Illite 
A 2:1 layer clay mineral. Silicates, such as feldspar, is weathered to form this clay mineral, with alkaline pH and high concentrations of potassium and aluminium being typical outcomes.
Illuvial Horizon
The B Horizon in a soil profile. It is where material from the A Horizon has been removed and accumulated in the lower layer of the B Horizon.
Infiltration
The ability of water to move into a soil surface.
Infiltration Capacity
The maximum rate, typically measured in millimetres per hour, that water can enter a soil. Also referred to as infiltration rate.
Interceptor Drain
Kaolinite
A 1:1 layer clay mineral. This can form following the weathering of feldspars under acid conditions, or by hydrothermal alteration.
L Horizon/Layer/Zone
The Litter horizon within a soil profile. Where present, it forms part of the O Horizon.
Layering
The formation of a distinct layer within a soil profile. Mostly this is due to the use, or misuse, of top-dressings: See Soil Layering. Aerating a surface to a consistent depth without varying the depth will also result in a layer developing; this is also called an Aeration Pan.
Leaching
The action of water, in the form of rain and irrigation, washing nutrients in a soil solution through a soil profile. High sand profile constructions and excessive irrigation or rainfall increase the potential for leaching. The leached nutrients find their way through drains and soil horizons to pollute water courses, including aquifers.
Lime Requirement
The quantity of calcium carbonate needed to raise a specified quantity of soil from one pH level to another pre-determined pH level. Typically, the calculation for many agricultural crops is to raise an acidic soil up to pH 6.5 to encourage maximum yield. This is not an aim for turfgrass swards, but altering soils in the soil cultivation stage might be considered for a very acidic soil on which a perennial ryegrass sports surface is to be produced.