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Part 652Irrigation Guide

CropsChapter 3

3–21(210-vi-NEH, September 1997)

Chapter 3 Crops

Contents:

3–i

652.0300 Purpose and objective 3–1

(a) Soil condition ................................................................................................ 3–1

(b) Nutrient management ................................................................................... 3–1

(c) Soil, water, pest, nutrient, and crop residue management ...................... 3–1

652.0301 Crop growth characteristics 3–2

(a) Response to water, crop yield, and quality ................................................ 3–2

(b) Critical growth periods ................................................................................ 3–2

(c) Irrigation related management .................................................................... 3–7

(d) Rooting depth and moisture extraction patterns ...................................... 3–8

652.0302 Crop and irrigation system water requirements 3–12

(a) Crop evapotranspiration ............................................................................ 3–12

(b) Irrigation frequency .................................................................................... 3–12

(c) Net irrigation requirement ......................................................................... 3–12

(d) Gross irrigation requirement ..................................................................... 3–13

652.0303 Reduced irrigation and restricted water supply 3–14

652.0304 Adapted irrigation systems 3–15

652.0305 Temperature—effects and management 3–16

(a) High temperatures ...................................................................................... 3–16

(b) Low temperatures ....................................................................................... 3–16

652.0306 Salinity and sodicity effects 3–17

(a) General ......................................................................................................... 3–17

(b) Measuring salinity and sodicity concentration ....................................... 3–17

(c) Effects of salinity on yields ....................................................................... 3–17

(d) Effect of salinity and sodicity on AWC .................................................... 3–17

(e) Management practices for salinity and sodicity control ........................ 3–18

(f) Toxic elements ............................................................................................ 3–18

652.0307 Crop data bases 3–19

652.0308 State supplement 3–20


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3–22 (210-vi-NEH, September 1997)3–ii

Tables Table 3–1 Critical periods for plant moisture stress 3–3

Table 3–2 Adapted irrigation methods 3–6

Table 3–3 Recommended Management Allowable Depletion (MAD) 3–7

for crop growth stages (% of AWC) growing in loamy soils

Table 3–4 Depths to which the roots of mature crops will extract 3–8

available soil water from a deep, uniform, well drained

soil under average unrestricted conditions

Table 3–5 Irrigation system vs. crops grown 3–15

Figures Figure 3–1 Root distribution systems—deep homogenous soils 3–9

with good water management and no soil restrictions

Figure 3–2 Typical water extraction pattern in uniform soil profile 3–10

Figure 3–3 Effect of root development on soils with depth 3–11

limitations


CropsChapter 3


Chapter 3 Crops

652.0300 Purpose andobjective

The purpose of irrigation is to supplement naturalprecipitation so that moisture requirements of cropsbeing grown are met. Crop response to irrigationvaries with soils, fertility, type of plants, stage ofgrowth, and local climate. Where crop stress causedby moisture shortage is prevented by proper andtimely irrigation, other factors can become inhibitorsto desirable yield and quality.

Knowledge of how plants respond to, and use, soilwater throughout their growing season is essential tosuccessfully design and manage an irrigation system.Continuous plant uptake of soil nutrients has a poten-tial for improving ground water quality. Profitable cropproduction is generally the objective of agriculture.With proper management, soils (or water) affected bysalinity or sodicity can sustain plant growth in perpe-tuity. Irrigation provides the insurance for high qualityand desirable quantity crops at reduced risk in semi-arid, subhumid, and humid areas. It is a necessity inarid regions. The effect of irrigation both onsite andoffsite on soil, water, air, plant, and animal resourcesalong with human considerations needs to be consid-ered.

(a) Soil condition

For desirable crop growth, good soil condition is keyto optimum soil aeration, water infiltration, permeabil-ity, and uniform root development. It also helps reducerunoff and potential soil erosion. Good soil conditioncan be maintained or improved by eliminating excesstillage operations, avoiding field operations while soil-water content is high, using organic material or cropresidue, and using grass and legumes in rotation. Toreduce opportunity of soil compaction on irrigatedpastures, livestock should be excluded during andafter irrigation until adequate soil surface dry-outoccurs.

(b) Nutrient management

A healthy plant uses water more efficiently than aplant that lacks nutrients and trace elements. Totalwater use by a healthy plant is greater than that for aplant deprived of nutrients. However, the yield per unitof water is much greater for healthy plants.

Soil fertility is maintained with proper nutrient man-agement by maintaining proper soil reaction (pH level)and by using an appropriate cropping system. Limingmay be needed on acid soil. On saline soils, leaching ofexcess salts is generally needed. On sodic soils, bothsoil amendments and leaching may be needed. Soiltests, field observations, planned yield and quality, andfield experience help determine the type and amountof fertilizers and other elements to use. Using excessfertilizer or poor application timing can result inmovement of chemicals below the root zone into theground water or off the field.

(c) Soil, water, pest, nutrient, andcrop residue management

Optimum production requires the operator to controlweeds and insects, use high quality seed of adaptedvarieties, apply fertilizer according to plant needs, andpractice good soil and water management during allparts of the growing season. Crops grown should beselected to fit the soil, water, climate, irrigation sys-tem, farm equipment, and market availability. Plantpopulation can generally be increased when practicinggood soil, water, pesticide, nutrient, and crop residuemanagement.


