Tractor Passes Research Articles

Evolution of Conservation Tillage Systems for Processing Tomato in California’s Central Valley

Traditional processing tomato (Solanum lycopersicum) production in California’s Central Valley relies heavily on tillage to produce high yields. However, recent research and farm innovation have produced a variety of conservation tillage (CT) management alternatives that cut costs, reduce soil disturbance, and produce fewer emissions. A 12-year study in Five Points, CA, demonstrated that CT methods reduced tractor passes by 40%, lowered tillage costs by ≈$80 per acre in 2011 dollars, and achieved comparable yields as standard tillage (ST) methods. As comparable yield performance and net profitability are further demonstrated, an array of CT systems will become increasingly attractive to producers and more common in Central Valley tomato growing areas.

Desenvolvimento de plantas de cobertura e produtividade da soja conforme atributos físicos em solo compactado

Com a intensa utilização de tecnologias voltadas à mecanização das operações agrícolas, a compactação do solo é um fator que tem limitado a produtividade. Para atenuar este problema, o uso de plantas de cobertura do solo é frequentemente recomendado. Com o objetivo de avaliar o efeito da compactação nas propriedades físicas do solo, no desenvolvimento de plantas de cobertura e na produtividade da soja, conduziu-se um experimento no delineamento em blocos casualizados no esquema fatorial 2 x 5, sendo duas espécies de plantas de cobertura (aveia preta - Avena strigosa e nabo forrageiro - Raphanus sativus) e cinco níveis de compactação (0, 2, 4, 6 e 8 passadas de trator com 5,0 Mg na superfície do solo), com quatro repetições. Intensidades de tráfego superiores a 2 passadas de trator alteram a densidade do solo, a porosidade total e a resistência mecânica à penetração, na camada de 0-0,10 m, e reduzem, de forma linear, a altura e a produção de matéria seca da parte aérea das plantas de cobertura. O cultivo da aveia preta ou nabo forrageiro antecedendo a soja, associado ao uso de sulcadores na operação de semeadura da soja minimiza os efeitos da compactação do solo, possibilitando obter-se produtividades superiores a 3,5 t ha-1.

Soil compaction distribution under land clearing in calden (Prosopis Caldenia Burkart) forest in Argentinean pampas

Although the virgin soil has a high organic matter content that reduces the soil's susceptibility to compaction, the high weight of the tractors used for land clearing cause a high degree of soil compaction. This study was performed in the Calden (Prosopis Caldenia Burkart) forest at La Pampa State, in West Argentina. The object of this study was to assess the soil compaction during mechanical land clearing with heavy machinery. Variables measured were (CI) cone index, (BD) dry bulk density, tractor rut depth (TRD) and Proctor test. The land clearing was performed with a Caterpillar tractor D6, with 0.7×2.3m metallic tracks. The engine power was 300kW and the weight was 18 Mg. The relevant results were as follows: in topsoil (0–0.2m) 1 and 2 passes of a tractor caused mean CI values of 1833 and 2437kPa, respectively. Dry bulk density mean values had a similar behaviour to those of CI, measuring 1.75 and 1.85Mgm−3 for 1 and 2 passes, respectively. In the subsoil (0.2–0.9m), two tractor passes caused higher CI and BD values than one tractor pass: CI mean values were between 1991 and 2439kPa for one and two tractor passes, respectively. Dry bulk density mean values were between 1.81 and 1.89Mgm−3 for 1 and 2 passes, respectively. Significant differences were found in TRD when the tractor passed one or two times, measuring 0.092m TRD for one tractor pass and 0.111m for two tractor passes. The main conclusions of this study are as follow: the level of soil compactability increases as the maximum BD attains a higher level and at a lower water content level. When soil with a moisture content level over the critical water content is compressed (for example, by one tractor pass with 18 Mg total load) it reaches saturation, and the compaction stress travels deeper in the soil than when the soil is drier. Then if the use of heavy machinery cannot be avoided, special attention should be given to traffic at a water content which the soil do not attain saturation by compression. Compared with most agricultural practices the high weight of the land clearing machinery generate greater compaction stresses in the soil, raising the maximum BD that develops at lower water content and subsequently reducing the work opportunities.

