Guidelines for safe trafficking and cultivation, and resistance–density–moisture relations of three disturbed soils from Alberta

The Atterberg limits and the Proctor compaction test are used by engineers for classifying soils and for predicting stability of building foundations. Field capacity and wilting point (agronomic limits) are used to indicate available water for plant uptake. Few studies have related the engineering criteria to the agronomic ones with regard to compaction hazard for soils. This study investigated the relationships between Atterberg limits, agronomic limits and the critical moisture content (moisture content at Proctor maximum density) for three disturbed soils (sandy loam and clay loam soils from a reclaimed Highvale mine site, and a silt loam soil from a grazing site at Lacombe) of different textures. Relationships between bulk density, moisture content and penetration resistance for these soils were also investigated. For the sandy loam and loam soils, the field capacity was close to the critical moisture content but lower than the plastic limit. Therefore, cultivation of these two soils at moisture contents close to field capacity should be avoided since maximum densification occurs at these moisture contents. Overall, the critical moisture content or field capacity would be a better guide for trafficking of sandy loam and loam textured soils than the Atterberg limits. For the clay loam, field capacity was within the plastic range. Thus trafficking this soil at field capacity would cause severe compaction. In conclusion, either field capacity or plastic limit, whichever is less, can be used as a guide to avoid trafficking at this moisture content and beyond. For the sandy loam and loam soils penetration resistance significantly increased only with increased bulk density (P≤0.05). For the clay loam soil, penetration resistance was positively related to bulk density and negatively related to moisture content.     

Critical soil bulk density and strength for pea seedling root growth as related to other soil factors

Bulk density and soil strength are two major soil physical factors affecting root growth of pea seedlings. This study was conducted to determine the influence of soil texture, organic carbon content and water content on critical bulk density and strength. Soil from the plough layer (PL) and beneath the sub-soil (SUB) was used. By soil packing and adjusting the water content between 30% and 100% of field water capacity (FWC) a wide range of bulk density (1.3–1.7 Mg m⁻³) and strength (0.24–6.66 MPa) were obtained. Pea (Pisum sativum L.) was grown in the packed cores of 100 cm³ for 72 h at 20°C. Regression models were developed to explain root growth in terms of bulk density, soil strength, silt and clay (<60 μm) content, organic carbon, and water content. The regression curve of root growth as a function of soil strength showed that 40% of maximum root length can be regarded as an indicator of very poor root growth. By substituting this value into the root growth equations we calculated a critical bulk density and strength in terms of fraction<60 μm, organic carbon percentage and water content. The values of critical bulk density in both layers and of critical soil strength in the sub-soil increased with a decreasing content of fraction<60 μm. Irrespective of fraction<60 μm content, the critical bulk density and strength decreased as soil water content decreased. Critical soil strength was more sensitive than critical bulk density to changes in fraction<60 μm content and water content. This study provides data and a method for predicting critical bulk density and soil strength in relation to other soil properties for pea seedling root growth.     

Bulk density by Proctor test as a function of texture for agricultural soils in Maputo province of Mozambique

Soil degradation processes may be of various kinds, including soil compaction. The present study was carried out with the objective of assessing the sensitivity of agricultural or recently abandoned soils in Maputo province of Mozambique to compaction. The assessment is based on the maximum of bulk density attained using the Proctor test (MBD).

In this study the soil texture is expressed by silt plus clay (S + C) or clay (C). The relations between the soil texture and MBD, and between soil texture and critical water content (CWC—soil water at which MBD is attained) were determined. Selected soils range from 10 to 74% of S + C and 9 to 60% of C.

The results suggest there is a relationship between the considered parameters, being that between S + C and MBD or CWC, the best. For MBD the relationship is represented by two quadratic equations with the boundary in between these being a S + C value of 25% and C value of 20%.

Based on the obtained results, one can conclude that the selected parameters may be a useful basis for estimation of the sensitivity to compaction of the Maputo province's soils. It is recommended that similar studies be carried out for soils under forest land and for soil of other provinces to establish the national physical degradation hazard as a function of soil parameters determined routinely and at low cost. The suggested parameters are texture and soil organic matter (SOM).     

