W. Ehlers

Penetration resistance and root growth of oats in tilled and untilled loess soil

Penetration resistance, bulk density, soil water content and root growth of oats were intensively studied in a tilled and an untilled grey brown podzolic loess soil. Bulk density and penetration resistance were higher in the top layer of the untilled soil compared with the tilled soil. In the latter, however, a traffic pan existed in the 25–30 cm soil layer which had higher bulk density and penetration resistance than any layer of the untilled soil. Above the traffic pan, rooting density (cm root length per cm3 of soil) was higher but below the pan it was lower than at the same depth in the untilled soil. Root growth was linearly related to penetration resistance. The limiting penetration resistance for root growth was 3.6 MPa in the tilled Ap-horizon but 4.6-5.1 MPa in the untilled Ap-horizon and in the subsoil of both tillage treatments. This difference in the soil strength-root growth relationship is explained by the build up of a continuous pore system in untilled soil, created by earthworms and the roots from preceding crops. These biopores, which occupy < 1% of the soil volume, can be utilized by roots of subsequent crops as passages of comparatively low soil strength. The channeling of bulk soil may counteract the possible root restricting effect of an increased soil strength which is frequently observed in the zero tillage system.

Ploughing effects on soil organic matter after twenty years of conservation tillage in Lower Saxony, Germany

Conservation tillage may concentrate organic matter and carbon in the soil, thus improving soil quality and counteracting CO2-increase in the atmosphere. In parts of Germany however, continuous conservation tillage can cause problems in soil and crop management, resulting in a need to shift to short-term conventional tillage, such as mouldboard ploughing. The objective of the present research was to follow the fate of soil organic matter, when soil is ploughed after long-term minimum tillage in the temperate climate of Lower Saxony. In minimum tillage, shallow cultivation was restricted to stubble cleaning and seedbed preparation, using a rotary harrow or rototiller. After 20 years of shallow cultivation, soil organic carbon, soil nitrogen and microbial biomass carbon were concentrated in the top 5 cm of a loess-derived silt loam (Orthic Luvisol). In the 50 cm soil profile, mass of soil organic carbon tended to be higher by about 5 Mg ha ˇ1 as compared to conventionally ploughed soil, which contained roughly 65 Mg ha ˇ1 . In the ploughed soil, soil nitrogen amounted to about 6.8 Mg ha ˇ1 , whereas in the minimum tilled soil it was roughly 1.0 Mg ha ˇ1 higher. Total microbial biomass carbon fluctuated between 800 and 1300 kg ha ˇ1 , the differences between tillage systems being less distinct. Ploughing the old minimum tilled land destroyed the stratification of soil organic matter. Moreover, during the winter months (November‐March) the surplus of soil organic carbon and nitrogen masses, enriched by conservation tillage, was completely decomposed, presumably a consequence of its labile quality. Inverting the minimum tilled soil did not increase concentrations of organic carbon and nitrogen in the lower part of the Ap-horizon, but it did increase concentration of microbial biomass carbon. We conclude that organic matter stratification and accumulation as a result of long-term minimum tillage were completely lost by a single application of inversion tillage in the course of a relatively mild winter. # 1999 Elsevier Science B.V. All rights reserved.

Water dynamics in plant production.

Water Dynamics in Plant Production , Water Dynamics in Plant Production , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Tillage and mulching effects on physical properties of a tropical Alfisol

