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SOIL REACTION AND ACID SOILS
AREAL EXTENT OF ACID SOILS
.
A soil with a neutral reaction contains
equal emount of H+ and OH- ions
To characterize these conditions the term
soil pH is used
In acid soils, more H+ than OH- ions are present. Tanah masam (pH < 7) biasa ditemukan pada kawasan humid. Most plants grow best in soils with a slightly acid reaction. In this pH range, nearly all plant nutrients are available in optimal amounts.
Tanah dengan pH <6 bisa mengalami kekahatan beberapa hara yang dibutuhkan tanaman, sebagai contoh Ca, Mg dan K.
Pada tanah masam kuat dan sangat kuat, Al, Fe dan Mn biasanya berada pada aras meracun (toxic level). Unsur-unsur tersebut juga bereaksi dengan fosfat membentuk fosfat tak larut.
Tanah-tanah dengan pH >7 (tanah alkaline) biasanya berada di dalam kawasan arid dan semiarid. In basic soils, more OH- ions than H+ are present.
Tanah alkaline mengandung Al, Fe dan Mn dalam jumlah rendah oleh karena mengalami presipitasi menjadi hidroksida Al, Fe dan Mn tak larut.
Pada tanah alkaline kuat, fiksasi fosfat juga bisa menjadi masalah oleh karena membentuk tricalsium fosfat yang tak larut.
FACTORS CAUSING SOILS TO BECOME ACIDIC
Loss of exchangeable bases from the soil CEC Leaching Removal from plant uptake
Production of organic acids from organic matter decay
Use of fertilizers, particularly ammonium sources: (NH4)SO4, NH4NO3, Anhydrous ammonia, Urea
Soil erosion: Loss of bases from surface runoff
Parent material: Presence of acidic materials that weather giving rise to acid soils
Weathering
Source of soil acidity1. Hydrolysis of Al3+ ions
Ion hidroksi aluminium adalah produk hidrolisis ion Al3+ yang melepaskan ion H+ selama reaksi. Ion aluminium dan produk hidrolisisnya merupakan sumber ion H+ untuk kemasaman aktif dan potensial.
As soon as an Al3+ ion is present in the soil solution, it is quckly surrounded by six moleculs of H2O in octahedral coordination, forming Al(H2O)6
3+. (aluminum hexahidronium ion)
Al hydrolysis species at various pH values and their relative contribution to total soluble Al
Hidrolisis(lihat BAB Prinsip Dasar Kimia tanah KIMTAN)
Ion Log K1 Log
1. Be2+ -6.52. Mg2+ -12.03. Ca2+ -12.54. Mn2+ -10.55. Fe2+ -7.06. Ni2+ -8.07. Cu2+ -7.58. Zn2+ -9.19. Cd -10.010.
Hg2+ -3.5
11.
Pb2+ -8.0
12.
Al3+ -5.0 -5.5
13.
Fe3+ -2.9 -3.3
14.
La3+ -9.0
15.
Ti4+ >-1
16.
Th4+ -4.1
2. Rainfall
Acid presipitation also know as acid rain is caused by conversion of nitrogen oxide and sulfur dioxide gases into strong acid. These gases are very harmful to the enviroenment, plant growyth and human health.
SO2 + O2 2SO3 3SO3 + H2O H2SO4 (sulfuric acid)
2NO + O2 2NO2 2NO2 _+ H2o HNO3 + HNO2 (nitric and
nitrous acid)
3. Crop Fertilization
4. PLANT RESIDUES / ORGANIC WASTE DECOMPOSITION
Ammonium fertilizer:
2NH4+ + O2 2NO2
- + H2O + 4 H+ + energy
2NO2- + O2 2NO3- + energy
Equivalent acidity : the amount of lime required per unit amount of fertilizer used to bring the soil pH back to prefertilizer application levels. Nilai bisa posirif atau negatif.
Dinamakan juga “Acidity Index”.