Soil physical properties of selected soil series in relation to compaction and erosion on farmers’ fields at Abeokuta,southwestern Nigeria

This study evaluated physical properties of selected soil series and their implications on the soil compaction and erosion in Abeokuta, southwestern Nigeria. Daily rainfall data (1999–2007) were collected to estimate the rainfall erosivity. Seven soil series (Iwo, Iseyin, Ekiti, Jago, Okemesi, Apomu, and Egbeda) were sampled from 0–15, 15–30, and 30–50 cm depths for particle size distribution, organic carbon, pH, upper plastic limit, and compactibility (Proctor test). Microtopographical changes along and across toposequences of two farmers’ fields cleared mechanically and manually, respectively, were monitored using the erosion pin method. Mean annual erosivity (EI30) was high (7646 MJ mm ha^?1 hr^?1). Particle size, organic carbon, and pH were similar (p ≥ 0.05), while upper plastic moisture was ≤?2% among different soil series. Soil-moisture density curves indicated a maximum bulk density of 1.77–1.99 g cm^?3 for a moisture range of 7.6–14%; while the soils were prone to compaction at low moisture content. Microtopographic changes were found between –2 and 0 cm and –8 and –2 cm on mechanically and manually cleared farmland, respectively. Spatial dependence showed that the soil erosion could be predicted within 5–8 m distance. To avoid erosion and compaction, soil water content should be less than 7.6% before the introduction of mechanical tillage.     

Reference bulk density and critical degree-of-compactness for no-till crop production in subtropical highly weathered soils

The concept of degree of compactness (DC), referred to as field bulk density (BD) as a percentage of a reference bulk density (BD_ref), was developed to characterize compactness of soil frequently disturbed, but for undisturbed soil such as under no-tillage critical degree of compactness values have not been tested. The objective of this study was to compare methods to determine BD_ref and limits of DC and BD for plant growth under no-tillage in subtropical soils. Data from the literature and other databases were used to establish relationships between BD and clay or clay plus silt content, and between DC and macroporosity and yield of crops under no-tillage in subtropical Brazil. Data of BD_ref reached by the soil Proctor test on disturbed soil samples, by uniaxial compression with loads of 200 kPa on disturbed and undisturbed soil samples, and 400, 800 and 1600 kPa on undisturbed soil samples, were used. Also, comparisons were made with critical bulk density based on the least limiting water range (BDc LLWR) and on observed root and/or yield restriction in the field (BDc Rest). Using vertical uniaxial compression with a load of 200 kPa on disturbed or undisturbed samples generates low BD_ref and high DC-values. The standard Proctor test generates higher BD_ref-values, which are similar to those in a uniaxial test with a load of 1600 kPa for soils with low clay content but lower for soils with high clay content. The BDc LLWR does not necessarily restrict root growth or crop yield under no-tillage, since field investigations led to higher BDc Rest-values. A uniaxial load greater than 800 kPa is promising to determine BD_ref for no-tillage soils. The BD_ref is highly correlated to the clay content and thus pedotransfer functions may be established to estimate the former based on the latter. Soil ecological properties are affected before compaction restricts plant growth and yield. The DC is an efficient parameter to identify soil compaction affecting crops. The effect of compaction on ecological properties must also be further considered.     

A method for predicting soil susceptibility to the compaction of surface layers as a function of water content and bulk density

Identifying the vulnerability of soils to compaction damage is becoming an increasingly important issue when planning and performing farming operations. Soil compaction models are efficient tools for predicting soil compaction due to agricultural field traffic. Most of these models require knowledge of the stress/strain relationship and of mechanical parameters and their variations as a function of different physical properties. Since soil compaction depends on the soil's water content, bulk density and texture, good understanding of the relations between them is essential to define suitable farming strategies according to climatic changes. In this work we propose a new pedotransfer function for 10 representative French soils collected from cultivated fields, a vineyard and forests. We investigate the relationship between soil mechanical properties, easily measurable soil properties, water content and bulk density. Confined compression tests were performed on remoulded soils of a large range of textures at different initial bulk densities and water contents. The use of remolded samples allowed us to examine a wide range of initial conditions with low measurement variability. Good linear regression was obtained between soil precompression stress, the compression index, initial water content, initial bulk density and soil texture. The higher the clay content, the higher the soil's capacity to bear greater stresses at higher initial water contents without severe compaction. Initial water content plays an important role in clayey and loamy soils. In contrast, for sandy soils, mechanical parameters were less dependent on initial water content but more related to initial bulk density. These pedotransfer functions are expected to hold for the soils of tilled surface layers, but further measurements on intact samples are needed to test their validity.     

