SEASONAL CROP COEFFICIENT OF GERBERA SOILLESS CULTURE A. Papadopoulosl. E. Maloupa^. F. Papadopoulos^ 1. National Agricultural Research Foundation, Soil Science Institute, 541 10 Thessaloniki, Greece. 2. National Agricultural Research Foundation, Agricultural Research Center of Northern Greece, 570 01 Thermi, Greece. Abstract Seasonal crop coefficients, for gerbera soilless culture grown in perlite, sand, zeolite and rockwool substrates in plastic greenhouses, were estimated during the growing season, summer 1991 - summer 1993. The crop coefficients were calculated on a monthly basis as the ratio of the actual évapotranspiration ETa, measured using the water balance formula, to the potential évapotranspiration ETp, as it is given by the solar radiation method. The monthly values of the crop coefficient showed that they can be grouped into four categories, one for each growing period. The first growing period is summer, soon after the transplantation, where the mean value of crop coefficient for sand - zeolite was found 0.47 and for perlite - rockwool 0.53. The second period is the flowering one (autumn - spring) with a mean value of 0.54 for sand - zeolite and 0.71 for rockwool perlite. The third one is summer, where the flowering is ceased for commercial reasons, with a mean value of 0.35 for sand and zeolite and 0.52 for rockwool and perlite. The fourth period is the flowering one of the second year with a mean value of 0.51 for sand and zeolite and 0.72 for rockwool and perlite. Additional index words: évapotranspiration, crop coefficient, substrate, soilless culture, greenhouse, gerbera. 1. Introduction The main objective of irrigation is to provide plants with sufficient water to prevent stress that may cause reduced yield or poor quality of harvest. The required timing and amount of applied water is governed by the prevailing climatic conditions, crop and stage of growth, soil moisture holding capacity, and the extent of root development as determined by type of crop, stage of growth, and soil. Water is transferred to the atmosphere by direct evaporation from soil surface as well as from plant stomata (transpiration). Since these processes each involve evaporation and are not easily separated, they are combined and called évapotranspiration, ET. The determination of évapotranspiration, and therefore, plant water irrigation requirements from meteorological data, using the Blaney-Criddle, Radiation, and Penman methods, do not take into account plant factors such as extent of root development, stage of growth etc. Therefore, these methods estimate potential rather than actual évapotranspiration, (Doorenbos & Pruitt, 1984). The relationship between the actual crop ETa, at a specific time in its growth stage, and potential ETp is of practical interest. It enables the farmer or extension service agronomist, as well as the designer or operator of an irrigation system, to define the irrigation water Acta Horticulturae 408, 1995 Soilless Cultivation Technology for Protected Crops 81
requirements of the crops. Actual évapotranspiration estimates are often made from potential évapotranspiration measurements using the following relationship: ETa = Kc. ETp (1) where: ETa is the actual évapotranspiration in mm, ETo is the potential ET in mm, Kc is the crop coefficient. The coefficient Kc is given by tables or is determined experimentally and reflects the physiology of the crop, the degree of crop cover, the location where the climatic data were collected, and the method used to compute ETp. In the experimental determination of crop coefficients, both actual and potential ET are measured concurrently. The crop coefficient is then calculated as the dimensionless ratio of the two measurements using the following formula: Kc = ETa / ETp (2) Values of Kc generally increase from an initial plateau to a peak plateau and then decline as the plant progresses through its growth stages. The distribution of crop coefficients with time is known as the crop curve. If the crop coefficient is known, it is possible to assess the actual évapotranspiration in regions different from that of the experiment, provided that there are sufficient climatic data of the area. The problem that arises for greenhouses is the lack of these climatic data for the particular greenhouse environment, that could be used in Penman or Blaney Criddle, methods. For this reason, it is accepted that the potential évapotranspiration, for the greenhouse environment, is equal to the solar radiation, penetrating the greenhouse, Rs6, expressed in equivalent evaporated water depth (mm/day), (Morris G.P. et al., 1957). The only climatic input required is the mean actual sunshine duration, measured at a nearby meteorological station. The purpose of this work is to determine the values of gerbera crop coefficients for each month by measuring the actual évapotranspiration and estimating the potential one by applying the solar radiation method in a plastic greenhouse. 