noun
rich soil; fertile soil
Refers to soil that is nutrient-rich and good for growing plants. Often used in agricultural or gardening contexts.
この畑の肥土は野菜作りに最適だ。
The rich soil in this field is ideal for growing vegetables.
肥土を入れて花壇を整えた。
I improved the flower bed by adding fertile soil.
肥沃 is an adjective meaning 'fertile' (of land), while 肥土 is a noun referring to the rich soil itself.
土壌 is a more technical term for 'soil' in general, whereas 肥土 specifically emphasizes fertility.
Compound of 肥え (koe, 'manure, fertilizer') + 土 (tsuchi, 'soil'). The kanji 肥 carries the meaning of 'fertilize' or 'enrich'.