What is the reason for the formation of fertile chernozems in the steppe?

The steppes of our planet are located in very favorable climatic conditions in the southern parts of the temperate heat zone. Adequate sunlight allows rich herbaceous vegetation to grow here, which has a powerful root system that allows grasses to receive water from the deep layers of the soil. The steppe vegetation is predominantly annual; leaves and roots that die off every season are converted into humus under the action of soil bacteria. The structure of the soil layer in these latitudes is mainly clayey, and heavy rains in the steppes are rare, therefore this highly fertile soil component is not washed out as in podzolic soils, but accumulates in the form of powerful and fat chernozems.