Assessing the compaction susceptibility of South African forestry soils. II. Soil properties affecting compactibility and compressibility

Factors affecting the compaction susceptibility of South African forestry soils were assessed. Two traditional measures of compaction susceptibility were used: maximum bulk density (ρ_mbd) determined by the standard Proctor test, defined compactibility, and the compression index using a simple uni-axial test, defined compressibility. Soils were chosen from a broad range of geological and climatic regions and they varied greatly in texture (8 to 66 g 100 g⁻¹ clay) and organic matter content (0.26 to 5.77 g 100 g⁻¹ organic carbon). Soils showed a wide range in ρ_mbd values, from 1.24 to 2.00 Mg m⁻³, and this reflected the wide range of particle size distributions and organic matter contents of the soils. Very good correlations were achieved between measures of particle size distribution, particularly clay plus silt and both compactibility and compressibility. Both compactibility and compressibility were significantly correlated with loss-on-ignition (L_OI) which is a measure reflecting the combined effects of soil texture and organic matter on soil physical properties. Indices of compaction susceptibility were influenced more by particle size distribution than by organic carbon content. Clear effects of organic carbon on compaction behaviour were only evident for soils with low clay contents (< 25 g 100 g⁻¹. No clear relationship between compactibility and compressibility was found. Compactibility generally increased with decreasing clay plus silt content, whereas compressibility increased up to about 70 g 100 g⁻¹ clay plus silt before decreasing again. It is difficult to define compaction susceptibility solely in terms of indices of compactibility or compressibility particularly as there is no clear relationship between these two properties. A classification system for compaction risk assessment is presented, based on the relationship between compactibility (ρ_mbd) and L_OI, and between clay plus silt content and compressibility.     

Critical limits of the degree of compactness and soil penetration resistance for the soybean crop in N Brazil

In agricultural fields soil compaction is a major cause of physical degradation. Degree of compactness (DC) is a useful parameter for characterizing compaction and the response of crops for different soils. The objectives of this study were: (1) to identify the critical DC and PR values for soybean [Glycine max (L.) Merrill] using plant growth variables and (2) to verify the relationship between DC and PR, and assess which parameter is recommended for the evaluation of soil compaction. The study was conducted in a greenhouse in a completely randomized factorial design of 4 textures × 5 compaction levels for sandy loam and sandy clay loam soils, and 3 compaction levels for the clayey and very clayey soils. Soil samples were collected from the surface of a Xantic Kandiudox from the NE region of the State of Pará, Brazil. The DC was calculated from the maximum bulk density obtained by the Proctor test, and the PR curve was determined in undisturbed samples equilibrated in different matric potentials. The growth and development of the soybean was favored in the DC range of 80 to 85%, regardless of soil texture. The critical degree of compactness for the growth of soybean was around 98% regardless of soil texture, while the critical values for penetration resistance at field capacity varied according to soil texture and bulk density and were 28.2, 5.6, 3.5, and 5.2 MPa for the sandy loam, sand clay loam, clayey and very clayey soils, respectively. The root length was the plant growth variable most susceptible to soil compaction. Change in soil penetration resistance was poorly related with change in degree of compactness showing that one parameter cannot be replaced by the other. Because PR is quickly determined in field and have a direct relationship with plant growth, for the soils evaluated in this study we recommend the use of soil PR to assess the state of soil compaction.     

Geostatistical analysis of soil mechanical properties in Ardabil plain of Iran

This study was conducted to estimate the spatial distribution of penetration resistance (PR), Proctor maximum bulk density (MBD) and critical water content (CWC) as soil mechanical indices. Soil samples to determine sand, silt, clay, organic carbon (OC), CaCO₃, bulk and particle density, total porosity, field water content, MBD and CWC values were collected. Field measurements of PR at 0–10 cm depth were taken from 105 geo-referenced points with 3000 × 3000 m intervals in agricultural lands of Ardabil plain, Iran. Ordinary kriging (OK) and inverse distance weighting (IDW) methods were used to analyze spatial variability of PR, MBD and CWC. The strongest spatial dependences with the lowest ranges of influence were found for OC (7560 m) and MBD (8370 m). The models of fitted semivariograms were Gaussian for PR and MBD, and exponential for CWC. The moderate spatial dependences with the ranges of 13,300 and 40,100 m were found for CWC and PR, respectively. The best prediction according to Lin’s concordance correlation coefficient was obtained by OK for PR (0.48) and IDW for MBD (0.09) and CWC (0.03). These results can be applied in programming of optimum tillage operations for reducing soil compaction risk in the studied region.     