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3–2 (210-vi-NEH, September 1997)

652.0301 Crop growthcharacteristics

(a) Response to water, crop yield,and quality

Water is only one component needed to achieve de-sired crop yield and quality. A practical definition forwater use efficiency is the amount of yield per unit ofarea per unit of water, e.g., 6 bushels of wheat per acreper acre-inch of applied water. Such yield water usecomparison units can provide a basis for comparisonwhen improvements are made.

Maintaining soil water within a desirable depletionrange (preferably less than 5 bars tension) generallyprovides the expected yield and quality. The effect onyield and quality depends on how severe and duringwhich period of crop growth water deficit occurs.Applying excess irrigation water over and above thatnecessary to grow a successful crop will not increaseyields and generally reduces yields.

Other factors, such as the lack of available nutrients,trace elements, and uncontrolled pest activity, maylimit crop yield. Excess irrigation water can leachessential plant nutrients and some pesticides and theirmetabolites below the root zone. This is especiallytrue with nitrates, which are quite mobile in water.Excess irrigation water percolating below the rootzone can pollute ground water.

(b) Critical growth periods

Plants must have ample moisture throughout thegrowing season for optimum production and the mostefficient use of water. This is most important duringcritical periods of growth and development. Mostcrops are sensitive to water stress during one or morecritical growth periods in their growing season. Mois-ture stress during a critical period can cause an irre-versible loss of yield or product quality. Critical peri-ods must be considered with caution because theydepend on plant specie as well as variety. Some cropscan be moderately stressed during noncritical periodswith no adverse effect on yields. Other plants requiremild stress to set and develop fruit for optimum har-vest time (weather or market).

The need for an irrigation should be determined by anonsite examination of the soil for water content or byany irrigation scheduling method for which basic datahave been established. Using only plant appearance asthe moisture deficit symptom can lead to misinterpre-tation, which generally results in reduction of yieldand product quality. When the plant appears to be dry,it may already be in a moisture stress condition. Someplants temporarily wilt to conserve moisture duringotherwise high evapotranspiration periods of the day.Dry appearance may also be caused by other problems(lack of nutrients, insect activity, disease, lack ofessential trace elements). Critical water periods formost crops and other irrigation considerations aredisplayed in table 3–1. Irrigation scheduling techniquesare described in more detail in Chapter 9, IrrigationWater Management.


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Table 3–1 Critical periods for plant moisture stress

Crop Critical period Comments

Alfalfa hay At seedling stage for new seedlings, just Any moisture stress during growth periodafter cutting for hay, and at start of reduces yield. Soil moisture is generallyflowering stage for seed production. reduced immediately before and during

cutting, drying, and hay collecting.

Beans, dry Flowering through pod formation. Sensitive to over-irrigation.

Beans, green Blossom through harvest.

Broccoli During head formation and enlargement.

Cabbage During head formation and enlargement.

Cauliflower During entire growing season.

Cane berries Blossom through harvest.

Citrus During entire growing season. Blossom and next season fruit set occursduring harvest of the previous crop.

Corn, grain From tasseling through silk stage Needs adequate moisture from germinationand until kernels become firm. to dent stage for maximum production.

Depletion of 80% or more of AWC may beallowed during final ripening period.

Corn, silage From tasseling through silk stage and Needs adequate moisture from germinationuntil kernels become firm. to dent stage for maximum production.

Corn, sweet From tasseling through silk stage untilkernels become firm.

Cotton First blossom through boll maturing Any moisture stress, even temporary, ceasesstage. blossom formation and boll set for at least 15

days after moisture again becomes available.

Cranberries Blossom through fruit sizing.

Fruit trees During the initiation and early development Stone fruits are especially sensitiveperiod of flower buds, the flowering and to moisture stress during last 2fruit setting period (maybe the previous weeks before harvest.year), the fruit growing and enlargingperiod, and the pre-harvest period.

Grain (small) During boot, bloom, milk stage, early head Critical period for malting barley is at softdevelopment and early ripening stages. dough stage to maintain a quality kernel.


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Table 3–1 Critical periods for plant moisture stress—Continued


Grapes All growth periods especially during fruit See vine crops.filling.

Peanuts Full season.

Lettuce Head enlargement to harvest. Water shortage results in a sour and stronglettuce. Crop quality at harvest is controlledby water availability to the plant, MAD15 – 20% is recommended.

Melons Blossom through harvest.

Milo Secondary rooting and tillering to bootstage, heading, flowering, and grainformation through filling.

Onions, dry During bulb formation. Maintain MAD 30 – 35% of AWC. Let soil drynear harvest.

Onions, green Blossom through harvest. Strong and hot onions can result from mois-ture stress.

Nut trees During flower initiation period, Pre-harvest period is not key because nutsfruit set, and midseason growth. form during midseason period.

Pasture During establishment and boot stage to Maintain MAD less than 50%. Moisture stresshead formation. immediately after grazing encourages fast

regrowth.

Peas, dry At start of flowering and when pods areswelling.

Peas, green Blossom through harvest.

Peppers At flowering stage and when peppers arein fast enlarging stage.

Potato Flowering and tuber formation to harvest. Sensitive to irrigation scheduling. RestrictMAD to 30 – 35% of AWC. Low quality tubersresult if allowed to go into moisture stressduring tuber development and growth.

Radish During period of root enlargement. Hot radishes can be the result of moisturestress.

Sunflower Flowering to seed development.


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Table 3–1 Critical periods for plant moisture stress—Continued


Sorghum grain Secondary rooting and tilling to boot stage,heading, flowering, and grain formationthrough filling.

