Alluvial soils are sedimentary soils that are deposited by rivers and streams. They are known for their fertile nature and are commonly found in river valleys and floodplains. These soils can vary in composition from sandy loam to clay, indicating different levels of sand, silt, and clay particles in the soil.

The given correct answer is option 4, which states that alluvial soils are rich in potash and poor in phosphorus. Potash refers to a group of minerals that are a source of potassium, an essential nutrient for plant growth. Alluvial soils generally have a good amount of potassium, making them rich in potash.

On the other hand, alluvial soils are poor in phosphorus. Phosphorus is another crucial nutrient required for plant growth, but it is not typically abundant in alluvial soils. Plants may need to supplement their phosphorus requirements through fertilizers or organic matter.

Therefore, it is important to note that although alluvial soils vary in nature from sandy loam to clay, they are generally rich in potash and poor in phosphorus, making option 4 the correct answer.