This plant hygrometer can be used just by placing it in a pot, without using batteries When water is sufficient, the indicator will turn blue and the blue color turns white when water is out of water The core material is 100% natural fibers, breaking down by bacteria in the soil Estimated life of the middle core is about 6-9 months on organic soil, about 9-12 months on inorganic soil, and hydroculture, and bark are about 1-2 years When the core reaches its life, it can be replaced with a separate refill