Water is a critical resource for agricultural production, and its availability and management can significantly impact crop yields, quality, and profitability. However, agriculture is also the largest consumer of freshwater resources, accounting for about 70% of global water withdrawals, and is facing increasing pressures from climate change, population growth, and competing demands from other sectors. To address these challenges and to ensure the sustainable and efficient use of water in agriculture, farmers and researchers are increasingly turning to precision tools and technologies, such as soil sensors and moisture monitoring systems.
Soil sensors and moisture monitoring systems are devices and techniques that enable the real-time and continuous measurement of soil moisture content and other related parameters, such as temperature, electrical conductivity, and water potential, at various depths and locations in the soil profile.
These tools can provide valuable information and insights for optimizing irrigation scheduling, improving water use efficiency, reducing water losses and environmental impacts, and enhancing crop growth and yield.
Principles and Types of Soil Moisture Sensors
Soil moisture sensors are devices that can measure the amount of water present in the soil, either directly or indirectly, based on various physical and electrical properties of the soil-water system. There are several types of soil moisture sensors, which differ in their measuring principles, accuracy, resolution, range, and cost. Some of the most common types of soil moisture sensors used in agriculture are:
Tensiometers
Tensiometers are one of the oldest and most widely used types of soil moisture sensors, which measure the soil water potential or tension, which is the force that holds water in the soil pores and that plants need to overcome to extract water from the soil. Tensiometers consist of a porous ceramic cup filled with water, connected to a vacuum gauge or pressure transducer via a water-filled tube.
When the ceramic cup is inserted into the soil, it establishes a hydraulic connection with the soil water, and the water in the cup comes into equilibrium with the soil water potential. As the soil dries out, the water in the cup is pulled out by the suction of the soil, creating a vacuum in the tube, which is measured by the gauge or transducer. The reading of the tensiometer, expressed in centibars or kilopascals, indicates the soil water potential, which can be related to the soil moisture content using a soil water characteristic curve.
The advantages of tensiometers are their simplicity, durability, and direct measurement of soil water potential, which is a more relevant parameter for plant water uptake than soil moisture content. However, tensiometers also have some limitations, such as their limited range of measurement (0 to -80 kPa), their sensitivity to temperature fluctuations and air bubbles in the water column, and their need for regular maintenance and refilling.
Electrical Resistance Sensors
Electrical resistance sensors, also known as gypsum blocks or granular matrix sensors, measure the electrical resistance of a porous material, usually gypsum or a similar matrix, that is in equilibrium with the soil water. The porous material acts as an electrical resistor, whose resistance changes with its water content, which in turn depends on the soil water potential.
The sensor consists of two electrodes embedded in the porous material, which are connected to a meter that applies a small alternating current and measures the voltage drop across the resistor. The resistance of the sensor, expressed in ohms, can be related to the soil water potential using a calibration curve.
The advantages of electrical resistance sensors are their low cost, simplicity, and ability to measure a wide range of soil water potentials (-10 to -1500 kPa). However, they also have some drawbacks, such as their sensitivity to soil salinity and temperature, their slow response time, and their need for site-specific calibration.
Capacitance Sensors
Capacitance sensors, also known as dielectric sensors or frequency domain reflectometry (FDR) sensors, measure the dielectric permittivity of the soil, which is a measure of its ability to store electrical charge, and which is strongly related to its water content. The sensor consists of two or more electrodes that form a capacitor, with the soil acting as the dielectric medium between the electrodes.
The sensor applies a high-frequency alternating current to the electrodes and measures the capacitance of the soil, which is proportional to its dielectric permittivity. The capacitance of the sensor, expressed in picofarads, can be related to the soil moisture content using a calibration equation that accounts for the soil type, density, and temperature.
The advantages of capacitance sensors are their high accuracy, precision, and resolution, their ability to measure a wide range of soil moisture contents (0 to 100%), and their fast response time. However, they also have some limitations, such as their sensitivity to soil texture, density, and salinity, their limited measurement volume, and their higher cost compared to other types of sensors.