Effects of tire inflation pressure and field traffic on compaction of a sandy clay soil as measured by cone index and permeability.

Chehaibi, S., M. Khelifi, A. Boujelban, K. Abrougui. 2012. Effects of tire inflation pressure and field traffic on compaction of a sandy clay soil as measured by cone index and permeability. Canadian Biosystems Engineering/Le genie des biosystemes au Canada 54: 2.92.15. Many external factors have an effect on soil compaction, including the contact pressure generated at the soil-tire interface and the inflation pressure of tires. The main objective of this study was to investigate the effects of number of wheeled tractor passes and tire inflation pressures on the compaction and permeability of a sandy clay soil. Tests of soil compaction were conducted at the Higher Institute of Agronomy of Chott-Mariem, Sousse, Tunisia. Soil cone indices and permeability were measured under different number of tractor passes (one, two, and three) and tire inflation pressure (50 kPa, 100 kPa, and 150 kPa). A tractor of 59 kW with a total mass of 2,910 kg (1,715 kg on the rear axle) was used for the tests. Results showed that repeated passes increased soil compaction essentially in the upper horizons. This compaction, as measured by cone index, was positively correlated with tire inflation pressure. Indeed, the compaction was less pronounced under low tire inflation pressure. However, soil cone index remains closely related to soil moisture content. Multiple tractor passes also significantly reduce the soil permeability (P < 0.05). Similarly, our results showed that the permeability was significantly affected by tire inflation pressure and considerably decrease when increasing the selected pressure (50, 100, and 150 kPa). However, the correct tire inflation pressure indicated by the manufacturer varies from 84 to 140 kPa.

Effects of different tillage regimes on soil compaction, maize ( Zea mays L.) seedling emergence and yields in the eastern Argentinean Pampas region

Crop establishment depends primarily on the methods used for seedbed preparation and sowing. Our main objective was to evaluate the soil compaction and crop residue effects on maize ( Zea mays L.) seedling emergence and yields in three tillage regimes: direct sowing (DS), chisel plough (ChP) and mouldboard plough (MP). The current study was performed in the eastern section of the humid subtropical Rolling Pampa region, Buenos Aires State (Argentina) between October 2004 and March 2007. The soil of the study site is a typical Argiudoll. We hypothesised that maize yields are affected by different tillage regimes used for seedbed preparation and that the traffic for the three tillage regimes caused subsoil compaction. The variables measured were: (1) cone index (CI) in the 0–450 mm depth profile; (2) root dry matter per plant (RDM); (3) dry matter per plant (DMP); (4) seedling emergence (SE); and (5) maize yields (MY). The results showed that in DS, the high soil compaction level causes smaller yields (decrease between 10.7 and 15.2%) compared with the other two tillage systems. SE was slower under ChP and MP compared with DS, but similar results were achieved 18 days after sowing. The highest RDM average values for three growing seasons were observed in ChP and MP (42.3 and 46.1 g plant −1, respectively) compared to DS (37.1 g plant −1). The study produced the following conclusions. Maize yield was directly related to root dry matter per plant, which was affected by soil compaction. The soil Ap horizon needs to be tilled to improve yields. Despite the greater number of tractor passes in ChP and MP, all systems caused subsoil compaction. SE was not affected by plant residue.

Effect of multiple passes of tractor with varying normal load on subsoil compaction

A field experiment was conducted on alluvial soil with sandy loam texture, in a complete randomized design, to determine the compaction of sub-soil layers due to different passes of a test tractor with varying normal loads. The selected normal loads were 4.40, 6.40 and 8.40 kN and the number of passes 1, 6, 11 and 16. The bulk density and cone penetration resistance were measured to determine the compaction at 10 equal intervals of 5 cm down the surface. The observations were used to validate a simulation model on sub-soil compaction due to multiple passes of tractor in controlled conditions. The bulk density and penetration resistance in 0–15 cm depth zone continuously increased up to 16 passes of the test tractor, and more at higher normal loads. The compaction was less in different sub-soil layers at lower levels of loads. The impact of higher loads and larger number of passes on compaction was more effective in the soil depth less than 30 cm; for example the normal load of 8.40 kN caused the maximum bulk density of 1.53 Mg/m 3 after 16 passes. In 30–45 cm depth layer also, the penetration resistance increased with the increase in loads and number of passes but to a lesser extent which further decreased in the subsoil layers below 45 cm. Overall, the study variables viz. normal load on tractor and number of passes influenced the bulk density and soil penetration resistance in soil depth in the range of 0–45 cm at 1% level of significance. However, beyond 45 cm soil depth, the influence was not significant. The R 2 calculated from observed and predicted values with respect to regression equations for bulk density and penetration resistance were 0.7038 and 0.76, respectively.

