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Presentation on theme: "STRUKTUR TANAH, DENSITAS, POROSITAS"— Presentation transcript:

Diabstraksikan oleh: smno.jursntnh.fpub.Jan2013

2 Definisi Soil structure is the arrangement of the primary soil particles (sand, silt, and clay) and other soil materials into discrete aggregates.

3 Peds = Gumpalan, Bongkahan, Agregat
Structural units are called peds, and have distinct boundaries and well-defined planes of weakness between the aggregates. Peds consist of primary particles bound together by cementing agents like organic matter, clay, and hydrous oxides of iron and aluminum. Ped / agregat tanah mempunyai bermacam-macam bentuk.

4 STRUKTUR TANAH Structure – the way soil particles clump together into large units called aggregates or peds

5 STRUKTUR TANAH Structure can alter the effects of texture
e.g. a fine-textured silty clay with good structure can be permeable!

6 STRUKTUR TANAH Structure is classified by three groups of traits:
Type – refers to shape of aggregates e.g. Granular, Platy, Blocky, Prismatic, Columnar Class – refers to size of peds e.g. very fine, fine, medium, coarse, very coarse Grade – refers to strength and distinction of peds e.g. weak/not visible vs. strong/easily distinguished

Dua-tahap Pembentukan Struktur: Individual soil particles loosely aggregate Weak aggregates are cemented to strengthen - clay - iron oxides - organic matter - microorganism gums

8 Bentuk-bentuk Struktur-tanah Ped Shapes
Granular Blocky Prismatic Columnar Platy Single-grained Massive

9 Granular Structure Resembles cookie crumbs and is usually less than 0.5 cm in diameter. Commonly found in surface horizons where roots have been growing.

10 Struktur Kubus: Blocky Structure
Irregular blocks that are usually cm in diameter. Can be subangular or angular blocky.

11 Struktur Prismatik Vertical columns of soil that might be a number of cm long. Usually found in lower horizons.

12 Struktur Columnar Vertical columns of soil that have a salt "cap" at the top. Found in soils of arid climates.

13 Struktur Pipih Thin, flat plates of soil that lie horizontally.
Usually found in compacted soil.

14 Struktur berbutir tunggal
Soil is broken into individual particles that do not stick together. Always accompanies a loose consistence. Commonly found in sandy soils.

15 Massive Structure Soil has no visible structure, is hard to break apart and appears in very large clods.

16 Grade of Soil Structure
The terms weak, moderate, or strong are used to describe the grade or how stable the peds are and how hard they are to break apart. What do you think the grade would be for this picture?

17 The size or class of the peds is described as fine, medium, or coarse.
Kelas Struktur Tanah The size or class of the peds is described as fine, medium, or coarse.

18 Struktur tanah mempengaruhi pergerakan air dalam tanah
In soils with good structure, the pore space that occurs between peds is relatively large and facilitates water and air movement. Struktur yang berkembang baik, sangat penting dalam tanah-tanah liat Tanah-tanah liat yang strukturnya jelek, pergerakan udara dan airnya sangat terbatas.

19 Pergerakan air tanah Structure Water Movement

20 Mengubah Struktur Tanah
Struktur tanah dapat diubah dengan jalan pengolahan tanah. Tilling soils that are too wet, or compacting soils with heavy equipment can break down the natural structural units.


Two densities in Soil: Particle Density – PD Bulk Density - BD

PD average soils ~ 2.65 gm/cu cm BD average range from 1.0 – 1.8 gm/cu cm Depends on amount of pore space BD = wt. dry soil = _g_ vol. dry soil cu cm e.g. BD = 650 g = g/cu cm 500 cu cm e.g.

24 Bobot Isi Tanah = Bulk Density
Soil bulk density is the mass per unit bulk volume of soil that has been dried to a constant weight at 105 °C.

25 Bobot Isi If we have a soil that weighs 50 grams after being oven-dried and has a volume of 30 cm3, what will be the bulk density? Nilainya BI sebesar 50 g/30 cm3 atau 1.67 g/ cm3.

26 Bobot Isi tanah di lapangan
Bobot isi tanah-tanah organik sangat rendah, misalnya 0.5 g/cm3. Tanah liat mempunyai BI lebih tinggi. Tanah liat yang dipadatkan mempunyai bobot isi sangat tinggi, antara 1.6 dan 1.8 g/cm3.

