Monitoring Soil Moisture for Optimal Crop Growth

Larry Pitts

Created: ; updated: .

A primer on using the data provided by soil moisture probes to more effectively manage your farm, this article lays out both the theory and practice of soil moisture based decision making. Observing and understanding the consequences of soil moisture fluctuations in your crops’ root zone will enhance your efforts to maximize yield and reduce wasted resources.

This article will discuss the following:

Introduction

Soil moisture and its availability to support plant growth is a primary factor in farm productivity. Too little moisture can result in yield loss and plant death. Too much causes root disease and wasted water.

Just as important, water is a delivery mechanism for any nutrients that are not tightly bound to the soil. Whether these nutrients are delivered to the field through the irrigation system or through other means, movement of water within the soil governs how they are delivered to the plant roots. Good water management is important within itself, but good water management also means good nutrient management.

Precise control over the root zone environment, in terms of both water and nutrient content,  leads to healthier crops and higher yields. The techniques described in this publication can help you gain control over the root zone environment by making measurements, observing trends over time, and using this information to make irrigation decisions. If applied correctly, the results can be significantly increased productivity and reduced cost.

Soil Moisture Basics

Key Points

  • Water is held in a soil mixture by action of surface tension attracting water molecules to soil particles.
  • The amount of water that can be stored by a soil and its availability to plants both depend on soil type.
  • Volumetric Water Content (VWC) is a measure of the amount of water held in a soil expressed as a percentage of the total mixture.
  • Tension is a measure of the amount of water held in a soil expressed as the amount of work required (for plants) to remove water from the soil.
  • The relationship between VWC and Tension depends on soil type.
  • Field Capacity is a soil water content that results in a state of balance between gravity force and surface tension force. At Field Capacity soil has a balance of air and water that results in good growing conditions.

Water Holding Capacity

Among other things, soil is a storage medium for farm water until it is used by plants. Water resides in the spaces between soil particles. The force of gravity constantly acts on water in the soil to move it downward and out of reach of plants. The counterbalancing force which keeps it from moving downward is surface tension, which causes the water to 'stick' to soil particles. The smaller the soil particles are, the more combined surface area they have, and the more they are able to hold onto water through its surface tension. Therefore, the ability of water to move through soil and be stored in soil depends heavily on soil type.

When water enters a soil with large sandy particles, only a small amount stays attached to the particles, and the remainder quickly drains downward. Sand has low 'water holding capacity.' Conversely, a volume of clay soil has huge numbers of small particles with large surface area. When water enters a clay soil, surface tension holds it tightly to the soil particles and only a small remainder drains downward. Clay has a high water holding capacity. A soil with a high water holding capacity can store large amounts of water relative to its own volume after a rain event and, under the right conditions, this stored water can remain available for plants to use.

In a soil with very small particles, the same surface tension forces that allow for a large water holding capacity also make it difficult for plants to extract and use the water. Water does not move easily through a fine-particle soil and requires a large amount of energy for plants to extract and use it. The force a plant must exert on water to separate it from soil particles and move it into the root system is referred to as 'tension'. In most on-farm applications tension is measured in centibars (1/100 bar) as a negative pressure or vacuum (plants 'suck' the water out of the soil to use it). Even if a soil mixture contains water, if the tension required to extract the water is greater than the plants can overcome, they will die.

Sandy soil has low water holding capacity due to its large particles. Both water and nutrients can easily drain out of reach of plants. However, while a sandy soil does not hold much water, what it holds is easily available to plants. A clay soil has large water holding capacity, but because it holds onto water tightly, tension is relatively high for a given amount of water. A certain amount of water in clay soil is not easily available to plants. And just as a clay soil tightly binds to water, it can also keep nutrients out of reach of plants. The ideal soil for most growing conditions is a loamy soil with a variety of particle sizes and ample structure which can hold a large amount of water that is easy for plants to withdraw.

