This is the 9628th most frequent Japanese word.
肥やし
"肥やし" means "fertilizer" or "manure."
In this sentence, "肥やし" figuratively means something that enriches one's inner self or intellect.
読書や旅行は心の肥やしになる。
Reading and traveling become nourishment for the soul.
The word "肥やし" here refers to "fertilizer" or manure that enriches the soil for farming.
この畑には肥やしが必要です。
This field needs fertilizer.
Here, "肥やし" metaphorically signifies something that contributes to personal development, like gaining wisdom through experiences.