Mechanical impedance in plant substrates at increasing water tension

Transcription

1 Mechanical impedance in plant substrates at increasing water tension Maria Helena Fermino Fundação Estadual de Pesquisa Agropecuária Porto Alegre-RS, Brasil Atelene N. Kämpf Jardim Botânico FZB/RS Porto Alegre-RS, Brasil Abstract The resistance of the soil or the substrate to the penetration of plant roots, known as mechanical impedance, can be a cause of the variation of the root growth. This study was performed to evaluate the influence of the usually most applied water tensions on the mechanical impedance of growing media. Six substrates were used: three types of the peat black (BP), brown (BrP) and red peat (RP) and three commercial mixtures based on pine bark, indicated for vegetables (V), forest (F) and tobacco plantlets (T). The samples were set in cylinders, saturated and submitted to five moisture tensions: 10, 30, 50, 80 e 100 hpa. The mechanical impedance was measured with a penetrometer five times in each sample, at 2 cm depth. Increasing water tension increased the mechanical impedance, with different curve responses for each material. The lower mechanical impedance was obtained at 10 hpa (corresponding to the concept of container capacity) and the higher difference in the curves was achieved between the tensions of 10 and 30 hpa. Key words: micropenetrometer, peat, pine bark, Brazil, growing media. INTRODUCTION The resistance of the soil or growing media to the penetration of plant roots, called mechanical impedance, can have negative effects on the root growth (BENGOUGH & MULLINS, 1990). After BENNIE (1991) the changes in the morphology of roots are so characteristic that they can be used to identify high levels of compaction in growing media. Roots growing under high mechanical impedance are shorter and thicker than roots growing under low pressure, and show an irregular growth pattern; lateral roots can be very short, atrophied or totally absent. BENGOUGH & MULLINS (1990) explain the relatively small number of papers measuring the pressure of penetration by plant roots, due to experimental difficulties. Comparing measurements done directly by roots and indirectly by penetrometers, the results show values of pressure of two to eight times higher by the equipment in comparison to roots. Whereas plant roots are flexible and grow through sinuous pathways in the soil or substrate, apparently searching for the lower resistance to penetration, penetrometers are metal probes that penetrate into the soil creating a rigid linear pathway. These differences between roots and penetrometers values led to the discussion of the practical utility of this equipment. Although, in spite of its limitation,

2 penetrometers remain the best available method to predict the soil or substrate resistance to root growth (HARTGE et al., 1985; BENGOUGH & MULLINS, 1990). For BENNIE (1991), the pressure needed to create a pathway in a plant substrate is mainly a function of an interparticle attraction and of the friction pattern among the particles in movement. The attraction force among the particles consists of the solid-to-water adhesion, water-to-water cohesion and solid-to-solid cementation. And the forces of adhesion and cohesion, so as the friction among particles, are dependent on the water content of the growing media. Mechanical impedance has been reviewed mostly considering crops on compacted soils. In the specialized literature there are few papers about this physical property related to growing media. Based on the results of KÄMPF et al. (1999a/b), who confirmed the influence of water on the penetrability of plant substrates, the aim of the present study was to evaluate the effect of increasing water tension on the mechanical impedance of growing media, using tension levels between 10 and 100 hpa, as recommended for potted crops. MATERIAL AND METHODS This study was performed in the Biotechnological Laboratory (Bio-Lab) of the Dept. of Horticulture and Forestry, Agronomy School of the Federal University of Rio Grande do Sul (UFRGS) in Porto Alegre, Brazil, with the following Brazilian materials: Peat in natura, from the Florestal Co. located in Ararangua, SC; the denominations used here are the same adopted commercially: Black peat (BP) a strongly mineralized peat, with a high content of colloids, higher density (407 g L -1 ) and lower total porosity (68% vol.), corresponding to H8 of the Von Post scale; Brown Peat (BrP) less mineralized, high content of fibbers, low density (147 g L -1 ) and high total porosity (85% vol.), corresponding to H6 of the Von Post scale; Red Peat (RP) also a less mineralized peat, H3 of the Von Post scale. Commercial Mixtures, based on Pine bark, from MecPrec Co., located in Telemaco Borba, PR. The three mixtures used in this study are recommended for vegetables (H), forest (F) and tobacco plantlets (T). Determination of bulk density (Bd): The bulk density of wet samples was determined using the methods regularly applied in the Bio-Lab, based on the recommendations of the VDLUFA (German Association of Agricultural Research) as in RÖBER & SCHALLER (1985). A graduated PVC-cylinder (250 ml), is softly fulfilled with a sample with the humidity level near to 50% (weight). The cylinder is let to fall 10 times from a height of 10 cm (vertically). The compacted volume in the cylinder was read (in ml) and weighted (in g), to determine the mass/volume relation. The dry density was calculated after drying the samples at 65 C till constant weight. The following formulas were used to calculate the values of bulk density (Bd wet and dry) and the dry matter content: Bd wet (g L - ¹) = 1000 [wet mass (g) / compacted Volume (ml)] Bd dry (g L - ¹) = Bd wet x Dry Matter (g g - ¹) 2

