National Drought Mitigation Center

Plant Growth and Development

Why is Soil Water Important to Plant Growth?

In order to fully comprehend how grasses respond to drought, one must understand what plants need for photosynthesis.

Soil Water is the Limiting Factor for Photosynthesis


Green plants convert solar energy to chemical energy by the process of photosynthesis. This reaction is directly or indirectly responsible for nearly all life on earth.

During photosynthesis, carbon dioxide, a gas, is combined with water and solar energy, and converted to carbohydrates, a solid. Formation of carbohydrates is a chemical way to store the sun’s energy as “food.” Carbohydrates produced from photosynthesis provide energy for all plant growth and maintenance.

Carbohydrate reserves are accumulated in crowns, stolons or rhizomes in the autumn and used to keep dormant buds and below-ground plant parts alive from the end of the growing season to spring green-up.

Carbon dioxide is constantly available in the air we breathe and energy is readily available from the sun. When air temperatures are favorable for plant growth, soil water is the limiting factor for photosynthesis.

The Importance of Carbohydrates for Forage Production

All living plant cells must have carbohydrates to continue life and function; however, photosynthesis occurs only in the presence of sunlight and only in cells with chlorophyll. Plant cells not directly involved in photosynthesis are completely dependent on the translocation of carbohydrates from green foliage or from storage areas known as sources. Recipient tissue is referred to as a sink.

Drought reduces the supply of available soil water. Overgrazing reduces the ability of plants to extract soil water. Additionally, reductions in plant cover caused by overgrazing and natural defoliation processes, like fire or severe hail, often reduce the amount of rain water that enters the soil because of runoff losses. Overgrazing has a profound effect on the total amount of carbohydrates produced per acre. Learn more about grazing and soil moisture.

Relationships between sources and sinks of carbohydrates are seasonally dynamic. Rapid growth of roots, new leaf area or seed heads is evidence of a strong sink. Rapid growth does not occur in all plant parts simultaneously. Carbohydrate sinks compete with one another for products of photosynthesis such as starch and sugar.

Growth of plant parts is reduced or delayed when green leaf area is removed. Grazing pastures every year at the same time, when root growth, bud formation, stolon growth or rhizome growth are seasonally most rapid, reduces subsequent-year forage production potential of most mid- and tall grasses.