PLASTIC. TOMATO ROOT DEVELOPMENT ON SAND MULCH t GREENHOUSE IN ALMERIA (SPAIN)

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1 TOMATO ROOT DEVELOPMENT ON SAND MULCH t GREENHOUSE IN ALMERIA (SPAIN) PLASTIC N. Castilla Caja Rural de Almeria Apdo Almeria, Spain C. Gimenez and E. Fereres SIA and Univ. of Cordoba Apdo Cordoba, Spain Abstract Most of the ha of plastic greenhouses in Almeria use beach sand as mulch on a stratified, artificial soil profile. Manure is placed on strips, about 1 m wide and 2 cm deep, between the sand and the 20 cm of loam or clay soil placed on top of the original, rocky, sandy loam soil. Under such conditions, there is great uncertainty on the pattern and extension of root development and on the role that each soil layer plays in supplying water and nutrients to the crop. The use of localized, drip irrigation systems adds an additional level of complexity, increasing the spatial variablity of soil water and nutrients. A three-year study on root development of tomato, under three levels of irrigation and of nitrogen fertilization was conducted at'the Experimental Farm of the "Caja Rural de Almeria". A modified trench-profile technique and core sampling using Newman's method to estimate root length densities, gave estimates of root length for each soil layer as affected by the nitrogen and water supply. Root growth was substantial in the interface between the sand layer and the manure, with root length density values between 10 and 100 cm/cm^. Root length in the sand-manure zone contributed nearly 25 percent of the total measured root length. I. Introduction Plastic greenhouses in Almeria use poliethylene (PE) sheet as covering material, placed on a very simple wood or metallic tube structure, between two grids of wire that fix the plastic sheet. Climate in Almeria is mediterranean semiarid subtropical although the microclimate inside these greenhouses is far from optimum for most horticultural crops. Anyhow, this cropping system has been successful and, in the last decade, the surface area under plastic greenhouses has exceeded II 00 0 ha. The original soil profile is normally modified. Once the original rocky sandy loam soil has been levelled, a 20cm-thick layer of loam or clay soil is placed over it, and manure is added on top, in a 2cm-thick layer, under Acta Horticulturae, 191, 1986 Solana:ea in Mild Winter 113

2 a beach siliceous sand mulch of about 8 cm. Manure is partly incorporated to soil, about 20%, in amounts reaching 100 mt/ha. Every 3 to 4 years, the sand mulch is removed to till the soil, applying new manure or other organic material, partly added to the soil with the tillage and most of it placed in a new 2 cm layer under the sand. This operation is normally done in strips, up to 1.0 m wide (where double rows will be planted). Castilla (1985) has described evaporation reduction from the soil by the sand mulch and the low rates of water infiltration into the soil under this stratified, artificial soil profile. In addition, there are great uncertainties in root development that make necessary to characterize and evaluate root systems of the principal horticultural crops. We report here results on root density patterns estimated under the conditions described above. 2. Materials and methods Three experiments were conducted between 1980 and 1983 using undeterminate tomato (Vemone hybrid), under three irrigation levels. Tomato plants were trained up to 7-8 flower trusses in a winter growing cycle. Planting was in early autumn with 2 plants/m 2, and finishing harvest up to 31 weeks later. The low irrigation treatment received 85% water of the normal irrigation treatment the first two years, and 70% the last one. The high irrigation level received 115% water of the normal irrigation treatment the first two years, and 130% the last one. Water was applied through drip irrigation (2 drippers/m 2 ) Applied water by drip irrigation exceed the crop évapotranspiration (ET C ) the last year (in 25 mm), while the other two years did not reach it (20 mm deficit the first and 60 mm the second), for the average irrigation treatment (ET C values of 290 to 350 mm). A fourth trial was conducted in , at the normal irrigation treatment with three levels of mineral nitrogen fertilization: low (200 kg N/ha), medium (400) and high (600). The last two experiments took place just after the sand mulch removement to till the soil, in 1 m wide strips, and manure applying (figure 1). The first two trials took place three years after this operation. All the experiments were located in the southern part of the greenhouses, covered with 200 microns thick thermal PE sheet, under homogeneous microclimate conditions. Mean monthly air temperatures varied from 15 to 31 C, with mean maximum from 23 C to over 36 C and mean minimum from 8 C to 1 9 C. Daily mean air temperature oscillations were between 12 C to 20 C. Air relative humidity (RH) mean values, 60 to 80%, are not representative of the variable conditions day-night, for the maximum monthly means reached 100% RH while the minimum reached 33% RH. Solar radiation inside the 114

