用戶:SusieHsia/沙盒

在土壤科學中, 陽離子交換容量，又稱陽離子交換量，（cation exchange capacity, CEC）是指在特定pH下，單位乾重的土壤持有的、能與土壤溶液進行交換的陽離子的最大量。^[1] CEC常被用來作為檢測土壤肥力、養分持留容量、以及保護地下水免受陽離子污染的指標。其單位是毫當量（英語：milliequivalent）氫離子每一百克土壤乾重(meq/100g)，其國際標準單位是厘摩每千克(cmol/kg)。在任何單位體系中，CEC的數值都是不變的。

粘土和腐殖質的表面帶負電，能夠吸附陽離子。每種粘土的吸附力不同，腐殖質的吸附力是具有最強吸附力的粘土的兩到三倍。

提高土壤CEC的一個方法是促進腐殖質的形成。一般來說，土壤CEC越高，土壤肥力越高。

CEC的計算

CEC是某土壤樣品能持有的正電荷（陽離子）的量，人們通常將它描述為中和100g干土所需要的全部H⁺的量，有時H⁺也可以換作Al³⁺或Ca²⁺。在土壤科學中，當量的定義為電荷量，但該電荷量是用等量氫離子表示的。因為氫離子只帶一個正電荷，用氫離子來表示會使計算更為簡便。一當量的Al³⁺相當於三分之一當量的氫離子，一當量的Ca²⁺相當於二分之一當量的氫離子。

我們可以將單位meq/100g轉化為lbs/acre，但是在計算中必須要考慮到原子質量、離子化合價，並且合理估計土壤深度和密度。Mengel給出了以下幾種營養元素將單位從1 meq/100g轉換為lb/acre的數值：^[2]

鈣, 400 lb/acre

鎂, 240 lb/acre

鉀, 780 lb/acre

銨根, 360 lb/acre

鹼基飽和度

和陽離子交換量相似的一個概念是鹼基飽和度^[3] which is the fraction of exchangeable cations that are base cations (Ca, Mg, K and Na)，用百分數來表示。可交換的鹼類陽離子越多，短時間內能夠被中和的酸就越多。所以與CEC較低的土壤相比，CEC較高的土壤通常都需要更長的時間來酸化（或者從酸化中恢復）（假設這兩種土具有相似的鹼基飽和度）。雨水降落到CEC較高的土壤上，氫離子會很快被緩衝，土壤pH恢復原值；而CEC較低的土壤如亞馬遜流域的酸性土，無法儲存大量的氫離子，在下雨后土壤pH會迅速降低並維持較長時間。

鹼基飽和比（英語：base-cation saturation ratio） (BCSR)是由National Sustainable Agriculture Information Service (ATTRA)所提倡的用來在可持續農業中描述土壤檢測結果的一種方法。^[4]該方法在全世界約4,000 km²的農業用地中都得到了廣泛使用。

pH和CEC

對許多土壤來說，CEC與土壤pH有關。這一特性在很大程度上由霍夫曼序列（英語：Hofmeister series）(又稱感膠離子序，lyotropic series)所決定。霍夫曼序列用以描述不同陽離子在膠質上的吸附能力，其大致順序為：

Al³⁺ > H⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH₄⁺ > Na⁺

當土壤酸度增加，土壤膠體會吸收更多的H⁺。這些H⁺會阻止其他陽離子吸附土壤膠體，迫使這些陽離子進入土壤溶液。當土壤鹼性增加（即pH增大）時，因為沒有了H⁺的阻礙，會有更多的陽離子吸附到土壤膠體上，土壤CEC增大。(CEC increases).^[5]

各種膠體及土壤CEC

不同的土壤和土壤組分的CEC差別很大。

**不同土壤、土壤質地及土壤膠體的陽離子交換容量**^[6]
土壤	州	CEC meq/100 g
Charlotte fine sand	佛羅里達	1.0
Ruston fine sandy loam	德克薩斯	1.9
Glouchester loam	新澤西	11.9
Grundy silt loam	伊利諾伊	26.3
Gleason clay loam	加利福尼亞	31.6
Susquehanna clay loam	阿拉巴馬	34.3
Davie mucky fine sand	佛羅里達	100.8
土壤質地	—	CEC meq/100 g
砂土(Sands)	—	1–5
細砂壤土(Fine sandy loams)	—	5–10
壤土(Loams)和粉砂壤土(silt loams)	—	5–15
粘壤土(Clay loams)	—	15–30
黏土(Clays)	—	over 30
土壤膠體	—	CEC meq/100 g
倍半氧化物(Sesquioxides)	—	0–3
高嶺土(Kaolinite)	—	3–15
伊利土(Illite)	—	25–40
蒙脫土(Montmorillonite)	—	60–100
蛭石(Vermiculite)	—	80–150
腐殖質(Humus)	—	100–300

標準值

礦物	CEC meq/100 g
高嶺土	3–15
埃洛石（英語：Halloysite）(多水高嶺土) (Halloysite 2H₂O)	5–10
埃洛石(Halloysite 4H₂O)	40–50
Illite\|伊利土	10–40
Chlorite	10–40
Glauconite	11–20+
Palygorskite-group	20–30
Allophane	~70
Montmorillonite-group	70–100
Vermiculite	100–150

These are the values reported by Carroll (1959)^[7] for the cation-exchange capacity of minerals in meq/100g at pH of 7.

