Why in Heavy Soils, even though they hold more Water, Sometimes this Water Is Not Enough, even if we feel enough moisture in our hand?

We have observed that in very heavy soils, although we feel moisture in our hands during a check, the moisture felt often isn't enough to irrigate the plant. In contrast, in sandy soils, whenever we feel moisture, it is usually sufficient to supply water to the plant. Why does this happen? This often leads producers, especially those with both light and heavy soils, to make frequent mistakes in management.

The Answer Lies in the Properties of the Soil

Among many others, each soil type has the following characteristics:

• Field Capacity (FC): Measured in %, this is the amount of water the soil can hold after drainage has ceased. It varies by soil type. In short, it's the maximum amount of water the soil can retain without creating standing water.
• Permanent Wilting Point (PWP): Measured in %, this determines the minimum moisture level needed for plants to survive, varying by soil type.
• Total Available Water (TAW): This is the amount of water available to plants between field capacity (FC) and the permanent wilting point (PWP). Only this water can be used by plants before they begin to suffer from lack of moisture. Managing TAW is critical for efficient water use and crop health.

How Do We Calculate TAW (Total Available Water)?

Let's look at two cases: one with heavy soil and one with sandy soil.

EXAMPLE 1: For Heavy / Clay Soil:

• Field Capacity (FC): 40%
• Permanent Wilting Point (PWP): 20%

The available water for plants is calculated as:

[TAW]=[FC]?[PWP]=>        

[TAW]=40%?20%=20%

Therefore, of the 40% moisture the clay soil can hold, only 20% is available for plant use, while the other 20% is retained by the soil and not released.

Why This is a Problem:

As the moisture gradually decreases day by day, it can eventually reach the 20% level in heavy soil, which equals the PWP. Even then, if we touch the soil, it feels moist, but this moisture isn't available to the plants, putting them in a stressed condition.

EXAMPLE 2: For Light / Sandy Soil:

• Field Capacity (FC): 10%
• Permanent Wilting Point (PWP): 4%

The available water for plants is calculated as:

[TAW]=[FC]?[PWP]=>        

[TAW10%?4%=6%

Therefore, of the 10% moisture the sandy soil can hold, 6% is available for plant use.

Perception of Moisture in Sandy Soil:

In sandy soils, it's hard to mistake the available water content by hand feel. As the moisture level drops towards 4%, we stop feeling significant moisture in the soil, which provides a safety buffer, unlike clay soil where plants are at risk while the soil still retains 20% of its moisture.

Practical Implications for Irrigation:

• In sandy soils, frequent but small irrigation events are needed to replenish the 10%, avoiding heavy irrigation due to high runoff and nutrient leaching.
• In clay soils, despite having 20% moisture, it may not be enough to irrigate the plants. Therefore, irrigation in heavy soils should not just meet the field capacity but exceed it to create runoff, leaching out potentially toxic accumulated salts and nutrients.

Recommendation for the difficult Heavy Soils:

We recommend that irrigation in heavy soils is better to be equal to the field capacity plus an additional percentage of water, equivalent to the permanent wilting point, also taking into account the evapotranspiration model, the Management Allowable Depletion (MAD) value, and for outdoor crops, any excess moisture due to rainfall. Additionally, this approach considers that in heavy soils, the 20% unavailable water might hold excessive sodium and fertilizer salts that need to be removed. This strategy ensures that irrigation addresses the soil's capacity to retain water and compensates / cleans the tightly held water that is not accessible to plants. It also accounts for environmental factors such as evaporation and rainfall. Importantly, in heavy soils, it helps flush out any accumulated sodium and fertilizer salts from the 20% unavailable water, which can be detrimental to plant health. Of course you must validate this approach taking all the safety measures.

Bibliography:

1. USDA Natural Resources Conservation Service (NRCS): Provides detailed information on soil moisture properties and plant-water relationships.
2. FAO Irrigation and Drainage Paper 56: Offers guidelines for computing crop water requirements and understanding soil moisture dynamics.
3. Soil Science Society of America (SSSA): Offers educational resources on soil properties and water management.

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