STRUKTUR TANAH, DENSITAS, POROSITAS

1. STRUKTUR TANAH, DENSITAS, POROSITAS Diabstraksikanoleh: 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 / agregattanahmempunyaibermacam-macambentuk.

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

7. PEMBENTUKAN STRUKTUR TANAH Dua-tahapPembentukanStruktur: • Individual soil particles loosely aggregate • Weak aggregates are cemented to strengthen - clay - iron oxides - organic matter - microorganism gums

8. Bentuk-bentuk Struktur-tanahPed 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. http://soil.gsfc.nasa.gov/pvg/granular.gif http://soils.usda.gov/technical/manual/images/fig3-30_large.jpg

10. Struktur Kubus: Blocky Structure • Irregular blocks that are usually 1.5 - 5.0 cm in diameter. • Can be subangular or angular blocky. http://soil.gsfc.nasa.gov/pvg/blocky.gif http://www.dpi.vic.gov.au/dpi/vro/glenimages.nsf/Images/gl167_profile/$File/gl167_profile.jpg

11. Struktur Prismatik • Vertical columns of soil that might be a number of cm long. • Usually found in lower horizons. http://soil.gsfc.nasa.gov/pvg/prismatic.gif http://soils.usda.gov/technical/manual/images/fig3-27_large.jpg

12. Struktur Columnar • Vertical columns of soil that have a salt "cap" at the top. • Found in soils of arid climates. http://soil.gsfc.nasa.gov/pvg/columnar.gif http://soils.usda.gov/technical/manual/images/fig3-28_large.jpg

13. Struktur Pipih • Thin, flat plates of soil that lie horizontally. • Usually found in compacted soil. http://soil.gsfc.nasa.gov/pvg/platy.gif http://soils.ag.uidaho.edu/soilorders/i/Arid_03.jpg

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. http://soil.gsfc.nasa.gov/pvg/singlegrained.gif

15. Massive Structure • Soil has no visible structure, is hard to break apart and appears in very large clods. http://soil.gsfc.nasa.gov/pvg/massive.gif http://soils.usda.gov/technical/manual/images/fig3-31_large.jpg

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? http://soils.usda.gov/technical/manual/images/fig3-27_large.jpg

17. 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 berkembangbaik, sangatpentingdalamtanah-tanahliat • Tanah-tanahliat yang strukturnyajelek, pergerakanudaradanairnyasangatterbatas.

19. Pergerakan air tanah Structure Water Movement http://ohioline.osu.edu/b905/images/006.jpg

20. Mengubah Struktur Tanah • Strukturtanahdapatdiubahdenganjalanpengolahantanah. • Tilling soils that are too wet, or compacting soils with heavy equipment can break down the natural structural units. http://www.ny.nrcs.usda.gov/programs/images/tractor-tillin.jpg

21. DENSITAS & PERMEABILITAS TANAH DENSITY: MASS per VOLUME D = M V

22. DENSITAS & PERMEABILITAS TANAH Two densities in Soil: Particle Density – PD Bulk Density - BD

23. DENSITAS & PERMEABILITAS TANAH PD average soils ~ 2.65 gm/cu cm BDaverage 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 = 1.3 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 atau1.67 g/ cm3.

26. BobotIsitanahdilapangan • Bobotisitanah-tanahorganiksangatrendah, misalnya0.5 g/cm3. • Tanah liatmempunyai BI lebihtinggi. • Tanah liat yang dipadatkanmempunyaibobotisisangattinggi, antara 1.6 dan1.8 g/cm3.

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

28. Bobot Isi vs. Bobot JenisBulk Density vs. Particle Density http://soil.gsfc.nasa.gov/pvg/pd3.htm

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 • Permeabilitastanahmencerminkankemudahanudara, air atauakartanamanmenembuskedalammassatanah. • Tanah-tanahdenganruangporibesardansalingberhubunganbiasanyalebihpermeabel. • 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 • Kekuatantanahdidefinisikansebagaibanyaknyagaya yang diperlukanuntukmenggerakkanataumenatapartikel-partikeltanah. • Soil strength is an important physical property affecting plant growth and seedling emergence. • Kekuatantanahdipengaruhiolehtigafaktor: kandunganlengastanah, teksturtanah, danbobotisitanah.

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 jenuhatautanah-tanahtergenangmempunyainilaikekuatan-tanah yang rendah.

33. Kadar Air Tanah (Lengas Tanah) • Manatanah yang kekuatannyalebihbesar? http://www.wy.blm.gov/botany/pics/cob-duckswamp-500wfo.jpg http://www.research.noaa.gov/spotlite/archive/images/drysoil.jpg

34. Tekstur & Kekuatan Tanah • Teksturtanahmempengaruhikekuatantanah. • Kekuatantanah-tanah yang agregasinyabagusakansemakinbesarkalaukandunganliatnyasemakinbanjyak. • Tanah-tanah yang agregasinyaburukatautanahbutirtunggal (sands, loamy sands, sandy loams) biasanyamempunyaikekuatan paling lemah, kecualikalaumerekadipadatkan. • 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 diperlukanuntukmendorong “a rod” menembustanahmerupakanukuranbagikekuatantanah. • Penetrometermerupakanperalatan yang digunakanuntukmengukurresistensisuatutanahterhadap penetration, untukmemperkirakanefekpemadatantanahterhadfappertumbuhantanaman, danuntukmendeteksi lap[isan-lapisan yang kekuatantanahnyaberbeda.

38. 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 0.36. Typical bulk density of clay soil is between 1.1 and 1.3 g/cm³. This calculates to a porosity between 0.58 and 0.51. 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. Diunduhdari: ….. http://en.wikipedia.org/wiki/Porosity

39. POROSITAS TANAH Usually expressed as a percentage; e.g. 50% Duacarauntukmenentukanporositas: • Calculate ratio water volume to total core volume • Calculate from bulk density and particle density

40. 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 500 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? Clayat about 50%; Sand is lower at about 30% Why?

45. TEXTURE AND SOIL PORES SUMBER: http://informedfarmers.com/more-ways-to-investigate-your-soils/

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. Diunduhdari: ….. http://www.swtafe.vic.edu.au/toolbox/turf/html/pages/office/grass_roots/soil_structure.html

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. Diunduhdari: …. http://www.swtafe.vic.edu.au/toolbox/turf/html/pages/office/grass_roots/soil_structure.html.

48. STRUKTUR TANAH – PORI TANAH 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. Diunduhdari: ….. http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_soil_structure

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 poresPores 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) Diunduhdari: ….. http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_soil_structure_porosity

50. Transmission poresTransmission 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 poresStorage 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 poresThese 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. Diunduhdari: ….. http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_soil_structure_porosity