The interaction between soil type, water holding capacity and water availability is illustrated in the Soil Texture Triangle shown in Figure 1. It is important to know what soil types are present in your field. Since any point in your field can contain different soil types at different depths, it can be useful to use core samples to determine soil type vs. depth profiles at key locations.

 

Figure 1.
Soil Texture Triangle

Water Content, Tension and Field Capacity

The amount of water held by a soil is referred to as its 'Volumetric Water Content' ('VWC') and is expressed as a percentage or a ratio (inches of water per inch of soil). For example, one cubic foot of soil with 30% Volumetric Water Content contains 0.3 cubic feet, or 2.25 gallons, of water. Because a soil with small particles holds on to water more tightly than a sandy soil, the same Volumetric Water Content means different levels of tension depending on soil type.

 

Figure 2.
Volumetric Water Content

There are three water content (or tension) levels that are important when planning irrigation: 'Saturation,' 'Field Capacity' and 'Permanent Wilting Point.' These can be understood by examining how water moves through the soil after a watering event.

Saturation

When water enters a soil volume more quickly than it is moved downward by gravity, it becomes saturated. Saturation is formally defined as the condition where all soil pores/voids are filled with water. Saturated soil is heavy, contains little air, and can be thought of as mud. Conditions in a saturated soil are anaerobic and are not conducive to healthy plant growth. Tension in saturated soil is very low, generally less than -10 centibar. VWC at Saturation can range between 15% and 60% depending on soil type.

Field Capacity

With time (the amount of time depends on soil type), substantially all of the water that will drain due to gravity has drained. The soil solution is now in balance, containing all of the water that can be held by surface tension. This condition is referred to as Field Capacity. At Field Capacity, water is easily available to plants, and the soil solution contains ample oxygen. Optimal growing conditions for most plants occur at Field Capacity or slightly drier than Field Capacity. Tension at Field Capacity is between -10 and -20 centibar. VWC can range from 10% to 50% depending on soil type.

Management Allowable Depletion

'Management Allowable Depletion' (MAD) is the lowest moisture level which can be sustained by plants without adverse stress effects. This is the moisture point at which irrigation should be initiated to avoid having stress affect plant growth. Tension at MAD is typically -50 to -70 centibar. VWC at this point can range from 5% to 40%. Any moisture content below this level is in the 'Stress' zones.

Permanent Wilting Point

As soil is subject to evaporation and withdrawals from plants, water content decreases and tension increases to a point where plants can no longer extract water. Maintaining soil at this level for any length of time can cause permanent damage to plants. Tension at PWP can be as great as -15 bar (-1,500 centibar). VWC ranges from 2% for sandy soils to 30% for high clay-content soils.



Figure 3. 
Soil Moisture Terms

Root Zone Moisture

In practice, the soil-water relationship within the root zone of a crop is complicated and continually changing. A root system can extend well below the soil surface through several soil horizons. Each horizon can have a different soil type or structure, so water holding capacity and the relationship between VWC and tension can vary throughout the root zone. Furthermore, since the root system grows throughout the season, the set of soil conditions it is subject to continually changes. This presents challenges when using root zone soil moisture information for making decisions. To draw meaningful conclusions, reliable measuring equipment is needed along with the ability to take remote readings and process large amounts of data.


Soil Moisture Measurement

Key Points

  • The most accurate method of soil moisture measurement is through weight ('gravimetric') measurements. While practical in a lab environment, gravimetric methods are too time consuming for farm water management.
  • Commercial Soil Moisture measurement devices can be classified as those that measure Tension and those that measure VWC.
  • Tension sensors include Tensiometers and Gypsum Block sensors.
  • VWC sensors include Neutron Probes and Dielectric Probes.
  • Most VWC sensors measure soil dielectric properties. To obtain actual VWC, these measurements must be scaled by a calibration curve that depends on soil type.
  • It is usually desirable to measure at several points in the root zone profile. Some moisture probes accommodate this by providing an array of sensors, in a single probe, positioned at different depths.