Time Domain Reflectometry (TDR) Sensors
Time domain reflectometry (TDR) sensors are another type of dielectric sensor that measures the propagation velocity of an electromagnetic wave along a waveguide or probe inserted into the soil. The propagation velocity of the wave depends on the dielectric permittivity of the soil, which is related to its water content.
The sensor consists of a pulse generator that sends a fast-rise-time electromagnetic pulse along the waveguide, and a receiver that detects the reflected pulse and measures the travel time of the pulse along the waveguide. The travel time of the pulse, expressed in nanoseconds, can be related to the soil moisture content using a calibration equation that accounts for the soil type, density, and temperature.
The advantages of TDR sensors are their high accuracy, precision, and repeatability, their ability to measure a wide range of soil moisture contents (0 to 100%), their fast response time, and their insensitivity to soil salinity and temperature. However, they also have some drawbacks, such as their higher cost, complexity, and power requirements compared to other types of sensors, and their limited measurement volume and spatial resolution.
Applications and Benefits of Soil Moisture Monitoring in Agriculture
Soil moisture monitoring has numerous applications and benefits in agriculture, across different scales, regions, and cropping systems. Some of the main applications and benefits of soil moisture monitoring are:
Irrigation Scheduling
Irrigation scheduling is one of the most important and widespread applications of soil moisture monitoring in agriculture, which involves determining the timing and amount of irrigation based on the soil moisture status and the crop water requirements.
Soil moisture sensors can provide real-time and continuous information on the soil water content and potential, which can be used to optimize irrigation decisions and to avoid over- or under-irrigation, which can lead to water stress, nutrient leaching, and yield losses.
By using soil moisture sensors for irrigation scheduling, farmers can:
- Improve water use efficiency: Soil moisture sensors can help farmers to apply the right amount of water at the right time, based on the actual soil moisture conditions and the crop water needs, rather than on fixed schedules or visual observations. This can reduce water losses through evaporation, runoff, and deep percolation, and increase the proportion of applied water that is used by the crop for growth and yield.
- Reduce energy and labor costs: Soil moisture sensors can help farmers automate and optimize irrigation systems, such as drip or sprinkler systems, by triggering or stopping irrigation based on pre-set soil moisture thresholds or algorithms. This can reduce the energy and labor costs associated with manual or timer-based irrigation, and allow farmers to irrigate more frequently and precisely, with smaller amounts of water.
- Enhance crop quality and yield: Soil moisture sensors can help farmers maintain the soil moisture within the optimal range for crop growth and development, which can vary depending on the crop type, growth stage, and environmental conditions. This can prevent water stress, which can reduce photosynthesis, nutrient uptake, and biomass accumulation, and lead to yield losses and quality defects, such as fruit cracking, blossom end rot, or tip burn.
For example, a study by the University of Georgia found that using soil moisture sensors for irrigation scheduling in cotton could reduce water use by 50-80%, while maintaining or increasing yield, compared to traditional irrigation methods based on evapotranspiration estimates or soil feel.
Another study by the University of California found that using capacitance sensors for irrigation scheduling in tomatoes could increase water use efficiency by 30-40%, and reduce nitrate leaching by 70-80%, compared to grower standard practices.
Nutrient Management
Nutrient management is another important application of soil moisture monitoring in agriculture, which involves optimizing the timing, placement, and amount of fertilizer application based on the soil moisture and nutrient status, and the crop nutrient requirements.
Soil moisture sensors can provide information on the soil water content and flow, which can affect the availability, transport, and uptake of nutrients by the crop, as well as the risk of nutrient losses through leaching, runoff, or volatilization.
By using soil moisture sensors for nutrient management, farmers can:
- Improve nutrient use efficiency: Soil moisture sensors can help farmers synchronize fertilizer application with crop nutrient uptake, by ensuring that the soil moisture is adequate for nutrient solubilization and transport to the roots, but not excessive for nutrient leaching or denitrification. This can increase the proportion of applied nutrients that are taken up by the crop, and reduce the amount of fertilizer needed to achieve the desired yield and quality.
