The question of whether plant-based crop monitoring methods, like the crop water stress index (CWSI), can replace soil moisture sensors is frequently debated. The short answer is: No.
Why monitor plant water status using a plant-based method?
Common reasons include detecting water stress for irrigation scheduling, perceived shortcomings of soil moisture sensors, or simply following established research. Whatever your reason, it's important to understand the limitations.
Plant-based methods, including those relying on canopy surface temperature (used in CWSI calculations) or dendrometers (and similar tools), are insufficient on their own.
Both biotic and abiotic stresses influence plant water status.
Biotic stresses originate from living organisms (e.g., pests, pathogens, weeds), while abiotic stresses are caused by non-living environmental factors (e.g., extreme temperatures, drought, salinity, nutrient deficiencies). This means a simple canopy feedback, like temperature or trunk diameter change, doesn't tell the whole story.
Consider these scenarios:
𝗦𝗰𝗼𝗿𝗰𝗵𝗶𝗻𝗴 𝗵𝗲𝗮𝘁: The plant closes its stomata to conserve water, resulting in elevated canopy temperature regardless of soil moisture.
𝗗𝗶𝘀𝗲𝗮𝘀𝗲/𝗽𝗲𝘀𝘁 𝗶𝗻𝗳𝗲𝘀𝘁𝗮𝘁𝗶𝗼𝗻: The plant's health is compromised, leading to increased canopy temperature even with sufficient soil water.
These examples highlight why relying solely on plant feedback is inadequate. So, what's a better approach?
If you're calculating the empirical CWSI it means you’re incorporating vapor pressure deficit (VPD) that includes two important microclimate parameters of air temperature and relative humidity into your index. For a detailed discussion of VPD and CWSI, see my previous article: "Decoding VPD and CWSI for Optimized Crop Water Management."
If you’re relying on the theoretical CWSI (which I'll cover in detail in a future post), you’ll be using all microclimate parameters. But even combining canopy temperature with weather data isn't sufficient.
Similarly, relying solely on soil moisture readings can be misleading. While soil moisture is undoubtedly crucial, it doesn't account for all stressors. Plants can experience stress even with adequate soil moisture due to factors like salinity, disease, or extreme heat.
A wilting plant, despite good soil moisture levels, might indicate a root problem, pests, or simply hot, dry air. This highlights the necessity of integrating soil moisture data with plant-based indicators, such as canopy temperature, and weather information for a comprehensive understanding of crop health.
Soil water status, monitored by a soil moisture or (preferably) soil water potential sensor, is crucial. Soil monitoring acts as a valuable check, especially when biotic stresses begin to impact plants.
A robust crop monitoring strategy integrates plant canopy feedback, soil water status, and microclimate measurements. This comprehensive approach provides a more accurate picture of plant health and water stress, enabling informed management decisions.
Happy Farming!
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