2. Materials and methods 2.1. Experimental layout The experiment was conducted in the Agricultural Research Center of Northern Greece (40 30' N and 22 55' E).The experiment has been established in a plastic greenhouse of 153 m2, that is 18.0m x 8.5m, with a forced air heating system and a double cover with 85 p, IR polyethylene inside during the cold season. It is estimated that the double cover absorbed 60% of the incoming radiation The temperature during winter (November - March) was kept above 12 C. During summer (May - September) the greenhouse was shaded using a net that absorbed 50% of the incoming radiation. The experimental design was a split-plot with the substrates as main plots and the varieties as subplots, in four replications. Every plot had 82 plants from each variety and the plant density was 5.1 plants/m. The planting date was the 21st June 1991. The substrates were in bags, covered with black and white polyethylene film. The volume was 10 1 substrate per plant. From the four substrates used, three were locally available, sand, perlite and zeolite (bulk density 1.85 g/ml) and one was imported, rockwool slabs 7.5 x 15 x 100 cm. The sand 82
came from a riverside and had a particle size from 0.002 to 2.0 mm. Perlite had a size of 3 to 5 mm. The four cultivars of gerbera grown were: Fame, Party, Regina and Chimena. The supplied nutrient solution contained (mmol/1) 2 N03; 1.5 P04; 0.5 NH4; 5.5 K ; 4 Ca ; 2 Mg. After transplanting the flowers in the substrate slabs, each plant was irrigated by a dripper of nominal discharge 2 1/h. A control unit, connected with 4 electrovalves, one for each substrate, regulated the plant nutrient solution on a daily basis. The number of water applications per day ranged from 5 to 10, according to the plant needs. The volume of the nutrient solution drained was measured for each slab after 24 hours of water application. The draining solution was collected in a tank and reused. This procedure was repeated for each substrate. During the two summer months July and August the plants were defoliated and the flower buds were cut before their development, in order to prevent their growth due to the small demand for flowers during this period. 2 2. Estimation of actual évapotranspiration The actual évapotranspiration ETa was determined by a direct measurement technique based on the conservation of mass principle, Eq. 3: ETa = inflow - outflow (3) where inflow, outflow are the total flow into and out of the control volume during the time interval (mm). It is obvious that in greenhouses, irrigated with drippers several times per day, in initially saturated substrates enclosed in plastic bags, equation (3) has the following form: ETa = Ir - D (4) where: Ir is the applied water depth in mm D is the draining water in mm. Equation 4 was used to estimate the actual évapotranspiration of gerbera, in the four substrates, during this experiment. 2.3. Estimation of potential évapotranspiration Potential ETp is the maximum rate at which water, if available, can be removed from soil and plant stomata. It depends on the amount of energy available for evaporation and varies from day to day. Many methods, with differing data requirements and levels of sophistication have been developed for computing ETp. Some of these methods require daily measurements of relative humidity, solar radiation, wind and air temperature data, (Penman method), while others need only, mean monthly temperatures (Blaney Criddle method). In this experiment the method of solar radiation, modified for greenhouses, was used to estimate the potential ET, (De Villele, 1974). The method is based on the following principles: The amount of solar radiation reaching the upper limit of the atmosphere of a region (Ra) depends on the latitude and month of the year. It can be expressed in cal/cm /day or in mm of evaporated water per day (table 1). A part of this radiation is absorbed and diffused into the atmosphere and the rest of it, along with a part of the diffused one, reaches the soil 83
