1991-P&S-Bengough&Mullins Root Elongation Penetration of Sandy Loam Soils

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  Plant and Soil 131: 59-66, 1991. 1991 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 8546 Penetrometer resistance root penetration resistance and root elongation rate in two sandy loam soils A.G. BENGOUGH 1 and C.E. MULLINS Plant and Soil Science Department, University of Aberdeen, Aberdeen AB9 2 UE, UK. I Present address: Soil-Plant Dynamics Group, Cellular and Environmental Physiology Department, Scottish Crop Research Institute, Dundee DD2 5DA, UK Received 2 January 1990. Revised August 1990 Key words: maize, mechanical stress, penetrometer, root growth, sandy loam bstract Root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy loam soils. Root elongation rate was negatively correlated with root penetration resistance, and was reduced to about 50 to 60 of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa. Resistance to a 30 ° semiangle, 1 mm diameter penetrometer was between about 4.5 and 7.5 times greater than the measured root penetration resistance. However, resistance to a 5 ° semiangle, 1 mm diameter probe was approximately the same as the resistnace to root penetration after subtracting the frictional component of resistance. The diameter of roots grown in the undisturbed cores was greater than that of roots grown in loose soil, probably as a direct result of the larger mechanical impedance in the cores. Introduction Experiments on the effect of mechanical impe- dance on rate of root growth have generally fallen into one of two categories. Firstly, those which use an empirical measure of soil strength such as penetrometer resistance e.g. Taylor and Ratliff, 1969) and those which attempt to apply a 'known' resistance to root growth by growing plant roots in a pressurised artificial medium, such as ballotini (Abdalla et al., 1969; Gill and Miller, 1956; Goss, 1977). However, both ex- perimental approaches are subject to consider- able uncertainty. Penetrometer resistance is a factor of between 2 and 8 times greater than the root penetration resistance (Whiteley et al., 1981) and in externally pressurised media, root penetration resistance does not have an easily predictable relation to the externally applied pressure (Bengough and Mullins, 1990a; Rich- ards and Greacen, 1986). We have found only two studies in which both the root penetration resistance and the root elon- gation rate were measured directly in the same soil (Eavis, 1967; Stolzy and Barley, 1968). Stol- zy and Barley (1968) used a proving ring to measure the force exerted by two individual pea radicles growing into remoulded cores of sandy loam soil. The roots grew at 0.44 mm h -1 in soil with root penetration resistance of 0.46 MPa as compared with 1 mm h -1 in loose soil crumbs. Eavis (1967) used a dead-load technique in which a weight was attached to a pea seedling such that if the root exerted more force on the soil than the attached weight, the weight was lifted due to the growth of the root. By using a series of increasing weights the approximate force exerted by the root was determined. Final root length and root force was measured for a range of soil bulk densities and matric potentials. The relation obtained (leaving out points where aeration was limiting) is shown in Figure 1.  60 Bengough and Mullins E o s I.IJ 1.0 0.8 0.6 0.4 0.2 oO 0 0 0 0 0 0.0 i i t i i 0 0 0 1 0 2 0 3 0 4 oot resistance MPa) Fig I Axial root force/root cross sectional area versus elongation rate of pea seedling roots in remoulded cores of sandy loam soil calculated from Eavis, 1967). The results reported by Eavis are of sufficient importance to merit an independent study using an improved technique for measuring root force on a different species. The first aim of this paper is, therefore, to quantify the effect of mechanical impedance on maize root elongation rate in un- disturbed soil cores by measuring root penetra- tion resistance directly and recording the average elongation rate of these roots. Root penetration resistance and penetrometer resistance are defined as the force exerted by the root or probe divided by its cross sectional area (Bengough and Mullins, 1990b). Greacen et al. (1968) suggested that both roots and 'sharp' penetrometers i.e. penetrometers with a small cone angle) deform the soil cylindrically, which requires less pressure than the spherical de- formation caused by 'blunt' penetrometers. They also suggested that roots experience virtually no frictional resistance, in contrast to penetrometers which often have a high component of frictional resistance. Greacen and Oh (1972) used the resistance to a 5 ° semi-angle probe minus the frictional resistance as an estimate of the root penetration resistance, while Voorhees et al. (1975) found better correlations between root elongation rate and penetrometer resistance (to a 5 ° semi-angle probe) after the frictional compo- nent of resistance was subtracted off. Our second aim was, therefore, to test the hypothesis that the soil resistance to root penetration is the same as that experienced by a sharp penetrometer, once the component of frictional resistance on the penetrometer has been subtracted off. aterials and methods Preparation of soil cores Undisturbed soil cores were collected from two fields (Big Ground and Plum Orchard) at the Institute of Horticultural Research, Welles- bourne. Two shallow pits several metres apart were dug at each site to provide replicate samples, and soil cores (56 mm diameter x 40 mm) were extracted from 0.12 to 0.20 m depth. The cores were sea- led at field moisture content and stored at 10°C until required for use. Details of the physical and chemical properties of the soils are given in Young (1987), and some of the soil properties are summarised in Table 1. The soil cores were saturated overnight with distilled water (containing 'Phorate' systemic in- secticide to kill small numbers of springtail in- sects which had been observed on the cores) and then equilibrated at -10 kPa matric potential on a tension table for 5 to 7 days. After equilibra- tion, cores were wrapped in a layer of plastic film containing a small (10 mm diameter) hole for the root. The rate of water loss from the polythene covered core was less than about 0.3 g day -1. To test whether the insecticide affected maize root elongation rates, root elongation rates were mea- sured for 56 seedlings grown in moist horticultur- al vermiculite which was watered with either distilled water or with insecticide solution. The average root elongation rate of the insecticide treated seedlings (averaged over 3 days) was within 3 of that of the controls, and this differ- ence was not statistically significant. Seed germination and pre-selection To reduce variability between replicate seed- lings, maize seeds (cv. Artus) were graded by weight, and only seeds with masses within one standard deviation of the mean were used. Seven seeds per core were sown on moist vermiculite, and left in the dark to germinate (at about 20°C).  