970); Saly- ani and Fox (1999) and Cerruto et al.(2016) and by digital micrometer (KINEX ABSOLUTE ZERO, 0-25 MM, 0,001MM, DIN 863, IP 65) produced by KINEX Measuring. Nozzles were set to produce spray plum consisting of relatively uniform droplets with diameter of 25 µm or 300 µm. 3 RESULTS AND DISCUSSION The results from conducted trials with measure- ments of the surface tension from different water sourc- es are presented bellow as tables indicated location of the taken water samples, geographical coordinates, height above the sea level (m), date of taking samples and the average value surface tension of each sample. 3.1 RESULTS OF TAP W ATER ANALYSIS The presented Table 1 shows that surface tension of water is absolutely changeable and of upredictable value and can vary in remote places or on a very short distance. For example – town of Plovdiv, Bulgaria, has approximately 102 square kilometers area. The tap water from different residential districts has different surface tension. The same situation is in the towns of Karlovo and Sopot in Central Bulgaria, where the distance be- tween towns is only 4 km or town of Stamboliiski and village of Joakim Gruevo, where the distance between them is 2 km. The surface tension value of tested tap water was completely different and unpredictable. Location Coordinates Height above the sea level (m) Date of taking sample Measured surface tension (mN m -1 ) Town of Karlovo 42°63′66″N 24°79′98″E 452 10.02.2018 64 Town of Karlovo 42°63′66″N 24°79′98″E 452 10.05.2018 58 Town of Karlovo 42°63′66″N 24°79′98″E 452 10.08.2018 71 Town of Karlovo 42°63′66″N 24°79′98″E 452 10.10.2018 57 Village of Vasil Levski, town of Karlovo district 42°60′80″N 24°88′95″E 444 10.02.2018 78 Village of Vasil Levski, town of Karlovo district 42°60′80″N 24°88′95″E 444 10.05.2018 55 Village of Vasil Levski, town of Karlovo district 42°60′80″N 24°88′95″E 444 10.08.2018 58 Village of Vasil Levski, town of Karlovo district 42°60′80″N 24°88′95″E 444 10.10.2018 64 Town of Stamboliiski 42°13′25″N 24°52′20″E 200 03.02.2018 63 Town of Stamboliiski 42°13′25″N 24°52′20″E 200 03.05.2018 72 Town of Stamboliiski 42°13′25″N 24°52′20″E 200 03.08.2018 71 Town of Stamboliiski 42°13′25″N 24°52′20″E 200 03.10.2018 65 Table 1: Surface tension of tap water samples taken in Bulgaria during 2018 in different seasons (winter, spring, summer and autumn) Acta agriculturae Slovenica, 117/3 – 2021 4 D. GANCHEV Village of Joakim Gruevo, town of Stambiliiski district 42°11′93″N 24°55′83″E 289 03.02.2018 69 Village of Joakim Gruevo, town of Stambiliiski district 42°11′93″N 24°55′83″E 289 03.05.2018 72 Village of Joakim Gruevo, town of Stambiliiski district 42°11′93″N 24°55′83″E 289 03.08.2018 68 Village of Joakim Gruevo, town of Stambiliiski district 42°11′93″N 24°55′83″E 289 03.10.2018 64 Town of Jambol 42°47′78″N 26°49′22″E 114 20.02.2018 70 Town of Jambol 42°47′78″N 26°49′22″E 114 20.05.2018 70 Town of Jambol 42°47′78″N 26°49′22″E 114 20.08.2018 74 Town of Jambol 42°47′78″N 26°49′22″E 114 20.10.2018 68 Town of Varna 43°22′20″N 27°88′18″E 80 25.02.2018 72 Town of Varna 43°22′20″N 27°88′18″E 80 25.05.2018 58 Town of Varna 43°22′20″N 27°88′18″E 80 25.08.2018 71 Town of Varna 43°22′20″N 27°88′18″E 80 25.10.2018 70 Agricultural University, town of Plovdiv 42°13′35″N 24°76′68″E 164 01.02.2018 71 Agricultural University, town of Plovdiv 42°13′35″N 24°76′68″E 164 01.05.2018 70 Agricultural University, town of Plovdiv 42°13′35″N 24°76′68″E 164 01.08.2018 72 Agricultural University, town of Plovdiv 42°13′35″N 24°76′68″E 164 01.10.2018 65 town of Plovdiv, residential district Trakia 42°13′68″N 24°79′46″E 164 01.02.2018 63 town of