Traffic effect on soil compaction and yields of wheat in Spain

The general objective of this paper was to quantify the changes in the physical properties of an Aridisol soil and theeffect on wheat yield due to agricultural tractors traffic in the Vélez Blanco District, Almería Spain. Parameters measured were cone index (CI) in the 0-600 mm depth profile, bulk density (BD) and rut depth; the variable wheat yields (WY) was measured too. The traffic treatments applied were: 0 (control plot), 1, 3, 5, and 7 tractor passes in the same tracks. Each experimental plot was trafficked with two tractors, one light (60 kN) and one heavy (80 kN). In topsoil (0-200 mm), up to five passes of the heavy (HT) and light tractors (LT), as in one and three passes, the BD and CI values responded to the ground pressure being higher in absolute value for LT. For the 200 to 400 mm depth range HT caused higher CI (1,570 to 2,200 kPa) and BD (1.38 to 1.68 Mg m–3) values than LT. Eight months later, WY was evaluated in tractor’s track areas and decreases in the range of 18-38%, were measured. For seven passes the applications of total loads of 80 and 60 kN increased BD up to 1.5 Mg m–3 at depths of 200-600 mm. Although soil had high bulk density prior to traffic treatments, a significant increment of subsoil compaction still occurred due to the high traffic intensities applied.

Soybean nodulation and nitrogen fixation in response to soil compaction and surface straw mulching

Soil compaction affects nodulation and nitrogen fixation of soybean ( Glycine max (L.) Merr.). The effects of compaction can be mediated by straw mulch applied at soil surface. To test these hypotheses a field experiment was carried out in 2008 on a Haplic Luvisol (south-eastern Poland). We grew soybean in treatments with different soil compaction levels: non-compacted (NC), moderately compacted (MC) (3 tractor passes) and strongly compacted soil (SC) (5 tractor passes). Each compaction level had treatments without and with surface chopped straw mulch at the rate of 0.5 kg m −2. Nodulation was evaluated by measurements of nodule-size distribution, number, weight, and diameter of nodules and symbiotic nitrogen fixation – by measurement of nitrogenase activity, nitrogen content in leaves, protein content in seeds and seed yield. Soil measurements included bulk density and nitrogen content in the form of N-NO 3 and N-NH 4. In general, nitrogenase activity decreased with increasing soil compaction level. At every compaction level, the nitrogenase activity was higher in the mulched when compared to the unmulched soil. Soil compaction and mulching enhanced the contribution of large nodules, those greater than 0.41 cm and dry weight of individual nodules. Total nodule number and weight decreased in sequence: moderately, strongly and non-compacted soil. Mulch influenced nitrogenase activity and nodulation to a higher extent than compaction. An optimum soybean response in terms of seed and protein yields was observed under mulched MC and the worst – under unmulched SC. Both N content in leaves and nitrate content in soil were lower in mulched than in unmulched soil, irrespective of compaction. Bulk density and N-NO 3 increased with soil compaction level and N-NH 4 remained nearly unchanged. Therefore, our results supported the hypotheses that soil compaction affects nodulation and nitrogen fixation, and that compaction can be mediated by straw mulch. Further studies are needed to assess variability related with different soil and weather conditions.