Soybeans Flowering and fruiting stage.

Strawberries Fruit development through harvest.

Sugar beets At time of plant emergence, following Frequent light applications during earlythinning, and about 1 month after growth period. Temporary leaf wilt on hotemergence. days is common even with adequate soil water

content. Excessive fall irrigation lowers sugarcontent, but soil moisture needs to beadequate for easy beet lifting.

Sugarcane During period of maximum vegetativegrowth.

Tobacco Knee high to blossoming.

Tomatoes When flowers are forming, fruit is setting,and fruits are rapidly enlarging.

Turnips When size of edible root increases Strong tasting turnips can be the result ofrapidly up to harvest. moisture stress.

Vine crops Blossom through harvest.

Watermelon Blossom to harvest.


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Table 3–2 Adapted irrigation methods

Crop Management - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Adapted irrigation methods - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -depth - - - - - - - - - - - - - - - - - - - - - Surface - - - - - - - - - - - - - - - - - - - - - Sprinkler Micro- Subirr.

- - - - - - level - - - - - - - - - - - - - - - - graded - - - - - - - - - -(ft) border furrow border furrow corrug.

Alfalfa 5 x x x x x xBeans, dry 3 x x x x xBeans, green 3 x x xCane berries 3 x x x x x xCitrus 3 x x x xCorn, grain 4 x x x xCorn, silage 4 x x x xCorn, sweet 3 x x x xCotton 3 x x x x

Grain, small 4 x x x x x x xCranberries 2 x xGrass, seed 3 x x x x x xGrass, silage 3 x x x x x xMilo (sorghum) 3 x x x xNursery stock 0-3 x x x x x x xOrchard 5 x x x x x x xPasture 3 x x x x xPeanuts 3 x x x xPeas 3 x x x x xPotatoes 3 x x x x

Safflower 5 x x x x xSugar beets 5 x x xSunflower 5 x x x x xTobacco 3 x xTomatoes 2 x x x x xTurf, sod 2 x x xTurf 2 x x x xVegetables 1/ x x x x x xVegetables 2/ x x x x x x xVegetables 3/ x x x x x x xVegetables 4/ x x x x x x x

1/ 1-foot depth—Lettuce, onions, spinach.2/ 2-foot depth—Cabbage, brussel sprouts, broccoli, cauliflower.3/ 3-foot depth—Turnips, parsnips, carrots, beets, green beans.4/ 4-foot depth—Squash, cucumber, melons.


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(c) Irrigation related management

Determining when to irrigate a specific crop requiresthe selection of a Management Allowable Depletion(MAD) of the available soil water. MAD is defined asthe percentage of the available soil water that can bedepleted between irrigations without serious plantmoisture stress. MAD is expressed as:

• a percentage of the total Available Water Con-tent (AWC) the soil will hold in the root zone,

• a soil-water deficit (SWD) in inches, or• an allowable soil-water tension level.

Different crops tolerate different soil-water depletionlevels at different stages of growth without going intomoisture stress. Some crops have critical growthperiods during only one stage of growth, while othershave critical periods during several stages of growth.

MAD should be evaluated according to crop needs,and, if needed, adjusted during the growing season.Values of MAD, during the growing season are typi-cally 25 to 40 percent for high value, shallow rootedcrops; 50 percent for deep rooted crops; and 60 to 65percent for low value deep rooted crops.

Recommended MAD values by soil texture for deeprooted crops are:

• Fine texture (clayey) soils 40%• Medium texture (loamy) soils 50%• Coarse texture (sandy) soils 60%

Table 3–2 displays adapted irrigation methods forvarious crops, and table 3–3 lists recommended MADlevels by crop development stages for a few crops.Caution: Medium to fine textured soils can reduceMAD values given in this table.

Table 3–3 Recommended Management AllowableDepletion (MAD) for crop growth stages(% of AWC) growing in loamy soils 1/,2/

Crop - - - - - - - - -- Crop growth stage - - - - - - - - - -Estab- Vege- Flowering Ripening

lishment tative yield maturityformation

Alfalfa hay 50 50 50 50Alfalfa seed 50 60 50 80Beans, green 40 40 40 40Beans, dry 40 40 40 40Citrus 50 50 50 50Corn, grain 50 50 50 50Corn, seed 50 50 50 50Corn, sweet 50 40 40 40Cotton 50 50 50 50Cranberries 40 50 40 40Garlic 30 30 30 30Grains, small 50 50 40 3/ 60Grapes 40 40 40 50Grass pasture/hay 40 50 50 50Grass seed 50 50 50 50Lettuce 40 50 40 20Milo 50 50 50 50Mint 40 40 40 50Nursery stock 50 50 50 50Onions 40 30 30 30Orchard, fruit 50 50 50 50Peas 50 50 50 50Peanuts 40 50 50 50Potatoes 35 35 35 50 4/

Safflower 50 50 50 50Sorghum, grain 50 50 50 50Spinach 25 25 25 25Sugar beets 50 50 50 50Sunflower 50 50 50 50Tobacco 40 40 40 50Vegetables1 to 2 ft root depth 35 30 30 353 to 4 ft root depth 35 40 40 40

For medium to fine textured soils:1/ (Most restrictive MAD) Some crops are typically not grown on

these soils.2/ Check soil moisture for crop stress point approximately one-

third of the depth of the crop root zone.3/ From boot stage through flowering.4/ At vine kill.