3 greenhouse represented 57% to 82% of incident solar radiation, varying along the season and influenced by the dust accumulated on the plastic roof. Soil temperature, at 20 cm depth under the manure layer, varied between 18 C and 20 C (monthly mean). Root length density in the soil, L v (cm of roots per cm^ of soil), was estimated from direct observations and from soil core samples. In the greenhouse, a modified trench-profile technique (Bohm et al., ; Bohm, ) was used digging a trench, smoothing the profile wall (as far as possible in the rocky zones of the soil), exposing the roots in a layer of 0.25 cm by removing this vertical soil layer with water pressure and counting the roots. In each profile, the vertical areas studied were 100x50 cm or 200x80 cm (figure 1), with two replicates on each treatment of all the experiments. As to the counting procedure, a 5x5 cm square grid net was placed against the profile wall and the number of intersections of roots with each side of the 5x5 cm sguare were counted. For each square, estimates of L v were calculated using Newman's procedure (Newman, 1966) and assuming a horizontal depth of view inside the soil of 0.25 cm. Soil core samples from the upper part of the profile (not rocky) were taken in the fourth trial, five per treatment, with a 2.0 cm diameter Veihmeyer auger, horizontally and perpendicularly to the profile wall. Soil cores were processed in the laboratory, separating the roots from the soil through a 100 mesh screen and counting root intersections with a grid, using Newman's method (Newman, 1966), modified by Henderson (D.W. Henderson, personal communication). Soil core root length density values were correlated with profile wall estimates of root length density. Root length densities in the sand and organic matter layers were estimated on volumetric samples, taken with a 5.3 cm diameter auger, in the last irrigation experiment ( ), with six replicates per treatment in the manured strips and other six replicates in the zones without manure. In the first irrigation experiment ( ) volumetric samples were taken in the organic matter layer (one per treatment), as well as in other crops (dwarf and climbing fresh beans and pepper) whose root density in soil were also estimated from direct observation of the profile. Organic matter and sand layers root length densities were estimated, as in soil core samples, by the Newman's method described above. All observations were made near the end of the growing season. 3. Results and discussion 3.1. Root length density in the sand and organic matter layers 115

4 Roots in sand layer, measured in laboratory, gave higher L v values in the low irrigation treatment, with a maximum value of 21.1 cm/cm 3 and mean Ly value of 11.7 cm/cm 3, while in the normal and high irrigation levels mean Ly values were 8.4 and 7.6 cm/cm 3 (table 1). The strips recently manured showed higher L v values than the strips not manured. Variability was higher in the low irrigation treatment (coefficient of variability, CV, from 59 to over 100%) than in the normal (CV from 45 to 67%) or high (CV from 40 to 67%) irrigation treatments. In all cases, root length density, which mean Ly values are referred to the whole thickness of the layer, was higher in the lower part of the sand. In the organic matter layer, where L v reached a maximum value of 79.7 cm/cm 3, root length density was higher in the manured strips, with mean L v values higher in the low irrigation treatment (24.5 cm/cm 3 ) than in the others (13.8 and 14.7 cm/cm 3 ). Table 1 presents L v values for the sand and organic matter which showed great variability (CV from 49 to over 100%). Other crops also showed high L v values in the organic layer (table 3), lower than tomatoes, where the maximum value recorded (first trial) of L v was cm/cm 3 (table 3), in the low irrigation treatment. The higher Ly values obtained in the strips manured, for both layers in all treatments, could possibly be influenced by being closer to the plant and to the dripper (figure 1). The very high L v values obtained in all cases, higher than L^ reported for tomato under field conditions (Taylor et al., 1970), could also be influenced by the very low mechanical resistance of these layers (Taylor, ) and better aeration (Huck, ). The higher Ly values in the low irrigation treatments were probably due to the high-frequency, deficit irrigation, that maintained water in the upper parts of the soil profile. These higher L v values lowered the shoot/root ratio in the low irrigation level, as it had been suggested previously (Bierhuizen, 1981; Brouwer, 1981). These sand-organic matter layers contribute about 25% of the total root length with adequate water supply, and could assist under waterlogging conditions (Bradford et al., 1981). Drip irrigation probably could have an effect on the high L v values in the interface sand-organic matter-soil by maintaining high moisture conditions Root le_njgth density in the soil profile Root length density values in the soil, estimated by direct observations in the soil profile, exhibited for all crops an exponential decrease in Ly with depth (figure 2) with higher L v values in the upper layers of soil, and with increased Ly values in the manured and tilled strips (0-20 cm). 116