3 Dry Matter (g g -1 ) = dry mass (g) / wet mass (g) Setting the water tension and determination of water volume in the samples Based on the work of GAULAND (1996), the samples were prepared after the following procedures: a) metallic rings of 150 ml of volume and 3 cm high were prepared closing the bottom with a fine tissue and a gummy band and weighed; b) five rings per substrate (total of 30 rings) were filled with the amount of sample according to the bulk density; c) each sample (prepared ring + substrate) was placed in a plate with water reaching 1/3 of the height of the rings to saturate for 24 hours; d) each sample was removed from the plate and immediately weighed (taking care to have no water loss from the samples). The weight of the saturated sample corresponds to the zero-point of tension; e) transference of the samples to the tension funnel (25 cm diameter with an internal porous plate of 1 bar); f) re-saturation of samples for 24 hours, with the water level in the funnel reaching 0,5 cm bellow the border of the ring; g) adjustment of the water tension to 10 hpa (10 cm water column); h) the samples stay under this level until equilibrium (no more drainage); i) remove one sample of each substrate to weight and determine the mechanical impedance; j) repeat items f, g, h and i, adjusting the tension to 30, 50, 80 and 100 hpa (respectively a 30, 50, 80 and 100 cm water column); k) dry the samples at 65 C till constant weight; l) weight the dry samples; m) calculate the water volume (Wv) by each sample and tension ( t ), according to the formulas: Wv t = wet mass (g) - dry mass (g) / volume of the ring (ml), where: wet mass (g) = weight of the drained substrate (item i); dry mass (g) = weight of the dried substrate (item l); t = water tension in the sample at 10, 30, 50, 80 or 100 hpa. Determination of the Mechanical Impedance: The mechanical impedance was measured with a micropenetrometer Chatillon, Greensboro, NC/USA. The equipment consist of a metal probe with a 6,5 mm diameter and a conical tip with a semi-angle of 30 o. Five measurements were made at a depth of 2,0 cm five times in each sample; the read values were calculated as follows (BENGOUGH & MULLINS, 1990 ; KÄMPF et al. 1999a/b): Q = F / A, where: Q = Penetrometer resistance, corresponding to the pressure needed to penetrate the medium (kpa); F = Force (N) required to push the metal probe through the sample, given as a Pic C (Compression) in Newton. It represents the maximal value among 600 individual lectures done by the penetrometer in a period of 120 milisseconds; A = transversal area of the cone (cm). The results were submitted to analysis of variance (6 substrates, 5 levels of tension, 5 determinations/sample). The effect of the applied water tensions on the penetrability of the substrates was determined by regression curves. The statistical analysis was performed with the Program SAS and the graphics by Sigma Plot. 3

4 RESULTS AND DISCUSSION The three types of peat showed different regression curves to describe the relation between the water tension and the mechanical impedance (Fig. 1): the highest impedance was found in the Black Peat, where these values increased by increasing the water tension till the maximal inflexion point (-b/2c) between 80 and 100 hpa. The Red Peat, less dense than the Black Peat, showed a linear positive relationship between the pressure of penetration and the water tension and till 100 hpa no inflexion point was observed. The regression curve of the Brown Peat was not significant (p 0,05), what can be related to its very low density (147 kg m -3 ). The three commercial mixtures based on Pine Bark also showed two different pattern of curves: linear positive for T and quadratic for the mixtures V (inflexion point between 50 and 80 hpa) and F (between 50 and 100). These mixtures have in common the same components (Pine Bark and Vermiculite in different proportions) and similar dry bulk densities (> 209 and < 216 g L -1 ). In this case, the impedance can be influenced by characteristics such as the architecture of the aggregates and cementation among the particles. For each material the lowest mechanical impedance was found at the lowest water tension (10 hpa) and the largest effect of the water tension on the penetrability of the probe was observed from 10 to 30 hpa. At 10 hpa the sample is considered to be saturated and represents the status called container capacity, in analogy to the concept of field capacity used for soils. There is a higher volume of free water in the medium (Fig. 2 and 3), permitting better movement among the particles when the probe passes trough. WHITE & MARTALERZ (1966) observed that also plant roots can find less resistance to penetrate substrates at container capacity, suggesting that growers should maintain potted plants at this level of moisture. On the other hand, higher water tension represents drier substrate, increasing the friction among the particles and the penetrating probe. CONCLUSION Under the same regular condition of packing the samples into the test rings, substrates showed the lowest mechanical impedance at the highest water content (at 10 hpa) considered as the status of Container Capacity by definition, the maximal water content after saturation and natural drainage. The same trend was found with Peat and commercial mixtures based on Pine Bark. AKNOWLEDGEMENTS The authors thank the Brazilian Companies Florestal and Mec Prec for providing the samples analyzes in this study. LITERATURE: BENNIE, A.T.P. Growth and Mechanical Impedance. In.: Waisel, Y.; ESHEL, A.; KAFKAFI, U. Plant Roots, the Hidden Half. New York: Marcel Dekker,

