土壌基質

アルミノシリカ粘土またはアルミノケイ酸塩粘土は、規則的な結晶構造または準結晶構造を特徴としています。 ^{[ 22 ]}酸素はシリコンとイオン結合して四面体配位（シリコンが中心）を形成し、それがシリカのシートを形成します。2枚のシリカシートは、上下のシリカシートの酸素と八面体配位（アルミナと呼ばれる）を形成するアルミニウムの平面によって結合されています。 ^{[ 23 ]}水酸化物イオン（OH ⁻）が酸素を置換することもあります。粘土形成プロセス中に、シリカ層では Al ³⁺が Si ^{4+を置換し、アルミナ層ではアルミニウム Al 3+}の最大 4 分の 1 がZn ²⁺、 Mg ²⁺または Fe ²⁺に置換されることがあります。低価数の陽イオンが高価数の陽イオンに置換されること（同形置換）により、粘土は酸素原子上に局所的な負電荷を帯びる^{[ 23 ]} 。この負電荷は水や正電荷を帯びた土壌陽イオンを引き付けて保持し、その一部は植物の成長に有益である^{[ 24 ]}。同形置換は粘土の形成中に起こり、時間とともに変化しない^{[ 25 ] 。}

• モンモリロナイト粘土は、2つのケイ素面と1つの中央アルミニウム面が介在する4つの酸素面から構成されています。アルミノケイ酸塩であるモンモリロナイト粘土は、ケイ素とアルミニウムの比率が2:1であると言われており、簡単に言うと2:1粘土鉱物と呼ばれています。^{[ 26 ]} 7つの面が合わさって、モンモリロナイトの単結晶を形成します。結晶は弱く結合しており、水が介在すると、粘土は乾燥時の体積の最大10倍まで膨張することがあります。^{[ 27 ]}浸出がほとんど行われていない土壌で発生するため、乾燥地域で見られますが、鉱物学的起源によっては湿潤気候でも発生することがあります。^{[ 28 ]}結晶は面と面が結合していないため、表面全体が露出しており、表面反応に利用できるため、陽イオン交換容量（CEC）が高くなります。^{[ 29 ]}
• Illite is a 2:1 clay similar in structure to montmorillonite but has potassium bridges between the faces of the clay crystals and the degree of swelling depends on the degree of weathering of potassium-feldspar.^[30] The active surface area is reduced due to the potassium ionic bonds. Illite originates from the modification of mica, a primary mineral. It is often found together with montmorillonite and its primary minerals. It has moderate CEC.^[31]
• Vermiculite is a mica-based clay similar to illite, but the crystals of clay are held together more loosely by hydrated magnesium and it will swell, but not as much as does montmorillonite.^[32] It has very high CEC.^[33]
• Chlorite is similar to vermiculite, but the loose bonding by occasional hydrated magnesium, as in vermiculite, is replaced by a hydrated magnesium sheet, that firmly bonds the planes above and below it. It has two planes of silicon, one of aluminium and one of magnesium; hence it is a 2:2 clay.^[34] Chlorite does not swell and it has low CEC.^[35]
• Kaolinite is a very common, highly weathered clay, and more common than montmorillonite in acid soils.^[36] It has one silica and one alumina plane per crystal; hence it is a 1:1 type clay. One plane of silica of montmorillonite is dissolved and is replaced with hydroxyls, which produces strong hydrogen bonds to the oxygen in the next crystal of clay.^[37] As a result, kaolinite does not swell in water and has a low specific surface area, and as almost no isomorphous substitution has occurred it has a low CEC.^[38] Where rainfall is high, acid soils selectively leach more silica than alumina from the original clays, leaving kaolinite.^[39] Even heavier weathering results in sesquioxide clays.^[40]

The carbonate and sulfate clay minerals are much more soluble and hence are found primarily in desert soils where leaching is less active.^[41]

Amorphous clays are young, and commonly found in recent volcanic ash deposits such as tephra.^[42] They are mixtures of alumina and silica which have not formed the ordered crystal shape of alumino-silica clays which time would provide.^[43] The majority of their negative charges originates from hydroxyl ions, which can gain or lose a hydrogen ion (H⁺) in response to soil pH, in such way as to buffer the soil pH. They may have either a negative charge provided by the attached hydroxyl ion (OH⁻), which can attract a cation, or lose the hydrogen of the hydroxyl to solution and display a positive charge which can attract anions. As a result, they may display either high CEC in an acid soil solution, or high anion exchange capacity in a basic soil solution.^[43]

Sesquioxide clays or sesquioxides are a product of heavy rainfall that has leached most of the silica from alumino-silica clay, leaving the less soluble oxides iron hematite (Fe₂O₃), iron hydroxide (Fe(OH)₃), aluminium hydroxidegibbsite (Al(OH)₃), hydrated manganese birnessite (MnO₂), as can be observed in most lateriticweathering profiles of tropical soils.^[44] It takes hundreds of thousands of years of leaching to create sesquioxide clays.^[45]Sesqui is Latin for "one and one-half": there are three parts oxygen to two parts iron or aluminium; hence the ratio is one and one-half (not true for all). They are hydrated and act as either amorphous or crystalline. They are not sticky and do not swell, and soils high in them (e.g. lateritic soils) behave much like sand and can rapidly pass water.^[46] They are able to hold large quantities of phosphates, a sorptive process which can at least partly be inhibited in the presence of decomposed (humified) organic matter.^[47] Sesquioxides have low CEC but these variable-charge minerals are able to hold anions as well as cations.^[48] Such soils range from yellow to red in colour according to the dominance of various iron oxides.^[49] Such clays tend to hold phosphorus so tightly that it is unavailable for absorption by plants, in proportion to their content in goethite.^[50]