深翻－旋耕轮耕与有机肥配施对黑土农田土壤物理性质的影响

针对东北松嫩平原中南部黑土区玉米带农田长期旋耕导致耕层变浅、容重增大等问题,开展深翻-旋耕轮耕模式改善土壤物理性质的研究。试验设置连年旋耕配施化肥（RT）、连年旋耕配施化肥与有机肥（RM）、深翻－旋耕轮耕配施化肥（DT）和深翻－旋耕轮耕配施化肥与有机肥（DM）4个处理,分析0 ~ 45 cm土壤含水量、容重、紧实度、团聚体的变化及10 cm、20 cm、30 cm各深度处土壤温度变化情况。结果表明,与RT处理相比,DT处理能够显著提高玉米苗期和拔节期20 cm、30 cm深度土壤温度,增加玉米各生育时期15 ~ 45 cm土层土壤含水量,并且显著降低土壤容重和紧实度,提高了30 ~ 45 cm土层 > 0.25 mm水稳性团聚体的比例;同时DM处理能够增加苗期、收获期各土层含水量,且对0 ~ 45 cm土壤容重均有显著降低作用;而RM处理仅使0 ~ 15 cm土层容重有降低,但并不显著,且对深层土壤容重无明显影响。相关分析表明,在0 ~ 15 cm土层中,土壤含水量、紧实度、容重与温度呈负相关关系（P < 0.05）;在0 ~ 45 cm土层中,土壤容重与土壤紧实度呈极显著正相关关系（P < 0.05）。DM的耕作模式能降低土壤容重和紧实度,有效提高土壤温度、土壤含水量以及 > 0.25 mm 水稳性团聚体的比例,能够较好的改善土壤耕层物理性质。     

A laboratory study of the residual influence of some amendments on the reconsolidation of disturbed Salisbury silt loam

The residual effect of various soil amendments on the reconsolidation of a strongly acidic Salisbury silt loam C horizon which contained 0.25% free aluminium was investigated in the laboratory. Limestone (CaCO₃), gypsum (CaSO₄) and peat moss were added at 0.5, 0.08 and 2.0% (w/w), respectively. In a fourth treatment, CaCO₃ (0.5%, w/w) was added 24 months previously. Results from these treatments were compared to those from an untreated control. All soils were redisturbed to simulate tillage. The soils were subjected to a standard Proctor test and a low-energy compaction test designed to simulate a 0.44 m soil overburden. Penetrometer studies were performed to examine soil strength.

The mineralogy of the Salisbury C-horizon was dominated by illite, with lesser amounts of chlorite occurring; kaolinite and vermiculite were present only in small amounts. Application of peat lowered the maximum dry density of the Proctor test and increased the concomitant optimum moisture content. Soil chemical amendments had no effect on compaction as measured by the Proctor test, but significantly increased the dry bulk density in the low-energy compaction test. Hence volumetric moisture content, at a moisture tension of 101 kPa, was increased by the chemical amendments. Soil strength, as measured by the cone index at a constant soil moisture tension, was decreased by the chemical amendments; however, after correction for volumetric moisture content, no treatment effect occurred. The chemical amendments significantly decreased the concentrations of inorganic and organically bound Al and inorganically bound Fe.

The results of the low-energy compaction test suggest that application of gypsum or limestone amendments to severely disturbed, highly acidic soils, which contain appreciable amounts of aluminium oxides and exchangeable aluminium, may increase the soil's propensity to reconsolidate. This was not revealed by the standard Proctor test.     

