Getting Started with Soil Moisture Monitoring: What You Really Need to Know

What is Soil Moisture?

Soil moisture refers to the amount of water stored between soil particles. This water is essential for plant growth, root development, and nutrient transport. However, not all water in the soil is available to plants.

Soil Texture and Water Holding Capacity

Soil is made up of different particle sizes: sand, silt, and clay. These textures influence how much water the soil can hold and how easily that water can be used by plants.

• Sand drains quickly and has low water retention.
  
• Silt hold more water and retain it moderately well.
  
• Clay soils have high water-holding capacity but release it slowly.

Texture determines both the total amount of water in the soil and how accessible that water is to plants.

Key Terms:

• Field Capacity (FC): is the amount of water remaining in the soil after excess water has drained away due to gravity. It is an important threshold because it indicates how water will move within the soil. When soil moisture is above field capacity, water continues to drain downward, potentially recharging the groundwater, or it may lead to surface runoff and erosion. When soil moisture is below field capacity, the water is held in place by capillary forces between soil particles. In this condition, water typically moves upward due to evaporation or plant uptake through evapotranspiration.
  
  
  
• Wilting Point (WP): The point at which plants can no longer extract water. Below this, crops will experience permanent wilting. (Wilting point = early signs of water stress, still reversible)

• Permanent Wilting Point (PWP): Refers to the level of soil moisture at which plants can no longer extract sufficient water to meet their physiological needs. At this point, the remaining water in the soil is held too tightly by soil particles for plant roots to access it.
  Event if the plant is placed in a humid environment or watered after reaching this stage, it will not recover. This will lead to permanent wilting and often death of the plant.
  This threshold varies depending on soil texture.
  (Permanent wilting point = critical moisture limit, no recovery possible)
  
  
  
• Available Water Capacity (AWC): The difference between field capacity and wilting point. This is the water that plants can actually use.

Types of Water in the Soil

• Gravitational Water: Drains freely and is only briefly held in the soil after rain or irrigation. Not available to plants.
  
• Plant-Available Water: The portion of water held in the soil that plants can access.
  
• Hygroscopic Water: Tightly bound to soil particles and unavailable to plants.

Effects of Salinity and Temperature

High salinity can reduce the plant’s ability to draw water from the soil, even if moisture levels appear adequate. Soil temperature also influences root activity and water uptake. Cold soils slow down root function and may affect sensor accuracy.

Most common soil moisture sensors (for example TEROS10) measure Volumetric Water Content (VWC). VWC indicates the percentage of the soil volume that is occupied by water.

For example, a reading of 30 percent means that 30 percent of the total soil volume is water.

This is a direct measurement of how much water is physically present, but it does not indicate how tightly that water is held. Understanding this difference is important when deciding when to irrigate.

Standard sensors that measure only VWC do not provide information about:

• Soil tension or suction (how hard roots must work to extract water)
  
• Soil salinity or electrical conductivity
  
• Soil temperature
  

These additional values often require separate or more advanced sensors, especially in systems where salinity or temperature fluctuations are common. For example SDI-12 Sensors

In addition to VWC sensors, there are other types of sensors that can provide complementary data:

• Tensiometers and Matric Potential Sensors: Measure the tension or force required for plants to extract water. Useful in fine-tuning irrigation triggers, especially in clay soils.
  
• Salinity Sensors (EC sensors): Measure electrical conductivity to monitor salt levels in the root zone. Useful in fertigation systems or salt-sensitive crops.
  
• Temperature Probes: Provide soil temperature, which influences plant metabolism and root activity.
  

These sensors can often be combined into one device using SDI-12 communication protocols, allowing for integrated irrigation monitoring.

Calculating Percentage of AWC Used

To use VWC for irrigation decisions, you first need to know your soil’s field capacity and wilting point. The difference between these two values is your available water capacity.

Example for a silt loam soil:

• Field Capacity = 35 percent
• Wilting Point = 18 percent
• AWC = 17 percent
  

If your sensor reads 25 percent VWC:

• Depletion = (35 - 25) ÷ 17 = 59 percent AWC used
  

This value helps determine whether irrigation is needed. Many growers choose to irrigate when 50 to 70 percent of the available water has been used.

Setting Irrigation Thresholds

To set effective irrigation thresholds, identify the VWC values at field capacity and wilting point for your soil type. Then choose a depletion level that aligns with crop needs and irrigation frequency.

For example, in the same silt loam:

• Trigger point at 60 percent depletion = 35 - (0.6 × 17) = 24.8 percent VWC
  

This becomes your action point for scheduling irrigation.

Values vary depending on local conditions and management. Lab analysis or soil survey data can provide more precise figures for your site.

Explore New Zealand Soils

You can access detailed soil information through the interactive Soils Map Viewer by Manaaki Whenua – Landcare Research:

https://soils-maps.landcareresearch.co.nz/

This tool allows you to explore soil types, classifications, and properties across New Zealand. It’s especially useful for landowners, researchers, and environmental planners.