Measuring soil moisture has always played a role in successful farm management. For many years farmers have relied on the 'look and feel' of soil to evaluate moisture content. In fact, studies have shown that experienced farmers can identify certain moisture levels, such as Field Capacity, with a very high degree of accuracy simply by feeling soil and visually observing its characteristics. However, monitoring soil moisture on an ongoing basis at several positions within the root zone and systematically using this information to make irrigation decisions requires measurement devices, computers and networked communication equipment.

Types of Soil Moisture Sensors

Commercial soil moisture sensors fall into two categories: those that measure VWC and those that directly measure tension. Each has its advantages and disadvantages as summarized below. See the Help Desk Article, Selecting Soil Moisture Monitoring Devices, for additional information on selecting the right monitoring device for your application.

Tension Sensors

  • Directly measure the stress felt by plants as a result of water conditions
  • Do not tell how much water is in the soil volume so do not indicate the reserve available until the next irrigation event
  • Allow easy determination, relatively independent of soil type, of Saturation, Field Capacity and Stress
  • Rely on simple technology and tend to be less expensive than VWC sensors
  • Often are not 'automation-ready,' due to their simple construction

VWC Sensors

  • Directly measure water content (or a quantity related to water content), indicating the water reserves available to the plant.
  • Can be used to predict the next time irrigation should occur.
  • Must be calibrated, depending on soil type, to determine VWC corresponding to Field Capacity and Stress.
  • Tend to be higher in cost than tension devices
  • Are usually microprocessor controlled and, therefore, easily integrated with networked systems
  • Can have highly repeatable readings through a wide range of moisture levels
  • Usually use a calibration curve, depending on soil type, to directly report VWC — This introduces errors since soil type may not be consistent throughout the field or the root zone.

Due to their low maintenance requirements, easy interface to digital networks and, in some cases, ability to sample at multiple depths, VWC sensors are most commonly used for automated soil moisture monitoring. As a result, the remainder of this publication focuses on VWC sensors. A summary of common methods and devices used for soil VWC measurement follows.

Gravimetric Methods

Gravimetric measurement is direct, exact and is the 'gold standard' for all other measurement methods. To measure VWC gravimetrically, a sample is taken from the field and brought to a lab in an air-tight container. In the lab, the sample is weighed, baked in an oven long enough to remove all water through evaporation then weighed again. This directly measures the proportion of water that was in the original sample:

The only accuracy limitations of gravimetric measurements are the accuracy of the scale and the amount of time available for drying. However, gravimetric measurements are manual and time consuming and are not practical for everyday use in farming.

Neutron Probes

A neutron probe uses a radioactive isotope, which emits high energy neutrons into the soil. Neutron energy is absorbed in collisions with water molecules, and a reading of the proportion of lower energy neutrons returning to the sensor can be calibrated to give a very accurate, and salinity-independent, measurement of VWC. Neutron probes are typically very expensive and require licensing in the US by the Nuclear Regulatory Commission. As a result of these factors, their use has become less common in recent years as the reliability of capacitance (dielectric) based sensors has improved.

Capacitance Probes

Capacitance probes are devices which measure the dielectric constant of a volume of soil, either surrounding a probe or located between a probe's two or three conductive spikes (waveguides). Since soil dielectric properties are directly impacted by water content, the dielectric measurements can be calibrated to give an accurate indication of volumetric water content (VWC).

Time Domain Reflectometry Probe

'Time Domain Reflectometry' (TDR) probes measure the amplitude of a reflected electrical pulse, which gives an indication of soil impedance. Circuit models are then used to derive dielectric properties from the impedance measurements. Single-frequency capacitance probes and 'Frequency Domain Reflectometry' (FDR) probes measure soil dielectric properties by measuring the attenuation of a high frequency electrical signal.