Plovdiv, residential district Trakia 42°13′68″N 24°79′46″E 164 01.05.2018 70 town of Plovdiv, residential district Trakia 42°13′68″N 24°79′46″E 164 01.08.2018 63 town of Plovdiv, residential district Trakia 42°13′68″N 24°79′46″E 164 01.10.2018 70 town of Plovdiv, residential district Komatevo 42°10′38″N 24°70′54″E 164 05.02.2018 72 town of Plovdiv, residential district Komatevo 42°10′38″N 24°70′54″E 164 05.05.2018 63 town of Plovdiv, residential district Komatevo 42°10′38″N 24°70′54″E 164 05.08.2018 68 town of Plovdiv, residential district Komatevo 42°10′38″N 24°70′54″E 164 05.10.2018 71 town of Plovdiv, residential district Karshiaka 42°16′16″N 24°74′23″E 164 01.02.2018 65 Acta agriculturae Slovenica, 117/3 – 2021 5 Investigation about wetting ability (surface tension) of water used for preparation of pesticide solutions town of Plovdiv, residential district Karshiaka 42°16′16″N 24°74′23″E 164 01.05.2018 72 town of Plovdiv, residential district Karshiaka 42°16′16″N 24°74′23″E 164 01.08.2018 70 town of Plovdiv, residential district Karshiaka 42°16′16″N 24°74′23″E 164 01.10.2018 65 Town of Sopot 42°65′04″N 24°76′51″E 417 10.02.2018 58 Town of Sopot 42°65′04″N 24°76′51″E 417 10.05.2018 70 Town of Sopot 42°65′04″N 24°76′51″E 417 10.08.2018 67 Town of Sopot 42°65′04″N 24°76′51″E 417 10.10.2018 70 Town of Kalofer 42°61′11″N 24°97′78″E 666 10.02.2018 67 Town of Kalofer 42°61′11″N 24°97′78″E 666 10.05.2018 66 Town of Kalofer 42°61′11″N 24°97′78″E 666 10.08.2018 69 Town of Kalofer 42°61′11″N 24°97′78″E 666 10.10.2018 70 3.2 RESULTS OF RIVER W ATER ANALYSIS From Table 2 we can see that just like in the case of tap water, the samples from river water also have differ - ent surface tension in different seasons. The water from river Nevolia in the village of Vasil Levski, in Febru- ary had surface tension of 71 mN m -1 , the sample taken from same river, at the same time in the town of Bania, which is situated lie about 7 km south, had surface ten- sion of 59 mN m -1 . The tap water however taken from house 5 m away from the river Nevolia during Febru- ary has 78 mN m -1 surface tension. The results confirm the unpredictable nature of the surface tension value towards river waters just like tap waters. Some early investigations reveal direct connection between natu- ral waters contaminations and changes in the surface tension (Pockels, 1893). Certain contaminants can alter water surface tension significantly. The value of surface tension is almost independent of pH of the water (Srid- har & Reddy, 1984; Beattie et al., 2014). 3.3 RESULTS OF LAKE W ATER ANALYSIS Just like river and tap water samples, the results show absolutely changeable and unpredictable surface tension values for lake. For example, the tap water of the village of Vasil Levski in February had 78 mN m -1 surface tension, the value of this index of river water sample taken from river 5 meters away was 71 mN m -1 , the surface tension of dam lake water taken from dam lake Vasil Levski situated approximately 1,8 km away from the river was 65 mN m -1 3.4 RESULTS OF WELL W ATER ANALYSIS The tap water of the village of Vasil Levski in Feb- ruary had 78 mN m -1 surface tension, the value of this index of river water sample taken from river 5 meters away was 71 mN m -1 , the surface tension of dam lake water taken from dam lake Vasil Levski situated ap- proximately 5 km away from the river was 65 mN m -1 , however the water sample taken from well situated 20 m away from place where river sample and tap water sample were taken, had 57 mN m -1 surface tension. The previously conducted research also confirmed that sur - face tension of water of soils (underground water) is