FERTILE SOIL. 4 letters. UK Daily Mail Crosword clue on May 3, 2017.
The answer is LOAM.
Loam is soil composed mostly of sand (particle size > 63 µm), silt (particle size > 2 µm), and a smaller amount of clay (particle size < 2 µm). Its composition is about 40–40–20% concentration of sand-silt-clay, respectively. These proportions can vary to a degree, however, and result in different types of loam soils: sandy loam, silty loam, clay loam, sandy clay loam, silty clay loam, and loam.In the USDA textural classification triangle, the only soil that is not predominantly sand, silt, or clay is called "loam".Loam soils generally contain more nutrients, moisture, and humus than sandy soils, have better drainage and infiltration of water and air than silty soils, and are easier to till than clay soils. The different types of loam soils each have slightly different characteristics, with some draining liquids more efficiently than others. For food production, a loam soil containing a small amount of organic material is considered ideal.The mineral in a loam soil ideally is about 40% sand, 40% silt and 20% clay by weight. The soil's texture, especially its ability to retain nutrients and water are crucial.Loam soil is suitable for growing almost all plant varieties.