27 Bobot Jenis = Berat Jenis = Kerapatan Partikel dan Porositas Tanah
Kerapatan partikel merupakan massa per satuan volume partikel tanah. Particle density is a relatively constant parameter and is sometimes assumed to be 2.65 g/cm3.

28 Bobot Isi vs. Bobot Jenis Bulk Density vs. Particle Density

29 Porositas Tanah Soil porosity is the percentage of a soil that is pore space or voids. The average soil has a porosity of about 50%, and the pores are filled with air or water depending on the moisture content. Sands have larger pores, but less total pore space than clays. If both bulk density and particle density are known, the total porosity can be calculated using these values.

30 Permeabilitas Tanah Permeabilitas tanah mencerminkan kemudahan udara, air atau akar tanaman menembus ke dalam massa tanah. Tanah-tanah dengan ruang pori besar dan saling berhubungan biasanya lebih permeabel. Rainwater soaks in readily and moves down through the soil profile. Clayey soils can have greater total porosity than sand and still be less permeable than sand since the pores are small.

31 Soil Strength = Kekuatan Tanah
Kekuatan tanah didefinisikan sebagai banyaknya gaya yang diperlukan untuk menggerakkan atau menata partikel-partikel tanah. Soil strength is an important physical property affecting plant growth and seedling emergence. Kekuatan tanah dipengaruhi oleh tiga faktor: kandungan lengas tanah, tekstur tanah, dan bobot isi tanah.

32 Kadar Air & Kekuatan Tanah
Water content is the most important factor determining soil strength. The lower the soil water content, or the drier the soil, the greater the soil strength. Tanah-tanah yang jenuh atau tanah-tanah tergenang mempunyai nilai kekuatan-tanah yang rendah.

33 Kadar Air Tanah (Lengas Tanah)
Mana tanah yang kekuatannya lebih besar?

34 Tekstur & Kekuatan Tanah
Tekstur tanah mempengaruhi kekuatan tanah. Kekuatan tanah-tanah yang agregasinya bagus akan semakin besar kalau kandungan liatnya semakin banjyak. Tanah-tanah yang agregasinya buruk atau tanah butir tunggal (sands, loamy sands, sandy loams) biasanya mempunyai kekuatan paling lemah, kecuali kalau mereka dipadatkan. Individual particles of single grain (sandy) soils are easy to rearrange, but these soils are susceptible to compaction, sometimes resulting in the formation of hard pans.

35 Bobot Isi & Kekuatan Tanah
Bulk density is the third factor affecting soil strength. As bulk density of a given soil increases soil strength also increases. Remember that soil is composed of solids and pores, and the greater the bulk density the greater the amount of solids, and the smaller the amount of pore space.

36 Problem Pengelolaan Tanah
Two examples of management problems caused by increasing soil strength are soil crusts and tillage pans. A soil crust is a thin soil layer that forms at the soil surface following heavy rains. Tillage or hard pans are high bulk density (>1.7 g/cm3) layers that occur within the Ap and E horizons. Compaction caused by traffic of equipment, vehicles, or even foot traffic often increases soil strength to levels that restrict root penetration and plant growth.

37 Penetrometers Gaya yang diperlukan untuk mendorong “a rod” menembus tanah merupakan ukuran bagi kekuatan tanah. Penetrometer merupakan peralatan yang digunakan untuk mengukur resistensi suatu tanah terhadap penetration, untuk memperkirakan efek pemadatan tanah terhadfap pertumbuhan tanaman, dan untuk mendeteksi lap[isan-lapisan yang kekuatan tanahnya berbeda.

38 Diunduh dari: …..
POROSITAS Porosity or void fraction is a measure of the void (i.e., "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%. The term is used in multiple fields including pharmaceutics, ceramics, metallurgy, materials, manufacturing, earth sciences, soil mechanics and engineering. Porosity of soil Porosity of surface soil typically decreases as particle size increases. This is due to soil aggregate formation in finer textured surface soils when subject to soil biological processes. Aggregation involves particulate adhesion and higher resistance to compaction. Typical bulk density of sandy soil is between 1.5 and 1.7 g/cm³. This calculates to a porosity between 0.43 and Typical bulk density of clay soil is between 1.1 and 1.3 g/cm³. This calculates to a porosity between 0.58 and This seems counterintuitive because clay soils are termed heavy, implying lower porosity. Heavy apparently refers to a gravitational moisture content effect in combination with terminology that harkens back to the relative force required to pull a tillage implement through the clayey soil at field moisture content as compared to sand. Porosity of subsurface soil is lower than in surface soil due to compaction by gravity. Porosity of 0.20 is considered normal for unsorted gravel size material at depths below the biomantle. Porosity in finer material below the aggregating influence of pedogenesis can be expected to approximate this value. Soil porosity is complex. Traditional models regard porosity as continuous. This fails to account for anomalous features and produces only approximate results. Furthermore it cannot help model the influence of environmental factors which affect pore geometry. A number of more complex models have been proposed, including fractals, bubble theory, cracking theory, Boolean grain process, packed sphere, and numerous other models. See also Characterisation of pore space in soil. Diunduh dari: …..

