AE1675 (Revised) Soil, Water and Plant Characteristics Important to Irrigation Revised by Thomas F. Scherer, Extension Agricultural Engineer David Franzen, Extension Soil Science Specialist Larry Cihacek, Associate Professor, Department of Natural Resource Sciences

North Dakota State University, Fargo, North Dakota

Reviewed December 2017 Introduction ...... 2

Soil Properties ...... 3

Soil Texture ...... 3

Soil Structure ...... 4

Soil Series ...... 4 Soil Depth ...... 5 rrigation is the application of water to ensure sufficient soil Soil Permeability and moisture is available for good plant growth throughout the Infiltration ...... 5 growing season. Irrigation, as practiced in North Dakota, is Saline and Sodic Soils . . .6 I Topography of the Field . .7 called "supplemental irrigation" because it augments the rainfall Irrigation Water Quality . .8 that occurs prior to and during the growing season. Irrigation Water Irrigation often is used on full-season agronomic or high- Classification ...... 8

Carbonates ...... 9 value specialty crops to provide a dependable yield every year.

Boron ...... 10 It also is used on crops such as potatoes, flowers, vegetables The Interaction Between and fruits where water stress affects the quality of the yield. Soil and Water ...... 10 Most years, some places in the state receive sufficient Water Holding Capacity rainfall for good plant growth. But in many of those years, of Soils ...... 11

Soil Moisture Tension . . 12 other areas of the state experience reduced yields and/or How Plants Get Water reduced quality on nonirrigated crops due to water stress from From Soil ...... 12 insufficient soil moisture. Crop Water Use ...... 13 For irrigation planning purposes, the average precipitation Irrigation Water during the growing season is not a good yardstick to determine Management ...... 14

Irrigation Scheduling . . .15 a need for irrigation. The timing and amounts of rainfall during Additional Sources of the season, the soil's ability to hold water and the crop's Information ...... 16 water requirements are all factors that influence the need for irrigation. Any location in the state can have what might be considered "wet or dry" weeks, months and even years. Under irrigation, soil and water compatibility is very important. If they are not compatible, the applied irrigation water could have an adverse effect on the chemical and physical properties of the soil. Determining the suitability of land for irrigation requires a thorough evaluation of the soil properties, the topography of the land in the field and the quality of water to be used for irrigation. A basic understanding of soil/water/plant interactions will help irrigators efficiently manage their crops, soils irrigation systems and water supplies.

2 Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu Soil Properties

The county soil survey con- organic (for example, mucky silt class is modified (for example, tains detailed soils information loam). Soil particles greater than gravelly sand). for any parcel of land in North 2 mm are not used to determine Separating and weighing Dakota. Soil surveys of every soil texture. However, when they the amount of sand, silt and county in North Dakota have make up greater than 15 percent clay in a sample determines the been completed by the Natural of the soil volume, the textural texture of a mineral soil. For Resources Conservation Service (NRCS). The official and most current of soil survey informa- tion is accessible on the NRCS’s Websoil Survey website (http:// websoilsurvey.nrcs.usda.gov/). Published copies can be found at local NRCS and NDSU Extension offices, but they may not have the latest soil survey information. The soil survey Figure 1. Classification by size of the primary soil particles database provides information that define a textural group based on the U.S. Department of on important soil properties Agriculture soil classification system. Under SAND, V.F. refers to such as texture, structure, depth, very fine and V.C. to very coarse. permeability and chemistry, all of which are important for irriga- tion management.

Soil Texture Soil texture is determined by the size and type of solid particles that make up the soil. Soil particles may be mineral or organic. Most irrigated soils in North Dakota are mineral soils. For mineral soils, the texture classification is based on the relative proportion of the particles less than 2 millimeters (mm) or 5/64th of an inch in size. As shown in Figure 1, the largest particles are sand, the smallest are clay, and silt is in between. The soil texture is based on the percentage of sand, silt and clay (Figure 2). Soil texture classes may Figure 2. U.S. Department of Agriculture (USDA) soil textural be modified if greater than 15 triangle. The percent (by weight) of the sand, silt and clay fraction percent of the particles are determines the texture of the soil. The dotted line depicts a loam soil that has 45 percent sand, 35 percent silt and 20 percent clay content.

Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu 3 example, if a 100-pound sample The processes of root penetra- with plant roots. Plant growth of soil were sifted through tion, wetting and drying cycles, depends on rapid rates of ex- screens and found to contain 45 freezing and thawing, and animal change. pounds of sand, 35 pounds of silt activity, combined with inorgan- Practicing beneficial soil and 20 pounds of clay, then the ic and organic cementing agents, management techniques such soil would be composed of 45 produce soil structure (Figure as using cover crops, reduced percent sand, 35 percent silt and 3). Structural aggregates that are tillage, crop rotations, organic 20 percent clay. As shown by the resistant to physical stress are matter additions and timely dotted lines in Figure 2, this soil important to the maintenance of tillage practices can maintain has a loam texture. soil tilth and productivity. Exces- good soil structure. In sandy Twelve basic soil textures sive cultivation or tillage of wet soils, aggregate stability is often are shown on Figure 2. Sand, soils disrupt aggregates and difficult to maintain due to low loamy sands and sandy loams accelerate the loss of organic organic matter, clay content and are the most common soil tex- matter, thus causing decreased resistance of sand particles to tures irrigated in North Dakota. aggregate stability. aggregation processes. The movement of air, water Soil Structure and plant roots through a soil Soil Series Soil structure refers to the is affected by soil structure. Soil is the layer of the grouping of particles of sand, silt Stable aggregates result in a Earth's surface that has been and clay into larger aggregates network of soil pores that allow changed by physical or bio- of various sizes and shapes. rapid exchange of air and water logical processes. The five soil- forming factors that control the process of change are parent material, climate, topography, biota (plants and animals) and time. Single grain Blocky Platy Soils are grouped into categories according to their observed properties. The U.S. Department of Agriculture’s classification system consists of six categories. The highest category (soil order) contains Rapid Moderate Slow 11 basic soil groups, each with a

Granular Prismatic Massive very broad range of properties. The lowest category (soil series) contains more than 12,000 soils, each defining a very narrow range of soil properties. North Dakota has 339 named soil series. A soil series Rapid Moderate Slow is unique due to a combination of properties such as texture, Figure 3. Examples of the most common soil structures. Also shown is the structures’ effect on downward movement structure, topographic position (infiltration) of water. (Courtesy of the NRCS, Section 15 of the National (on the side of a hill or in a val- Engineering Handbook)

4 Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu ley) or depth to the water table. ure 4), the majority of the plant The depth to a contrast- A particular soil series roots will be in the top 18 inches ing soil layer of sand and gravel describes areas in which these because the gravel below is a (Figure 4) can affect irrigation soil conditions are similar. These poor rooting environment. This management decisions. If the locations may be in the same type of information is important depth to this layer is less than field, section, county, state or for irrigation management. 3 feet, the rooting depth and even region. Soil delineations available soil water for plants on county soil survey maps are Soil Depth is decreased. Soils with less based on the soil series. Soil depth refers to the available water for plants require A soil series generally is thickness of the soil materials more frequent irrigations. named after a town near the site that provide structural support, that represents the typical prop- nutrients and water for plants. Soil Permeability and erties for that soil. For example, In North Dakota, soil series that Infiltration the site with typical properties have bedrock 10 to20 inches A measure of the ability of for the Embden soil series is near below the surface are described air and water to move through Embden, N.D. as shallow. Bedrock from 20 to soil is its permeability. It is Many soil series do not have 40 inches is described as moder- influenced by the size, shape and a deep, uniform soil profile. Re- ately deep. continuity of the pore spaces, strictive subsurface layers often Most soil series in North which in turn are dependent on interfere with root penetration. Dakota have bedrock at depths the soil bulk density, structure In these soils, plant roots will be greater than 40 inches and are and texture. concentrated in the upper part of described as deep. Depth to con- Most soil series are as- the soil profile. For example, in trasting textures is given in the signed to a single permeability the Renshaw loam profile (Fig- soil series descriptions of the soil class based on the most restric- survey reports. tive layer in the upper 5 feet of the soil profile (Table 1). How- ever, soil series with contrasting textures in the soil profile are as- signed to more than one perme- ability class. In most cases, soils with a slow, very slow, rapid or very rapid permeability classifi- cation are considered poor for irrigation. Infiltration is the downward flow of water from the surface through the soil. The infiltration rate (sometimes called intake rate) of a soil is a measure of its ability to absorb an amount of rain or irrigation water during a given time period. It commonly is expressed in inches per hour. Figure 4. Soil horizon depths for four representative North Dakota soil series. A, B and C refer to the different soil horizons It is dependent on the permeabil- and IIC indicates a different parent material (for these soil ity of the surface soil, moisture series, it is sand and gravel).

Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu 5 Saline and Sodic Soils Salt-affected soils are of sodium by calcium improves grouped according to their con- the structure of the soil. Calcium tent of soluble salts and sodium soil amendments can be helpful in Table 1. Soil permeability (Table 2). Saline and sodic soils situations where land with a ma- classes. usually occur in areas where jority of unaffected irrigable soils ground water moves upward contains pockets (inclusions) of Infiltration Rate from a shallow water table close sodium-affected soils. Under irri- Classification (inches/hour) to the soil surface. The water gation, calcium soil amendments Very slow less than 0.06 carries dissolved minerals (salts) will help where surface crusting Slow 0.06 to 0.2 that accumulate in the soil as the has become a problem. Special Moderately slow 0.2 to 0.6 water is evaporated from the soil irrigation management practices Moderate 0.6 to 2.0 surface or transpired through may be required on these soils. Moderately rapid 2.0 to 6.0 the plants to the atmosphere. In Leaching or controlling the Rapid 6.0 to 20.0 general, these soils are not water table elevation can manage Very rapid greater than 20.0 recommended for irrigation. salt concentrations. Leaching is Saline and sodic soils accomplished by applying more may be of natural or man-made water than the soil will hold in the content of the soil and surface origins. One of the man-made root zone. Large rainfall events, conditions such as roughness processes is related to irriga- applying additional irrigation wa- (tillage and plant residue), slope tion. Under certain combinations ter or both will carry some of the and plant cover. of irrigation water quality and salts below the root zone. Coarse-textured soils, soils, salts and/or sodium may Planting a deep-rooted crop, such as sands and gravel, usu- accumulate in the root zone and such as alfalfa, or installing sub- ally have high infiltration rates. have an adverse effect on plant surface drainage can accomplish The infiltration rate of medium- growth. water table control. Deep ditches and fine-textured soils, such as Under some conditions, and tiling are methods of subsur- loams, silts and clays, is lower sodium can be controlled in the face drainage that have been used than coarse-textured soils and is upper part of the soil through successfully in many parts of the influenced by the stability of the the use of soluble calcium world to control the level of the soil aggregates. amendments. The replacement water table. Water and plant nutrient losses may be greater on coarse- textured soils. Thus, the timing and quantity of chemical and Table 2. Soil chemistry measurements used to classify irrigation water applications saline, sodic and saline-sodic soils. is particularly critical on these soils. Sodium Electrical Adsorption Conductivity* Ratio* (mmhos/cm) pH (SAR) Saline soil greater than 4 less than 8.5 less than 13 Sodic soil less than 4 8.5 to 10 greater than 13 Saline-sodic soil greater than 4 less than 8.5 greater than 13 *Measured from a saturated soil extract

