Permanent Wilting Point

Introduction

The Permanent Wilting Point (PWP) or Wilting Point is a term in soil science that refers to the minimal amount of water in the soil that a plant can extract and use. When the soil moisture content falls below this level, the plant can no longer obtain sufficient water from the soil and it wilts permanently, often leading to the plant's death.

A close-up image of dry, cracked soil, indicating a state of extreme water deficiency.

Understanding Soil Moisture Content

Soil moisture content is a critical factor in plant growth and survival. It is the measure of the amount of water present in the soil, which is available for plant uptake. The soil moisture content can range from completely dry, with no available water, to saturated, where all the soil pores are filled with water. The soil moisture content at which a plant wilts and can no longer recover its turgidity is known as the Permanent Wilting Point.

Factors Influencing Permanent Wilting Point

The Permanent Wilting Point varies depending on several factors. These include the type of plant, the soil texture, the soil structure, and the soil's organic matter content.

Plant Type

Different plant species have different abilities to extract water from the soil. Some plants, such as cacti and other succulents, are adapted to survive in arid conditions and can extract water from the soil even when the soil moisture content is very low. Other plants, such as most crop plants, require a higher soil moisture content to survive and grow.

Soil Texture

Soil texture, which refers to the proportion of sand, silt, and clay particles in the soil, significantly influences the soil's water holding capacity and thus the Permanent Wilting Point. Soils with a high clay content have a higher water holding capacity and a higher Permanent Wilting Point than sandy soils.

Soil Structure

The structure of the soil, which refers to the arrangement of soil particles into aggregates or peds, also affects the soil's water holding capacity. Well-structured soils have a higher water holding capacity and a higher Permanent Wilting Point than poorly structured soils.

Organic Matter Content

Soils with a high organic matter content have a higher water holding capacity and therefore a higher Permanent Wilting Point than soils with low organic matter content. Organic matter improves the soil's structure and increases its ability to retain water.

Measurement of Permanent Wilting Point

The Permanent Wilting Point is typically measured in the laboratory using a pressure plate apparatus. The soil sample is saturated with water and then placed on the pressure plate. The pressure is gradually increased until the amount of water that a plant can extract from the soil is equal to the weight of the water lost from the soil. This point is the Permanent Wilting Point.

Importance of Understanding Permanent Wilting Point

Understanding the Permanent Wilting Point is crucial for effective water management in agriculture. It helps farmers and agronomists determine when to irrigate and how much water to apply. By avoiding irrigation when the soil moisture content is above the Permanent Wilting Point, water can be saved and used more efficiently.

See Also

• Field Capacity
• Available Water Capacity
• Soil Water Retention Curve