EFFECT OF THE PACKING DENSITY ON THE MECHANICAL IMPEDANCE OF ROOT MEDIA Atelene N. Kämpf UFRGS, Faculdade de Agronomia C.P.776-90001.970 P. Alegre / RS Brazil P. Allen Hammer & Terri Kirk Purdue University - Dep. of Horticulture 1165 Hort. Bldg - W. Lafayette, IN 47907.1165 USA Abstract The force applied to fill containers with growing media in greenhouses results in varying packing densities. The physical properties of the medium also vary with packing densities. The influence of the packing density on the penetrability of five media (sand, perlite, Sphagnum peat, coir and a commercial mixture Fafard 2P (60% S. peat + perlite) was studied. Four levels of compaction were applied: 1. loose (no compaction); 2. firmed (the bulk density determined in lab); 3. compacted (middle point between levels 2 and 4); 4. maximal compaction that could be obtained by hand. The substrates were analyzed at the moisture content as received and at container capacity. The pressure needed to penetrate a pointed metal probe of 6.5 mm diameter vertically 3.5 cm into the medium was measured by a penetrometer. The mechanical impedancesignificantly increased for all substrates with increasing packing density. A significant interaction between the moisture level and the applied packing density was observed. Loosely packed samples showed higher mechanical impedance when the moisture content was at container capacity as compared with the drier samples, with exception of Fafard 2P ; maximal compacted samples showed higher mechanical impedance at the lower moisture content. Additional index words: physical properties of substrates; compaction; penetrometer; sand; perlite; Sphagnum peat; coir; Fafard Mixture 2P. 1. Introduction Greenhouse containers often have different amounts of root medium with variable packing pressure applied. This variability in filling can be measured as packing density of the filled container. Greenhouse operations can also affect packing density. Bouncing filled container during movement, pressing root medium during transplanting, and watering can all increase packing density. Burés et al. (1993 and 1995), by means of computer simulation, demonstrated that pore size decreased in pine bark/sand mixtures with increased compaction. Plant roots easily grow through the large soil pores but must displace soil particles when pores are lacking (Bennie, 1991). The resistance of root medium to displacement by roots has been termed mechanical impedance (Bengough & Mullins, 1990). Increased mechanical impedance is a major factor influencing poor root growth and reduced crop yields in compacted field soils (Gerard et
al.1982; Bengough & Mullins, 1990). The difficulty of measuring soil resistance directly by using actively growing roots has led to a widely accepted procedure of using a penetrometer to measure the mechanical impedance in soils (Dexter 1987; Bengough & Mullins, 1990; Bennie, 1991). Penetrometers are force measurement instruments equipped with a metal cylindrical probe with a conical tip. It measures the force required to push the probe into the soil. This resistance estimates the pressure required by the root to displace the soil particles. Root growth occurs when the root pressure exceeds the soil resistance. Whiteley et al. (1981) compared the ratio of pressure required for soil penetration by roots and penetrometers and found this ratio can vary from 2 to 8. Although metal probes overestimate the resistance to root growth in soil, penetrometers remain the best available method for predicting root resistance, according Bengough & Mullins (1990). In this study the packing density effects on the penetrability of several important horticulture root media constitutes were determined. 2. Materials and Methods Bedding plant containers (5.5 x 5 x 6 cm) were filled with perlite, Sphagnum peat, coir and Fafard Mixture 2P (Table 1) at four levels of compaction, and with sand at three levels of compaction. Five single pot replicates with five penetrometer reading per pot in a complete random experimental design was used. The mechanical impedance of the five media was measured at two levels of moisture content: at container capacity (White & Mastalerz, 1966) and at the moisture level as received in the commercial product (Table 1). The following levels of compaction were applied: 1. loose (no compaction); 2. firmed (the bulk density determined in lab following Röber & Schaller, 1985); 3. compacted (mid-point between levels 2 and 4); 4. "maximal" compactness obtained by hand packing. The applied packing densities used as treatments are listed in Table 2. The pressure (Q) needed for a conical probe (semiangle = 30 º) of 6.5 mm diameter to penetrate 3.5 cm vertically into the samples was measured with a penetrometer ( Chatillon Digital Force Gauge). The Equation Q = F / A (Bengough & Mullins, 1990), where F is the force required to push the probe through the substrate and A is the cross-sectional area of the probe cone, describes this force. The F value, displayed on the instrument as Peak C (compression), represents the maximum value of the 600 individual instantaneous readings over a 120 millisecond time period which the gauge captures. The moisture content was measured on samples dried at 80 ºC until constant weight. 3. Results and Discussion The original data are listed in Table 2. These data failed in the normality test; therefore, the statistical analysis was performed with log 10 transformed data.