- Reduce environmental impacts: Soil moisture sensors can help farmers to minimize nutrient losses from the root zone, by avoiding fertilizer application during periods of high soil moisture, such as after heavy rainfall or irrigation, or in areas of the field with poor drainage or high leaching potential. This can reduce the risk of groundwater contamination, eutrophication of surface waters, and greenhouse gas emissions, such as nitrous oxide, which are major environmental concerns associated with agricultural nutrient management.
- Enhance crop nutritional quality: Soil moisture sensors can help farmers optimize nutrient uptake and translocation to the harvested parts of the crop, by maintaining the soil moisture and nutrient status within the optimal range for each growth stage and plant organ. This can increase the concentration and bioavailability of essential nutrients, such as vitamins, minerals, and antioxidants, in the edible portions of the crop, and enhance its nutritional quality and value.
For example, a study by the University of Florida found that using capacitance sensors for irrigation and fertilization scheduling in strawberries could reduce nitrogen leaching by 30-50%, and increase fruit yield and quality, compared to traditional irrigation and fertilization methods based on grower experience or general recommendations.
Another study by the University of Nebraska found that using TDR sensors for fertigation management in corn could increase nitrogen use efficiency by 20-30%, and reduce residual soil nitrate by 50-70%, compared to uniform fertigation or split application methods.
Crop Water Stress Detection
Crop water stress detection is another emerging application of soil moisture monitoring in agriculture, which involves using soil moisture sensors, along with other plant and environmental sensors, to detect and quantify the degree of water stress experienced by the crop, and to guide precision irrigation and crop management decisions.
Crop water stress can occur when the soil moisture is insufficient to meet the crop water demand, due to factors such as drought, heat waves, salinity, or root diseases, and can lead to reductions in leaf water potential, stomatal conductance, photosynthesis, and growth.
By using soil moisture sensors for crop water stress detection, farmers can:
- Identify and map water stress zones: Soil moisture sensors, especially those with high spatial and temporal resolution, such as wireless sensor networks or mobile platforms, can help farmers to identify and map the areas of the field or orchard that are experiencing water stress, based on the soil moisture distribution and variability. This can allow farmers to target irrigation, fertilization, or other interventions to the zones that need them most, and to avoid over- or under-applying resources to the rest of the field.
- Quantify and monitor water stress levels: Soil moisture sensors, along with plant sensors that measure leaf water potential, sap flow, or canopy temperature, can help farmers quantify and monitor the degree and duration of water stress experienced by the crop, and relate it to the soil moisture status and the environmental conditions. This can allow farmers to determine the critical soil moisture thresholds or plant water stress indices that trigger irrigation or other management actions, and to optimize the timing and amount of water application based on the crop water stress levels and the irrigation system capacity.
- Assess and mitigate the impacts of water stress: Soil moisture sensors, along with yield and quality sensors, can help farmers assess and mitigate the impacts of water stress on crop growth, development, and productivity, and make informed decisions on harvest timing, postharvest handling, and marketing. This can allow farmers to minimize the yield and quality losses associated with water stress, and to enhance the resilience and profitability of their cropping systems under water-limited conditions.
For example, a study by the University of Adelaide found that using soil moisture sensors and thermal infrared sensors for precision irrigation in grapevine could reduce the variability in leaf water potential and berry sugar content, and increase the water use efficiency and wine quality, compared to uniform irrigation based on evapotranspiration estimates or soil water balance.
Another study by the University of Sao Paulo found that using soil moisture sensors and sap flow sensors for irrigation scheduling in sugarcane could increase the stalk yield and sugar content, and reduce the water use and energy costs, compared to fixed irrigation intervals or crop coefficients.
Challenges and Opportunities for Soil Moisture Monitoring in Agriculture
Despite the numerous applications and benefits of soil moisture monitoring in agriculture, several challenges and limitations need to be addressed for its effective and widespread adoption and use.
