كليدواژه :
حد خميرايي , خاكدانه , مقاومت كششي , هوا-خشك , كشت
چكيده فارسي :
تمایل یك توده خاك محصور نشده به خرد شدن تحت تنش¬های بكار رفته و شكستن آن به قطعه¬های كوچك¬تر كه دامنه اندازه مشخصی دارند، تردی خاك نامیده می¬شود. هدف از این پژوهش اندازه¬گیری و بررسی مقاومت كششی و تردی دو خاك با بافت¬های لوم¬رسی و لوم¬شنی تحت كشت¬های گندم و یونجه بود. آزمایش¬ها در سه شرایط رطوبتی هوا-خشك، رطوبت نظیر مكش ماتریك 80 و 50 كیلوپاسكال، برای سه سری اندازه خاكدانه و كلوخه انجام شد. برای جداكردن خاكدانه¬ها و كلوخه¬های با اندازه 10-8، 25-15 و 38-30 میلی¬متر، نمونه¬های خاك به آهستگی از الك-های مربوطه در شرایط هوا-خشك عبور داده شدند. فاكتورهای مورد بررسی شامل بافت خاك و نوع كشت، در رطوبت¬های مختلف و اندازه¬های متفاوت خاكدانه و كلوخه، بر مقاومت كششی و تردی خاك اثر داشتند. خاك لوم¬رسی- یونجه بیشترین مقدار مقاومت كششی را نسبت به خاك¬های لوم¬رسی-¬گندم (21 درصد)، لوم¬شنی-یونجه (57 درصد) و لوم¬شنی-گندم (70 درصد) نشان داد كه احتمالاً به دلیل رس و ماده آلی بیشتر در این خاك¬ها می¬باشد. نتایج نشان دهنده ارتباط معكوس مقاومت كششی خاكدانه¬ها با اندازه و میزان رطوبت آن¬ها بود. تركیب بافت خاك و نوع كشت روند تغییرات تردی را به صورت لوم¬رسی-یونجه (06/0) < لوم¬رسی-گندم (09/0) < لوم¬شنی-یونجه (15/0) < لوم¬شنی-گندم (20/0) تحت تأثیر قرار داد. نتایج نشان داد كه بیشترین مقدار تردی محاسبه شده (16/0) در حدود رطوبت حد خمیرایی (رطوبت نظیر مكش ماتریك 80 كیلوپاسكال) مشاهده شد.
چكيده لاتين :
Introduction: Soil friability is defined as the tendency of a mass of unconfined intact soil in bulk to crumble and break up under applied stress into similar fragments, aggregates and individual soil particles with specific size range. Tensile strength is a term which defined as the stress, or force per unit area, required to cause soil to fail in tension. The stated parameters are almost considered as the key physical properties of agricultural soils, because the friable condition is a desirable feature for establishing adequate seedbeds during tillage practice. In spite of the relevance of the subject, information on the effects of intrinsic soil properties on the tensile strength and friability is limited in Iran. The objective of this study was to quantify and to relate tensile strength and friability of two texturally different soils of clay loam and sandy loam under two different cropping systems of wheat and alfalfa.
Materials and methods: The soil samples were collected from the 0–30 cm horizon of two sites of sandy loam (SL) and clay loam (CL) soils which were located inHamadan province in western Iran. Each soil had been under cultivation of either wheat (conventionally tilled) or alfalfa for 11 years. At the laboratory, the soils were gently dry-sieved to separate 8-10, 15-25 and 30-38 mm fractions. The tensile strength was calculated as suggested by Dexter and Kroesbergen, (1985) and the soil friability was calculated through the coefficient of variation method as proposed by Watts and Dexter (1998). The experiment was carried out at the air-dry water content and soil matric suctions of 80 and 50 kPa for three ranges of aggregate size (8-10 mm, 15-25 mm and 30-38 mm). Then the impacts of soil texture (clay loam and sandy loam) and cultivation types (alfalfa and wheat) were assessed in a factorial design at each water content. Regression analyses were carried out to evaluate the relationship between soil intrinsic properties (clay content and organic matter) and tensile strength and friability.
Results and discussion: The considered factors in this study i.e. soil texture and cultivation, in different water content and aggregate size, have a pronounced influence on the tensile strength and friability. The soil of clay loam-alfalfa displayed a higher increase in tensile strength than clay loam-wheat (21%), sandy loam-alfalfa (57%), and sandy loam-wheat (70%) that may be related to differences in organic matter content and clay amount. Both organic matter and clay content have been mentioned as aggregating agents that affected soil strength. The results indicated negatively correlation of tensile strength of soils aggregate with aggregates size and water content. In the other word at low water contents, smaller aggregates of all soil treatments have a small friability value and a large tensile strength, that is, the soils are very difficult to crush and at high water contents the soils have relatively small strengths. Soil texture and cultivations ' combination affected friability in the order of CL-A (0.06) < CL-W (0.9) < SL-A (0.15) < SL-W (0.20). The results showed that the calculated amount of friability reaches maximum (0.16) at water content around the plastic limit (matric suction of 80 kPa). This is in good agreement with some earlier workers found that the water content giving the maximum soil crumbling on tillage is around 0.9-1.0 of the plastic limit.
Conclusion: Tensile strength and friability are influenced by several factors such as water content, clay content and soil organic matter. The influence of these factors on soil tensile strength and friability depends on climatic conditions, management practices, and soil composition. Since the formation of cracks in large aggregates occurs more intensively than in small aggregates, the decrease in strength in the large aggregate occurs more rapidly than that in the smaller aggregates; this resulted in greater value of friability of large aggregates compared to small aggregates. Friability on its own does not define the tensile strengths of aggregates, only the way (or except the condition) that the tensile strength changes with aggregate size. Soils may have high friability but also have very high strengths over a wide range of aggregate sizes. Our result showed that these two parameters could be considered as useful indicators of the soil structural condition and the friability of a soil is an important factor in determining soil response to tillage.