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(d) Rooting depth and moistureextraction patterns

The soil is a storehouse for plant nutrients, an environ-ment for biological activity, an anchorage for plants,and a reservoir for water to sustain plant growth. Theamount of water a soil can hold available for plant useis determined by its physical properties. It also deter-mines the frequency of irrigation and the capacity ofthe irrigation system needed to ensure continuouscrop growth and development.

The type of root system a plant has is fixed by geneticfactors. Some plants have tap roots that penetratedeeply into the soil, while others develop many shal-low lateral roots. The depth of the soil reservoir thatholds water available to a plant is determined by thatplant’s rooting characteristics and soil characteristicsincluding compaction layers and water management.The distribution of the plant roots determines itsmoisture extraction pattern. Figure 3–1 shows typicalroot distribution for several field and vegetable crops.Typical rooting depths for various crops grown on adeep, well drained soil with good water and soil man-agement are listed in table 3–4.

For annual crops, rooting depths vary by stage ofgrowth and should be considered in determining theamount of water to be replaced each irrigation. Allplants have very shallow roots early in their develop-ment period; therefore, only light and frequent irriga-tions are needed. Because roots will not grow into adry soil, soil moisture outside the actual root develop-ment area is needed for the plant to develop a full rootsystem in the soil profile. Excess moisture in this areawill also limit root development.

For most plants, the concentration of moisture absorb-ing roots is greatest in the upper part of the root zone(usually in the top quarter). Extraction is most rapid inthe zone of greatest root concentration and where themost favorable conditions of aeration, biologicalactivity, temperature, and nutrient availability occur.Water also evaporates from the upper few inches ofthe soil; therefore, water is diminished most rapidlyfrom the upper part of the soil. This creates a high soil-water potential gradient.

Table 3–4 Depths to which the roots of mature cropswill extract available soil water from a deep,uniform, well drained soil under averageunrestricted conditions (depths shown arefor 80% of the roots)

Crop Depth Crop Depth(ft) (ft)

Alfalfa 5 Peas 2 - 3Asparagus 5 Peppers 1 - 2Bananas 5 Potatoes, Irish 2 - 3Beans, dry 2 - 3 Potatoes, sweet 2 - 3Beans, green 2 - 3 Pumpkins 3 - 4Beets, table 2 - 3 Radishes 1Broccoli 2 Safflower 4Berries, blue 4 - 5 Sorghum 4Berries, cane 4 - 5 Spinach 1 - 2Brussel sprouts 2 Squash 3 - 4Cabbage 2 Strawberries 1 - 2Cantaloupes 3 Sudan grass 3 - 4Carrots 2 Sugar beets 4 - 5Cauliflower 2 Sugarcane 4 - 5Celery 1 - 2 Sunflower 4 - 5Chard 1 - 2 Tobacco 3 - 4Clover, Ladino 2 - 3 Tomato 3Cranberries 1 Turnips 2 - 3Corn, sweet 2 - 3 Watermelon 3 - 4Corn, grain 3 - 4 Wheat 4Corn seed 3 - 4Corn, silage 3 - 4Cotton 4 - 5 Trees

Cucumber 1 - 2 Fruit 4 - 5Eggplant 2 Citrus 3 - 4Garlic 1 - 2 Nut 4 - 5Grains & flax 3 - 4Grapes 5 Shrubs & misc. trees

Grass pasture/hay 2 - 4 for windbreaks

Grass seed 3 - 4 < 10 ft tall 2 – 3+Lettuce 1 - 2 10 – 25 ft tall 3 – 4+Melons 2 - 3 > 25 ft tall 5+Milo 2 - 4Mustard 2 Other

Onions 1 - 2 Turf (sod & lawn) 1 - 2Parsnips 2 - 3 Nursery stock 1 - 3Peanuts 2 - 3 Nursery stock pots


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Figure 3–1 Root distribution systems—deep homogenous soils with good water management and no soil restrictions

Wheat

CucumberTomato

Lima bean

Strawberry

Red clovermature

Mature cornSugar beetirrigated

Sugar beetsandy soil withlayers of clay2nd and 4th ft.

PotatoOnion

Spinach

Alfalfairrigated

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2' 3'1' 5' 6'4' 8' 9'7'

2' 3'1' 5' 6'4'2' 3'1' 5' 6'4' 7'

2' 3'1' 5' 6'4' 7'


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In uniform soils that are at field capacity, plants usewater rapidly from the upper part of the root zone andmore slowly from the lower parts. Figure 3–2 showsthe typical water extraction pattern in a uniform soil.About 70 percent of available soil water comes fromthe upper half of a uniform soil profile. Any layer orarea within the root zone that has a very low AWC orincreased bulk density affects root development andmay be the controlling factor for frequency of irriga-tions.

Figure 3–3 illustrates the effect on root developmentof some limitations in a soil profile. Variations andinclusions are in most soil map units, thus uniformityshould not be assumed. Field investigation is requiredto confirm or determine onsite soil characteristicsincluding surface texture, depth, slope, and potentialand actual plant root zone depths.

Soil texture, structure, and condition help determinethe available supply of water in the soil for plant useand root development. Unlike texture, structure andcondition of the surface soil can be changed withmanagement.