6 Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu Topography of the Field Soil salt and sodium con- Topography, or the "lay of The shape of the slope is tents need to be measured to the land," has a large impact on another important characteristic. determine precisely the severity whether a field can be irrigated. A convex slope curves outward of the problem. The salt content Relief is a component of topog- like the outside surface of a ball, of the soil is estimated from an raphy that refers to the differ- a concave slope curves inward electrical conductivity measure- ence in height between the hills like the inside surface of a saucer, ment using one of the following: and depressions in the field. The and a plane slope is like a tilted a soil water extract, soil water topographic relief will affect the flat surface. slurry or soil paste. The sodium type of irrigation system to be Slopes are described as content of the soil often is used, the water conveyance sys- simple or complex. Simple slopes measured on a soil water ex- tem (ditches or pipes), drainage have a smooth appearance, with tract and expressed as the ratio requirements and water erosion surfaces extending in one or per- between the sodium and calcium control practices. haps two directions. For example, plus magnesium; it is given the The shape and arrange- slopes on alluvial fans and foot term sodium adsorption ratio ment of topographic landforms slopes of river valleys are re- (SAR). and the type of surface water- garded as simple. Complex areas Soil sampling the surface way network also will influence have short slopes that extend in layer (top 6 inches) on a periodic irrigation management. For several directions and consist of basis (every three to five years) example, a low spot in the field convex and concave slopes much will track the change in accumu- where water typically accumu- like the knoll and pothole topog- lated salt or sodium. The SAR of lates after a rain may become a raphy found on glacial till plains. the soil samples will indicate if a place that is continually wet with Gravity (surface) irrigation buildup of sodium has occurred. the addition of irrigation water. can be used only on simple slopes Generally, soils with an With some crops, such as of 2 percent or less. In general, SAR of 13 from the saturated potatoes, a wet low spot could simple and complex slopes great- extract will exhibit significant become a source of disease. For er than 1 percent should be ir- physical problems due to dis- a center pivot, a tower that trav- rigated only with sprinkler or drip persal of clay particles. Usually els through the low spot could systems. Center pivot sprinkler a soil with an SAR of 6 or lower become stuck. irrigation systems can operate on from the saturated extract will slopes up to 15 percent, but gen- ■ Slope not have physical problems erally simple slopes greater than 9 Slope is important to soil associated with dispersed clay. percent are not recommended. formation and management be- However, if periodic sampling To accommodate gravity or cause of its influence on runoff, indicates that the SAR is increas- sprinkler irrigation systems, land soil drainage, erosion, the use of ing, say from 6 to 9, then you smoothing can be used to modify machinery and choice of crops. may need to consider corrective the slope in a field. However, land Slope is the incline or gradient action. smoothing may cause yield reduc- of a surface and commonly is tions for one to three growing expressed in percents. seasons. The places where topsoil The percent of slope is was removed are most likely to determined by measuring the have yield reductions. Special difference in vertical elevation in management using increased feet over 100 feet of horizontal organic matter may be required to distance. For example, a 5 per- accelerate soil building in these cent slope rises or falls 5 feet per areas. 100 feet of horizontal distance.

Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu 7 Irrigation Water Quality

The quality of some water sourc- test report, you might see one of ticular piece of land is irrigated es is not suitable for irrigating the following units:: one year out of three or more), crops. Irrigation water must be millimhos per centimeter (mmhos/cm) lower-quality water may be used. compatible with the crops and micromhos per centimeter (μmhos/cm) However, the lower-quality water soils to which it will be applied. deci-Siemens per meter (dS/m) should not have an EC that ex- The Soil and Water Testing Labo- micro-Siemens per centimeter (μS/cm) ceeds 3,000 μmhos/cm or an SAR ratory in the Soil Science Depart- greater than 10. ment at NDSU provides soil and where: Calcium added to irriga- water compatibility recommen- 1,000 μmhos/cm = 1 mmho/cm = 1 dS/m tion water can lower the SAR dations for irrigation. A water = 1,000 μS/cm and reduce the harmful effects analysis and legal description of The SAR of a water sample of sodium. The effectiveness of the land proposed for irrigation is the proportion of sodium rela- added calcium depends on its are required before a recommen- tive to calcium and magnesium. solubility in the irrigation water. dation can be made. Because it is a ratio, the SAR has Calcium solubility is controlled The quality of water for no units. by the source of the calcium (for irrigation purposes is deter- Laboratories that perform example, calcium carbonate, gyp- mined by its total dissolved salt irrigation water analysis may sum, calcium chloride) and the content. An analysis of water for provide a suitability classifica- concentration of other ions in the irrigation should include the cat- tion based on a system devel- irrigation water. ions (calcium, magnesium and oped at the U.S. Salinity Labora- Compared with calcium sodium) and the anions (bicar- tory in California (Figure 5). This carbonate and gypsum, calcium bonate, carbonate, sulfate and classification system combines chloride additions will result in chloride). Because some crops salinity and sodicity. For exam- higher concentrations of soluble are sensitive to boron, it often is ple, a water sample classified as calcium and be the most effective included in the analysis. C3-S2 would have a high salinity at lowering irrigation water SAR. rating and a medium SAR rating. However, calcium chloride is con- Irrigation Water The scale for sodicity is not siderably more expensive than Classification constant because it depends on calcium carbonate and calcium The two most important the level of salinity. For example, sulfate (gypsum). factors to look for in an irriga- an SAR of 8 is in the S1 category tion water quality analysis are if the salinity is from 100 to 300 Carbonates the total dissolved solids μmhos/cm; S2 if the salinity is Carbonate and bicarbonate (TDS) and the sodium adsorp- from 300 to 3,000 μmhos/cm and ions in the water combine with tion ratio (SAR). The TDS of a S3 if the salinity is greater than calcium and magnesium to form water sample is a measure of the 3,000 μmhos/cm. compounds that precipitate out of concentration of soluble salts in Much of the water in North solution. The removal of calcium a water sample and commonly is Dakota is classified in the C2 and magnesium increases the so- referred to as the salinity of the to C3 salinity range and the S1 dium hazard to the soil due to the water. to S2 sodium hazard range. In irrigation water. The increased The electrical conductivity general, any water with an EC sodium hazard often is expressed (EC) of a water sample often is greater than 2,000 or an SAR as "adjusted SAR." The increase used as a proxy for TDS. EC can value greater than 6 is not of "adjusted SAR" from the SAR be expressed in many differ- recommended for continuous is a relative indication of the ent units, and this often causes irrigation in North Dakota. increase in sodium hazard due to confusion. On an irrigation water In cases where sporadic the presence of these ions. irrigation is practiced ( a par- (continued on page 10)