Capacitance probes are repeatable and do not require a large amount of maintenance. Since soil dielectric properties are affected by salt content, their readings can be affected by salinity. However, many commercial capacitance probes independently measure EC and use this to compensate for the effect of salinity on VWC readings. A convenient byproduct of this is that many capacitance probes provide EC readings along with VWC readings. Capacitance probes are typically microprocessor controlled and, as a result, interface easily with automated monitoring systems through standard communication protocols.

Some commercial capacitance probes contain multiple VWC-measurement sensors in a single probe, at preset depths. This allows a single probe to measure VWC at several depths within the root zone. Commercial probes that measure VWC at multiple depths include the EnviroPro Soil Probe by EnviroTek Solutions, Inc. and the AquaCheck subsurface probe by AquaCheck, Inc. Single point capacitance probes include the Decagon GS3 and the Stephens Hydra Probe II.

Using Soil Moisture to Make Irrigation Decisions

Key Points

  • Each moisture probe should be located at a point that represents the area being irrigated.
  • For most crops, moisture should be measured at several locations throughout the depth of the root zone and averaged together into a single 'Root Zone Summary.'
  • Irrigation decisions can be made with raw data direct from moisture probes instead of calibrated VWC. This avoids errors that can be introduced through calibration curves that depend on soil type.
  • Plan irrigation by tracking soil moisture relative to preset 'Management Lines,' which define five root zone moisture regions: 'Very Full', 'Full', 'Optimal', 'Refill' and 'Stress'. The goal is to schedule irrigation to keep moisture in the Optimal region. 
  • Soil moisture level and its rate of change can be used to predict the time and duration of the next irrigation cycle.

Using soil moisture measurements to determine irrigation involves identifying the lowest and highest root zone moisture you wish to permit, then scheduling irrigation events to keep moisture levels between those values.

In this section we discuss how to put this concept into practice by selecting the right sensor locations, using profile measurements to evaluate average root zone moisture, correctly setting the high and low moisture points ('Management Lines') and maintaining optimum soil moisture throughout the season.

Sensor Location

Each farm can be thought of as a collection of logical 'Management Units', where each covers an area of uniform soil type, crop, climate and irrigation method. Examples of a Management Unit are a single block or zone in a drip irrigated field, a single center pivot circle, or a section in a large, uniform dryland farm. Each Management Unit should contain a moisture probe in the crop root zone at a location that represents the average properties (climate, soil type, etc.) of the entire unit.

Ideally, each irrigation zone can be classified as a Management Unit, allowing moisture levels to be independently measured and used to control each zone. This may not be practical in farms with smaller irrigation zones, since the resulting large number of sensors can become cost prohibitive. In these farms, irrigation decisions for several zones may rely on information from a single moisture probe. If farm areas vary dramatically in soil type or crop type, each zone should be designated as a Management Unit and monitored separately.

Sensor Depth

Constructing a clear picture of moisture conditions within the root zone requires measurements to be taken at several depths at each probe location. Multiple depth readings provide a good indication of average moisture throughout the root zone, and can also provide important information about moisture at specific depths. For example, a group of sensors within the root zone can be used to evaluate moisture available to the plant, while a deeper sensor below the root zone can measure the deep percolation of water (below its availability to the plant).

Multiple depth readings can be taken either by using a multi-depth soil moisture probe ('moisture profile probe') or several point-measurement probes. The shallowest sensor should be about 4” (10 cm) below the surface; the deepest should be at least 20% deeper than the bottom of the mature plant's root zone. Additional sensors should be placed at 4-6” (10-15 cm) increments within that range.

Root Zone Summary

For the purposes of irrigation, the moisture measurements at all depths within the root zone need to be combined into a single number, which represents average root zone water content. This may be done by:

  • Adding together the measured values from each sensor level
  • Averaging those measured values
  • Weighted-averaging the measured values

However obtained, this number becomes the 'Root Zone Summary'.

Root Zone Summary calculations should only include data from sensors within the root zone. In a weighted average calculation, set weights to zero for sensors below the root zone. Since the size of the root zone changes as the plant grows, it may be necessary to change a probe's weighting factors as the season progresses.