Figure 3–2 Typical water extraction pattern in uniformsoil profile

��40% extraction here

30% here

20% here

10%

D/4

D/4

D/4

D/4

Roo

t zo

ne w

ater

extr

acti

on d

epth

-D

Note: Approximately 70 percent of water used by plants isremoved from the upper half of the plant root zone.Optimum crop yields result when soil-water tensions inthis area are kept below 5 atmospheres. Very thin tillagepans can restrict root development in an otherwisehomogenous soil. Never assume a plant root zone.

Observe root development of present or former crops.

Numerous soil factors may limit the plant’s geneticcapabilities for root development. The most importantfactors are:

• soil density and pore size or configuration,• depth to restrictive layers and tillage pans,• soil-water status,• soil condition,• soil aeration,• organic matter,• nutrient availability,• textural or structural stratification,• water table,• salt concentrations, and• soil-borne organisms that damage or destroy

plant roots.

Root penetration can be extremely limited into drysoil, a water table, bedrock, high salt concentrationzones, equipment and tillage compaction layers, densefine texture soils, and hardpans. When root develop-ment is restricted, it reduces plant available soil-waterstorage and greatly alters irrigation practices neces-sary for the desired crop production and water con-servation.

Root penetration is seriously affected by high soildensities that can result from tillage and farm equip-ment. Severe compacted layers can result from heavyfarm equipment, tillage during higher soil moisturelevel periods, and from the total number of operationsduring the crop growing season. In many medium tofine textured soils, a compacted layer at a uniformtillage depth causes roots to be confined to the upper 6to 10 inches. Roots seek the path of least resistance,thus do not penetrate a compacted dense layer exceptthrough cracks. Every tillage operation causes somecompaction. Even very thin tillage pans restrict rootdevelopment and can confine roots to a shallow depth,thereby limiting the depth for water extraction. This isprobably most common with row crops where manyfield operations occur and with hayland when soils areat high moisture levels during harvest.


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Compaction layers can be fractured by subsoilingwhen the soil is dry. However, unless the cause ofcompaction (typically tillage equipment itself), thenumber of operations, and the method and timing ofthe equipment’s use are changed, compaction layerswill again develop. Only those field operations essen-tial to successfully growing a crop should be used.Extra field operations require extra energy (tractorfuel), labor, and cost because of the additional wearand tear on equipment. The lightest equipment withthe fewest operations necessary to do the job shouldbe used.

For site specific planning and design, never assume aplant root zone depth. Use a shovel or auger to ob-serve actual root development pattern and depth withcultural practices and management used. The previouscrops or even weeds will generally show root develop-ment pattern restrictions. See NEH Part 623, (Section15), Chapter 1, Soil-Plant-Water Relationship, andChapter 2, Irrigation Water Requirements, for addi-tional information.

Figure 3–3 Effect of root development on soils withdepth limitations

��Dry soil at or belowwilting percentage��

��

Severe compacted layer(may be < .25 inch thick)

�

High water table

Rock


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652.0302 Crop and irriga-tion system water require-ments

(a) Crop evapotranspiration

Plants need water for growth and cooling. Smallapertures (stomata) on the upper and lower surfacesof the leaves allow for the intake of carbon dioxiderequired for photosynthesis and plant growth. Watervapor is lost to the atmosphere from the plant leavesby a process called transpiration. Direct water evapo-ration also occurs from the plant leaves and from thesoil surface. The total water used by the specific crop,which includes direct evaporation from plant leavesand the soil surface and transpiration, is called cropevapotranspiration (ETc). Processes to determinelocal crop evapotranspiration are described in NEH,Part 623, Chapter 2, Irrigation Water Requirements,and in Chapter 4, Water Requirements, of this guide.

(b) Irrigation frequency

How much and how often irrigation water must beapplied depends on the soil AWC in the actual plantroot zone, the crop grown and stage of growth, therate of evapotranspiration of the crop, the planned soilManagement Allowable Depletion (MAD) level, andeffective rainfall. More simply put; it depends on thecrop, soil, and climate.

Never assume a plant root zone for managementpurposes. Check actual root development pattern anddepth. See section 652.0301(d).

Once a MAD is selected, determining when to irrigatesimply requires estimation or measurement of whenthe soil moisture reaches that level. Coarse texturedand shallow soils must be irrigated more frequentlythan fine textured deep soils because fine textureddeep soils store more available water. The moistureuse rate varies with the crop and soil. It increases asthe crop area canopy increases, as humidity decreasesand as the days become longer and warmer.

Frequency can be estimated by dividing the MAD bythe estimated or measured evapotranspiration of thecrop as follows:

Irrigation frequency (days)MAD inches

Crop ET rate (in/day)=

( )

A much higher quality product is produced if the MADlevel is kept less than 35 percent in some crops, suchas potatoes, pecans, vegetables, and melons. This isalso true for mint.

Several methods are available for irrigation scheduling(determining when to irrigate and how much to apply).They are described in Chapter 9, Irrigation WaterManagement.