Aluminium ions and CEC

Many heavily leached or oxidized soils, especially in the wet tropics, have a high concentration of Al³⁺ occupying the soil colloids cation exchange sites. Since aluminium is toxic in high quantities for most plants, there are certain advantages to this. Due to the relatively high adsorption rate of aluminium to soil colloids, it will be taken out of the soil, hence the plant cannot be adversely affected by it. On the other hand, because it has three positive charges, it takes up a large amount of charge on a colloid. For example, Al³⁺ fills the same space as three NH₄⁺ ions. As a result, the ammonium is left in the soil water solution where it can be washed away by a heavy rain. This makes many aluminium heavy soils relatively infertile. There is no easy way to remove aluminium ions from the soil colloid and free the CEC for other ions.

Organic matter

Organic materials in soil increase the CEC through an increase in available negative charges. As such, organic matter build-up in soil usually positively impacts soil fertility. However, organic matter CEC is heavily impacted by soil acidity as acidity causes many organic compounds to release ions to the soil solution.

Anion exchange capacity

Similar to the CEC, the anion exchange capacity (AEC) is a measurement of the positive charges in soils affecting the amount of negative charges which a soil can absorb. There are relatively few anions that are restrictive in agriculture, but they are important, such as sulfur or phosphorus. The anion lyotropic series is:

H₂PO₄⁻ > SO₄⁻² > NO₃⁻ > Cl⁻

Converse to CEC, AEC increases with the number of positive charges formed on the silanol and aluminol groups present on the lateral edge of clay minerals particles when pH drops. The number of positive charges decreases when pH rises and they disappear at high pH. The formation and disappearance of the positive charges is due to the autoprotolysis of S–OH surface groups according to the following reaction involving the capture or the departure of a proton (H⁺):

>S–OH + H⁺ —> >S–OH₂⁺ (acidic conditions)

>S–OH + OH^– —> >S–O^– + H₂O (high pH)

Laboratory determination

There are two standardised International Soil Reference and Information Centre methods for determining CEC:

extraction with ammonium acetate; and
the silver-thiourea method (one-step centrifugal extraction).

There exist slightly conflicting ideas on which mechanisms to include in the term, "cation exchange", in soil chemistry. From a theoretical point of view, one should distinguish cation exchange from ligand exchange, and exchange of diffuse layer adsorbed cations. On the other hand, from a practical point of view, e.g. in forest and agricultural management, what is important is the soils' ability to replace one cation with another rather than the exact mechanism by which this replacement occurs. What is included in the term, "cation exchange", in soil science thus varies with the scientific context.

參考文獻

^ Robertson, G. Philip; Sollins, Phillip; Ellis, Boyd G.; Lajtha, Kate. 1999. Exchangeable ions, pH, and cation exchange capacity. In: Robertson, G. Philip; Coleman, David C.; Bledsoe, Caroline S.; Sollins, Phillip, eds. Standard soil methods for long-term ecological research. New York, NY: Oxford University Press: 106-114. (PDF). [2015-01-22].
^ Mengel, David D., Department of Agronomy, Purdue University. Fundamentals of Soil Cation Exchange Capacity. [2011-05-03]. （原始內容存檔於2011-04-11）.
^ Turner, R.C. and Clark J.S., 1966, Lime potential in acid clay and soil suspensions. Trans. Comm. II & IV Int. Soc. Soil Science, pp. 208–215
^ NCat Soil Management^{[永久失效連結]}
^ Havlin, Tisdale; Beaton, Nelson. Soil Fertility and Fertilizers. New Delhi: PHI. 2011.
^ Donahue, Miller, Shickluna. Soils: an introduction to soils and plant growth 4. Inglewood Cliffs, New Jersey 07632: Prentice- Hall. 1977: 115, 116. ISBN 0-13-821918-4.
^ Carroll, Dorothy. Ion exchange in clays and other minerals. Geological Society of America Bulletin. 1959, 70 (6): 749‐780. doi:10.1130/0016-7606(1959)70[749:IEICAO]2.0.CO;2.

外部連結

[1] Robertson, G. Philip; Sollins, Phillip; Ellis, Boyd G.; Lajtha, Kate. 1999. Exchangeable ions, pH, and cation exchange capacity. In: Robertson, G. Philip; Coleman, David C.; Bledsoe, Caroline S.; Sollins, Phillip, eds. Standard soil methods for long-term ecological research. New York, NY: Oxford University Press: 106-114. (PDF). [2015-01-22].

[2] Mengel, David D., Department of Agronomy, Purdue University. Fundamentals of Soil Cation Exchange Capacity. [2011-05-03]. （原始內容存檔於2011-04-11）.

[3] Turner, R.C. and Clark J.S., 1966, Lime potential in acid clay and soil suspensions. Trans. Comm. II & IV Int. Soc. Soil Science, pp. 208–215

[ll-4] NCat Soil Management^{[永久失效連結]}

[5] Havlin, Tisdale; Beaton, Nelson. Soil Fertility and Fertilizers. New Delhi: PHI. 2011.

[6] Donahue, Miller, Shickluna. Soils: an introduction to soils and plant growth 4. Inglewood Cliffs, New Jersey 07632: Prentice- Hall. 1977: 115, 116. ISBN 0-13-821918-4.

[7] Carroll, Dorothy. Ion exchange in clays and other minerals. Geological Society of America Bulletin. 1959, 70 (6): 749‐780. doi:10.1130/0016-7606(1959)70[749:IEICAO]2.0.CO;2.

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