For a more accurate measurement of root zone conditions, use different weighting factors throughout the root zone. This deals more effectively with the multiple soil-types of some soil profiles. 

Take a core sample through the root zone, as deep as the moisture probe, and have a soil texture analysis performed for each sensor depth. Smaller weights can then be assigned to soils with low water availability (such as clay soils), since higher VWC does not necessarily mean more water available to plants. Conversely, larger weights can be assigned to sandier soils.  

Water Measurement Units

The range of numeric raw data obtained from each moisture sensor depends on sensor type and brand. For most capacitance probes, their raw data is a numeric value, proportional to soil dielectric constant which, in turn, can be calibrated to VWC using calibration tables. Calibration tables typically depend on soil type, which may not be precisely known. Furthermore, it may vary throughout the field or even throughout a plant's root zone.

The Root Zone Summary is typically a weighted average of readings at several points in the root zone, each of which may be a different soil type. Consequently, measurements of absolute VWC (even using a calibrated probe) may have significant errors.

Luckily, absolute readings of VWC are not needed to plan irrigation. As outlined previously, irrigation is planned by identifying sensor readings corresponding to the highest and lowest tolerable moisture levels during the growing season, then planning irrigation to maintain sensor readings within those values. The readings can be raw numeric readings from the probe, or calibrated VWC. Either value has the same effect.

Management Lines

In essence, the objective of soil moisture based irrigation is to keep the measured value of the Root Zone Summary between predefined high ('Full') and low ('Refill') levels. Full and Refill moisture levels may be plotted on a graph of soil moisture vs. time as two horizontal 'Management Lines' — sometimes referred to as 'Budget Lines.' The region between Full and Refill defines an 'Optimal' moisture zone (Figure 5). The goal is to keep soil moisture summary measurements within that zone.

Figure 4.
Management Lines

 

Figure 5.
Root Summary Plot Showing Management Lines

The Refill line represents the Root Zone Summary reading at the Management Allowable Depletion (MAD) level of water content. The MAD is simply the dryest we wish the soil to be on a sustained basis between irrigations. In most cases the Full line is equal to the reading at Field Capacity. Because there may be uncontrolled events, such as rainfall, that drive soil moisture out of the Normal zone, it is good practice to define additional Management Lines to show moisture levels at which plants can be harmed. These are shown in Figure 5 as the 'Too Full' and Stress (or Permanent Wilting Point) lines. 

Use the following procedure to set the Full Management Line to Field Capacity*.

  1. Run irrigation until the top portion of the root zone is saturated, as indicated by muddy conditions and/or surface ponding.
  2. Allow the soil to drain for at least 24 hours until it is moist and dark throughout the root zone but not saturated. Soil is now at Field Capacity.
  3. Set the Full Management Line to be equal to the Root Zone Summary reading at that point in time.

*Field Capacity may not be an appropriate Full level for all crops and growing conditions. Consult with a qualified agronomist to determine the appropriate Full point for your crop.

After steps A—C above, the line should be aligned as shown here:

Figure 6.
Setting Fill Line to Most Current Root Summary Value

Use the following procedure to set the Refill Management Line to the Management Allowable Depletion level:

  1. After irrigating, allow soil to dry to a level where you would normally initiate irrigation again. It may be necessary to dig into the root zone and use the 'look and feel' method to confirm soil is becoming dry, but not dry enough to cause significant plant stress.
  2. Set the Refill management line to be equal to the current Root Zone Summary reading.

When using additional Management Lines, follow the procedures described above to set the Stress (Permanent Wilting Point) and Too Full (severe Saturation) lines. You can measure and set these values using a confirming soil tension sensor. If it is not practical to bring the soil profile completely to Saturation and Permanent Wilting Points, set the Stress and Too Full levels on the Root Summary Plot intuitively to be slightly above and below the Normal (green) region.