(c) Net irrigation requirement

The net amount of water to be replaced at each irriga-tion is the amount the soil can hold between fieldcapacity and the moisture level selected when irriga-tion is needed (MAD). Maintaining the same soilmoisture level throughout the growing season is notpractical and probably not desirable. Ideally, an irriga-tion is started just before the selected MAD level isreached or when the soil will hold the irrigation appli-cation plus expected rainfall. The net amount of waterrequired depends on soil AWC in the plant root zoneand the ability of a particular crop to tolerate moisturestress. If the MAD level selected is 40 percent of AWCin the root zone (Soil-water Deficit = 40%), it is neces-sary to add that amount of water to bring the root zoneup to field capacity. For example if the total soil AWCin the root zone is 8 inches and MAD = 40%:

Net irrigation = ×=

40 8

3 2

%

.

in

in

In semihumid and humid areas, good water managersdo not bring the soil to field capacity with each irriga-tion, but leave room for storage of expected rainfall.When rainfall does not occur, the irrigation frequencymust be shortened to keep the soil moisture within theMAD limit. It is a management decision to let MADexceed the ability of an irrigation system to applywater. For example, if a center pivot sprinkler systemapplies a net of 1 inch per cycle, let MAD be equal to 1inch plus expected rainfall. MAD for a surface irriga-tion system will be typically greater as heavier applica-tions are required for best uniformity across the field.


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(d) Gross irrigation requirement

The gross amount of water to be applied at eachirrigation is the amount that must be applied to assureenough water enters the soil and is stored within theplant root zone to meet crop needs. No irrigationsystem that fully meets the season crop evapotranspi-ration needs is 100 percent efficient. Not all waterapplied during the irrigation enters and is held in theplant root zone. Also, all irrigation systems have adistribution uniformity less than 100 percent. Applyingtoo much water too soon (poor irrigation water man-agement) causes the greatest overuse of water. Irriga-tion systems and management techniques are availablethat reduce the avoidable losses. They are described inchapters 5, 6, 7, 8, and 9 of this guide.

Unavoidable losses are caused by:• Unequal distribution of water being applied

over the field.• Deep percolation below the plant root zone in

parts of the field.• Translocation or surface runoff in parts of the

field.• Evaporation from the soil surface; flowing and

ponded water.• Evaporation of water intercepted by the plant

canopy under sprinkler systems.• Evaporation and wind drift from sprinklers or

spray heads.• Nonuniform soils.

For a given irrigation method and system, irrigationefficiency varies with the skill used in planning, de-signing, installing, and operating the system. Localclimatic and physical site conditions (soils, topogra-phy) must be assessed. To assure that the net amountof soil water is replaced and retained in the root zoneduring each irrigation, a larger amount of water mustbe applied to offset the expected losses. The grossamount to be applied is determined by the equationshown at the bottom of this page.

For more information on irrigation and system require-ments, see Chapter 4, Water Requirements; NEH Part623, Chapter 2, Irrigation Water Requirements; and theWest National Technical Center publication, FarmIrrigation Rating Index (FIRI), A method for planning,evaluating and improving irrigation management.

Gross irrigation amount (in)Net amount to be replaced in

Overall irrigation efficiency of system imcluding management (%)

Management Allowable Depletion (MAD)Overall irrigation efficiency

=( )

=


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652.0303 Reduced irriga-tion and restricted watersupply

Several opportunities are available to the irrigator insemiarid, subhumid, and humid areas for reducedirrigation water application:

• Maximizing effective rainfall.• Deficit or partial season irrigation.• Selection of crops with low water requirements

during normal high water use periods; i.e.,small grains, (or accept the risk of droughtperiods).

• Selection of drought resistant crops and variet-ies that provide yields based on water availabil-ity, i.e., alfalfa hay, grass pasture (accept thereduced yields caused by drought periods).

• Irrigate just before critical growth period(s) ofthe crop to minimize critical plant moisturestress during those periods.

• Use state-of-art irrigation scheduling tech-niques that use local area climate and onsiterainfall data, and field-by-field soil moisturestatus monitoring.

• Use tillage practices that allow maximumsurface storage and infiltration of rainfallevents, reducing runoff and soil surfaceevaporation.

• Follow an intensive crop residue managementand mulch program and minimize tillage toreduce soil surface evaporation.

• Reduce irrigated acreage to that which can beadequately irrigated with the available watersupply.

Risk is less when growing crops on deep, high AWC,loamy soils and in climatic areas that have adequaterainfall for the crop. The risk is greater when growingcrops on low AWC soils even in areas that have ad-equate rainfall during the growing season. Whengrowing high value crops, an irrigation system andadequate water supply are highly desirable for insur-ance against potential crop loss. A detailed economicanalysis should be completed to provide estimates ofoptimum net benefits. The analysis should include costof water, pumping costs, reduced yields caused byreduced crop water use, and reduced tillage operationcosts. Subsequent management decisions should bebased on this analysis. See chapter 11 for additionaldiscussion.

In some areas irrigation water delivery systems, in-cluding management, limit on-farm water managementimprovements. Rotational delivery systems have thelowest on-farm water management potential, while on

demand delivery systems have the highest.

Improving both management and the irrigation systemcan reduce the amount of water applied and moreeffectively use existing water supplies. Improvingwater management, including irrigation schedulingand adequate water measurement, is always the firstrecommended increment of change. Improving exist-ing irrigation systems is the next. Unless the existingirrigation system is unsuitable for the site, crop grown,or water supply, converting to another irrigationmethod seldom produces benefits equal to improve-ments in water management. See Chapter 5, Selectingan Irrigation Method, for additional information onselecting and applying the best method or system forthe site.


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652.0304 Adapted irriga-tion systems

All crops can be efficiently irrigated by more than oneirrigation method and system. Crops grown and theircultural requirements aid in determining the irrigationmethod and system used. Crops can be placed inbroad categories as follows:

Category 1. Row or bedded crops:

sugar beets, sugarcane, potatoes,pineapple, cotton, soybeans, corn,sorghum, milo, vegetables, vegetableand flower seed, melons, tomatoes, andstrawberries.

