Soil Moisture Management & Monitoring

Objectives:

1. Increase understanding of soil physical properties that affect soil moisture storage and permeability.
2. Increase familiarity with local soils and their characteristics, as well as information resources addressing local soils.
3. Apply these concepts to optimizing water management in crop production.

Key Points:

1. Soil permeability is affected by soil texture, structure, and moisture.
2. Plant available water in the root zone is that which can be stored in the soil between field capacity and permanent wilting point. Plant available water is soil-specific.
3. Water in soil is subjected to gravity, osmotic potential (suction), and matric (or capillary) potential (suction).
4. There are several methods available for measuring or estimating soil moisture. These include gravimetric (oven dry), soil feel and appearance, resistance (gypsum blocks or WaterMark™ sensors), tensiometry, capacitance, and other methods. Factors affecting selection of soil moisture monitoring method include costs, convenience, ease of use, precision and accuracy required, and personal preference of the operator.

Assess your knowledge:

1. Describe three methods for measuring soil moisture. Discuss advantages and limitations of each.
2. Describe how soil structure can affect permeability.
3. Describe how cultural practices (tillage, cropping patterns, etc.) can affect permeability.
4. Estimate the total water available in the following example:

(Example problem based upon local soils.)

Soil moisture storage capacity

Soil moisture characteristics: A soil’s capacity for storing moisture is affected by soil structure and organic matter content, but it is determined primarily by soil texture.

Field capacity is the soil water content after soil has been thoroughly wetted when the drainage rate changes from rapid to slow. This point is reached when all the gravitational water has drained. Field capacity is normally attained 2-3 days after irrigation and reached when the soil water tension is approximately 0.3 bars (30 kPa or 4.35 PSI) in clay or loam soils, or 0.1 bar in sandy soils.

Permanent wilting point is the soil moisture level at which plants cannot recover overnight from excessive drying during the day. This parameter may vary with plant species and soil type and is attained at a soil water tension of 10-20 bars. Hygroscopic water is held tightly on the soil particles (below permanent wilting point) and cannot be extracted by plant roots.

Plant available water is retained in the soil between field capacity and the permanent wilting point. It is often expressed as a volumetric percentage or in inches of water per foot of soil depth. Approximate plant available water storage capacities for various soil textures are shown below.

Compiled by Dana Porter, PhD, PE, Department of Biological and Agricultural Engineering and Texas A&M AgriLife Research and Extension Center – Lubbock.

If the goal is to apply water to moisten the root zone to some target level (75% field capacity, for instance, depending upon local factors), it is essential to know how much water the soil will hold at field capacity, and how much water is already in the soil. Estimating soil moisture can be accomplished through direct methods (gravimetric soil moisture determination) or indirect methods. Soil moisture monitoring instruments, including gypsum blocks and tensiometers, provide the means to estimate soil moisture quickly and easily. Alternately, a soil's moisture condition can be assessed by observing its feel and appearance. A soil probe, auger, or spade may be used to extract a small soil sample within each foot of root zone depth. The sample is manually gently squeezed to determine whether the soil will form a ball or cast, and whether it leaves a film of water and/or soil in the hand. Pressing a portion of the sample between the thumb and forefinger allows one to observe whether the soil will form a ribbon. Results of the sample are compared with the following guidelines.

Table 1. How soil feels and looks at various soil moisture levels

Soil moisture level	Fine sand, loamy fine sand	Sandy loam, fine sandy loam	Sandy clay loam, loam, silt loam	Clay loam, clay, silty clay loam
0 – 25% available soil moisture	Appears dry; will not retain shape when disturbed or squeezed in hand.	Appears dry; may make a cast when squeezed in hand but seldom holds together.	Appears dry. Aggregates crumble with applied pressure.	Appears dry. Soil aggregates separate easily, but clods are hard to crumble with applied pressure.
25 – 50% available soil moisture	Slightly moist appearance. Soil may stick together in very weak cast or ball.	Slightly moist. Soil forms weak ball or cast under pressure. Slight staining on finger.	Slightly moist. Forms a weak ball with rough surface. No water staining on fingers.	Slightly moist; forms weak ball when squeezed, but no water stains. Clods break with applied pressure.
50 – 75% available soil moisture	Appears and feels moist. Darkened color. May form weak cast or ball. Leaves wet outline or slight smear on hand.	Appears and feels moist. Color is dark. Forms cast or ball with finger marks. Will leave a smear or stain and leaves wet outline on hand.	Appears and feels moist and pliable. Color is dark. Forms ball and ribbons when squeezed.	Appears moist. Forms smooth ball with defined finger marks; ribbons when squeezed between thumb and forefinger.
75 – 100% available soil moisture	Appears and feels wet. Color is dark. May form weak cast or ball. Leaves wet outline or smear on hand.	Appears and feels wet. Color is dark. Forms cast or ball. Will smear or stain and leaves wet outline on hand; will make weak ribbon.	Appears and feels wet. Color is dark. Forms ball and ribbons when squeezed. Stains and smears. Leaves wet outline on hand.	Appears and feels wet; may feel sticky. Ribbons easily; smears and leaves wet outline on hand. Forms good ball.

Root zone depth: Roots are generally developed early in the season, and will grow in moist (not saturated or extremely dry) soil. Soil compaction, caliche layers, perched water tables, and other impeding conditions will limit the effective rooting depth. Most crops will extract most (70% – 85%) of their water requirement from the top one to two feet of soil, and almost all of their water from the top 3 feet of soil, if water is available. Deep soil moisture is beneficial primarily when the shallow moisture is depleted to a water stress level. Commonly reported effective root zone depths by crop are listed in Table 2.