Table 1 Details of soils used Root resistance and elongation rate 61 Soil property Soil Big Ground Plum Orchard mineral matter a by mass of 2-0.2 mm 200-60 ttm 60-20 p.m 20-2 m <2/xm % by mass organic matter ~ % air filled porosity (at - 10 kPa matric potential) Dry bulk density (Mg m -3) % moisture content (at - 10 kPa matric potential) 46 22 7 10 15 1.67 10 1.77 13.3 49 20 6 9 16 2.29 10 1.67 16.3 a From Young (1987). Root lengths were measured 2.0 and 2.5 days after planting and root growth rates during this interval calculated. One seedling with a straight main root and having an elongation rate near the mean for the seven was selected for each experi- ment. Root diameter was measured 2 3 and 4 mm behind the tip using a Vernier microscope. The average of these three readings was taken as the initial root diameter. Technique for measuring root resistance The method for measuring root force was similar to that of Whiteley et al 1981) in which roots were grown into a soil core placed on a top pan balance Fig. 2). The experiment was designed such that each root was held vertically and an- chored rigidly behind the zone of elongation. The air gap between the root holder and the soil core surface was made as small as possible to reduce the likelihood of the root buckling. The seedling root was slid gently through a short length of plastic tubing into a narrowly tapered performed hole 1.5 to 2 mm deep) in the surface of a prepared soil core Fig. 2). The root was then fixed in position with a small quantity of plaster of Paris, and the whole seed- ling covered with moist horticultural vermiculite, previously equilibrated at -0.8 kPa matric potential, using a Haines apparatus. The soil core itself was on the balance pan of a tared electronic balance accurate to within ( ) b) ram lm4< -_- I ~ p T V s 1 P i i I I '1 Fig 2 Schematic diagram of apparatus used to measure root force. A maize seedling (S) root was inserted through a short plastic tube (T) so that the root tip pushed down into a small hole in the soil core (SC), registering a reading on the electronic balance (E). The root was anchored in position with plaster of Paris (P) and the seedling covered with moist expanded vermiculite (V). (a) Root in tube. (b) Final ar- rangement. 0.01 g). The whole apparatus was then covered with a double thickness of black polythene to exclude light. Seedlings were only briefly exposed to ordi- nary levels of room lighting during the anchoring of the roots with plaster of Paris. Balance readings were taken automatically every 5.3 minutes during the first 12 hours by a  62 Bengough and Mullins microcomputer interfaced to the balance output and stored on cassette tape. There was negligible drift in the balance readings during each ex- perimental run. The root was allowed to elon- gate for a further 8 hours and then was exca- vated out of the soil core. Final root diameter was measured 2 to 5 mm behind the apex by hand sectioning the root using pith and a fine razor blade. Because the root circumference was often quite distorted and irregular, several sec- tions were cut and two perpendicular diameters were measured for each section using a mi- croscope with graticule scale. The average of these readings was taken as the final root diam- eter, and root length was also recorded. Because it was not possible to maintain accur- ate temperature control, temperature was con- tinuously monitored using a thermograph. After completing each experiment, part of the soil was passed through a 2-mm sieve and repacked ve T loosely to a bulk density of about 0.7g cm-. Maize root elongation rates were measured for eight seedlings in sieved Big Ground soil and seven seedlings in the sieved Plum Orchard soil to obtain a reference rate of elongation in soil with negligible root penetration resistance. Penetrometer resistance Penetration resistance to 0.5 and 1 mm diameter 30 ° semi-angle penetrometer probes was meas- ured in each intact soil core, both before and after each root resistance experiment. All pene- trations were performed at a penetration rate of 4mm min -1. In a separate experiment, soil re- sistance to a 5 ° 1 mm diameter penetrometer was measured on a replicate set of equilibrated cores, and the angle of soil-metal friction estimated using an inclined plane Bengough, 1988). esults Root force diameter and elongation rate Seven out of eight maize roots penetrated cores of both soils. Average root force was calculated over the growth period when the root was be- tween 2 and 5 mm long assuming a constant rate of elongation). The mean root penetration resist- ance was calculated by dividing the root force by either the initial or the final root cross sectional area averaged between 2 and 5 mm behind the tip. Average values of root resistance, diameter and elongation rate for both soils are given in Table 2. Root elongation rates and diameters did not differ significantly between the two soils, and neither did the root resistances based on the initial root cross sectional area. The root resist- ance based on the final root cross sectional area was significantly greater for Plum Orchard than for Big Ground soil. Final root diameter was significantly greater than the initial diameter in each soil. Table 2 Mean maize root penetration resistances, elongation rates and diameters in undisturbed soil cores standard errors of the mean are shown in brackets) Soil Big Ground Plum Orchard Root penetration resistance Force/initial area MPa) 0.38 0.08) 0.47 0.06) Force/final area MPa) 0.26 0.03) 0.38 0.04) Root diameter: Initial mm) 1.14 0.02) 1.12 0.02) Final mm) 1.35 0.07) 1.36 0.08) Root elongation rates In undisturbed cores a mm/hour) sieved soils b mm/hour) 1.06 0.09) 0.88 0.11) 1.32 0.07) 1.32 0.07) a Temperature = 22-24.5°C. b Temperature = 20-21°C.
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