Abstract The mechanization of field operations like seeding, spraying and harvesting in continuous zero-tillage may lead to a severe compaction of the surface layer of coarse textured tropical soils, especially when mulch is sparse or missing. Therefore, a 2 year (1982–1984) field experiment was initiated on an Alfisol in Nigeria to study the effect of tillage, mechanization and mulch on soil structure and physical properties. Three zero-tillage treatments and a plough treatment were compared. The disk-plough and one of the no-till treatments were highly mechanized: all the field work was performed with tractors and machines, and consisted of secondary bush clearing, crop cultivation and harvest. On the other two no-till treatments, the impact of machine load was reduced, wither by hand harvesting or by performing all field operations manually. These four tillage-traffic systems were either treated with mulch or left unmulched. There were four growing seasons, with maize ( Zea mays L.) as a test crop. After 2 years of zero-tillage the bulk density (BD) and penetration resistance (PR) were significantly greater on plots with high mechanization compared with hand treated plots. Plots with hand harvest but otherwise mechanized were in between. Because of the hard-setting nature of the soil, the plougheed plots with and without mulch exhibited a dramatic change in PR and BD during the season. On no-till the infiltration transmissivity ( A ) was greater and BD and PR were less in the mulched compared with the unmulched treatments. The gravel content of the topsoil was negatively correlated with BD and positively correlated with A . Geostatistical analysis revealed that within the experimental area there was a similar spatial distribution of gravel content and A after the first season. Because of the superimposing effect of gravel on BD, which cannot be accounted for by considering the gravel content per se, BD was adjusted by means of covariance analysis for evaluation of the treatment effects already mentioned. It was concluded that mechanization of a no-till system on sandy Alfisols may only be successful in the long run if appropriate measures like mulching, crop rotation and fallow systems are applied to regenerate soil structure and to enhance macroporosity.

Wirkung mechanischer Belastung auf Gefüge und Ertragsleistung einer Löss‐Parabraunerde mit zwei Bearbeitungssystemen

In Germany farmers are committed to caring for the land by a soil protection law. Yet vehicles with ever increasing axle load endanger productivity and environmental quality of arable soils. In spring of 1995 a field experiment was started on a wet silty Luvisol to test the effect of single mechanical loading on soil and crop characteristics, when managed by mouldboard ploughing (PL) or conservation tillage (CT). CT soils are considered to be more resistant against compactive stresses and to recover from degeneration more rapidly than PL soils. Beside an unwheeled control the loading treatments were light (2 X 2.5 t; number of wheel passes times wheel load); medium (2 X 5 t) and high (6 X 5 t). In 1995 even light loading of the PL soil caused a significant yield decline by 50% in spring barley, but this happened on CT soil only with high loading. In subsequent years with winter wheat and winter barley yield decline was less distinct. Loading of PL soil reduced total root length (from 4 to 1 km m -2 ) and rooting depth (from 70-90 to 40-70 cm), but on CT soil only root length was diminished by high loading. A tillage-traffic pan (30-35 cm) hindered subsoil rooting in PL, which was favored in CT by earthworm channels. High loading caused compaction to at least 50 cm depth. Within the pan of the PL soil, penetration resistance attained 5 MPa and bulk density 1.65 g cm In the CT soil the zone of maximum compaction was closer to the surface (15-25 cm). In PL soil the saturated hydraulic conductivity and the O 2 diffusion coefficient gradually decreased with loading, but in CT soil only with heavy loading. The compacted top soil was broken in subsequent years by ploughing (PL: 25 cm) or rotary implements (CT: 5-8 cm). With PL, structure in the pan layer and subsoil did not recover, and rooting depth was still limited. Some restoration, however, was indicated with CT. Here transmitting properties increased in time, which was attributed to the reconstruction of root and earthworm channels, as demonstrated by computer tomography. We conclude that in silty soils compacted layers below ploughing depth will hardly be regenerated by internal processes. CT soils are less susceptible to loading but high stresses are harmful too. Therefore recommending CT as a measure for protecting soil from compaction would not be enough, considering the present development towards heavy field machinery.

