Features Environment Soil
Water, water everywhere, but what is actually available?

As many are aware, the maritime provinces experienced higher-than-normal amounts of rainfall during the months of June, July and August in 2023. Not only were there high amounts; there were also heavy downpours. This resulted in erosion, increased leaching and runoff, and flooding in some areas. However, prior to these heavy rains, there was very little rainfall in the spring, which caused some drought-like conditions. Even in less variable years, water management on farms is often a major concern for maritime producers. 

Water management on farms can be challenging, but understanding how water behaves in the soil may help improve water management. Soil can hold an amount of water equal to the volume of its pores. Therefore, the inherent texture of the soil plays a large role in the capacity of the soil to retain water. Sandier soils have larger pores, but the total pore volume is actually smaller than in soils with higher clay content; therefore, clay soils have the capacity to retain more water. 

Water creates forces between itself and other materials, which impacts water retention and movement in the soil. When water is attracted to itself, this is cohesion force; when water is attracted to soil particles, this is adhesion force. Typically, adhesion forces are stronger than cohesion forces. Also, adhesion forces are stronger the closer the water is to the soil particle. Therefore, adhesion forces are stronger in clay soils because the pore sizes are smaller, and water is typically in closer proximity to the soil. 

When a field receives water, either by rainfall or irrigation, several different types of water occur. 

Gravitational water: This is water that can be freely drained by gravity. It is the water that is not tightly held by adhesion or cohesion forces. This water leaves the soil first and is not typically taken up by plants or soil microbes. 

Plant-available water: This is the water that plants can take up. This water is held in the soil against gravitational forces but not held so tightly that plant roots can’t extract it. It is the water between field capacity and the permanent wilting point (Figure 1).

Hydroscopic water: This is water mainly held by adhesion forces, but plants cannot exert enough force to extract the water. This is a small portion of water in the soil and is not plant-available. 

As a field drains or plants use these different types of water, certain terms describe the field conditions. For example, if a field is flooded, all soil pores will be full of water, and this is known as saturated water content. Gravitational water will then be drained from the soil; the speed at which this water is drained depends on the soil texture and management. Once gravitational water is drained, you reach field capacity; water is plant-available at field capacity. It can be loosely defined as the amount of water left in a field two to three days after being saturated. If rain does not continue and plants take up the available water and there is no longer any available, this is the permanent wilting point. After this point, plants cannot recover, even if water is added after. 

Figure 1

Soil texture and structure impact the movement of water as well as management. As previously stated, soil texture impacts how tightly water can be held in soil and the amount of water that can be held. Soil structure is the arrangement of primary soil particles (sand, silt and clay). Soil structures with poor aggregation, such as single-grained (very sandy) or massive (high clay content) structures, can affect the movement of water. Because water movement is largely dependent on the number of pores and pore size, management practices which promote compaction can have a sizable impact on infiltration and internal drainage. 

There are several ways you can measure soil water in the field, including soil water meters, tensiometers, using hand-feel method, or collecting samples to be analyzed in the lab. Soil moisture meters can use several different measurements, but most often are represented as a percentage. The ideal soil moisture percentage range for plant-available water measured by a soil moisture meter will depend on the crop, the soil type and the type of meter. Conversely, a tensiometer measures the tension of water between soil particles. The units are typically centibars, and the lower the centibars, the wetter the soil. If the centibar reading is between zero and 10, that means that soil is typically saturated. However, these methods are not always available to producers, and water measurements are most effective if you can monitor water over time. Care should be taken when using soil water measurements for irrigation or drainage management. Without equipment, producers can get a general sense of water holding capacity by knowing your soil texture, soil structure and management practices. 

Soil water management can vary and depends on the soil type, commodity and climate. More extensive practices such as drain tile are common ways to help control the amounts of water on the field. Some less invasive practices, such as building soil organic matter and keeping off fields in wet weather, can go a long way to help manage water in your fields or at least prevent crop damage from water stress. Overall, water management is a bit of a goldilocks situation: you don’t want too much, but you don’t want too little either, and finding that perfect plant-available water content can be difficult. Understanding your soil and how water is made plant-available can give a better sense of how to manage water on your farm.