Soil compaction distribution under tractor traffic in almond ( Prunus amigdalus L.) orchard in Almería España

Almond orchards in Almería require frequent traffic with farm machinery, about 1–8 times a year. Our main objective was to evaluate the vertical distribution of soil compaction induced by traffic of two tractors with different weights, one light (LT = 15 kN) and one heavy (HT = 50 kN), passing 0, 1, 3, 5 and 8 times over the same track on Aridisol soil. The work was performed in the Vélez Blanco District of Almería in southeast Spain. Outlined hypothesis were: (a) subsoil compaction distribution due to tractor traffic on recently tilled soils in almond orchard depends on total axle load and tractor passes, (b) topsoil compaction produced by tractor traffic depends on tractor passes and ground pressure. Variables measured were (CI) cone index, (BD) bulk density, (TSP) total soil porosity and (RD) rut depth. The relevant results were: in topsoil (0–200 mm), 1, 3, 5 and 8 passes of a LT caused mean values of CI of 1420, 1825, 1950 and 2050 kPa respectively, while for the HT with the same number of passes the values were of 1235, 1520, 1630 and 2510 kPa respectively. BD mean values had a similar behavior: 1.35, 1.38, 1.51 and 1.55 Mg m −3 for 1, 3, 5 and 8 passes of a LT and 1.30, 1.32, 1.41 and 1.52 for the HT with the same number of passes. In the subsoil (200–600 mm) the HT caused higher CI and BD values than the LT. CI mean values of the LT were between 1705 and 2490 kPa, while the HT produced 2100–2790 kPa of CI. BD mean values were between 1.58 and 1.7 Mg m −3 for the LT and 1.65–1.77 Mg m −3 for the HT. Hence, the data support both hypotheses. No significant differences were found in RD between HT and LT when they passed 1, 3 or 5 times, but there was a difference when traffic raised up to 8 passes (143 mm RD for HT). The main conclusions were: (a) this work has shown that soil compaction resulting from tractor traffic increases CI and BD and decreases total soil porosity. The data of CI and BD indicated that Almond orchard soil is unable to limit subsoil compaction under moderate traffic intensity. (b) Up to the fifth pass of either a FWA or 2WD tractor, ground pressure is responsible for topsoil compaction.

Compactação causada pelo tráfego de trator em diferentes manejos de solo

The objective of this work was to evaluate soil compaction induced by tractor traffic in a Red Latosol (Typic Acrustox), submitted to different soil managements. The experiment was arranged in a subdivided parcel scheme, with types of soil tillage as the primary soil treatments and the number of tractor passes as secondary treatments. In addition to a control treatment without soil tillage, the treatments were: tilling to a depth of 18 cm, a tilling to a depth of 18 cm followed by harrow use at a depth of 10 cm, and a 40-cm deep subsoil tilling. After preparing the parcels, soil penetration resistance was measured with an electronic penetrometer, before tractor passes and after 1, 3, 5 and 7 tractor passes on the same track. Soil density was also evaluated. It was concluded that the greater the pulverization during soil tillage, the greater the potential for compaction. Recently tilled soils (plough + harrow) were more susceptible to compaction than soils that were just tilled or deeply tilled. The effect of tractor traffic was chiefly concentrated on a superficial layer. The first tractor pass caused the greatest soil compaction, while the effect of subsequent passes was smaller.

Spatial distribution of soil penetration resistance as affected by soil compaction: The fractal approach

Using semivariograms and fractal dimension ( D), we identified the spatial variation of penetration resistance (PR) in variously compacted silty loam in the Lublin region, South-East Poland. Four compaction treatments were as follows: zero traffic (0p), one pass of tractor (1p), three passes of tractor (3p), and (8p) eight passes. Penetration resistance was measured in a square net of 0.6 m × 0.6 m with grid density of 0.05 m, in horizontal planes at depths of 0.05, 0.15, 0.25, 0.35, 0.45 and 0.55 m in each traffic treatment. The data were analysed in 6 horizontal planes and 12 vertical planes. The vertical planes were obtained by transforming the measured data along one side of the square at every 0.05 m. Total number of penetrations was 864. Fractal dimension ( D) was estimated from the slope of the log–log semivariogram plots. The semivariograms showed spatial autocorrelation of penetration resistance in the horizontal and vertical planes. Direction in space was important in this study. In the horizontal planes the differentiation of penetration resistance semivariance at different depths was considerable and not clearly related with traffic intensity. In the vertical planes the semivariograms showed spatial dependence of the PR and evident decreasing of semivariance with increasing traffic intensity. Kriging-interpolated maps revealed that the differentiation of penetration resistance was higher in the vertical than in the horizontal planes. The vertical differentiation was higher in 0p than in the remaining treatments. The overall mean fractal dimensions in the vertical planes increased with increasing compaction levels and can be a useful indicator of the compaction level. The opposite courses of fractal dimension in the vertical and horizontal planes indicate spatial anisotropy in distribution of penetration resistance.