5 BENGOUGH, A.G.; MULLINS, C. E. Mechanical impedance to root growth: a review of experimental techniques and root growth responses. Journal of Soil Science, n.41, p , GAULAND, D.C.S.P. Relações hídricas em substratos à base de turfas sob o uso dos condicionadores casca de arroz carbonizada ou queimada. Dissertação (Mestrado). UFRGS, CPG Ciência do Solo. Porto Alegre, HARTGE, K.H.; BOHNE, H.P.; SCHREY, H.P.; EXTRA, H. Penetrometer measurements for screening soil physical variability. Soil & Tillage Research, Amsterdam, n.5, p , KÄMPF, A.N.; HAMMER, P.A.; KIRK, T. Impedância mecânica em substratos hortícolas. Pesquisa Agropecuária Brasileira, Brasília, v.34, n.11, p , 1999 (a). KÄMPF, A.N.; HAMMER, P.A.; KIRK, T. Effect of the packing density on the mechanical impedance of root media. Acta Horticulturae, Wageningen, n.481, v.2, p , 1999 (b). RÖBER, R.; SCHALLER, K. Pflanzenernährung im Gartenbau Stuttgart, Ulmer WHITE, J. W.; MASTALERZ, J. W. Soil moisture as related to Container Capacity. Proceedings of the American Society for Horticultural Science, Geneva, v. 89, p ,

6 Mechanical Impedance (kpa) 4000 BP RP 3500 V F 3000 T Water Tension (hpa) Figure 1. Relation between the moisture level (measured as Water Tension) and the Mechanical Impedance for Peat in natura - Black Peat (BP) and Red Peat (RP) and for commercial mixtures based on Pine Bark for vegetable crops (V), forest (F) and tobacco plantlets (T). Note: there is no regression curve for Brown Peat because it was not significant. (n=5) Table 1 Equation and significance of the regression curves related to Figure 1 (* = p 0,05) Materials Equation of Regression R² BP Y = -0,47x² + 82,78x - 88,53 0,98 * RP Y = 15,99x ,60 0,80 * V Y = -0,53x² + 70,45x + 336,29 0,99 * F Y = -0,15x² + 23,81x + 977,23 0,96 * T Y = 13,81x ,5 0,86 * 6

7 Mechanical Impedance (hpa) B P B rp R P W ater V o lu m e (% ) Figure 2. Relation between the moisture level (measured as Water Volume) and the Mechanical Impedance for Peat in natura - Black Peat (BP), Brown Peat (BrP) and Red Peat (RP). (n = 5) Table 2. Equation and significance of the regression curves related to Figure 2 (* = p 0,05) Materials Equation of Regression R² BP Y = 7,39x² - 840,32x ,99 * BrP Y = -32,55x ,96 * RP Y = -50,91x ,97 * 7

8 Mechanical Impedance (kpa) V F T W a te r v o lu m e ( % ) Figure 3. Relation between the moisture level (measured as Water Volume) and the Mechanical Impedance for commercial mixtures based on Pine Bark used for vegetable crops (V), forest (F) and tobacco plantlets (T). (n=5). Table 3. Equation and significance of the regression curves related to Figure 3 (* = p 0,05; ** = p 0,01) Materials Equation of Regression R² V Y= -6,51x² + 559,75x ,96 * F Y = -2,27x² + 146,19x - 461,50 0,99 ** T Y= -113,02x ,90 * 8