Soil is more than just dirt. The substrate underfoot in which your plants grow consists of minerals, organic matter, water and a good bit of air. Soil consists of particles separated by air spaces called pores. All of these factors affect the availability of nutrients, water and air to your growing plants and can cause seeds to grow differently in different soils.
Soil type generally refers to the size of the soil particle, which in turn affects the size of the air space between particles. At either extreme, you find sand and clay. Sand consists of large particles with large air spaces between each. Clay consists of many tiny particles and, subsequently, many tiny air spaces. Silt, a third soil type, falls in the middle between the two. Topsoil refers to the upper layer of soil that is generally rich with nutrients and organic matter and drains easily.
Different soil particle sizes affect garden conditions in a way that impacts the seeds you plant as well. Larger pore spaces found in sandy soils allow water to move quickly through the soil, which can cause dry soils and nutrient leaching. Tiny pore spaces found in clay soils, on the other hand, tend to hold water, causing poor drainage and aeration.
Before a seed begins to grow, it absorbs large quantities of water. This helps the plant embryo break out of the hard seed coat and helps the seedling to grow rapidly. As the University of Minnesota Extension points out, the seed's need for water makes tight soils--such as clay soils--advantageous during germination because they hold water close to the seed and allow it to absorb what it needs. Topsoil also holds water well, but because sand drains so quickly, seeds may dry out before germination.
The first part of the seed to emerge is the root, which anchors the plant. As it begins to branch into the soil, it can extract water and nutrients needed for the plant to survive. The pore size in the soil affects the ability of roots to expand, according to the Warnell School of Forestry. Roots cannot force their way into pores smaller than they are, so roots grow more easily in topsoil and sand than clay. However, the University of Minnesota Extension states that tighter soils, such as those containing clay, may encourage the development of smaller, secondary roots and broader root networks.
Roots also need oxygen to grow, according to the Warnell School of Forestry. Because sandy soils contain few, large pores, more of them may have depleted oxygen. Also, because clay soils hold water so well, they can easily become saturated to where the plant can no longer extract the oxygen it needs and drowns.