Effect of the number of tractor passes on soil rut depth and compaction in two tillage regimes

The initially high level of soil compaction in some direct sowing systems might suggest that the impact of subsequent traffic would be minimal, but data have not been consistent. In the other hand on freshly tilled soils, traffic causes significant increments in soil compaction. The aim of this paper was to quantify the interaction of the soil cone index and rut depth induced by traffic of two different weight tractors in two tillage regimes: (a) soil with 10 years under direct sowing system and (b) soil historically worked in conventional tillage system. Treatments included five different traffic frequencies (0, 1, 3, 5 and 10 passes repeatedly on the same track). The work was performed in the South of the Rolling Pampa region, Buenos Aires State, Argentina at 34°55′S, 57°57′W. Variables measured were (1) cone index in the 0–600 mm depth profile and (2) rut depth. Tyre sizes and rut depth/tyre width ratio are particularly important respect to compaction produced in the soil for different number of passes. Until five passes of tractor (2WD), ground pressure is responsible of the topsoil compaction. Until five passes the tyre with low rut depth/tyre width ratio reduced topsoil compaction. Finally, the farmer should pay attention to the axle load, the tyre size and the soil water content at the traffic moment.

Soil compaction by machine traffic and least limiting water range related to soybean yield

The research aimed to evaluate machine traffic effect on soil compaction and the least limiting water range related to soybean cultivar yields, during two years, in a Haplustox soil. The six treatments were related to tractor (11 Mg weight) passes by the same place: T0, no compaction; and T1*, 1; T1, 1; T2, 2; T4, 4 and T6, 6. In the treatment T1*, the compaction occurred when soil was dried, in 2003/2004, and with a 4 Mg tractor in 2004/2005. Soybean yield was evaluated in relation to soil compaction during two agricultural years in completely randomized design (compaction levels); however, in the second year, there was a factorial scheme (compaction levels, with and without irrigation), with four replicates represented by 9 m² plots. In the first year, soybean [Glycine max (L.) Merr.] cultivar IAC Foscarim 31 was cultivated without irrigation; and in the second year, IAC Foscarim 31 and MG/BR 46 (Conquista) cultivars were cultivated with and without irrigation. Machine traffic causes compaction and reduces soybean yield for soil penetration resistance between 1.64 to 2.35 MPa, and bulk density between 1.50 to 1.53 Mg m-3. Soil bulk density from which soybean cultivar yields decrease is lower than the critical one reached at least limiting water range (LLWR =/ 0).

Soil compaction produced by tractor with radial and cross-ply tyres in two tillage regimes

The aim of this paper was to quantify soil compaction induced by tractor traffic on two tillage regimes: conventional tillage and direct drilling. Traffic was simulated with one pass of a conventional 2WD tractor, using four configurations of cross-ply rear tyres: 18.4–34, 23.1–30, 18.4–38 and 24.5–32, and four configurations of radial tyres 18.4R34, 23.1R 30, 18.4R 38 and 24.5R 32, with two ballast conditions used in each configuration. The experiment was conducted in the east of the Rolling Pampa region, Buenos Aires State, Argentina at 34°25′S, 59°15′W; altitude 22 m above sea level. Rut depth after traffic and soil bulk density and cone index in a 0–450-mm profile were measured before and after traffic. Considering topsoil level, in two tillage regimes, all treatments induced significant values of soil compaction as compared to the control plot without traffic. Subsoil compaction increased as total axle load increased and was independent of ground pressure. For the same tyre configuration, radial tyre caused less soil compaction than the cross-ply.