Tillage effects on root development, water uptake and growth of oats

Abstract The experiment was carried out in 1976 on a well-drained, loess-derived soil. The general objective was to study the interrelation between root development, water uptake and shoot growth of oats (Avena sativa L.) under field conditions during one growing season. A specific purpose was to determine if regular tillage induces differences in rooting pattern, water uptake and plant growth as compared to untilled soil. In tilled soil a plough-sole layer at 20–30-cm depth induced higher rooting densities within the 10–20-cm layer, but restricted proliferation of roots in deeper layers. Accordingly, total water uptake from the 10–20-cm layer was greater and from the 20–60-cm layer it was less than from untilled soil. Water uptake was particularly limited in the plough-sole layer. The water uptake rate was functionally related to rooting density and soil water potential. Relative growth rate of root length decreased with increasing soil water tension and ended at approximately 19 bar. Tillage favored initial shoot growth, but in June accelerated shoot growth on untilled soil was associated with higher evapotranspiration and a deeper soil exploration by roots. Shoot growth rate was linearly related to transpiration rate. One mm of water use corresponded to a production of 40 kg/ha dry matter.

Root system parameters determining water uptake of field crops

SummaryThe distribution of a crop rooting system can be defined by root length density (RD), root length (RL) per soil layer of depth Δz, sum of root length (SRL) in the soil profile (total root length) or rooting depth (zr. The combined influence of these root system parameters on water uptake is not well understood. In the present study, field data are evaluated and an attempt is made to relate a daily “maximum water uptake rate” (WUmax) per unit soil volume as measured in different soil layers of the profile to relevant parameters of the root system. We hypothesize that local uptake rate is at its maximum when neither soil nor root characteristics limit water flow to, and uptake by, roots. Leaf area index and the potential evapotranspiration rate (ETp) are also important in determining WUmax, since these quantities influence transpiration and hence total crop water uptake rate. Field studies in Germany and in Western Australia showed that WUmax depends on RD. In general, there was a strong correlation between the maximum water uptake rate of a soil layer (LWUmax) normalized by ETp and RL normalized by SRL. The quantity LWUmax · ETp-1was linearly related to (RL/SRL)1/2. The data show that the single root model will not predict the influence of RD on WUmax correctly under field conditions when water-extracting neighboring roots may cause non-steady-state conditions within the time span of sequential observations. Since the rooting depth zr was linearly related to (SRL)1/2, the relation: LWUmax · ETp-1= f (RL1/2/zr) holds. Furthermore it was found that the maximum “specific” uptake rate per cm root length URmax was inversely related to RD1/2 and to SRL1/2 or zr of the profile. Observed high specific uptake rates of shallow rooted crops might be explained not only by their lower RD-values but also by the additional effect of a low zr. The relations found in this paper are helpful for realistically describing the “sink term” of dynamic water uptake models.Growing plants extract water from the soil to meet transpiration needs. Rates of transpiration and of water uptake are set by evaporative demand and by plant and soil factors which influence capacity to meet that demand. These factors include crop canopy size and leaf characteristics, root system characteristics and hydraulic properties of the soil and the soil-root interface. Soil and root system properties vary with depth and all factors vary in time, so that parameters related to them require constant updating over a crop season.Dynamic simulation models describe water uptake by root systems under field conditions as a function of soil depth and time. Many of these simulation approaches are based on Gardners (1960) single root model (Feddes 1981). These simulation procedures follow the assumption that water uptake is proportional to a difference in water potential between the bulk soil and the root surface or the plant interior, to the hydraulic conductivity of the soil-plant system and to the “effectiveness” of competing roots in water uptake. The effectiveness factor accounts more or less empirically for the influence of various root system parameters on water uptake such as percentage of “active” roots absorbing water, root surface permeability, root length density determining the distance between neighbouring roots, or total root length and depth of the root system. Such models however, will not always reflect correctly the influence of root system characteristics on water uptake since these assumptions have rarely been tested under field conditions. In many instances, there is better agreement between simulated and measured total water use of plants than between predicted and observed water depletion by roots within individual layers of the soil profile (Alaerts et al. 1985).Water uptake by an expanding root system as a function of depth and time has been studied under field conditions for several crops (listed in Herkelrath et al. 1977a; Feddes 1981; Hamblin 1985). They show that the dynamics of water uptake depend on root length density and the “availability” of soil water. However, the combined influence of root length density, total root length and rooting depth on the water uptake pattern has not been assessed. An evaluation of root system parameters with respect to soil water extraction should aid our understanding of how roots perform under field conditions and may assist our efforts to formulate the water uptake function of roots in dynamic simulation studies more realistically.The aim of the present investigation is to develop an approach that relates measured water uptake rates to relevant parameters of the root systems. This approach will be confined to situations where water uptake in a soil layer is not restricted by unfavorable soil conditions, such as in wet soil, by insufficient aeration and, in dry soil, by reduced water flow towards roots or by increased contact resistance (Herkelrath et al. 1977b). We will define a maximum water uptake rate WUmax that is neither soil-limited nor appreciably limited by the decreasing permeability of aging roots. This WUmax will be related to relevant root system parameters as they exist when WUmax is observed. Hence, water uptake by roots in a very wet, as well as in a dry soil, has been excluded from consideration.

