Silt

Silt refers to minerals larger than clay and smaller than sand. Silt is commonly moved by water and deposited as sediment. Silt is what makes the alluvial soil surrounding rivers so fertile. Silt is also fine enough to be carried surprisingly long distances on the wind as dust.

Silt deposits forming along river (USDA) Public Domain

How silt is formed

As rocks and regolith are eroded by weather, frost, and other processes, larger particles are ground down into smaller, rounded bits. Those smaller pieces become silt. Silt typically measures 0.05-0.002 mm and is usually composed of quartz and feldspar. Because silt moves so easily in water, construction and clear-cutting often result in silt levels that pollute waterways. This type of pollution is called siltation. In home gardens, over-watering can cause similar leaching problems and urban-drool. But silt is good for your plants.

Silt in garden soil 

Sandy garden soil loses water and nutrients too quickly, while clay soil holds on too tightly. Loamy soil, in the middle, is ideal for garden plants. Loam consists of 40% sand, 20% clay, and 40% silt. ​

Dams built where the Mississippi and Atchafalaya Rivers meet facilitate agriculture in silty soil (U.S. Army Corps of Engineers) Public Domain

Silt particles tend to be round, so they can retain a lot of water. This high water holding capacity is made even better because silt particles cannot hold on to the water very tightly. The same is true for mineral nutrients. Roots and microorganisms have an easy time pulling water and food from silty soil. Silt can be beige to black, depending on how much organic material it contains.

Silt is prone to compaction, but not nearly as much as clay. If your soil feels slippery when it is wet, it contains a lot of silt.