surface (Rs). The amount of Ra that reaches the earth, that is Rs, depends on the actual sunshine duration, measured with the heliograph of Campbell-Stokes, and it can be estimated from the following formula: Rs = (a + b.n/n).ra (5) Where: Rs is the incoming shortwave radiation that reaches the soil surface in mm/day, Ra is the extraterrestrial radiation in mm/day (table 1 ), n is the mean actual sunshine duration in hr/day measured with the heliograph of Campbell-Stokes, N is the maximum possible sunshine duration in hr/day (table 2), a, b are two constants which can be estimated experimentally for each region. For Thessaloniki their values are, (Hatzigiannakis and Giakoumakis, 1988): a = 0.35 and b = 0.45. The radiation that infiltrates through the greenhouse Rs9 is considered as the potential évapotranspiration ET and is given ETp = Rsne = Rs C1,C2 (6) Where: C1 is the absorption rate of the greenhouse cover, C2 is the absorption rate of the net (artificial shading). 3. Results Figures 1 and 2 show the monthly and seasonal variation of gerbera crop coefficient for sand-zeolite and rockwool-perlite, respectively. The monthly values of the crop coefficient showed that they can be grouped into four categories, one for each growing period. The first growing period is the summer, soon after the transplantation, where the values of the crop coefficient for perlite-rockwool have a mean value of 0.53 while for sandzeolite have a mean value of 0.47. The second period is the flowering one (autumn - spring) with a mean value of 0.54 for sand - zeolite and 0.71 for rockwool perlite. The third one is the summer, where the flowering is ceased for commercial reasons, with a mean value of 0.35 for sand and zeolite and 0.52 for rockwool and perlite. The fourth period is the flowering one of the second year with a mean value of 0.51 for sand and zeolite and 0.72 for rockwool and perlite. 4. Discussion The high yield and quality of the produced flowers, figure 3, reveals the adequate nutrition and moisture supply of the plants. The variation between the four gerbera varieties is attributed to the genetic potential of each one or the physicochemical properties of the 84
substrates. The seasonal distribution of crop coefficient kc for rockwool and perlite was higher than that of sand and zeolite, figure 4. This is due to the fact that the actual ET, for rockwool and perlite, was higher than that for the other two substrates. The actual ET from perlite and rockwool was higher than that of sand and zeolite because of the lower hydraulic conductivity of sand and zeolite. Therefore, the total water consumption was lower than that of perlite and rockwool. It is evident that the proposed kc values are valid if similar cultivation techniques are followed, especially shading of the greenhouse during summer months. Therefore, the proposed gerbera crop coefficients, kc, could be applied in regions with Mediterranean type climate and in connection with the solar radiation method, for estimating potential évapotranspiration in soilless greenhouse cultures. Acknowledgements This work has been partially supported by the contract 8001-CT90-0015 of the Commission of the European Communities. References De Villele 0., 1974. Besoins en Eau de Cultures Sous Serres, Acta Horticulturae 35:123-135. Doorenbos J. and Pruitt W.O., 1984. Guidelines for Predicting Crop Water Requirements, FAO, Rome, pp.144. Hatzigiannakis S. and Giakoumakis E., 1988. Water Requirements for Greenhouse Tomatoes and Cucumbers, Technical Report 96, Greek Ministry of Agriculture, Institute of Land Reclamations, Thessaloniki, Greece, pp. 17, (in Greek). Morris G. P., Neale E. and Postlethwaire, 1957. The Transpiration of Glasshouses Crops and its Relationship to the Incoming Solar Radiation, J. Agr. Eng. Research 2:111-122. 85