Effect of Continuous Rice–Wheat Rotation on Soil Properties from Four Agro‐ecosystems of Indian Punjab

In Indian Punjab, rice–wheat is a dominant cropping system in four agro‐ecosystems, namely undulating subregion (zone 1), Piedmont alluvial plains (zone 2), central alluvial plains (zone 3), and southwestern alluvial plains (zone 4), varying in rainfall and temperature. Static and temporal variabilities in soil physical and chemical properties prevail because of alluvial parent material, management/tillage operations, and duration of rice–wheat rotation. A detailed survey was undertaken to study the long‐term effect of rice–wheat rotation on soil physical (soil separates, bulk density, modulus of rupture, saturated and unsaturated hydraulic conductivities, soil water content, and suction relations) and chemical (organic carbon, pH, electrical conductivity) properties of different textured soils (sandy clay loam, loam, clay loam, and silty clay loam) in these four zones of Punjab. Soil samples (of 0‐ to 30‐cm depth) from 45 sites were collected during 2006 and were analyzed for physical and chemical properties. The results showed that sand content and pH increased whereas silt and organic carbon decreased significantly from zones 1 to 4. Compared to other textures, significantly greater organic carbon, modulus of rupture, and pH in silty clay loam; greater bulk density in clay loam, and greater saturated hydraulic conductivity in sandy clay loam were observed. Irrespective of zone and soil texture, in the subsurface soil, there was a hard pan at 15–22.5 cm deep, which had high soil bulk density, modulus of rupture, more silt and clay contents (by 3–5%) and less organic carbon and hydraulic conductivity than the surface (0–15 cm) layer. These properties deteriorated with fineness of the soil texture and less organic carbon content. Continuous rice–wheat cropping had a deleterious effect on many soil properties. Many of these soils would benefit from the addition of organic matter, and crop yields may also be affected by the distinct hardpan that exists between 15 and 22.5 cm deep.     

毛乌素沙地东南缘沙漠化过程中土壤理化性质分析

采用空间代替时间的方法,对毛乌素沙地东南缘沙漠化过程中土壤理化性质变化进行了分析.结果表明,沙漠化过程中土壤理化性质呈规律性变化,随沙漠化程度的加剧,土壤黏粒含量、含水量、有机质、有机碳和全氮含量减少,土壤砂粒含量、容重和C/N增加,土壤质量下降;相关性分析结果表明,容重和C/N比与各指标呈负相关关系,其余各指标间呈正相关关系,其中容重与有机质、全氮含量之间以及有机质与全氮含量之间存在极显著相关性,黏粒含量与含水量、容重、有机质和全氮含量之间存在显著相关性,含水量与容重、有机质和全氮含量之间也存在显著相关性,说明沙漠化过程中土壤各组分之间存在着密切的内在联系.     

Predicting soil workability and fragmentation in tillage: a review

Soil workability and friability are required parameters to consider when creating suitable seedbeds for crop establishment and growth. Knowledge of soil workability is important for scheduling tillage operations and for reducing the risk of tillage‐induced structural degradation of soils. A reliable evaluation of soil workability implies a distinctive definition of the critical water content (wet and dry limits) for tillage. In this review, we provide a comprehensive assessment of the methods for determining soil workability, and the effects of soil properties and tillage systems on soil workability and fragmentation. The strengths and limitations of the different methods for evaluating the water content for soil workability, such as the plastic limit, soil water retention curve (SWRC), standard Proctor compaction test, field assessment, moisture‐pressure‐volume diagram, air permeability and drop‐shatter tests are discussed. Our review reveals that there is limited information on the dry limit and the range of water content for soil workability for different textured soils. We identify the need for further research to evaluate soil workability on undisturbed soils using a combination of SWRC and the drop‐shatter tests or tensile strength; (i) to quantify the effects of soil texture, organic matter and compaction on soil workability; and (ii) to compare soil water content for workability in the field with theoretical soil workability, thereby improving the prediction of soil workability as part of a decision support system for tillage operations.     

