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