8 Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu ■ Salinity C2 - Medium-salinity bility. Even with adequate perme- C1 - Low-salinity water: water: Can be used if a moderate ability, special management for Can be used for irrigation with amount of leaching occurs. Plants salinity control may be required most crops on most soils with with moderate salt tolerance can and plants with good salt toler- little likelihood that soil salinity be grown in most cases without ance should be selected. will develop. Some leaching is special practices for salinity con- C4 - Very high salinity required, but this occurs un- trol. water: Is not suitable for irriga- der normal irrigation practices C3 - High-salinity water: tion under ordinary conditions except in soils of slow and very Cannot be used on soils with mod- but may be used occasionally slow permeability. erately slow to very slow permea- under very special circumstanc- es. The soils must have rapid permeability, drainage must be adequate, irrigation water must be applied in excess to provide considerable leaching, and very salt-tolerant crops should be selected. ■ Sodium S1 - Low-sodium water: Can be used for irrigation on almost all soils with little danger of the development of harmful levels of exchangeable sodium. S2 - Medium-sodium water: Will present an appreciable sodium hazard in fine-textured soils, especially under low leaching conditions. This water may be used on coarse-textured soils with moderately rapid to very rapid permeability. S3 - High-sodium water: Will produce harmful levels of exchangeable sodium in most soils and requires special soil management, good drainage, high leaching and high organic matter additions. S4 - Very high sodium water: Generally is unsatisfactory for irrigation Figure 5. Diagram showing the classification of irrigation water. purposes except at low and (From Agriculture Handbook No. 60, USDA Salinity Laboratory in Riverside, perhaps medium salinity. Calif.)

Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu 9 The Interaction Between Soil and Water

Nozzles of sprinkler sys- Soil is a medium that stores and Gravitational water tems have been plugged by car- moves water. If a cubic foot of generally moves quickly bonate minerals in some states a typical silt loam topsoil were downward in the soil due to the but this has not been observed in separated into its component force of gravity. Capillary water North Dakota. However, carbon- parts, about 45 percent of the is the most important for crop ate minerals have plugged the volume would be mineral matter production because it is held by emitters in drip irrigation sys- (soil particles), organic residue soil particles against the force of tems in North Dakota. To control would occupy about 5 percent of gravity. this problem, add a mild acid to the volume and the rest would be As water infiltrates into lower the pH of the irrigation pore space. a soil, the pore spaces fill with water. The pore space is the voids water. As the pores are filled, between soil particles and is water moves through the soil Boron occupied by air or water. The by gravity and capillary forces. Boron is essential for the quantity and size of the pore Water movement continues normal growth of all plants, spaces are determined by the downward until a balance is and the quantity required is low soil's texture, bulk density and reached between the capillary compared with other miner- structure. forces and the force of gravity. als. However, some plants are Water is held in soil in two Water is pulled around sensitive to even low boron ways: as a thin coating on the soil particles and through small concentrations. Dry beans are outside of soil particles and in pore spaces in any direction very sensitive to small amounts the pore spaces. Soil water in the by capillary forces. When of boron, but corn, potatoes and pore spaces can be divided into capillary forces move water alfalfa are more tolerant. In fact, two different forms: gravitational from a shallow water table the concentration of boron that water and capillary water (Figure upward, salts may precipitate will injure the sensitive plants 6). and concentrate in the soil as often is close to that required for water is removed by plants and normal growth of tolerant plants. evaporation.. Although no problems with boron in water used for irriga- tion in North Dakota have been documented, testing for this element in irrigation water is a precautionary practice. Boron does occur in some North Da- kota ground water at concentra- tions that are theoretically toxic to some crops. Gravitational A boron concentration water. The pore spaces are greater than 2 parts per million filled with water in excess Capillary water is held in the pore (ppm) may be a problem for cer- of their capillary capacity, space against the force of gravity. tain sensitive crops, especially in and the excess, or gravitational wa- ter, drains downward. years that require large quantities of irrigation water. Figure 6. The two primary ways that water is held in the soil for plants to use are capillary and gravitational forces.