城市土壤压实对土壤水分特征的影响——以南京市为例

通过对南京市不同土地利用下的土壤容重、孔隙度和土壤水分特征曲线的测定，研究了压实对土壤水分特征参数的影响。结果表明城市土壤存在严重的压实退化现象，土壤容重和孔隙度能够很好地反映土壤的压实程度。随着压实程度的增加，土壤的田间持水量增加，萎蔫点含水量增加，而土壤的最大有效水含量却明显减少。所以，压实土壤对水分的调节能力下降，使其上生长的植物更不容易获得水分供应。     

灰色关联及非线性规划法构建传递函数估算黑土水力参数

土壤水分特征曲线和饱和导水率是重要的水力参数,为了简便准确获取这些参数,以松嫩平原黑土区南部为研究区域,采集136个采样点土样用于测定不同土层土壤水分特征曲线、饱和导水率以及土壤理化性质,并运用灰色关联分析确定影响土壤水力参数的主要土壤理化性质,采用非线性规划构建土壤分形维数、有机质、干容重、土壤颗粒组成与土壤水分特征曲线、饱和导水率之间的土壤传递函数,并通过与现有土壤传递函数对比分析进行精度验证。结果表明:1)土壤分形维数是估算土壤水分特征曲线模型参数和饱和导水率的主要参数之一,同时,干容重和有机质含量也在不同土层土壤传递函数中起到重要的作用;2)通过验证分析,不同土层各参数平均绝对误差接近于0,均方根误差值也都较小,其中在不同土层土壤传递函数估算的土壤含水率均方根误差分别为0.022、0.017cm~3/cm~3;3)对比分析其他已存的土壤水分特征曲线和饱和导水率的土壤传递函数,该文构建的土壤传递函数均方根误差值均较小,决定系数值都在0.66以上,表明估算精度较高,均好于其他方法估算精度,具有良好的区域适应性。综上,所构建的土壤水分特征曲线和饱和导水率土壤传递函数可以用于松嫩平原黑土区土壤水力参数估算。     

Complexed organic matter controls soil physical properties

It is shown that, for mineral soils, it is not the total amount of organic carbon (or organic matter) that controls soil physical behaviour but the amount of complexed organic carbon (COC). We assume that this complex is formed by the association of unit mass (i.e. 1 g) of organic carbon with n grams of clay. Analysis of data from two French and two Polish databases shows that, for these soils, n = 10. A consequence of this is that in soils with small contents of organic carbon (OC), such as arable soils, COC is proportional to OC. However, in soils with large contents of organic carbon, such as pasture soils, COC is proportional to the clay content. This explains why we find that soil bulk density is significantly correlated with OC in French arable soils but with the clay content in French pasture soils. The use of COC instead of OC enables the arable and pasture soils to be considered on the same scale.

Water retention data were fitted to a double-exponential equation which allows both the matrix and structural porosities to be estimated. It is shown that in soils with low contents of organic carbon, the carbon content is positively correlated with the matrix porosity. In contrast, in soils with high contents of organic carbon, the matrix porosity is constant at its maximum value and the structural porosity is not significantly correlated with either the total organic carbon or the non-complexed organic carbon (NCOC). It is suggested that the complexed organic carbon can be considered as being sequestered. The soil clay content can similarly be partitioned between clay that is complexed with organic carbon and clay that is not complexed. It is shown that non-complexed clay is more easily dispersed in water than clay that is complexed with organic carbon. These findings indicate how improved pedo-transfer functions for the prediction of soil physical properties may be produced. Such functions need to use the values of complexed and non-complexed organic carbon and clay which must be determined by algorithms. The values produced by the algorithms may then be used in the improved pedo-transfer functions.     

Effects of additions of organic matter on the penetration resistance of glacial till for the entire water tension range

Hardsetting soil properties are undesirable in agricultural soils because they hamper crop production by limiting seedling emergence and root growth via increased mechanical soil resistance at low moisture contents. The objective of this study was to determine the effect of additions of organic matter on the penetration resistance of a hardsetting soil for the entire water tension range. Investigations were carried out on Saalian glacial till, which is used as a reclamation substrate in post-lignite-mining reclamation. Proportions of 0%, 1%, 2%, 3% and 4% by mass of organic matter (OM) were used. The remoulded samples were saturated under a constant load of 2.4 kPa to achieve bulk densities equivalent to a soil depth of 15–20 cm via water-induced consolidation. Subsequently, the mixtures were adjusted to water tensions between 10⁰ and 10⁷ hPa and penetrated using a small cone penetrometer. Compared to 0% OM, the addition of 1% OM led to a very small but significant (P < 0.01) increase in the bulk density, while between 1% and 4% OM bulk density was seen to decrease in a linear fashion. At moisture contents greater than field capacity, penetration resistance values were consistent with the observed changes in bulk density, leading to an increase in the samples containing 0–1% OM to critical values for root-growth and a decrease for samples containing 2% and more organic matter reaching to values non-critical for roots. At moisture contents smaller than field capacity, penetration resistance values were inversely related to the bulk density, supporting the concept that the type of organic matter added contributed to soil cohesion. Modeling the relation between water tension and penetration resistance using a sigmoidal equation showed a high consistency between the observed data and the model.     

