An anti-caking agent for soil includes, as effective ingredients: a lignin having a molecular weight peak, as measured by GPC molecular weight analysis using a UV detector at a wavelength of 254 nm, within the molecular weight range of 10,000 to 40,000; and a diatomaceous earth. A method of improving the growth of a plant uses the above described anti-caking agent for soil. The anti-caking agent for soil has the effect of reducing the caking of soil and improving the poor growth of a plant.