Leaf water potential and stomatal conductance of field-grown faba beans (Vicia faba L.) and oats (Avena sativa L.)

SummaryFaba beans are known to be susceptible to water stress. The aim of the present research was to find out, if this sensitivity is related to an incapability of the plants to close the stomata effectively during times of water stress. For reasons of comparison oats were included in the investigation, as oat plants are known to respond less sensitively to water shortage than faba beans. The experiment was conducted on a loess-derived soil during a relatively dry vegetative season. Leaf area development and soil water use of beans was later in the season as compared to oats. Maximum leaf area and water extraction rates were attained end of June to beginning of July during pod development. Total leaf water potential ι of beans was always higher than in oats. At a given ι the osmotic potential π was less and the pressure (turgor) potential P was higher as compared to the cereal crop. To changing ι beans responded much more pronounced than oats in reducing adaxial and abaxial leaf conductance. The sensitivity in stomata regulation of faba beans became also apparent by a distinct oscillation of ι and conductance during the course of a day. It is concluded that the water-stress susceptibility of faba beans is brought about by a reduced CO2 diffusion into the leaves, thus lowering net assimilation rates.

Floods and Other Possible Adverse Environmental Effects of Meadowland Area Decline in Former West Germany

6 ha, or 6% of the countrys total area. The environmental implications of this large-scale conversion have so far received little attention; the present study examined some of these implications. A review of research on soil physical and chemical aspects of the conversion of permanent grassland into arable land reveals that such a large-scale conversion may have considerable effects upon the environment. For example, due to the mineralization of soil organic matter a release of NO3 and CO2 into the environment can be expected on the order of 10 t N and 100 t C per hectare. Environmentally equally severe, if not worse, is the increased amount of surface runoff that can be expected from converted grassland soils in arable land during winter because of surface sealing and soil compaction. This increased surface runoff, in combination with the runoff from other farmland, may be one of the reasons for the growing frequency of floods along major German rivers in recent years. In view of the lasting adverse environmental effects of permanent grassland conversion and the subsidized agricultural surpluses in Germany today, we conclude that a reconversion of arable land into permanent grassland may be beneficial both environmentally and economically.

Flow resistance in soil and plant during field growth of oats

Abstract During the vegatative period of oats (Avena sativa L.), soil-water content, soil-water potential and conductivity as well as root-length density and plant-water potential were intensively studied. The data were analysed to find out where the dominant flow resistance to water is localized, in soil or in plant. Emphasis is placed on the mode of calculation of average resistances when root distribution and water uptake are not uniform within the profile. It was found that all times plant resistance was much higher than soil resistance. When the soil-water potential decreased to −15 bar and the hydraulic conductivity to 7·10−6 cm/day, soil resistance became appreciable. Calculations on root conductance show that the plant resistance attributed to the root tissue may change not only with soil-water potential but also with root age and anatomical modifications.