less than that of pure water (Tschapek et al., 1978). From the results for well waters in present study, is obvious that this value sometimes is less, sometimes is bigger. For sure is completely unpredictable and changeable. However, in present study only freshwater sources were examined. Other study shows that according to the seawater sample analysis there is similar uncertainty in prediction on surface tension as sweet water (Nayar et al., 2014). Acta agriculturae Slovenica, 117/3 – 2021 6 D. GANCHEV Table 2: Surface tension of river water samples taken in Bulgaria during 2018 in different seasons (winter, spring, summer and autumn) Location Coordinates Height above the sea level (m) Date of taking sample Measured surface tension (mN m -1 ) River Nevolia, village of Vasil Levski, town of Karlovo district 42°60′80″N 24°88′99″E 444 10.02.2018 71 River Nevolia, village of Vasil Levski, town of Karlovo district 42°60′80″N 24°88′99″E 444 10.05.2018 56 River Nevolia, village of Vasil Levski, town of Karlovo district 42°60′80″N 24°88′99″E 444 10.08.2018 74 River Nevolia, village of Vasil Levski, town of Karlovo district 42°60′80″N 24°88′99″E 444 10.10.2018 62 River Nevolia, town of Banija 42°56′33″N 24°83′02″E 295 10.02.2018 59 River Nevolia, town of Banija 42°56′33″N 24°83′02″E 295 10.05.2018 63 River Nevolia, town of Banija 42°56′33″N 24°83′02″E 295 10.08.2018 68 River Nevolia, town of Banija 42°56′33″N 24°83′02″E 295 10.10.2018 72 River Strjama, town of Banija 42°54′20″N 24°82′08″E 295 10.02.2018 60 River Nevolia, town of Banija 42°56′33″N 24°83′02″E 295 10.05.2018 74 River Nevolia, town of Banija 42°56′33″N 24°83′02″E 295 10.08.2018 75 River Nevolia, town of Banija 42°56′33″N 24°83′02″E 295 10.10.2018 57 River Jantra, town of Gabrovo 42°89′09″N 25°32′42″E 392 27.02.2018 71 River Jantra, town of Gabrovo 42°89′09″N 25°32′42″E 392 27.05.2018 68 River Jantra, town of Gabrovo 42°89′09″N 25°32′42″E 392 27.08.2018 65 River Jantra, town of Gabrovo 42°89′09″N 25°32′42″E 392 27.10.2018 65 River Osam, town of Trojan 42°91′52″N 24°71′01″E 380 27.02.2018 64 River Osam, town of Trojan 42°91′52″N 24°71′01″E 380 27.05.2018 65 River Osam, town of Trojan 42°91′52″N 24°71′01″E 380 27.08.2018 55 River Osam, town of Trojan 42°91′52″N 24°71′01″E 380 27.10.2018 65 Acta agriculturae Slovenica, 117/3 – 2021 7 Investigation about wetting ability (surface tension) of water used for preparation of pesticide solutions Table 3: Surface tension of dam lake water samples taken in Bulgaria during 2018 in different seasons (winter, spring, summer and autumn) Location Coordinates Height above the sea level (m) Date of taking sampels Measured surface tension (mN m -1 ) Dam lake Vasil Levski, village of Vasil Levski, Karlovo district 42°61′48″N 24°91′16″E 444 10.02.2018 65 Dam lake Vasil Levski, village of Vasil Levski, Karlovo district 42°61′48″N 24°91′16″E 444 10.05.2018 60 Dam lake Vasil Levski, village of Vasil Levski, Karlovo district 42°61′48″N 24°91′16″E 444 10.08.2018 67 Dam lake Vasil Levski, village of Vasil Levski, Karlovo district 42°61′48″N 24°91′16″E 444 10.10.2018 70 Dam lake Murla, town of Sopot 42°64′74″N 24°77′21″E 417 10.02.2018 55 Dam lake Murla, town of Sopot 42°64′74″N 24°77′21″E 417 10.05.2018 55 Dam lake Murla, town of Sopot 42°64′74″N 24°77′21″E 417 10.08.2018 65 Dam lake Murla, town of Sopot 42°64′74″N 24°77′21″E 417 10.10.2018 68 Dam lake Kovatchevo, village of Kovatchevo, town of Stara Zagora district 42°21′83″N 24°20′90″E 133 20.02.2018 59 Dam lake Kovatchevo, village of Kovatchevo, town of Stara Zagora district 42°21′83″N 24°20′90″E 133 20.05.2018 70 Dam lake Kovatchevo, village of Kovatchevo, town of Stara Zagora