Water as a Limiting Factor in Plant Growth

Moisture in the Soil and Air is Critically Important to Plants

Jul 10, 2009

While plant growth is regulated and controlled internally, it is greatly influenced by external conditions such as the availability of moisture.

Soil moisture is difficult to define because it means different things in different disciplines. For example, a farmer's concept of soil moisture is different from that of a water resource manager or a weather forecaster. Generally, however, soil moisture is the water that is held in the spaces between soil particles.

Moisture and Air in the Soil are Critical to Plant Survival

The importance of air for plant roots is often underestimated. Roots respire as do other parts of the plant, and oxygen is as necessary for these organs as it is for stems and leaves. This necessary oxygen diffuses into the roots from tiny air pockets or soil pores around the roots.

In some situations plants cannot grow very well or at all because of inadequate soil pores resulting from the soil being puddled. That is, a heavy soil that has been compacted when wet, so that the soil pores are broken down. The bare soil of a playground of my school can often be as hard and bare as an asphalt parking lot. All those little feet over the years compact the soil until physically there are no soil pores left to contain the water and air necessary to support plant roots.

Continuous cropping of cultivated plants such as corn, cotton, sugar beets, and many types of vegetables breaks down the soil structure and reduces the pore spaces of the soil. Plus, farm implements like tractors driving over these fields tends to compact the soil, further reducing pore spaces.

A very dire situation for plants exists when water stands on the soil due to flooding or inadequate drainage. In such situations, all the soil spaces fill with water and plant roots literally suffocate from lack of oxygen.

However, many species, both woody and herbaceous, flourish in bogs and swamps even though there are very few if any air spaces in the soil around them. The ability of plants to grow under such conditions appears to be dependent upon structural and/or physiological adaptations. One example are the mangroves. Horizontal roots, buried in the mud, extend outward from the parent mangrove tree. Extending upward 10 to 15 inches off these horizontal roots grow air roots. Sticking up into the air, these aerial roots act like ventilation chimneys permitting interchange of gases between the atmosphere and the submerged roots.

On the other hand, cacti and other types of desert plants survive in soils that contain very little water and mainly air in the soil pores around their roots. Desert plants adapt to such limited water by reducing the size of their leaves or eliminating their leaves altogether in order to reduce transpiration. Also, the roots of many desert plants spread horizontal a great distance often reaching many tens of feet from the plant.

Humidity

Moisture in the soil is critical to the survival of a plant but the moisture in the air around a plant is an important factor in the survival of a plant as well.

Water is lost off leaves as it evaporates out through open stomates in a process called transpiration. A single corn plant may transpire more than two quarts of water daily and an acre of corn may give off more than 300,000 gallons of water during a single growing season. If humans used water at the same rate as a single corn plant, we would have to drink 10 to 15 gallons of water a day to keep up with the loss. Lost water means fewer raw products for photosynthesis resulting in less food to support growth and development.

The most important environmental factors affecting the rate of transpiration are temperature and humidity of the air, light, wind, and the water content of the soil.

Under natural conditions the air above plant leaves always contains some water vapor, typically in a concentration of 1 to 3 percent. Some of these water vapor molecules move in through the stomates of the leaf in a process of reverse transpiration. The rate at which this inward movement occurs is proportional to the concentration of water vapor in the air – that is, to the humidity. The movement of water vapor from the air into the leaf will reduce the net rate at which water is lost. Thus, other things being equal, the rate of transpiration will decrease as the humidity of the air increases.

The copyright of the article Water as a Limiting Factor in Plant Growth in Botany is owned by Dennis Holley. Permission to republish Water as a Limiting Factor in Plant Growth in print or online must be granted by the author in writing.