Mapping of penetrometer resistance in relation to tractor traffic using multivariate geostatistics

The traffic of agricultural machines can cause soil compaction and high variability of soil structure, both along normal lines and along those parallel to the field plane. The aim of this work was to investigate the potential of an electronic penetrometer, a GPS, a GIS and geostatistical techniques for mapping soil compaction. In July 2003 soil cone penetrometer resistance was measured using a semi-automatic electronic penetrometer in a sandy-silt soil (Vertic Xerochrept) of inland Sicily where a three-year rotation wheat ( Triticum durum Desf.)–wheat–tomato ( Solanum lycopersicum L.) was practiced. The measurements were carried out along three parallel 3-m long transects, from the soil surface to a depth of 0.70 m. A multivariate geostatistical approach, including exploratory analysis, variography, stochastic simulation and post-processing of simulations, was applied to produce thematic maps of penetrometer resistance and probability exceeding a critical value, in correspondence of a different number of tractor passes. Penetrometer resistance variation resulted erratic at the surface but showed high spatial correlation between data measured at different depths. The probabilistic maps of compaction risk showed that the soil volume, exceeding the penetrometer resistance of 2.5 MPa, critical for root growth, at the probability level of p > 0.40 increased from 20% to 40% after five tractor passes.

Soil Compaction Induced by Small Tractor Traffic in Northeast China

With the introduction of the small four‐wheel tractors (ST), agricultural mechanization has developed rapidly in China. In this study, we investigated the impact of these relatively small tractors on soil compaction of a silty clay loam (fine‐silty, mixed, superactive, mesic Typic Argiboroll) in northeast China using soil penetration resistance (PR) and bulk density as soil compaction indicators. The relationship between soil water content and PR, the comparison of soil compaction induced by ST and the medium power tractor (MT), the effect of tractor mass on compaction, the effect of number of tractor passes and tillage on PR, and the effect of compaction on crop yield were studied. Compared with MT‐powered tillage, the ST‐powered tillage system created a more compacted plow layer due to increased passes required with this system. Small four‐wheeled tractors had a statistically significant higher PR than MT in the topsoil and subsoil. The PR had a significant negative correlation (r = − 0.640, P &lt; 0.01) with soil water content at time of PR measurement. After trafficking in a wheat (Triticum aestivum L.) field, the highest PR values were found in the 5‐ to 14‐cm depth interval. Crop yield decreased with increasing numbers of tractor passes. There were different yield‐loss responses to compaction for spring wheat and corn (Zea mays L.). With the development of mechanical tillage in China, the problem of soil compaction may be significantly increased, causing potential yield reductions.

Light tractor traffic frequency on soil compaction in the Rolling Pampa region of Argentina

Direct seeding production in Argentina requires frequent traffic with farm machinery, in example 6–12 passes. The initially high level of soil compaction in some no till systems might suggest that the impact of subsequent traffic would be minimal, but data have not be consistent. Our objective was to quantify soil mechanical behavior related to compaction from farm machinery traffic on Typic Argiudoll under wheat ( Triticum aestvium L. Merr.), soybean ( Glycine max L. Merr.) rotation in the northeastern Rolling Pampa region at Argentina. Treatments included five different traffic frequencies (0, 6, 8, 10 and 12 passes of a two wheel-drive, 52 kW tractor running repeatedly on the same track). Total weight of the tractor was 3.1 Mg, with 1.0 and 2.1 Mg on the front and rear axles using 7.50 × 16 and 18.4 × 34 tyres, respectively. The speed of tractor traffic was 5.5 km h −1. Variables measured were: (1) soil bulk density, (2) cone index in the 0–600 mm depth profile and (3) rut depth. Only 10 and 12 tractor passes produced significant increases in cone index and dry bulk density throughout the soil profile. Dry bulk density and cone index increased with number of passes. All frequencies compacted the top soil generating physical soil conditions that would be unsuitable for seedling emergence. Although soil had high bulk density prior to traffic treatments, a significant increment of compaction still occurred due to the high traffic frequencies applied.