39 Usually expressed as a percentage; e.g. 50%
POROSITAS TANAH Usually expressed as a percentage; e.g. 50% Dua cara untuk menentukan porositas: Calculate ratio water volume to total core volume Calculate from bulk density and particle density

40 Water Volume to Core Volume
POROSITAS TANAH Water Volume to Core Volume Porosity = wet weight (g) – dry weight (g) soil volume (cu cm) x 100%

41 POROSITAS TANAH An oven-dry soil core, volume 500 cu cm, weighs 650g. When wet, it weighs 900g. Find it’s % porosity. Porosity = 900g – 650g x 100% = 250g x100% = 50% 500 cu cm cu cm Note: the unit cancellation is made possible by the metric system which defines 1 cu cm of water as weighing 1g.

42 POROSITAS TANAH Bulk Density to Particle Density
Defines the percentage of the soil that is solid matter The percent solid matter is subtracted from 100% to give percent porosity: Porosity = 100% - (BD/PD x 100%)

43 POROSITAS TANAH An undisturbed oven-dry soil, BD of 1.3 g/cu cm, consists of average mineral composition (PD 2.65 g/cu cm). Find its % Porosity: Porosity = 100% - (1.3 g/cu cm/2.65 g/cu cm x 100%) Porosity = 100% - (0.49 x 100%) = 100% - 49% = 51%

44 SOIL POROSITY Which has greater porosity, Sand or Clay?
Clay at about 50%; Sand is lower at about 30% Why?


46 PORI TANAH . Pore spaces Soil particles do not fit together snugly. There are spaces between particles. These spaces are called pore spaces and contain water and air. The pore spaces provide the route for the downward movement of water and allow roots to grow into them. They also provide air space, which is essential for plant growth. The larger the pore spaces the better the drainage of water and the less water retained in the soil. Conversely, the smaller the pore spaces the less water drains away and the more water is retained in the soil. Diunduh dari: …..

47 . What actually happens to the soil?
Soil needs a balance between large pore spaces for aeration and water infiltration and small pores for water retention. When the soil becomes compacted, soil particles are pushed together and broken down so that pore spaces are reduced and filled in by smaller particles. Smaller pore spaces means less aeration, water infiltration and poorer drainage. Diunduh dari: ….

Soil structure Only about 50% of soil is solid material. The remainder is pore space. It is in these spaces that the action happens. Water is stored there. Organisms live there. Organic matter and nutrients accumulate there. The diagram (magnified about 20 times) demonstrates how solids and pores might arrange in soil to give a porosity of 50 %. Small pores within the aggregates provide storage and refuge. The larger pores (and fissures) between the aggregates are the pathways for liquids, gases, roots and organisms. Diunduh dari: …..

49 POROSITAS TANAH Porosity is the pore space in soil between mineral particles (and solid organic matter) filled with either air or water. The pore space both contains and controls most of the functions of soil. It is not just the total amount of pore space that is important, but the size distribution of the pores, and the continuity between them which determines function and behaviour of soil. Size distribution of pores Pores range in diameter from a few millimetres right down to just a fraction of a micron (i.e. one thousandth of a millimeter). The following table gives some detail of this range. Pore sizes in soil (Rowell 1994) Pore diameter (µm) Nature of the pore 20000 A 20 mm crack 4000 An earthworm channel (4 mm) 300 The diameter of a grass root 60-30 The smallest pore that will be air filled at field capacity 10 A fungal hypha 2 The size of a bacterial cell. The largest clay particle. The smallest pore from which a plant can readily obtain water 0.2 The smallest pore that will give up water to the suction exerted by a plant root. The pore size corresponding to the permanent wilting point 0.003 The largest pore filled with water in an "air dry" soil. The pore size is approximately ten times the diameter of a water molecule Diunduh dari: …..

