TYPES OF SOIL COLLOIDS

a. Layer silicate clays (Crystalline)

• These are the dominating type in most soils. Their crystalline structure is layered much like pages in a book.
• Each layer consists of two to four horizontal sheets of Silicon, Aluminium, Magnesium or Iron atom surrounded and held together by O2and -OH group.
• Although all are predominantly negatively charged, silicate clay minerals differ widely with regards to their particles shapes (Kaolinite[Si2Al2O5(OH)4], a fine grained mica and a smectite), intensity of charge, stickiness, plasticity and swelling behavior.

b. Iron and Aluminum oxide clay

• These clays are dominant in highly weathered soils of tropics and subtropics; also in temperate soils.
• They consist mainly of either iron or aluminum atoms coordinated with oxygen atoms (often associated with hydrogen ions to make hydroxyl groups). Some, like gibbsite (an Al-oxide) and goethite (a Fe-oxide) consist of crystalline sheets.
• The oxide colloids are relatively low in plasticity, and stickiness. Their net charge ranges from slightly negative to moderately positive. Gibbsite Al(OH)3 an Al-oxide, and Geothite FeOOH an Fe-oxide.

c. Non Crystalline clay (Allophane and associated clays)

• It also consists of tightly bonded silicon, aluminum, and oxygen atoms, but they do not exhibit ordered, crystalline sheets -amorphous in nature.
• Two principal clays of this type, allophaneand imogolite, usually form from volcanic ash. They have high amounts of both positive and negative charge, and high water holding capacities.
• Although malleable (plastic) when wet, they exhibit a very low degree of stickiness.

d. Organic (humus) colloids

• Organic colloids are important in nearly all soils, especially in the upper layer of soil profile. The humus colloids are not mineral and not crystalline.
• They are basically composed of C, H, O and N2rather than Si, Al, Fe, and other (OH) groups. The organic colloidal particles are often among the smallest of soil colloids and exhibit very high capacities to adsorb water, but almost no plasticity or stickiness. Humus has high amounts of both negative and positive charge per unit mass, but the net charge is always negative and varies with soil pH.
• The negative charge on humus is extremely high in neutral to alkaline soils. The negative charges of humus are associated with partially dissociated enolic(-OH), Carboxyl (-COOH) and phenolic (-OH) groups.