10 Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu Water Holding Capacity of Soils The four important levels of soil moisture content reflect the availability of water in the soil. These levels commonly are referred to as 1) saturation, 2) field capacity, 3) wilting point and 4) oven dry. When a soil is saturated, the soil pores are filled with water and nearly all of the air in the soil Field capacity. The capillary pores Wilting point. The water available to has been displaced by water. The are full and the remaining pore space plants is exhausted. water held in the soil between is filled with air. saturation and field capacity is Figure 7. Soil moisture available to plants is the amount held gravitational water. Frequently, between field capacity and wilting point. gravitational water will take a few days to drain through the soil profile and, thus, some can be absorbed by roots of plants. used to provide a reference for For example, the water Field capacity is defined measuring the other three soil available can be calculated for as the level of soil moisture left moisture contents. a soil with fine sandy loam in in the soil after drainage of the When discussing the water- the first foot, loamy sand in the gravitational water (Figure 7). holding capacity associated with second foot and sand in the third Water held between field capacity a particular soil series, the water foot. The top foot would have and the wilting point is available available for plant use in the root about 2 inches, the second foot for plant use. zone commonly is given (Table would have about 1 inch and the The wilting point is defined 3). Available soil water content third foot would have about 0.75 as the soil moisture content at commonly is expressed as inches inch for a total of 3.75 inches of which most plants cannot exert per foot of soil. available water for a crop with a enough force to remove water 3-foot root depth. from small pores in the soil. Most crops will be damaged perma- nently if the soil moisture content Table 3. Available soil moisture holding capacity for is allowed to reach the wilting various soil textures. point. In many cases, yield reduc- tions may occur long before this Available Soil Moisture point is reached. Soil Texture inches/inch inches/foot Capillary water held in the Coarse sand and gravel 0.02 to 0.06 0.2 to 0.7 soil beyond the wilting point can Sands 0.04 to 0.09 0.5 to 1.1 be removed only by evaporation. Loamy sands 0.06 to 0.12 0.7 to 1.4 When dried in an oven, nearly all Sandy loams 0.11 to 0.15 1.3 to 1.8 water is removed from the soil. Fine sandy loams 0.14 to 0.18 1.7 to 2.2 "Oven dry" moisture content is Loams and silt loams 0.17 to 0.23 2.0 to 2.8 Clay loams and silty clay loams 0.14 to 0.21 1.7 to 2.5 Silty clays and clays 0.13 to 0.18 1.6 to 2.2

Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu 11 How Plants Get Water From Soil Soil Moisture Tension Water is essential for plant The degree to which wa- tensions around -0.10 bars, while growth. Without enough water, ter clings to the soil is the most clayey soil will have field capac- normal plant functions are dis- important soil water character- ity at a tension around -0.33 bars. turbed, and the plant gradually istic to a growing plant. This At field capacity, removing water wilts, stops growing and dies. concept often is expressed as soil from the soil is relatively easy for Plants are most susceptible to moisture tension. Soil moisture a plant. damage from water deficiency tension is negative pressure and The wilting point is reached during the vegetative and repro- commonly expressed in units of when the maximum energy ductive stages of growth. Also, bars. exerted by a plant is equal to the many plants are very sensitive to During this discussion, tension with which the soil holds salinity during germination and when soil moisture tension the water. For most agronomic early growth stages. becomes more negative, it will crops, this is about -15 bars of Most of the water that be referred to as "increasing" soil moisture tension. To put this enters the plant roots does not in value. Thus, as soil mois- in perspective, the wilting point stay in the plant. Less than 1 ture tension increases (the soil of some desert plants has been percent of the water withdrawn water pressure becomes more measured to be between -50 and by the plant actually is used in negative), the amount of energy -60 bars of soil moisture tension. photosynthesis ( assimilated by exerted by a plant to remove the The presence of high the plant). The rest of the water water from the soil also must amounts of soluble salts in the moves to the leaf surfaces, where increase. One bar of soil mois- soil reduces the amount of water it transpires (evaporates) to the ture tension is nearly equivalent available to plants. As dissolved atmosphere. The rate at which a to -1 atmosphere of pressure (1 salts increase in soil water, the plant takes up water is controlled atmosphere of pressure is equal energy expended by a plant to by its physical characteristics, to 14.7 pounds per square inch at extract water also must increase the atmosphere and soil environ- sea level). even though the soil moisture ment. A soil that is saturated has tension remains the same. In es- As water moves from the a soil moisture tension of about sence, dissolved salts decrease soil into the roots, through the -0.001 bars or less, which requires the total available water in the stem, into the leaves and through little energy for a plant to pull soil profile. the leaf stomata to the air, it water away from the soil. At field moves from a low water tension capacity, most soils have a soil to a high water tension (Figure moisture tension between -0.05 8). The water tension of the air is and -0.33 bars. Soils classified as determined in large part by the sandy may have field capacity relative humidity and always is greater than the water tension in the soil. Plants can extract only the soil water that is in contact with their roots. For most agronomic crops, the root distribution in a deep, uniform soil is concentrat- ed near the soil surface (Figure 9). Thus, during the course of a

12 Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu growing season, plants gener- ally extract more water from the upper part of their root zone than from the lower part. Plants such as grasses, which have a high root density per unit soil volume, may be Figure 8. Illustration of the energy differentials able to absorb all available soil that drive the water water. Other plants, such as movement from the vegetables, which have a low soil into the roots, root density, may not be able to up the stalk, into the obtain as much water from an leaves and out into the equal volume of the same soil. atmosphere. The water moves from a less Thus, vegetables are generally negative soil moisture more sensitive to water stress tension to a more than high-root-density agronomic negative tension in the crops such as alfalfa, corn, wheat atmosphere. and sunflower.

Crop Water Use Crop water use, also called evapotranspiration or ET, often is given as a daily estimate of the combination of the amount of water transpired by plants and the amount of evaporation from the soil surface around the plants. A plant's water use chang- es with a predictable pattern from germination to maturity. All agronomic crops have a similar water use pattern (Figure 10). However, total crop wa- Figure 9. During the ter use will vary from growing course of a growing season to growing season due to season, plants will changes in climatic variables (air extract about 40 percent of their water from the temperature, amount of sunlight, top quarter, 30 percent humidity, wind), and soil differ- from the second quarter, ences among fields (root depth, 20 percent from the third soil water holding capacities, quarter and 10 percent texture, structure, etc.). from the bottom quarter Many years of research of the root zone. have produced equations that allow accurate calculated esti- mates of crop water use values

Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu 13 Irrigation Water Management

for the major irrigated crops in Knowledge of the water One of the most difficult parts North Dakota using measured use patterns during the differ- of irrigation water management daily weather variables. These ent growth stages has a major is deciding when to turn on the equations have been combined influence on how an irrigation irrigation system and how much with the weather data collected system is designed and managed. to apply. Good irrigation manage- by the North Dakota Agricultural Failure to recognize the water ment begins with the accurate Weather Network (NDAWN) to use patterns of a crop may result measurement of the rain amount provide daily crop water use es- in poorly managed water applica- received on each irrigated field. timates for each weather station tions. Crop water stress, fertil- Ideally, each irrigated field should in the network. The crop water izer and pesticide leaching, and have at least one and possibly use values can be found in table increased pumping costs are just two rain gauges (at least 2 inches or map format on the NDAWN a few of the results of poor irriga- in diameter) mounted on posts website (http://ndawn.ndsu.no- tion water management. next to the field. dak.edu/) under “Applications” The estimation of soil mois- in the left-hand menu. ture is the most common method of irrigation water management; however, it must be done on a regular basis throughout the growing season. During the heat of the summer, checking the soil moisture two to three times each week is common. The oldest and most com- monly used is the "feel method," which estimates soil moisture by taking a soil sample in hand and squeezing it into a ball. By ob- serving the appearance of the ball and creating a ribbon of soil be- tween the thumb and forefinger, an estimate of the soil moisture content can be determined. This method requires practice and ex- perience to become accurate at predicting irrigation water needs. To determine the complete soil water status, soil samples Figure 10. Typical water use curve for most agronomic crops. should be obtained from sev- eral levels in the root zone. This method is popular because it can be combined with other field activities, such as scouting for insects, soil sampling for nitrogen or petiole sampling.