50 Transmission pores Transmission pores are the large pores which enable root growth, air movement and water movement. They are visible to the naked eye, indeed even with a x5 hand lens, and range between 30 to 60 µm. They are often called macropores. The volume of a soil occupied by transmission pores should be >10 % if plant roots are to get adequate oxygen. Coarse textured, sandy soils, and well structured soils with a lot of biological activity, have a large proportion of pores in this size class. Storage pores Storage pores retain water (ie. they do not drain under the force of gravity) which is then available for use by plant roots and soil organisms. The proportion of these pores in a soil controls the plant available water capacity. They (along with even smaller pores) are termed micropores. They have diameters between 0.2 and 60 µm. The volume of a soil occupied by them might range from <10% in a loamy sand to >20% in a good loam. Residual pores These hold water so tightly that it cannot be extracted by roots or soil organisms – they are less than 0.2 µm in diameter. Fine textured or clayey soils have the larger proportion of their pores in this class. A heavy clay might well have 25 % of its volume as residual pores. Diunduh dari: …..

51 POROSITAS Diunduh dari:

52 What physical properties are required for soil to “work”?
These But also these Strong: self-supporting & load-bearing Permeable to air and water Conduct water fast to prevent erosion Lots of surface area for reactions, microbes, etc. Low thermal conductivity (moderate temps. at depth) Weak enough for roots to penetrate Retaining both air and water Prevent water from leaving, so plants don’t wilt Too much surface area lowers the permeability High thermal conductivity (moderate depth of extremes) Soils don’t decide to have certain properties. Their transport properties emerge from other properties.

What is a porous medium? A composite of solid and fluid (liquid and/or gas) The volume fraction of non-solid is called the porosity (f). Note: Hillel uses f for porosity. f is the Greek f. What solids are not porous? (few if any)

54 The f continuum f = 0 f = 1 pure fluid solid Continuous solid
PERHATIAN! Porosity isn’t the only thing affecting continuity! There can be fluid continuity at just 0.1% porosity! f = 0 f = 1 pure fluid solid Continuous solid permeable porous medium suspension impermeable porous medium Continuous fluid In permeable media (like soils), both the solid and fluid are continuous

55 The f continuum at work f = 0 f = 1

56 Fractured rock Most porosity is not from fractures, but almost all flow is in fractures.

57 Little pores (soil matrix)
Soil with drying cracks Dual-porosity: Big pores (cracks) Little pores (soil matrix) Nothing in between

58 Slice through a 3D packing model with 4 sphere sizes
Virtual (Granular) Slice through a 3D packing model with 4 sphere sizes

59 Soils are porous media Porosity varies widely (60% > f > 30%)
Particle sizes vary widely (sand, clay) Geological and/or organic materials of varying mineralogy and composition Permeability varies widely Granular, fractured, and/or amorphous Vary spatially & temporally Most complex & widespread biomaterial on the planet so it is hard to generalize!

60 but sometimes it’s useful to think about here
Multiple scales We concentrate here, but sometimes it’s useful to think about here

61 Irwin Fatt asked (Petr. Trans. AIME, 1956):
What are the available models for porous media? Capillary tubes are too simplistic. Glass beads are intractable, and they’re still too simple. Real porous media have multiply connected pores (topology & connections again).

62 SIFAT FISIKA TANAH Diunduh dari: ….

63 SIFAT FISIKA TANAH Soil Consistence – the behavior of soil when pressure is applied; measured at three different moisture levels: Wet, Moist, Dry (fig 4-15, p. 59) Soil Tilth – ease of tillage, seedbed preparation, and seedling/root movement Compaction – results from pressure applied at the soil surface

64 SIFAT FISIKA TANAH Puddling, Clods, and Crusts:
Puddling occurs when pressure is applied to very wet soils (esp. plowing) Crusts occur when bare soil is struck by raindrops; disperses soil then dries to a hardened crust

65 SIFAT FISIKA TANAH Improving Tilth:
Best accomplished by improving structure - tilth relates to texture, structure, permeability, and consistence; however, texture and consistence cannot be altered easily therefore, improve tilth by improving structure and avoiding compaction.

66 SIFAT FISIKA TANAH Soil Channels – continuous macropores leading from surface to deep subsoil Soil Pans – any layer of hardened soil; includes: - claypans - fragipans (clays) - plinthite (tropics) - caliche (Ca cemented)

67 SIFAT FISIKA TANAH Soil Temperature (varies w/color, texture, O.M.)
Soil Color Munsell soil color chart Hue, Value, Chroma e.g. 10YR 3/6


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