Adjusting for Plant Growth

The Root Zone Summary indicates the average moisture content within the root zone volume. As plants grow and roots increase in size, the root zone volume increases. The weights used to calculate the Root Zone Summary need to be changed to reflect this. Additionally, soil properties can change throughout the growing season and increasing root sizes can introduce new soil types into the root zone volume. The Fill and Refill Management Lines need to be adjusted to reflect that. A farm is a complex environment which changes with time, and your management parameters need to change accordingly. 

Root Zone Summary weights and Management Lines should be adjusted a few times during the growing season using the previously described procedures. As you continue to readjust Management Lines, you will notice patterns in the rate of change of Root Zone Summary moisture as the soil transitions from Saturation to Field Capacity, then into Stress. Eventually, you will become proficient in making these adjustments without having to confirm with the look and feel method. Figure 7 shows examples of transition points. Note that the appearance of transition points depends on soil type and structure, so the moisture curves in your field may appear different than the one shown here. Experience matters.

Figure 7.
Transition Points

Using Soil Moisture to Plan Irrigation

Plan each irrigation event by setting its Start Time and Duration. The Start Time should be when the Root Zone Summary plot approaches the Refill Management Line. The Duration should be long enough to bring the Root Zone Summary close to the Full point without exceeding it.

Start Time

Setting the Start Time is simple. Turn on the irrigation system when the Root Zone Summary plot approaches the Refill Line. Depending on the tools you have available, you can find this point by

  1. Frequently watching the Root Zone Summary value
  2. Setting your system to automatically alert you when moisture approaches the Refill point, or
  3. By predicting the Initiation Time from observations of rate of change of the Root Zone Summary curve.

Whichever method is used, irrigation should be initiated before the Root Zone Summary drops below Refill.

Duration

Setting duration requires you to know how much time the irrigation system must run to bring the Root Zone Summary from the Refill point to Full (amount of time to 'fill' the root zone profile). The duration depends on irrigation-system application rate, root zone volume, soil type and proximity of the irrigation device or drip tube from the plants. Determine the amount of time required to fill the profile by closely observing the first few irrigation cycles. Run each cycle until the profile is nearly full, then continue to observe the Root Zone Summary as water continues to percolate into the root zone. Once you have done this a few times and you have a good feel for the amount of time needed to fill the profile, you will be able to set durations using a timer.

Changes

Any irrigation schedule needs to be periodically adjusted as the crop grows. This reality is reflected by the requirement to periodically adjust Root Zone Summary weights and Management Lines as the root zone changes sizes (previous section). When adjustments are made to the Root Zone Summary weights to reflect a larger root zone volume, the duration used to fill the root zone should also be reevaluated.

Deficit Irrigation

The procedures described in this section can be used for deficit irrigation. However, in deficit irrigation the Full Management Line is set well below Field Capacity, and the Refill line is set at a level of mild stress. The details of deficit irrigation are beyond the scope of this publication.

The plot in Figure 8 shows a typical Soil Moisture Profile illustrating several of the concepts covered in this section.

Figure 8.
Example of a Root Summary Plot

Limitations of Soil Moisture Management

Key Points

  • Irrigation management using soil moisture measurement is highly effective if the management zones are logically selected.
  • Microclimates within a farm or a large management zone can make it difficult for a single point reading to represent a large area.
  • Soil type variations within a Management Unit can further complicate the meaning of soil moisture measurements.
  • Weather station ET data can supplement soil moisture measurements to form a complete picture.

Climate, soil type and irrigation practices interact in complex ways to determine the distribution of water over a farm or a Management Unit. If Management Units are small and soil type is highly uniform, a point measurement with a moisture probe is a very accurate representation of the Management Unit as a whole. However, soil type is rarely uniform and choosing arbitrarily small Management Units increases cost. Effective moisture-based management requires careful design decisions. Observation throughout the growing season is needed to validate irrigation procedures and prompt changes when needed.

Moisture Probe Limitations

Management based on moisture probe information faces two fundamental limitations.

