Guideline Index

Chapter 2: Limits to Plant Growth

2.2 Water

Water is an essential compound for so many plant functions; both at the cellular level and as a medium through which all biological substances move. Growing plants are not in equilibrium with their environment regarding water, as there is a continual gradient or upward movement of water from the soil solution to that lost from the above ground parts of plants.

The availability of soil moisture for root uptake and plant growth is such a basic requirement that practically all other plant functions will be impaired if soil moisture is lacking.

Soils hold and provide varying amounts of water to plants depending on many factors; for example texture and structure. The amount of water held in a soil varies with time and is called its water holding capacity (WHC) or soil moisture content. The soil’s water holding capacity is usually expressed as the millimetres of water per metre of soil depth or sometimes as a % of a cubic metre of soil.

Evaporation from the pores (stomates) chiefly in leaves; called transpiration, creates a large water deficit within the leaf. This deficit is replenished by soil water pulled up from the root system by the cohesion of water molecules or “transpirational pull”. Under normal or optimal soil/plant conditions, as the soil water is absorbed by the root system from the larger soil pores it is replenished. This situation is ideal for plant growth and is referred to as “field capacity”. If the soil moisture is not replenished, the plants root system will continue to extract soil moisture from the smaller soil pores until the point where the remaining soil moisture is so tightly held that it is no longer available. At this point there is not sufficient water available to meet the plants needs so the plant wilts. Plants can exhibit what is called a “temporary wilt” which can occur as a result of the heat of the day. From a temporary wilt the plant will recover in the evening or early morning. However if the situation progresses, the plants normal turgor does not recover and the leaves wilt and change colour. This point is called the “permanent wilting point”. For more information on soil water, refer to Chapter 4.2.4 .

Both the water content of the soil and rainfall vary throughout the year and are not easily manipulated on dryland farms. In order to conserve soil moisture, a healthy soil with good internal drainage and a friable surface, will allow greater acceptance and storage of water than a compacted or sealed soil. Crops or pastures that are irrigated can achieve more consistent growth and production, as long as the extra nutrient demands for the increased dry matter production are met.