For making dorodango, <Good soil>has two meanings. One is that the soil is appropriate for forming a spherical shape. The other is that the soil gives a shining surface. These two may seem close to each other, but they actually differ considerably. While micro particles (powdery soil with a particle size of 1/1,000 or 1/10,000 mm; dust that is stirred up when the ground is beaten with a hand; it is a component contained in all soil in which plants can grow, i.e. soil that can retain moisture) are absolutely necessary to obtain a shining capsule, diversity of the particle size is important in shaping a round ball without roughness. Soil that is an even mixture of particles of diverse sizes-1/10,000 mm, 1/1,000 mm, 1/100 mm, 1/10 mm...-resulting from being wet by rain into mud and dried, being wet by rain into mud and dried again, and, in the mean time, being tramped by human feet thousands and tens of thousands of times.... Such is the best soil. The soil of the playgrounds of most of the Japanese nursing schools and kindergartens is like this.
Talking about “soil consisting of diverse particles including micro particles”, if you carefully observe the playground, you will see that the nature of soil varies from area to area: (A) Areas where the soil seems to contain more sand than powdery particles, (B) areas where the soil seems to contain them nearly by halves, (C) areas where powdery soil seems relatively dominant, (D) areas where excellent powdery soil seems abundant. In “shaping the ball”, it is reasonable to start with (A), then go to (B), and then to (C). If an inexperienced person tries to shape a ball using soil from a (C) area, for example, from the beginning, the ball often gets irregular and may never become a smooth sphere. Soil in a (D) area is used for “capsule making”, but you will see that there are fine differences in the quality of this “excellent” powdery soil from the very best to fair if you carefully test it with your eyes, hands, and tongue. In “capsule making”, which is described below, it is also a routine to start with fair powder soil, advance to good powder soil, and eventually go to the best powder soil by moving from one area of the playground to another depending on the stage of the process. The ability of judgment of the quality of soil is another basic qualification required for making a shining mud ball.