Category 2. Close-growing crops (sown, drilled,or sodded):small grain, alfalfa, pasture, and turf.

Category 3. Water flooded crops:

rice and taro.

Category 4. Permanent crops:

orchards of fruit and nuts, citrusgroves, grapes, cane berries, blueber-ries, cranberries, bananas and papayaplantations, hops, and trees and shrubsfor windbreaks, wildlife, landscape,and ornamentals.

A comparison of irrigation system versus crops thatcan be reasonably grown with that system is displayedin table 3–5.

See Chapter 5, Selecting an Irrigation Method, Chapter6, Irrigation System Design, and chapters 3, 4, 5, 7, and11 of the National Engineering Handbook, Part 623(section 15) for more information on adapted irriga-tions systems.

Table 3–5 Irrigation system vs. crops grown

Irrigation system - - - Crop category - - -1 2 3 4

Surface

Basins, borders x x x

Furrows, corrugations x x x

Contour levee - rice x x

Sprinkler

Side (wheel) roll lateral x x

Hand move lateral x x x

Fixed (solid) set x x

Center pivot, linear move x x

Big guns - traveling, stationary x x

Micro

Point source x

Line source x x

Basin bubbler x

Mini sprinklers & spray heads x

Subirrigation x x x x


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652.0305 Temperature—effects and management

Crop yield and quality can be negatively affected bytemperature extremes, both cold and hot. Applicationof water in a timely manner can provide some degreeof protection. Water can also be applied to cool plantsto maintain product quality, to delay bud development,and to provide frost protection of buds, flowers, andyoung fruit.

(a) High temperatures

Extremely high temperatures can• put plants into a temporary plant moisture

stress,• hasten untimely fruit development and ripen-

ing,• cause moisture stress in ripening fruit,• sunburn berries and other fruit,• overheat bare soils during seed germination

(i.e., lettuce), and• overheat standing water in basin irrigation.

Water used for temperature modification as a crop andsoil coolant is typically applied with a sprinkler/spraysystem.

(b) Low temperatures

When temperatures drop below the critical tempera-ture, damage can occur to both annual and perennialplants. If ambient air temperature and humidity areseverely low, permanent damage to fruit, citrus, andnut trees can occur. When it drops below freezing, thedeveloping buds and flowers on fruit and berry plantscan be damaged. Temporary freeze back of newgrowth in grasses and legumes can occur, and healthyannual plants can be killed or damaged beyond recov-ery.

Water can be applied to provide frost protection toabout 25 °F. Sprinkler/spray systems that apply wateroverhead onto the plant canopy are typically used.This allows a protective layer of ice to build up on theleaves, blossoms, and buds. Frost protection involvesheat release caused by changing water to ice. Theprocess must be understood to determine the applica-tion rate and timing of water for adequate frost protec-tion. Some limited success has been attained withunder-tree spray systems and surface flooding sys-tems.

See NEH, Part 623 (Section 15), Chapter 2, IrrigationWater Requirements, for further information.


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652.0306 Salinity and sod-icity effects

(a) General

In arid areas nearly all irrigation water and soils con-tain salts, some of which are toxic to plants and ani-mals. When water is removed from the soil profile byplant transpiration and soil surface evaporation, saltsremain in the soil profile and on the soil surface. If thesoil-water solute is high in sodium, the soil becomessodic. All other ionic concentrations in solution (i.e.,calcium, magnesium, potassium) cause salinity. Theseconditions are particularly common where most of thecrop water requirement comes from irrigation. Theproblem eventually becomes serious if

• irrigation or natural precipitation is notsufficient to leach the accumulating salts,

• water and soil management are less thanadequate,

• soil and water amendments are inadequate, or• the soil is poorly drained.

Salinity and sodicity problems can also develop as aresult of saline seeps, use of poor quality irrigationwater including flooding by brackish water near theocean, or by using drainage water from upslope irriga-tion. As salt concentrations increase above a thresholdlevel, the growth rate, mature size of crops, and prod-uct quality progressively decrease.

Principal objectives of water management are tomaintain soil tilth, soil-water content, and salinity andsodicity levels suitable for optimum plant growth. Anatural occurring internal drainage or an installeddrainage system within the usable soil profile is essen-tial. See Chapter 13, Quality of Water Supply, foradditional information.

(b) Measuring salinity andsodicity concentration

A method has been developed to measure and quantifysalinity and sodicity levels in soils. Thus, the salinity ofa soil can be determined by measuring the electricalconductivity, ECe, of the soil-water extract expressedin millimhos per centimeter or decisiemens per meter,

corrected to a standard temperature of 77 °F (25 °C).1 mmho/cm = 1 dS/m.

Salt molecules in solution produce electrically-chargedparticles called ions. Ions can conduct an electricalcurrent. The greater the concentration of ions in asolution, the greater the electrical conductivity of thesolution.

To measure sodicity in soil, the Sodium AdsorptionRatio (SAR) is used. It is a measure of the ratio ofsodium to calcium plus magnesium present.