5 Low irrigation treatments in tomato induced higher Ly values in the higher layers of soil, specially in the 0-20 cm depth soil layer (under the organic matter layer), with mean values of 4.8 to 6.8 cm/cm 3 (table 2) and great variability (CV values of 56% to over 100%, for 5cm-thick soil layers), while the deepest layers of soil had lower L^ values (0.8 cm/cm 3, in depths cm, table 2). This pattern could be induced by the higher content of available soil water in the soil near the surface, as it has been described (Newman, 1966). Normal and high irrigation treatments showed similar Ly values in all depths of soil (table 2), with great variability in each 5cm-thick layer (CV values from 38% to over 100%). Root density estimates below 80 cm soil depths were negligible. Shoot/root ratio was decreased by low irrigation treatment (on an L v basis in soil). Low level of mineral nitrogen fertilization (200 kg N/ha) induced higher L v values at depths studied (table 2), 7.2 and 3.2 cm/cm 3, than the medium (400 kg N/ha) and high treatments (600 kg N/ha), also with great variability. Low N treatment induced, on an L v basis, lower shoot/root ratio than other treatments as it has been suggested (Bierhuizen, 1981). Other crops showed similar values of L v (pepper, table 2) or lower values (beans, table 2) than well irrigated and fertilized tomato, with higher root length densities in the 0-20 cm depth soil layer. Root length densities estimated on soil core samples of the profile wall, measured in laboratory, were higher than the L v values obtained by direct observation on the profile wall, as it has been observed (Bohm et al., 1977), with lineal regression equation: Y = X being X = L v estimated by direct observation on the profile, and Y = L v estimated on soil core samples (correlation coefficient, r = 0.75). The upper 0-20 cm layer of soil contributes to about 35% of total root length in well-irrigated tomatoes. The estimated total root length was about 20 kilometers per plant, a value lower than the 44 km measured in a rhizotron under different soil conditions (Taylor et al., 1970). 4. Conclusions Even though substantial variabillity existed in the L v estimates, it can be concluded that a substantial fraction (around 2 5%) of the tomato root zone in the sand mulch systems of Almeria is located in a few cm of sand and in the interface between the sand and the soil. Another 35% of the total root length is located in the first 20cm of the artificial soil, while the root system of tomato extends down to about cm. Low applications of water (75-85% ET C ) and nitrogen (200 kg/ha) increased L v and therefore decreased the 117

6 root/shoot ratio of tomatoes. High L v densities in the interface between the sand and the soil could be of importance to the crop, when low hydraulic conductivity limit drainage in these artificial soil profiles. Acknowledgements To E. Gutierrez-Rave, R. Cuevas, J.I. Montero, M. Jimenez and F. Bretones for technical assistance. References Bierhuizen, J.F Plant-water relationships. Acta Hort. 119: Böhm, W Methods of studying root systems. Springer Verlag. Berlin. 188 PP Böhm, W., Maduakor, H. and Taylor, H.M Comparison of five methods for characterizing soybean rooting density and development. Agron. J. 69: Bradford, K.J. and Yang, S.F Physiological responses of plants to waterlogging. Hortscience 16: Brouwer, R Effects of environmental conditions on root functioning. Acta Hort. 119: Castilla, N Contribución al estudio de los cultivos enarenados en Almería: necesidades hidricas y extracción de nutrientes del cultivo de tomate de crecimiento indeterminado en abrigo de polietileno. Tesis Doctoral. Univ. Politécnica de Madrid. 195 pp. Huck, M.G Variation of the taproot elongation rate as influenced by composition of the soil air. Agron. J. 62: Newman, E.I A method of estimating the total length of root in a sample. J. Appi. Ecol. 3: Taylor, H.M Root behaviour as affected by soil structure and strenght. In: The plant root and its environment. Univ. of Virginia Press: Taylor, H.M., Huck, M.G., Klepper, B. and Lund, Z.P Measurement of soil-grown roots in a rhizotron. Agron. J. 62:

7 Table 1 - Mean values of root length density, (cm/cm 3 ) measured in laboratory in the sand and organic layers of the sand mulched soil, on the strips of soil manured (A) and not manured (B), in the irrigation treatments ( ). Treatment Strip Organic Sand Normal A B Mean High A B Mean Low A B Mean Table 2 - Mean root length density values, L v (cm/cm 3 ), estimated from direct observations on soil profile at 0-20 cm, cm, cm depth, under the organic layer of sand mulched soil on tomato (in the four experiments) and in pepper, dwarf beans and climbing beans. Crop Exper iment Treatment Tomato Normal _ Irrig. High Low Tomato Normal Irrig. High Low Tomato Normal _ Irrig. High Lo w Tomato Normal _ N fert. High Low Dwarf bean CI. bean Pepper

8 Table 3 - Root length density, (cm/cm 3 ), measured in the laboratory on volumetric samples of the organic layer in the sand mulched soil. Samples taken in high root density zones at the end of the cropping season ( ). Irrigation Tomato Dwarf Pepper Climbing beans beans Conventional Drip ( low) Drip (medium) Drip (high) * r «f Figure 1 - Profile walls studied for root length densities in the four experiments with tomatoes, in sand mulch soil (up). Strips on soil manured, A, and not manured, B (down), of 100 cm width. Distances in cm. 120

9 Figure 2 - Root length density, Ly (cm/cm 3 ) plotted versus depth, D (cm), for 5cm-thick layers of soil, under the organic layer of the sand mulched soil, in the recently manured strips (A) and not-manured strips (B) of the soil, with medium (MI), high (HI) and low irrigation (LI) treatments, in the last irrigation trial ( ). 121