Misal equivalen acidity ammonium nitrat = 593, berarti penggunaan 1000 kg ammonium nitrat menyebabkan penurunan pH tanah yang dapat dikembalikan ke pH semula dengan memberikan 593 kg kapur CaCO3.
Pupuk fosfat Hidrolisis pupuk fosfat akan
menghasilakan orthophosphoric acid:
Ca(H2PO4)2 CaHPO4 + H3PO4 (orthophosphoric acid)
H3PO4 H+ + H2PO4-
H3PO4 2H+ + HPO4-2
H3PO4 3H+ + PO4-3
Elemental sulfur: dapat digunakan untuk merekayasa lingkungan tumbuh tanam supaya menjadi masam. Oksidasi sulfur menghasilkan asam sulfat yang dapat meningkatkan konsentrasi ion H+ di dalam tanah.
2So + 3O2 + H2O 2H2SO4
Pyrite (FeS2)
Oksidasi pirit, menghasilkan sulfuric acid (H2SO4).
FeS2 + 7H2O + 71/2 O2 4SO42- +
8H+ + Fe(OH)3
Biological Nitrogen Fixation:
Nitrogen fixation alone does not contribute to directly toward increasing the H+ ion concentration in soils./Ammonia levels increas considerably in the soil due to ammonification of organic N from nitrogen fixation. In a soil ecosystem where the level of ionorganic nitrogen is in excess to that needed by the growing plants, this excess of ammonium will be nitrified, adding significant amount oh H+ ions to the soil solution (Kennedy)
TYPE OF SOIL ACIDITY
Potential, Reserve or Exchange Acidity Adsorbed H+ to colloid surfaces or other un-dissociated H+ sources, that will react with water to yield H+ (e.g. Al3+)
Active Acidity Presence of H+ in soil solution soil pH
Total Acidity = Potential Acidity + Active Acidity
CLASSIFICATION OF ACIDITY
pH
pH can be viewed as an abbreviation for power of concentration of hydrogen ion in solution
pH = - [log (H+)] in solution
kw = pH + pOH kw = 14 14 = pH + pOH pH = 14 - pOH
Menetapkan pH tanah dengan pH (H2O) = kemasaman aktif = aktifitas H+ pada larutan tanah
-H+ H+
-H+ H+
-H+ H+ H+
-H+ H+
Soil Colloid Soil Solution
Considerations for correcting acidity
Original soil pH Type of liming material and its fineness Availability of liming materials Type of soil (sandy, clayey, loamy, organic)
and CEC Crop to be grown Depth of mixing in the soil Soil moisture
Type of lime material
Limestone: Calcite type (CaCO3) Dolomite type (CaMgCO3) Wood ashes (oxides of Ca, K and Mg)
The type of lime to use depend on availability, price, and type of soil. Dolomite type are prefered when Mg is also deficient.
The size of the limestone (coarse vs. fine) define the degree of reactivity. Fine limestone is MORE reactive than coarse limestone.
SOIL BUFFERING CAPACITY = DAYA SANGGA TANAH
Buffering capacity in soils “Ability of a soil to resist a change in
pH” Direct correlated with CEC of a soil, a
high CEC is associated with a large number of exchange sites
Example: High buffered soils are organic soils, and 2:1clay soils. Low buffered soils are low organic matter soils and 1:1 clay soils.
DERAJAT KEJENUHAN BASA (PERCENT BASE SATURATION)
CEC (Cation Exchange Capacity > 25 cmol(+)/kg atau 25 me%/100 tinggi
CEC sedang 15 -25 cmol (+)/kg sedang
Derajat kejenuhan basa kapasitas tanah menyediakan basa-basa tertukar untuk tanaman
Derajat kejenuhan basa = jumlah kation tertukar (Ca, Mg, K, Na) / KPK x 100 %
KPK ditetapkan dengan metode NH4OAc ammonium acetat pada pH 7
KPK: kemampuan tanah menukar kation
KPK efektif = penjumlahan Ca+Mg+Na+K
Kation asam Al3+ dan H+
Kation basa Ca, Mg, Na dan K