Some of the main challenges and opportunities for soil moisture monitoring in agriculture are:
Technical and Operational Challenges
- Sensor accuracy and reliability: Soil moisture sensors can vary in their accuracy, precision, and reliability, depending on their measuring principle, calibration, installation, and maintenance. Some sensors, such as tensiometers or gypsum blocks, can be sensitive to temperature, salinity, or air gaps, and require frequent maintenance and recalibration. Other sensors, such as capacitance or TDR sensors, can be affected by soil texture, density, or organic matter, and require site-specific calibration and correction factors. The lack of standardization and interoperability of soil moisture sensors can also make it difficult to compare and integrate data from different sensors or networks.
- Sensor placement and representativeness: Soil moisture sensors provide point measurements of soil moisture, which can vary spatially and temporally, depending on the soil type, topography, vegetation, and climate. The placement and number of sensors needed to represent the soil moisture status of a field or orchard can be a challenge, especially in heterogeneous or large-scale systems. The optimal sensor placement and density can depend on the spatial variability of soil properties, the irrigation system design, and the management objectives, and may require a trade-off between the cost and the resolution of the monitoring network.
- Data management and interpretation: Soil moisture sensors can generate large amounts of data, especially if they are deployed in high-density networks or connected to wireless or cloud-based platforms. The storage, processing, and visualization of soil moisture data can be a challenge, especially for farmers or advisors who may lack the technical skills or resources to handle big data. The interpretation and use of soil moisture data for decision-making can also be a challenge, as it requires an understanding of the soil-plant-water relationships, the crop water requirements, and the irrigation system performance, and may involve the integration of soil moisture data with other data sources, such as weather, crop, or economic data.
Socioeconomic and Institutional Challenges
- Cost and affordability: Soil moisture sensors and monitoring systems can vary in their cost and affordability, depending on their type, accuracy, durability, and connectivity. The initial investment and maintenance costs of soil moisture sensors can be a barrier for small-scale or resource-poor farmers, who may not have access to credit, subsidies, or technical support. The cost-benefit analysis of soil moisture monitoring can also be a challenge, as the economic returns may depend on the crop value, the water price, the energy cost, and the environmental regulations, and may not be immediately apparent or measurable.
- Knowledge and capacity: Soil moisture monitoring requires a certain level of knowledge and capacity, both for the installation and maintenance of the sensors and for the interpretation and use of the data for decision-making. Many farmers, especially in developing countries or remote areas, may lack the awareness, education, or training on soil moisture monitoring and its benefits, and may rely on traditional or intuitive methods for irrigation scheduling. The lack of extension, advisory, or support services for soil moisture monitoring can also limit its adoption and impact, especially for farmers who may not have access to reliable or affordable sources of information and technology.
- Policy and institutional support: Soil moisture monitoring can be influenced by the policy and institutional environment, which can create incentives or barriers to its adoption and use. For example, water pricing, allocation, or conservation policies can affect the motivation and ability of farmers to invest in soil moisture monitoring and precision irrigation. Agricultural subsidies, insurance, or credit programs can also affect the adoption and profitability of soil moisture monitoring, depending on their design and targeting. The lack of coordination, regulation, or standardization of soil moisture monitoring systems and data can also limit their compatibility, scalability, and impact, especially across different regions, crops, or stakeholders.
Opportunities and Way Forward
Despite these challenges, there are also several opportunities and ways forward for soil moisture monitoring in agriculture, which can leverage its potential and overcome its limitations.
Some of the main opportunities and recommendations for soil moisture monitoring in agriculture are:
- Sensor and data integration: The integration of soil moisture sensors with other sensors, such as plant, weather, or remote sensing sensors, can provide a more comprehensive and accurate picture of the soil-plant-water system, and improve the precision and reliability of irrigation and crop management decisions. The integration of soil moisture data with other data sources, such as crop models, irrigation schedulers, or decision support tools, can also facilitate the interpretation and use of soil moisture information for different purposes and users. The development of standard protocols, APIs, or platforms for sensor and data integration can also enhance the interoperability, scalability, and value of soil moisture monitoring systems.
