A horizon is the uppermost layer of the soil profile, and it is where most of the organic matter and nutrients are found. The R horizon, on the other hand, is the bedrock or unweathered parent material that lies below the soil.
The A horizon and the R horizon are related in that the A horizon is formed from the weathering of the R horizon. As the R horizon is weathered, the minerals and organic matter that make up the rock break down and form the soil. The A horizon is also constantly being added to by the accumulation of plant litter and other organic matter.
The thickness of the A horizon varies depending on a number of factors, including the climate, the type of rock that is being weathered, and the amount of time that has passed since the weathering process began. In some cases, the A horizon can be very thick, while in other cases it may be only a few inches deep.
The A horizon is an important part of the soil profile because it is where most of the plant growth occurs. The organic matter and nutrients that are found in the A horizon provide the plants with the resources that they need to grow.