  1. Probes sense moisture at a single location within the large area of a Management Unit. If the Unit has different soil characteristics or is subject to different conditions than the probe location, the probe cannot accurately represent the entire management zone.
  2. Even if the Management Unit has multiple probes, when irrigated as a single zone, moisture management for all probe locations are likely to be optimized.

Soil Type

Soil type consistency within a management zone has a significant effect on the reliability of moisture probe measurements. Figure 9 shows a soil survey map of a single irrigation zone (center pivot circle) containing multiple soil types  

Figure 9.
Irrigation Zone with Varying Soil Types

Placing a single moisture probe in the Consistent Area shown in Figure 9 and interpreting its reading to represent the entire zone may result in stress conditions in the Weak Area. Conversely, locating a probe in the Weak Area may result in ponding elsewhere.

While varying soil types can limit the effectiveness moisture data, if properly interpreted, having such information is still an advantage. For example, if the soil in the 'Consistent Area' of Figure 9 is known to have a greater holding capacity than the rest of the Management Unit, the Refill Management Line can be set higher than normal, avoiding stress the 'Weak Area' between irrigation events.

Monitoring probes at the two locations with the greatest difference in soil type, may provide the information needed to adjust Management Lines in a way that keeps both sites between Field Capacity and maximum sustainable depletion — even when controlled by a single irrigation zone. 

Although varying soil types can pose a significant challenge to moisture management, strategic location of sensors and creative assignment of Management Lines can often overcome such obstacles. 

Microclimates

In most cases, the effects of microclimates are negligible. Soil reacts to local weather conditions for hours or days. Even when weather conditions at different locations in a management zone vary considerably, such effects usually average out over time. In very large Management Units, however, there may be sites with consistently different microclimate effects. In that case, a single probe may not be able to adequately represent that Management Unit.

Other Uses of Soil Moisture Information

So far we have focused on using soil moisture information for scheduling irrigation events; however, there are numerous other uses for soil moisture information in both irrigated and dryland farming. These include the following.

Runoff and percolation management

Moisture probes can be used to ensure that irrigation water only goes where it is intended to. Detect deep percolation with multi-depth probes extending below the root zone by monitoring individual readings from the deepest sensors. If irrigation events affect readings from these sensors, it is an indication that water is being wasted and potentially contaminating groundwater by leaching beyond the reach of the plants. Similarly, moisture probes can be installed at the perimeter or tail of a field to document that irrigation activities are not affecting areas outside of the field.

Germination with subsurface drip irrigation (SDI)

Individual moisture sensor readings from a multi-depth probe are also useful when germinating with deeply buried driplines. Germination with drip can only be effective if water can be pushed to the surface through successive irrigations. The top few sensors of a multi-depth probe can show the position of the wetted front and indicate whether it is shallow enough to germinate.

Salinity management

When salt sensitive crops are grown in semi-arid climates, soil salinity must be managed. One way is to periodically leach salts past the root zone with sprinkler or drip irrigation. Since salts typically accumulate on the wetted front of a heavy irrigation event, tracking the front with a multi-depth probe gives a quantitative indication of when salts have safely traveled beyond the reach of plants. EC sensors packaged with many moisture probes can also help monitor salt leaching.

Planning mechanized operations

Finally, in irrigated or dryland farming, moisture probe readings can indicate when it is safe to enter the field with heavy equipment to perform operations.

 

Conclusion

This document has laid out the basics of soil moisture analysis as it applies to managing root zone water content. The main point are:

  1. Managing root zone soil moisture is critical to optimum crop growth.
  2. Accurate control of soil moisture enables accurate control of nutrients and other inputs.
  3. Modern sensing and networking technology enables automated tracking of soil moisture.
  4. Soil moisture can be managed through irrigation by keeping water content between field capacity and a defined allowable depletion.

Putting these concepts to work can increase the yield and input-efficiency of your farming operation, while saving water and managing runoff. The end result can be increased farm profits with reduced environmental impact.

 

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