Table 1 - Extra Terrestrial Radiation (Ra) expressed in equivalent evaporation in mm/day. Lat. J F M A M J J A S O N D N 42 5..9 8.1 11.0 14,.0 16..2 17. 3 16. 7 15. 0 12.2 9,.1 6.5 5.2 40 6..4 8.6 11.4 14,.3 16..4 17. 3 16. 7 15. 2 12.5 9,.6 7.0 5.7 38 6..9 9.0 11.8 14,.5 16..4 17. 2 16. 7 15. 3 12.3 10..0 7.5 6.1 36 7..4 9.4 12.1 14..7 16..4 17. 2 16. 7 15. 4 13.1 10..6 8.0 6.6 34 7,.9 9.8 12.4 14,.8 16..5 17. 1 16. 8 15. 5 13.4 10..8 8.5 7.2 Table 2 - Mean Daily Duration of Maximum Possible Sunshine Hours (N) for different months and latitudes. Lat. J F M A M J J A S O N D N 42 9..4 10.6 11,.9 13..4 14..6 15.2 14.9 13..9 12..6 11,.1 9..8 9.,1 40 9,.6 10.7 11,.9 13..3 14..4 15.0 14.7 13..7 12..5 11,.2 10..1 9.,3 38 9,.8 10.8 11,.9 13,.2 14,.3 14.8 14.6 13,.6 12..5 11,.2 10..1 9.,5 36 10,.0 10.9 11,.9 13,.1 14,.1 14.6 14.4 13,.5 12..4 11,.3 10..2 9..7 34 10.2 11.0 11.9 13,.1 13,.9 14.4 14.2 13,.4 12,.4 11.3 10,.4 9..9 86
Table 3 - Mean évapotranspiration of gerbera in l/m 2 /day for sand and zeolite (Thessaloniki, Macedonia, Greece). Month n N Ra ETp Irrig. Drain. ETa* Kc Remarks h/day h/day mm mm mm % i mi Jun91 10.7 15.0 17.3 3.71 0.6 30 2,.14 0,.46 shaded Jul91 9.7 14.7 16.7 3.46 0.6 30 2,.14 0,.50 shaded Aug91 8.8 13.7 15.2 3,.11 0.5 30 1..78 0..46 shaded Sep91 7.9 12.5 12.5 2..54 0.45 28 1,.65 0,.52 shaded 0ct91 3.7 11.2 9.6 3.83 0.255 26 0,.96 0,.40 - Nov91 3.4 10.1 7.0 2.81 0.210 22 0..84 0..48 heated Dec91 3.6 9.3 5.7 2.40 0.265 25 1..01 0..67 heated Jan92 4.3 9.6 6.4 2.82 0.225 25 0,.86 0..49 heated Feb92 5.5 10.7 8.6 4.00 0.285 28 1..05 0..42 heated Mar92 5.2 11.9 11.4 4.99 0.416 30 1..49 0..48 heated Apr92 6.5 13.3 14.3 6.52 0.533 25 2..04 0..50 - May92 8.5 14.4 16.4 8,.08 0.666 25 2..55 0..71 shaded Jun92 10.5 15.0 17.3 3.68 0.420 20 1..71 0..40 shaded Jul92 9.8 14.7 16.7 3.47 0.325 15 1..41 0..32 shaded Aug92 7.7 13.7 15.2 2.93 0.315 18 1..32 0..32 shaded Sep92 7.9 12.5 12.5 6.34 0.25 23 0..98 0..31 shaded 0ct92 4.1 11.2 9.6 3.95 0.24 23 0..94 0..38 Nov92 4.0 10.1 7.0 2.96 0.25 22 0..99 0.,54 heated Dec92 3.7 9.3 5.7 2.41 0.245 22 0..97 0.,64 heated Jan93 6.2 9.6 6.4 3,.28 0.24 23 0..94 0..46 heated Feb93 5.3 10.7 8.6 4,.00 0.28 25 1..07 0.,43 heated Mar93 5.0 11.9 11.4 4,.92 0.35 26 1..32 0.,43 heated Apr93 7.3 13.3 14.3 6..83 0.50 25 1..91 0. 45 May93 7.1 14.4 16.4 7..50 0.6 25 2.,29 0. 61 shaded * it is referred to 5.1 plants/m 2 87
Table 4 - Mean évapotranspiration of gerbera in l/m 2 /day for perlite and rockwool (Thessaloniki, Macedonia, Greece). Month n N Ra ETp Irrig. Drain. ETa Kc Remarks h/day h/day n im m n mm X im i Jun91 10.7 15..0 17..3 4..64 0.6 30 2.14 0.,46 shaded Jul91 9.7 14..7 16..7 4..32 0.6 30 2.14 0..50 shaded Aug 91 8.8 13..7 15..2 3..89 0.5 30 1,.78 0..46 shaded Sep91 7.9 12..5 12..5 3.17 0.45 28 1,.65 0..52 shaded 0ct91 3.7 11. 2 9..6 2..39 0.435 26 1..64 0..69 - Nov91 3.4 10.,1 7..0 1,.76 0.285 22 1,.13 0.,64 heated Dec91 3.6 9..3 5..7 1,.50 0.335 25 1,.28 0.,85 heated Jan92 4.3 9..6 6.,4 1..77 0.315 25 1,.20 0.,68 heated Feb92 5.5 10..7 8..6 2..50 0.333 28 1,.22 0..49 heated Mar92 5.2 11..9 11..4 3.12 0.666 30 2,.38 0.,76 heated Apr92 6.5 13..3 14,.3 4.07 0.720 25 2,.75 0..68 - May92 8.5 14..4 16..4 3,.59 0.800 25 3,.06 0..85 shaded Jun92 10.5 15..0 17..3 4,.25 0.575 20 2..35 0. 55 shaded Jul92 9.8 14..7 16..7 4..42 0.510 15 2..21 0..50 shaded Aug92 7.7 13..7 15..2 4,.09 0.425 18 1..78 0..44 shaded Sep92 7.9 12..5 12..5 3.17 0.45 22 1,.77 0.,56 shaded 0ct92 4.1 11..2 9..6 2.47 0.35 24 1,.37 0..55 Nov92 4.0 10,.1 7..0 1.85 0.32 25 1,.27 0..69 heated Dec92 3.7 9,.3 5..7 1.51 0.33 23 1.31 0..87 heated Jan93 6.2 9.6 6.4 2.05 0.40 25 1.57 0..77 heated Feb93 5.3 10,.7 8..6 2,.50 0.50 26 1,.91 0.,76 heated Mar93 5.0 11,.9 11,.4 3,.07 0.50 28 1,.89 0.,62 heated Apr93 7.3 13,.3 14.3 4.27 0.65 26 2.49 0..58 May93 7.1 14.4 16.4 3.75 0.75 26 2.87 0..77 shaded * it is referred to 5.1 plants/m 2 * 88
CROP COEFFICIENT K 1991 I 1992 I 1993 I MONTH/YEAR MONTHLY VARIATION - 4 ~ MEAN SEASONAL VAR. Figure 1: Variation of Crop Coefficient (Rockwool, Perlite). CROP COEFFICIENT K MONTH/YEAR MONTHLY VARIATION -+- MEAN SEASONAL VAR. Figure 2: Variation of Crop Coefficient in Sand - Zeolite. 89
D Rockwool U Zeolite Sand S Perlite Figure 3: Commercial Production per Variety and Substrate in Gerbera Soilless Culture. CROP COEFFICIENT K 0.8 0.6 0.4 0.2 -J I. I I I I I I J J A S O N D J F M A M J J A S O N D J F M A M I 1991 I 1992 I 1993 I MONTH/YEAR SAND - ZEOLITE ROCKWOOL - PERLITE Figure 4: Seasonal Variation of Gerbera Crop Coefficient. 90