Table 2. Root zone depths reported for various crops.^*

Crop	Approximate Effective Rooting Depth (feet)
Alfalfa	3.3 – 6.6+
Corn	2.6 – 5.6
Cotton	2.6 – 5.6
Peanut	1.6 – 3.3
Sorghum	3.3 – 6.6

^* These values represent the majority of feeder roots.

Permeability is the ability of the soil to take in water through infiltration. A soil with low permeability cannot take in water as fast as a soil with high permeability; the permeability therefore affects the risk for runoff loss of applied water. Permeability is affected by soil texture, structure, and surface condition. Generally speaking, fine textured soils (clays, clay loams) have lower permeability than coarse soils (sand). Surface sealing, compaction, and poor structure (particularly at or near the surface) limit permeability.

Using soil moisture information to improve irrigation efficiency

Deep percolation losses are often overlooked, but they can be significant. Water applied in excess of the soil's moisture storage capacity can drain below the crop's effective root zone. In some cases, periodic deep leaching is desirable to remove accumulated salts from the root zone. But in most cases, deep percolation losses can have a significant negative impact on overall water use efficiency - even under otherwise efficient irrigation practices such as low elevation precision application (LEPA) and subsurface drip irrigation (SDI) irrigation. Furrow irrigation poses increased deep percolation losses at upper and lower ends of excessively long runs. Surge irrigation can improve irrigation distribution uniformity, and hence reduce deep percolation losses. Coarse soils are particularly vulnerable to deep percolation losses due to their low water holding capacity. Other soils may exhibit preferential flow deep percolation along cracks and in other channels formed under various soil structural and wetting pattern scenarios.

Runoff losses occur when water application rate (from irrigation or rainfall) exceeds soil permeability. Sloping fields with low permeability soils are at greatest risk for runoff losses. Vegetative cover, surface conditioning (including furrow dikes), and grade management (land leveling, contouring, terracing, etc.) can reduce runoff losses.   Irrigation equipment selection (nozzle packages) and management can also help to minimize runoff losses.

Soil Moisture Monitoring

Soil water measurement

Methods used to measure soil water are classified as direct and indirect. The direct method refers to the gravimetric method in which a soil sample is collected, weighed, oven-dried and weighed again to determine the sample’s water content on a mass percent basis. The gravimetric method is the standard against which the indirect methods are calibrated. Some commonly used indirect methods include electrical resistance, capacitance and tensiometry.

Electrical resistance methods include gypsum blocks or granular matrix sensors (more durable and more expensive than gypsum blocks) that are used to measure electrical resistance in a porous medium. Electrical resistance increases as soil water suction increases, or as soil moisture decreases. Sensors are placed in the soil root zone, and a meter is connected to lead wires extending above the ground surface for each reading. For most on-farm applications, small portable handheld meters are used; automated readings and controls may be achieved through use of dataloggers.

Capacitance sensors measure changes in the dielectric constant of the soil with a capacitor, which consists of two plates of a conductor material separated by a short distance (less than 3⁄8 of an inch). A voltage is applied at one extreme of the plate, and the material that is between the two plates stores some voltage. A meter reads the voltage conducted between the plates. When the material between the plates is air, the capacitor measures 1 (the dielectric constant of air). Most solid soil components (soil particles), have a dielectric constant from 2 to 4. Water has higher dielectric constant of 78. Hence, higher water contents in a capacitance sensor would be indicated by higher measured dielectric constants. Changes in the dielectric constant provide an indication of soil water content. Sensors are often left in place in the root zone, and they can be connected to a datalogger for monitoring over time.

Tensiometers measure tension of water in the soil (soil suction). A tensiometer consists of a sealed water-filled tube equipped with a vacuum gauge on the upper end and a porous ceramic tip on the lower end. As the soil dries, soil water tension (suction) increases; in response to this increased suction, water is moved from the tensiometer through the porous ceramic tip, creating a vacuum in the sealed tensiometer tube. Water can also move from the soil into the tensiometer during or following irrigation. Most tensiometers have a vacuum gauge graduated from 0 to 100 (centibars, cb, or kilopascals, kPa). A reading of 0 indicates a saturated soil. As the soil dries, the reading on the gauge increases. The useful limit of the tensiometer is about 80 cb. Above this tension, air enters through the ceramic cup and causes the instrument to fail. Therefore, these instruments are most useful in sandy soils and with drought-sensitive crops because they have narrower soil moisture ranges.

Soil water monitoring methods have advantages and limitations. They vary in cost, accuracy, ease of use, and applicability to local conditions (soils, moisture ranges, etc.) Most require calibration for accurate moisture measurement. Proficiency of use and in interpreting information results from practice and experience under given field conditions.

Excerpts from Enciso, Juan, Dana Porter, and Xavier Peries,. 2007. Irrigation Monitoring with Soil Water Sensors. TCE Fact Sheet B-6194. Texas AgriLife Extension Service (formerly Texas Cooperative Extension), Texas A&M System, College Station, TX.

References

• Soil Moisture Management (B-1670)
• Irrigation Monitoring with Soil Water Sensors (B-6194)
• Estimating Soil Moisture by Feel and Appearance (1619)