- Participatory and adaptive approaches: The adoption and impact of soil moisture monitoring can be enhanced by participatory and adaptive approaches, which involve the active engagement and feedback of farmers, advisors, and researchers in the design, testing, and improvement of soil moisture monitoring systems and practices. Participatory approaches, such as on-farm demonstrations, farmer field schools, or innovation platforms, can help to tailor soil moisture monitoring to the local needs, preferences, and contexts, and to build the trust, ownership, and capacity of the users. Adaptive approaches, such as iterative learning, flexible management, or multi-criteria assessment, can help to adjust soil moisture monitoring to the changing conditions, goals, and performance, and to optimize its benefits and minimize its risks.
- Business models and services: The development of innovative business models and services for soil moisture monitoring can help to overcome the cost and access barriers, and to create value and incentives for the users and providers. For example, soil moisture monitoring can be offered as a subscription-based or pay-per-use service, which can reduce the upfront costs and risks for farmers, and provide a recurring revenue stream for the providers. Soil moisture monitoring can also be bundled with other products or services, such as irrigation equipment, fertilizers, or crop insurance, which can create synergies and economies of scale. Soil moisture monitoring can also be integrated with carbon farming or water trading schemes, which can monetize the environmental benefits and externalities of precision irrigation and water conservation.
- Policy and institutional reforms: The enabling policy and institutional environment for soil moisture monitoring can be strengthened by reforms and investments that support its development, adoption, and impact. For example, water pricing, allocation, or conservation policies can be designed to incentivize the efficient and sustainable use of water and to reward the adoption of soil moisture monitoring and precision irrigation. Agricultural subsidies, insurance, or credit programs can be targeted to the farmers or regions that have the highest potential or need for soil moisture monitoring and can be linked to the performance or outcomes of the monitoring systems. The standardization, regulation, or certification of soil moisture monitoring systems and data can also enhance their quality, reliability, and compatibility, and facilitate their use and exchange across different stakeholders and scales.
Conclusion
Soil moisture monitoring is a critical tool for sustainable water management in agriculture, which can help farmers optimize irrigation scheduling, improve water and nutrient use efficiency, reduce environmental impacts, and enhance crop productivity and quality. Soil moisture sensors, such as tensiometers, electrical resistance sensors, capacitance sensors, and TDR sensors, can provide real-time and continuous information on the soil moisture status and dynamics, which can inform precision irrigation and crop management decisions.
Soil moisture monitoring has numerous applications and benefits in agriculture, such as irrigation scheduling, nutrient management, and crop water stress detection, which can vary depending on the crop, climate, soil, and management factors.
However, soil moisture monitoring also faces several challenges and limitations, such as the accuracy and reliability of the sensors, the placement and representativeness of the measurements, the management and interpretation of the data, the cost and affordability of the systems, the knowledge and capacity of the users, and the policy and institutional support.
To overcome these challenges and realize the full potential of soil moisture monitoring in agriculture, there is a need for sensor and data integration, participatory and adaptive approaches, business models and services, and policy and institutional reforms, which can leverage the opportunities and innovations in technology, science, and stakeholder engagement.
By advancing and scaling soil moisture monitoring in agriculture, we can contribute to the sustainable intensification and resilience of food systems, and to the achievement of the Sustainable Development Goals, such as ending hunger, achieving food security and improved nutrition, promoting sustainable agriculture, and ensuring availability and sustainable management of water and sanitation for all.
Soil moisture monitoring is not a silver bullet or a one-size-fits-all solution for sustainable water management in agriculture, but it is a valuable and versatile tool that can complement and enhance other water management strategies and practices, such as drought-tolerant crops, efficient irrigation systems, water harvesting and storage, and water governance and institutions.
The successful adoption and impact of soil moisture monitoring in agriculture require a holistic and integrated approach that considers the technical, economic, social, and environmental dimensions of water management, and that engages and empowers the diverse stakeholders and beneficiaries, from farmers to researchers to policymakers.
As we face the growing challenges of water scarcity, food insecurity, and climate change, soil moisture monitoring offers a promising and transformative pathway for sustainable water management in agriculture, which can help us to produce more food with less water, to conserve and restore the water resources and ecosystems, and to adapt and thrive in a changing and uncertain world. By embracing the potential and the responsibility of soil moisture monitoring in agriculture, we can create a more water-wise and resilient future for all.