14 Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu Mechanical devices such ment” method. Irrigators who fol- ing by the Checkbook Method.” as tensiometers and soil mois- low this method record (usually The other two are electronic ture blocks also are used for on a calendar) rain and irrigation methods. A site-specific Web- irrigation water management. amounts and daily crop water based application is available These devices are particularly use. Irrigation timing and amount through the NDAWN website; helpful with fruit and vegetable are adjusted to replace the soil look under the “Applications” crops. For these crops, they have water used by the crop. menu. proven to be accurate, reliable Another form of the re- A spreadsheet version of and inexpensive. Other more placement method is to assume the checkbook also has been sophisticated instrumentation the average daily crop water use developed. It includes the check- can be used for irrigation sched- is 0.25 inch per day. The average book methods from NDSU and uling but generally are not used total for a week in July and Au- the University of Minnesota. For for irrigation management due to gust is 1.75 inches. Rain amounts North Dakota, the program can the expense. during the week are subtracted be used to schedule irrigation from 1.75 and that amount of on corn, wheat, barley, potato, Irrigation Scheduling irrigation water is applied. With alfalfa, soybean, sunflowers, Irrigation scheduling is the access to the Internet, this sugar beets and sunflowers. For science and/or art of applying method could be improved sig- Minnesota, the program can be the proper amount of water at nificantly by using the crop water used to schedule irrigation on the proper time to provide the use amounts from the NDAWN corn, wheat, soybean, sunflower, maximum useable soil moisture website. potato, sugar beets and dry bean. in a plant’s root zone without An irrigation scheduling A copy of AE792, along causing harmful stress. Irriga- procedure called the "check- with user manuals for the elec- tion scheduling is a balancing book" method has been used suc- tronic versions, is available at act between applying too much cessfully for many years in North this website: www.ag.ndsu.edu/ water or not enough to meet Dakota. The checkbook method irrigation/irrigation-scheduling. plant needs at a particular stage is a soil moisture accounting of growth. method that uses daily crop Applying too much water water use values and soil water- can lead to higher pumping holding capacities to predict the costs and more disease pressure time and amount of water need- on the crop. Applying too little ed to replenish what has been water at the wrong time will removed from the root zone. cause crop stress and reduced Rain and irrigation amounts are yields. The practice of irrigation “deposits” for storage in the root scheduling is basically a decision zone, and crop water use is a methodology. Each irrigator has withdrawal of water from that had to learn to develop an ir- storage. rigation scheduling method that Three irrigation-scheduling works in his or her situation. tools are available to irrigators in A common form of irriga- North Dakota. A manual method tion scheduling is the “replace- is outlined in Extension publica- tion AE792, “Irrigation Schedul-

Soil, Water and Plant Characteristics Important to Irrigation | www.ag.ndsu.edu 15 Additional Sources of Information

Extension Publications (online copies at: www.ag.ndsu.edu/publications/crops) AE1360, “Irrigation Water Sample Analysis” AE1637, “Compatibility of North Dakota Soils for Irrigation” SF1087, “Managing Saline Soils in North Dakota” AE92, “Planning to Irrigate ... A Checklist” AE91, “Selecting a Sprinkler Irrigation System” AE792, “Irrigation Scheduling by the Checkbook Method”

NRCS Publications North Dakota Irrigation Guide, available on the NRCS website at http://efotg.sc.egov.usda.gov/references/public/ND/irrigation.pdf

Web Soil Survey: http://websoilsurvey.nrcs.usda.gov/app/

This publication was authored by Thomas F. Scherer, Extension agricultural engineer; Bruce Seelig, former Extension water quality specialist; and David Franzen, Extension soil science specialist, NDSU, 1996.

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