渭北果园土壤物理退化特征及其机理研究

【目的】针对我国渭北苹果主产区出现的随植果年限增加,果园土壤质量严重退化,树势衰弱、树体过早衰老、抗性降低、腐烂病及早期落叶病频繁发生,果品产量与品质下降等问题,开展了渭北苹果园土壤物理质量退化特征、退化机理及危害程度等问题的研究,以期查明制约果业可持续发展的因素,为果园土壤科学管理提供依据。【方法】在渭北黄土塬区选取了10 a、10 20 a、20 a 3个园龄段果园各4个,并以土壤条件相同的农田作对照,在果树冠层投影范围内距树干2/3处采取土样,测定土壤剖面不同层次容重、紧实度、孔隙度、饱和导水率、粘粒含量等物理性指标。【结果】渭北果园土壤容重和紧实度随园龄和土层深度的增加而增大,尤其在表层(20 cm)以下,土壤容重已经达到了1.45 1.61 g/cm3,紧实度达到933 2433 k Pa,严重超出果树健康生长的阈值。土壤孔隙度仅在0—20 cm土层能够保持在50%以上,属于良好状态,而20—60 cm土层维持在40%46%,已处于紧实和严重紧实状态。土壤饱和导水率在果园表层和紧实层均表现出随植果园龄的增大而减小的趋势,尤其是10 20 a和20 a的果园亚表层土壤饱和导水率低至46.88 cm/d和20.89 cm/d,制约着降水入渗和土壤蓄墒。3个园龄段果园土壤剖面上粘粒含量随土层深度呈递增趋势,且在0—30 cm土层随园龄的增加而明显减少,而在30 cm以下则随园龄的增加而呈递增趋势。进一步分析发现,粘粒含量与土壤容重、紧实度以及孔隙度之间呈极显著的相关关系。以压实密度(PD)为指标,对渭北果园土壤压实程度进行评估,发现渭北果园20 cm土层以下的土壤压实密度都在1.40 g/cm3以上,均达到了中度压实的程度,严重影响果树根系的健康生长及对养分的吸收。【结论】渭北果园20 cm以下的亚表层土壤孔隙密实、容重和紧实度增大,土壤饱和导水率递减是其土壤物理性质退化的主要特征,表层土壤粘粒的深层移动与淀积是土壤物理退化的主要过程和机理,果园土壤翻耕扰动少、对物理退化干预少是其土壤物理退化程度逐渐加剧的外在原因,土壤团聚体稳定性差是土壤物理状态退化的根本原因。     

Structural damage and recovery determined by the colloidal constituents in two forest soils compacted by heavy traffic

The role of colloidal constituents in soil structure and its resistance to compaction was studied in two acid forest soils of contrasting pH, clay type and texture. The soils were trafficked with an eight‐wheel‐drive forwarder, and undisturbed topsoil samples were taken on trafficked and control plots. Shrinkage analysis was used to assess the soil's physical behaviour, and in addition texture, organic carbon content and exchangeable Al³⁺ (Al_ex) and amorphous Al oxide (Al_oxa) contents were determined. The effect of each constituent on the soil's physical properties was assessed with covariance analysis. The hydro‐structural stability and coarse pore (> 150 µm radius) and structural pore volumes of control samples were strongly determined by organic carbon and the forms of Al, whereas the plasma porosity was determined by clay content only. Organic carbon and Al_oxa increased the structural pore and coarse pore volumes and modified their susceptibility to compaction; organic carbon provided a protecting effect, whereas it was the opposite with Al_oxa. We observed contrasting effects of the colloidal constituents and of the behaviour of the pore systems on compaction. The situation is complex and we need to take into account the effects of the colloidal constituents to determine the effects of compaction on the soil's porosity. A simplified approach in which we used the water content at ?10 hPa as a covariate predicted soil bulk density as accurately as with all the analytical covariates, and it seems to be an inexpensive way to assess compaction.