Then, all media showed a linear increase in mechanical impedance with increasing packing density (Fig. 1-5). A statistically significant interaction between the applied packing density and moisture level (Table 2) was found for all media. Loose samples (with the exception of Fafard 2P ) showed higher mechanical impedance when at container capacity than at the level of moisture as received in the commercial product. Maximal compacted samples (with the exception of sand) showed higher mechanical impedance at the lowest level of moisture. According to the Soil Survey Manual (USDA, 1993), the resistance to penetration of field soils is classified as: 1. small (resistance pressure below 100 kpa); 2. intermediate (pressure between 100 and 2,000 kpa) and 3. large (pressure higher than 2,000 kpa). In wet soils, the limit of 2,000 kpa indicates strong root restriction for several crops; between 2,000 and 1,000 kpa root restriction may be assumed to decrease linearly; below 1,000 kpa, root restriction may be assumed to be small. Compaction causes reduction in vegetative growth, and this effect varies with the kind of medium (Bunt, 1987). A decrease in root elongation with increasing soil mechanical resistance was reported by Bengough & Mullins (1990); they reported a reduction of 50 and 90% in the root elongation rates of maize at root penetration resistance of 390 to 480 kpa. Although no data are available for ornamental plants at this stage, the present results indicate that common horticultural substrates may reach similar critical levels of mechanical impedance as in field soils. 4. Conclusion The mechanical impedance of a root media depends strongly on the applied packing density and on its moisture content. Differences between the penetration resistance measured in loose samples and in high compacted samples are smaller when the moisture content of the media is at container capacity. Increasing packing density of a medium increases the content of solids per volume. Consequently, important physical properties can be modified, as porosity, water retention and air space. Hence, descriptions of physical properties of media should inform about the used packing density or dry solids content per volume. To avoid limiting to the root growth in container plants, mechanical impedance should be considered when developing and using greenhouse root media. References Bengough, A.G. and C.E. Mullins. 1990. Mechanical impedance to root growth: a review of experimental techniques and root growth responses. J. Soil Sci 41:341-358. Bennie, A.T. P. 1991. Growth and mechanical impedance. In: Waisel, Y., A. Eshel and U. Kafkafi. Plant roots, the hidden half. Marcel Dekker, N.Y.
Bunt, A.C. 1987. Media and mixes for container-grown plants. Unwin Hyman, London. Burés, S., D.P. Landau, A. M. Ferrenberg and F. A. Pokorny. 1993. Monte Carlo computer simulation in Horticulture: a model for container media characterization. HortScience 28(11):1074-1078. Burés, S., A. M. Ferrenberg, F. A. Pokorny and D. P. Landau. 1995. Computer simulation to understand physical properties of substrates. Acta Hort. 401:35-39. Dexter, A. R. 1987. Compression of soils around roots. Plant and Soil 97:401-406. Gerard, C. J., P. Sexton and G. Shaw. 1982. Physical factors influencing soil strength and root growth. Agron. J. 74:875-879. Röber, R. and K. Schaller. 1985. Pflanzenernährung im Gartenbau. Ulmer, Stuttgart. USDA. 1993. Soil Survey Manual. United States Department of Agriculture. Handbook 18. White, J. W. and J. W. Mastalerz. 1966. Soil moisture as related to container capacity. Proc. Amer. Soc. Hort. Sci. 89:758-765. Whiteley, G. M., W. H. Utomo and A. R. Dexter, A. R. 1981. A comparison of penetrometer pressures and the pressures exerted by roots. Plant and Soil 61:351-364. Tables Figures
Table 1 - Origin and moisture of the media. Media Origin Moisture (%w) in product sand Sand Silica/Weldrom, Illinois, USA (particle size of 125 µm) 0.1 perlite Coarse horticultural perlite, manufactured by Silbrico Co., Illinois, 0.5 USA; Sphagnum peat Coarse yellow Sphagnum peat moss/sogevex, Quebec, Canada 55.0 coir originally from Sri Lanka (compressed sample was wetted, expanded, 80.0 then dried at open air) Fafard 2P C. Fafard Inc., Massachusetts, USA (60% Sphagnum peat + perlite) 55.0 Table 2 - Effect of packing density and two levels of moisture content on the mechanical impedance (kpa) of five root media. Media Pack. dens. (kg.m 3 ) a Pressure (kpa) delivered medium b Pressure (kpa) at container capacity ANOVA : P > F Pack. dens. Moist. level. Interaction (A) (B) (AxB) sand 1560 91 405 < 0.001 < 0.001 < 0.001 1670 175 493 1740 207 611 perlite 100 186 358 < 0.001 < 0.001 < 0.001 125 666 707 150 1035 968 175 1497 1041 S. peat 48 70 196 < 0.001 < 0.001 < 0.001 70 272 268 92 756 503 114 1691 609 coir 51 18 134 < 0.001 0.058 < 0.001 76 181 183 102 749 735 128 2110 1834 Fafard 2P 55 62 57 < 0.001 < 0.001 < 0.001 77 170 125 100 519 360 123 1140 692 a = dry solids; b = corresponding to the moisture level in the product as received