Clay, sand and silt are definitions of textures. The following picture show respective sizes of these 3 different particles;
Sand, by its comparative large size does not have a chemical valence (capability to interact with charge particles) therefore does not hold minerals in the soil. Water molecules find as well easily their way through sand and get away by the action of gravity. The advantage of sand is its permeability to plants roots which can develop without obstacles. The inconvenient of sand is its poor water and plants nutrients retention. When cultivating with a sandy soil a lot of organic matter needs to be present in the soil to store moisture and nutrients. Biochar helps as well by its structure to mechanically increase water retention in sandy soils.
Clay in agriculture
Clay, on the opposite, is very closed in size to elementary charged particles and react with ionized minerals. Different types of clay have different capabilities to exchanges cations (read more about CEC and measures of fertility) in the soil, playing a role in plant nutrients storage and availability. Usually clays in soils which have been weathered intensely and seen high bacterial activity for long geological periods (e.g. in the tropics) have structures less prone to interacts with minerals. It is one of the reason soils have a lack of minerals in the tropics. These minerals not being held by clay have been washed away by intense rains. In this case organic matter or more precisely humus play the role of nutrients storage with a higher electrostatic capability to interact with minerals than clay. The density of organic matter although is located in the top soil and reduce quickly in lower layers. The greater volume of clay compensate with the reduced capacity to hold nutrients. Plants with deep roots and symbiosis with fungus allow to access this reserve of nutrients in the subsoil.
Clay in building and waterproofing
Clay expand when in contact with water and shrink when getting dry. This particularity impact the different techniques of clay building. Mixing sand and clay using water create a solid aggregate when drying. Sand can be compared to stone and clay to mortar in this mixture.
The flexible expanding and self sealing properties of bentonite make this clay suitable for pond waterproofing. Other techniques using gley (organic material transformed by anaerobic reaction) are a less expensive alternative to bentonite.
Silt is a sediment material with an intermediate size between sand and clay. Carried by water during flood it forms a fertile deposit on valleys floor. Silt is easily compacted.
Loam is a mixture of clay, sand and silt and benefits from the qualities of these 3 different textures, favoring water retention, air circulation, drainage and fertility.
Humus is a highly complex substance still not fully understood. It is a stable and uniformly dark, spongy and amorphous material which come from the mechanical degradation of organic matter. Humus is fertile and gather all properties suitable for optimal plant growth. It is formed by complex chemical compounds, of plant, animal and microbial origin. Humus cannot form in the presence of high levels of inorganic nitrogen, due to the inhibition of the microbes essential to sequestration.