SARNa

Ca Mg

=

+

2

1

2

(c) Effects of salinity on yields

Crop yields and quality are reduced when salinitylevels exceed a certain threshold. See NEH, Part 623,Irrigation, Chapter 1, Soil-Plant-Water Relationships(table 1–8), and Chapter 2, Irrigation Water Require-ments. The information presented provides two essen-tial parameters for expressing salt tolerance:

• The salinity threshold level above which re-duced yield will occur

• The percent yield reduction per unit salinityincrease beyond the threshold level

(d) Effect of salinity and sodicityon AWC

Plants extract water from the soil by exerting anadsorptive force or tension greater than the attractionof the soil matrix for water. As the soil dries, remain-ing water in the soil profile is held more tightly by soilparticles. Salts also attract water. The combination ofdrying soils and elevated salt concentrations results inless water at a given tension being available for plantuptake. The reduction in water available to the crop assalinity increases is evident in figure 2–7, chapter 2,which shows the volumetric water content versus soil-water potential for a clay loam soil at various degreesof soil salinity, ECe. Table 2–4 provides a process toestimate AWC based on texture and ECe of 0 to 15mmho/cm.


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If the salt content of the soil cannot be maintained orreduced to a point compatible with the optimum yieldof a crop, a more salt-tolerant crop should be grown orthe operator must accept reduced yields. Most often,salinity or sodicity is not maintained below plantthresholds because less than adequate soil and watermanagement practices are followed.

(e) Management practices forsalinity and sodicity control

The major objective of salinity management is to keepsoil salinity and sodicity below thresholds for seedgermination, seedling establishment, crop growth, andquality while minimizing the salt loading effects ofdrainage outflow. Procedures that require relativelyminor changes in management are:

• Improved irrigation water management• Improved crop residue management• Adding soil and water amendments• Selection of more salt-tolerant crops• Leaching with additional irrigation water• Preplant irrigations• Changing of seed placement on the furrow bed

Maintaining a higher soil-water content decreases soil-water tension; thereby, increasing water available toplants. Alternatives that require significant adjust-ments are:

• Changing the water supply• Changing irrigation methods• Land leveling for improvements to surface

drainage and irrigation water distribution• Modifying the soil profile• Providing for internal drainage

ASCE Report No. 71, Agricultural Salinity Assessmentand Management (1990) gives specific recommenda-tions regarding salinity and sodicity assessment andmanagement.

Irrigated agriculture cannot be sustained withoutadequate leaching and internal drainage to controlbuildup of calcium, sodium, and other toxic ions in thesoil profile. Where subsurface drainage systems areinstalled to improve downward water movement andremoval of the required leaching volume, soluble saltsplus other agricultural chemicals and fertilizers movewith the drainage water. They have the potential tomove to streams, wetlands, estuaries, and lakes.

Where possible, leaching events should be plannedwhen soil nitrate levels are low. The leaching require-ment for salinity control can be minimized with goodirrigation water management and with adequatelydesigned, installed, and operated irrigation waterdelivery and application systems.

Drainage outflow with high salt concentrations can bedisposed of through use of evaporation ponds (thesalts remain), or often water can be directly reused asan irrigation water supply for applications wheresaline water is acceptable, such as irrigation of salt-tolerant plants or for industrial uses. In some areasdrainage outflow with high salt concentration may notbe allowed to be released to public waters without apoint-discharge permit, or it must be desalted. In mosthigh salt content water reuse operations, the saltmoves and precipitates out at another spot. It does notgo away.

When irrigating with high salt content water, internalsoil drainage and leaching are required to maintain anacceptable salt balance for the plants being grown.The salt concentration in drainage outflow can bequite high, and concern for safe disposal still exists.Some saline and sodic tolerant crops require highquality water for germination and establishment. Oncethe crop is established, poorer quality water can beused. Generally, water containing different saline-sodic concentrations should not be mixed.

(f) Toxic elements

Toxicity problems can be the same or different fromthose of salinity and sodicity because they can occurbetween the plant and the soil and may not be causedby osmotic potential or water stress. Toxicity normallyresults when certain ions are present in the soil orabsorbed with soil-water, move with the plant transpi-ration stream, and accumulate in the leaves at concen-trations that cause plant damage. It also can resultfrom water sprayed directly on leaf surfaces. Theextent of the damage depends on the specific ionconcentration, crop sensitivity, crop growth stage, andcrop water use rate and time.

The usual toxic ions in irrigation water include chlo-ride, sodium, and boron. Excessive chlorine in domes-tic water systems and salts from water softeners inhome systems can also be a problem. Not all crops are


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sensitive to these ions, but some crops are very sensi-tive. Chemical analysis of plant tissue, soil-waterextract, and irrigation water is most commonly used toidentify toxicity problems.

The affect of toxic elements (i.e., selenium) on water-fowl from drainage outflow has also been observed inseveral areas. Toxic elements that occur naturally inthe soil (i.e., selenium and boron) in high concentra-tions or were used as pesticide control in past years(i.e., arsenic and mercury) are of great concern. Irriga-tion water that deep percolates below the plant rootzone can potentially carry these dissolved toxic ele-ments downslope into the ground water and caneventually flow into wetlands, estuaries, streams, andlakes.

See NEH Part 623, Chapter 2, Irrigation Water Require-ments, for further information on management of soilsalinity and sodicity and on assessment of boron andother toxic elements.

652.0307 Crop data bases

The Field Office Computer System (FOCS) includes aplant data base that is site specific and can be useddirectly by applications for planning and designingirrigation systems.


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652.0308 State supplement

nig ch 3 - irrigationtoolbox.comirrigationtoolbox.com/.../nehpart652ch3.pdf · 652.0307 crop data...

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