district 42°21′83″N 24°20′90″E 133 20.08.2018 67 Dam lake Kovatchevo, village of Kovatchevo, town of Stara Zagora district 42°21′83″N 24°20′90″E 133 20.10.2018 65 Dam lake Zrebchevo, town of Nikolaevo 42°63′19″N 25°84′95″E 274 20.02.2018 58 Dam lake Zrebchevo, town of Nikolaevo 42°63′19″N 25°84′95″E 274 20.05.2018 68 Dam lake Zrebchevo, town of Nikolaevo 42°63′19″N 25°84′95″E 274 20.08.2018 57 Dam lake Zrebchevo, town of Nikolaevo 42°63′19″N 25°84′95″E 274 20.10.2018 73 Dam Lake Radetski, village of Radetski, town of Nova Zagora District 42°28′51″N 26°09′65″E 156 20.02.2018 66 Dam Lake Radetski, village of Radetski, town of Nova Zagora District 42°28′51″N 26°09′65″E 156 20.05.2018 70 Dam Lake Radetski, village of Radetski, town of Nova Zagora District 42°28′51″N 26°09′65″E 156 20.08.2018 70 Dam Lake Radetski, village of Radetski, town of Nova Zagora District 42°28′51″N 26°09′65″E 156 20.10.2018 65 Dam Lake Matza, village Matza, town of Polski Gradetz district 42°22′14″N 26°15′20″E 190 20.02.2018 71 Dam Lake Matza, village Matza, town of Polski Gradetz district 42°22′14″N 26°15′20″E 190 20.05.2018 74 Dam Lake Matza, village Matza, town of Polski Gradetz district 42°22′14″N 26°15′20″E 190 20.08.2018 71 Dam Lake Matza, village Matza, town of Polski Gradetz district 42°22′14″N 26°15′20″E 190 20.10.2018 70 Acta agriculturae Slovenica, 117/3 – 2021 8 D. GANCHEV 3.5 RESULTS ON ANALYS OF REQUIREMENTS FOR DIFFERENT PLANTS ABOUT WETTING ABILITY OF SPRAYS IN PESTICIDES APPLYCATION Figures 2, 3 and 4 shows the wetting requirements of the leaves of different cultures in four different grow- ing stages. From presented results above can be clearly seen that wetting ability of the sprayed liquid does not de- pend on plant surface structures wax content, but on the roughness of plant surface texture. The cabbage leaves although have a high wax content but are relative smooth (Lee et al., 1988), require to be sprayed with solutions with surface tension 25 mN m -1 for full wet- ting (Figure 4). However, the bean leaves with less wax content but more rough textures for full wetting require solutions with surface tension 21 mN m -1 (Figure 4). The tomato leaves in earlier growing stages when are more rough require solutions with 22 mN m -1 surface tension, while in late growing stages when they develop more smooth texture need to be sprayed with solutions with 30 mN m -1 surface tension. The leaves of apples (Malus domestica Borkh.), potato (Solanum tuberosum L.), maize (Zea mays L.) and cauliflower (Brassica ol - eracea L. ssp. oleracea convar. Botrytis (L.) Alef) which are relativelly smooth in all phenophases require to be sprayed with solutions with surface tension close to 22 mN m -1 (Figure 3). Table 4: Surface tension of well water samples taken in Bulgaria during 2018 in different seasons (winter, spring, summer and autumn) Location Coordinates Height above the sea level (m) Date of taking sampels Measured surface tension (mN m -1 ) Village of Radetski, town of Nova Zagora district 42°28′39″N 26°11′45″E 156 20.02.2018 71 Village of Radetski, town of Nova Zagora district 42°28′39″N 26°11′45″E 156 20.05.2018 58 Village of Radetski, town of Nova Zagora district 42°28′39″N 26°11′45″E 156 20.08.2018 72 Village of Radetski, town of Nova Zagora district 42°28′39″N 26°11′45″E 156 20.10.2018 70 Village of Vasil Levski, town of Karlovo district 42°60′84″N 24°88′83″E 444 10.02.2018 57 Village of Vasil Levski, town of Karlovo district 42°60′84″N 24°88′83″E 444 10.05.2018 68 Village of Vasil Levski, town of Karlovo district 42°60′84″N 24°88′83″E 444 10.08.2018 55 Village of Vasil Levski, town of Karlovo district 42°60′84″N 24°88′83″E 444 10.10.2018 71 There is obvious difference between leaves of cab- bage and cauliflower, leaves of the second have more rough texture and for full wetting required solutions with lower surface tension. The test with bark also reveals that for good wetting of more rough textured plant surfaces – liquids with lower surface tension is required. 