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在土壤科学中, 阳离子交换容量，又称阳离子交换量，（cation exchange capacity, CEC）是指在特定pH下，单位干重的土壤持有的、能与土壤溶液进行交换的阳离子的最大量。[1] CEC常被用来作为检测土壤肥力、养分持留容量、以及保护地下水免受阳离子污染的指标。其单位是毫当量氢离子每一百克土壤干重(meq/100g)，其国际标准单位是厘摩每千克(cmol/kg)。在任何单位体系中，CEC的数值都是不变的。
粘土和腐殖质的表面带负电，能够吸附阳离子。每种粘土的吸附力不同，腐殖质的吸附力是具有最强吸附力的粘土的两到三倍。
提高土壤CEC的一个方法是促进腐殖质的形成。一般来说，土壤CEC越高，土壤肥力越高。
CEC的计算
[编辑]CEC是某土壤样品能持有的正电荷（阳离子）的量，人们通常将它描述为中和100g干土所需要的全部H+的量，有时H+也可以换作Al3+或Ca2+。在土壤科学中，当量的定义为电荷量，但该电荷量是用等量氢离子表示的。因为氢离子只带一个正电荷，用氢离子来表示会使计算更为简便。一当量的Al3+相当于三分之一当量的氢离子，一当量的Ca2+相当于二分之一当量的氢离子。
我们可以将单位meq/100g转化为lbs/acre，但是在计算中必须要考虑到原子质量、离子化合价，并且合理估计土壤深度和密度。Mengel给出了以下几种营养元素将单位从1 meq/100g转换为lb/acre的数值：[2]
- 钙, 400 lb/acre
- 镁, 240 lb/acre
- 钾, 780 lb/acre
- 铵根, 360 lb/acre
碱基饱和度
[编辑]和阳离子交换量相似的一个概念是碱基饱和度[3] which is the fraction of exchangeable cations that are base cations (Ca, Mg, K and Na)，用百分数来表示。可交换的碱类阳离子越多，短时间内能够被中和的酸就越多。所以与CEC较低的土壤相比，CEC较高的土壤通常都需要更长的时间来酸化（或者从酸化中恢复）（假设这两种土具有相似的碱基饱和度）。雨水降落到CEC较高的土壤上，氢离子会很快被缓冲，土壤pH恢复原值；而CEC较低的土壤如亚马逊流域的酸性土，无法储存大量的氢离子，在下雨后土壤pH会迅速降低并维持较长时间。
碱基饱和比 (BCSR)是由National Sustainable Agriculture Information Service (ATTRA)所提倡的用来在可持续农业中描述土壤检测结果的一种方法。[4]该方法在全世界约4,000 km²的农业用地中都得到了广泛使用。
pH和CEC
[编辑]对许多土壤来说，CEC与土壤pH有关。这一特性在很大程度上由霍夫曼序列(又称感胶离子序，lyotropic series)所决定。霍夫曼序列用以描述不同阳离子在胶质上的吸附能力，其大致顺序为：
当土壤酸度增加，土壤胶体会吸收更多的H+。这些H+会阻止其他阳离子吸附土壤胶体，迫使这些阳离子进入土壤溶液。当土壤碱性增加（即pH增大）时，因为没有了H+的阻碍，会有更多的阳离子吸附到土壤胶体上，土壤CEC增大。(CEC increases).[5]
各种胶体及土壤CEC
[编辑]不同的土壤和土壤组分的CEC差别很大。
| 土壤 | 州 | CEC meq/100 g |
|---|---|---|
| Charlotte fine sand | 佛罗里达 | 1.0 |
| Ruston fine sandy loam | 德克萨斯 | 1.9 |
| Glouchester loam | 新泽西 | 11.9 |
| Grundy silt loam | 伊利诺伊 | 26.3 |
| Gleason clay loam | 加利福尼亚 | 31.6 |
| Susquehanna clay loam | 阿拉巴马 | 34.3 |
| Davie mucky fine sand | 佛罗里达 | 100.8 |
| 土壤质地 | — | CEC meq/100 g |
| 砂土(Sands) | — | 1–5 |
| 细砂壤土(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 |
| 土壤胶体 | — | CEC meq/100 g |
| 倍半氧化物(Sesquioxides) | — | 0–3 |
| 高岭土(Kaolinite) | — | 3–15 |
| 伊利土(Illite) | — | 25–40 |
| 蒙脱土(Montmorillonite) | — | 60–100 |
| 蛭石(Vermiculite) | — | 80–150 |
| 腐殖质(Humus) | — | 100–300 |
标准值
[编辑]| 矿物 | CEC meq/100 g |
|---|---|
| 高岭土 | 3–15 |
| 埃洛石(多水高岭土) (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
[编辑]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
[编辑]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
[编辑]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
[编辑]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.
参考文献
[编辑]- ^ 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]. （原始内容存档于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[永久失效链接]
- ^ 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.
外部链接
[编辑]- ISRIC (International Soil and Reference Information Centre)
- Robert Lippert, Clemson University Extension Service
- David B. Mengel, Department of Agronomy, Purdue University
- "One Hundred Harvests Research Branch Agriculture Canada 1886–1986". Historical series / Agriculture Canada – Série historique / Agriculture Canada. Government of Canada
- "A study of the Lime Potential, R. C. Turner, Research Branch, Canadian Department of Agriculture, 1965