The Sponge and the Crop: How Much Dryness Can Soil Endure Without Losing Yield?

One of the most common anxieties among gardeners and farmers is the fear of yield loss due to insufficient watering. This fear often leads to overwatering, a practice that can be just as detrimental as underwatering. But how dry can soil become before a crop's yield is significantly impacted? Let's delve into the science of soil moisture and its relationship to crop production.

Lesson 1: The Sponge Analogy—Understanding Soil Water Dynamics

Imagine a wet sponge. When you first squeeze it, water flows out freely. As you continue to squeeze, the effort required to extract water increases, and eventually, no amount of pressure will yield more water. This is analogous to soil water dynamics.

Figure 1. Soil behaves like a sponge. The drier the soil, the more difficult it is for plants to extract water.

Initially, when soil is near field capacity (FC), water is readily available to plants. However, as the soil dries, the remaining water becomes increasingly difficult for plants to extract. This point where plants struggle to access water is closely related to the concept of the Management Allowable Deficit (MAD).

Lesson 2: The Production Reduction Function—Mapping Yield to Soil Moisture

The relationship between crop yield and soil water availability is described by the "production reduction function." This function illustrates how yield decreases as the Soil Water Deficit (SWD) increases (meaning moisture levels decrease). Crucially, this relationship is not linear. This means that crops possess a degree of resilience to water stress before yield is significantly impacted.

Figure 2. The production reduction function illustrates that "the higher the soil water deficit (SWD) (the lower the moisture level), the higher the yield loss." It also shows that maintaining soil moisture above the MAD prevents yield loss.

As depicted in Figure 2, crop production remains relatively stable as soil water depletes from FC (100% available water) towards the Permanent Wilting Point (PWP) (0% available water). However, at a certain threshold, production begins to decline sharply.

This threshold is typically defined as the MAD. The specific MAD and the shape of the production reduction curve vary depending on the plant species and soil type. Soil water depletion below the MAD results in substantial yield reductions.

Lesson 3: Maintaining Soil Moisture Within Optimal Limits

To optimize crop production, it is essential to maintain SWD between the MAD and FC. Exceeding FC leads to deep percolation, where water drains beyond the root zone, carrying away valuable nutrients. Conversely, allowing SWD to fall below MAD results in yield and quality reductions.

Soil has a finite capacity for water retention. Overwatering leads to water loss through deep percolation, along with the loss of fertilizers and increased labor and pumping costs. Conversely, insufficient watering compromises yield and quality. Therefore, precise irrigation management is crucial.

• Note: Trees, with their deeper root systems, generally exhibit greater tolerance to water stress compared to row crops. Consequently, a higher MAD may be acceptable for trees.

Summary:

Understanding the relationship between soil moisture and crop yield is vital for efficient irrigation management. The soil's ability to hold and release water, as illustrated by the sponge analogy, highlights the importance of maintaining soil moisture within optimal limits.

The production reduction function reveals that crops can tolerate a degree of water stress before significant yield loss occurs. By maintaining soil moisture between the MAD and FC, we can maximize crop production while minimizing water waste and nutrient loss. Trees, with their deeper roots, have a greater tolerance to dry soil.