How Do Different Soil Variables Affect Root Growth?

Nutrients

• Plants need at least 13 mineral nutrients in order to carry out their growth and metabolic processes. When one or more of these minerals is deficient in the soil, growth of the plant slows, including growth of the roots. Nitrogen and phosphorus are two of the most important nutrients for plant growth -- and two that can become easily depleted in the soil. Low availability of one of these nutrients can cause inadequate root growth. After correcting the deficiency, normal root growth resumes.

Moisture

• Plants also use their roots to take in water, so water availability in the soil also affects growth. Roots go where they can find water, so plants that receive water only in the top few inches of soil subsequently develop shallow root systems. For this reason, gardeners should water plants infrequently but deeply to encourage a deep root system that can better withstand drought. Plants that live in dry environments, like deserts, tend to develop roots that spread farther than the roots of plants grown in moist environments. These far-reaching roots allow the plant to absorb more water from infrequent rainfalls than if the roots covered only a limited surface area.

Soil Texture

• Soil texture refers to the composition of the soil in terms of particle size. Particles range from tiny clay particles to grains of sand you can see with the unaided eye. The spaces between particles, called pores, range in size as well, depending on the size of the soil particles. Clay soils, therefore, have many tiny pore spaces. Sandy soils have fewer but larger pore spaces. This affects root growth because oxygen may become limited deep in soil with small pore spaces. Since roots need oxygen to live, they grow shallow and spread wide, similar to how they behave in dry soils. Clay soils also tend to compact easily, and this can prevent roots from penetrating deeply, again causing shallow root systems to develop.

Soil Structure

• Soil structure refers to how soil arranges itself. For example, clay soils high in organic matter form small clumps called aggregates; this loosens the soil and creates large pore spaces. Roots move easily through this type of soil, and plants develop deep, extensive root systems. Other soil structures cause soil to form columns or plates in the soil. Roots have trouble penetrating these obstacles, so root growth is inhibited.