3.6 RESULTS OF ANALYSIS OF IMPACT OF W A- TER SURFACE TENSION ON NOZZLE FLOW RATE Figure 5 shows flow rates obtained with Matabi sprayer with solutions with different surface tensions. The nozzle of the sprayer was adjusted to spray with droplets with the size of 25 and 300 microns (di- ameter). The size of the droplets was visually deter - mined by water sensitive papers and digital microm- eter. The difference is obvious - the same sprayer with the same nozzle adjusted to a constant position and sprayed droplets with the same size but with solutions with different surface tension (wetting ability) achieved different flow rates. When sprayer works with distilled water (66 mN m -1 surface tension) 500 ml m -1 flow rate was achieved. The same sprayer, with the same nozzle adjustment working with the same water but with ad- dition of organosilicone surfactant Silwet L 77 at 0.01 % concentration (22 mN m -1 surface tension), achieved Acta agriculturae Slovenica, 117/3 – 2021 9 Investigation about wetting ability (surface tension) of water used for preparation of pesticide solutions 900 ml m -1 flow rate – 80 % more solution pass through nozzle. With increase of the droplet size from 25 to 300 microns, and addition of organosilicone surfactant Silwet L 77 to the distilled water again at 0.01 % (22 mN m -1 surface tension ), only 37 % more solution pass from the nozzle. In both cases (25 and 300 microns size of droplets) the addition of the same amount of organosilicone surfactant (0.01 % concentration) to the distilled wa- ter decreased the surface tension to the same level (22 mN m -1 ) and dramatically increased the flow rate of the sprayer nozzle. Figure 2: Wetting ability requirements of the crop leaves – on y axis are presented different plant leaves taken in different growth stages (BBCH stages); on x axis is measured surface tension (mN m -1 ) of pesticides spray needed for full coverage and penetra- tion in the texture of the leaves. Figure 3: Wetting ability requirements of the crop leaves – on y axis are presented different plant leaves taken in different growth stages (BBCH stages); on x axis is measured surface tension (mN m -1 ) of pesticides spray needed for full coverage and penetration in the texture of the leaves Acta agriculturae Slovenica, 117/3 – 2021 10 D. GANCHEV Figure 4: Wetting ability requirements of the crop leaves – on y axis are presented different plant leaves taken in different growth stages (BBCH stages); on x axis is measured surface tension (mN m -1 )of pesticides spray needed for full coverage and penetration in the texture of the leaves Figure 5: Flow rates (ml min -1 ) of solutions with different surface tensions - left two columns small droplets (25 µm ) and right two big droplets (300 µm) 4 CONCLUSION The conducted tests proved that surface tension of the water used for preparation of the pesticide spray was not of constant value and depended not only from temperature, but also on many other factors and could be completely different from different sources and time periods (season, weather) without any logical corre- lation with location, time and type of water source. Sprayed surfaces with more rough texture require to be treated with pesticide solutions with lower surface tension. 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