Effects on some soil qualities in central Italy following the passage of four wheel drive tractor fitted with single and dual tires

Field tests were carried out using the same four wheel drive tractor (113 kW engine power) fitted with two different tire arrangements: single and dual. Tests were conducted on arable soil previously ploughed and harrowed in order to check the compacting effects resulting from one to four passes of the tractor in the same track, fitted with two different tire configurations. Soil parameters studied were: penetration resistance, bulk density, soil shear strength and their increment ratio, soil macroporosity, pore size distribution and saturated hydraulic conductivity. Multiple passes made by the tractor fitted with the two-tire arrangements caused different effects on the soil with regard to the studied soil parameters. Mean values of penetration resistance (0–0.20 m depth) were 2.02 MPa for the single tires tractor and 1.86 MPa for the dual tires tractor; mean values of penetration resistance (0.21–0.35 m depth) were 2.72 MPa for the single tires tractor and 1.72 MPa for the dual tires tractor. The decrease in macroporosity, in particular that of elongated pores in the soil layers (0–0.10 m and 0.11–0.20 m depth) was greater in treatments involving the single tire arrangement of the tractor (from 25.3 to 1.1% and from 15.9 to 2.0%, respectively) than for the dual tire arrangement (from 25.3 to 2.8% and from 15.9 to 5.2%, respectively, for 0–0.10 m and 0.11–0.20 m depth). Following traffic of the tractor with the different tire arrangements, hydraulic conductivity decreased and the following values were found for the five treatments: control, 15.8 mm h −1; single tires one and four passes, 2.8 and 1.6 mm h −1, respectively; dual tires one and four passes, 5.9 and 5.8 mm h −1, respectively). Highly significant correlations between hydraulic conductivity and elongated pores and total macroporosity were found. Significant relationships between macroporosity and penetration resistance for one and four passes of both tractor tire arrangements were found in the soil layers (0–0.20 m). However, in the soil layer examined, with respect to the higher degree of macroporosity and hydraulic conductivity and the low values of penetration resistance, treatments involving tractor fitted with dual tires (one and four passes) showed a lower degree of soil compaction than was observed after the same passes of the tractor fitted with single tires.

Soil compaction and fertilization in soybean productivity

Soil compaction and fertilization affect soybean development. This study evaluated the effects of soil compaction and fertilization on soybean (Glycine max cv. Embrapa 48) productivity in a Typic Haplustox under field conditions in Jaboticabal, SP, Brazil. A completely randomized design with a 5 x 2 factorial layout (compaction vs. fertilization), with four replications in each treatment, was employed. Each experimental unit (replicate) consisted of a 3.6 m² useful area. After the soil was prepared by cultivation, an 11 Mg tractor passed over it a variable number of times to create five levels of compaction. Treatments were: T0= no compaction, T1= one tractor pass, T2= two, T4= four, and T6= six passes, and no fertilizer and fertilizer to give soybean yields of 2.5 to 2.9 Mg ha-1. Soil was sampled at depths of 0.02-0.05, 0.07-0.10, and 0.15-0.18 m to determine macro and microporosity, penetration resistance (PR), and bulk density (Db). After 120 days growing under these conditions, the plants were analyzed in terms of development (plant height, number of pods, shoot dry matter per plant and weight of 100 seeds) and seed productivity per hectare. Soil compaction decreased soybean development and productivity, but this effect was decreased by soil fertilization, showing that such fertilization increased soybean tolerance to soil compaction.

MASS-SIZE SCALING IN SOIL AGGREGATES AS AFFECTED BY AGGREGATE WATER CONTENT AND SOIL COMPACTION

Determining aggregate size distribution is a common way of characterizing soil structure. Information about soil structure can also be derived from examining the aggregate mass or density of aggregate size. There have been several reports indicating that the density-size relationships in air-dry aggregates follow predictions of a model assuming aggregates to be mass fractals. It was demonstrated recently that such model is applicable to wet aggregates if parameters of this model are assumed to be linear functions of gravimetric water contents. The objective of this study was to evaluate sensitivity of fractal parameters to soil compaction caused by wheel traffic. Irrigated and nonirrigated plots were laid out at silty clay Greyzem under fallow, and treatments of one tractor pass and three tractor passes were applied. The volume of individual aggregates for four depths in the plow layer was measured by kerosene method at air-dry water content, at two intermediate water contents between saturation and air-dry, and at saturation. The mass fractal model fitted data in a satisfactory manner within the range of water contents from air-dry to saturation with a R 2 of 0.999. Both the slope and the intercept of the dependence of fractal dimension and reference aggregate mass on water content were more sensitive to compaction than to soil bulk density and aggregate size distributions. Parameters of fractal scaling showed promise for diagnosing compaction in studied soil.