Soil-water-plant relationships

I. Rationale

The main objective of irrigation is to provide the plant with sufficient water to prevent stress that may cause yield reduction or poor quality of harvest. This is particularly important when irrigating nonflooded rice or upland (non-rice) crops, which require a higher degree of water control because these crops (particularly the non-rice crops) are more sensitive to inadequate or oversupply of water. Excessive water in the soil retards plant growth due to poor aeration and causes water logging of the soils. On the other hand, inadequate moisture will retard plant growth due to water stress.

To effectively control water and optimize its productivity, one must understand (a) the basic soil parameters that affect water availability, retention, storage capacity, and movement of water through the soil, and (b) the response of rice and non-rice crops to different rates and timing of water application.

II. Learning objectives

At the end of this module, you will be able to:

1. enumerate and describe the composition and function of soils in crop production; 2. describe the importance of organic matter in enhancing the physical, chemical and

biological properties of the soil; 3. understand the soil moisture relations, such as the mechanisms of soil moisture

retention, movement, availability, and frequency of irrigation; 4. describe the effects of water application intensity on the growth and yield of rice and

non-rice crops;

III. Content outline

The lesson is divided into two parts.

Part 1. Soil-water relations

1. Definition of soils2. Function of soils3. Major components of soils in relation to plant growth

3.1. The organic matter of the soil3.2. The mineral composition of the soil 3.3. The water and air composition of the soil

4. Depth of root zone.

Part 2. Plant-water relations

5. Function of water on plants5.1. Effect of water application rates on the growth and yield of rice

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5.2. Effect of water deficit on rice yield

6. Summary

IV. Support information

Brady, Nyle C. and Ray R. Weil, 1999. The Nature and Properties of Soils. 12th ed. Prentice Hall. Upper Saddle River, New Jersey 07458. 881 pp.

International Institute for Land Reclamation and Improvement (IILRI), 1979. Drainage Principles and Applications. I. Introductory Subjects. Vol 1. No. 16. 239 pp. The Netherlands.

Israelsen, O.W. and V.E. Hansen, 1980. Irrigation Principles and Practices. 4th ed. John Wiley and Sons, Inc. N.Y. 417 pp.

Sanchez, P.A., 1976. Properties and management of soils in the tropics. A Wiley-interscience publication. John Wiley and Sons, New York, 618 pp.

Thompson, Louis M. and Frederick R. Troeh, 1978. Soils and Soil Fertility. 4th ed. Publications in Agricultural Sciences. McGraw-Hill Book Company. N.Y. 516 pp.

University of the Philippines at Los Baños, 1983. Rice production manual; Philippines. Rev.ed. University of the Philippines at Los Baños, College, Laguna, 523 pp.

Water Use Seminar Damascus, 1972. Irrigation and Drainage Paper No. 13. Food and Agriculture Organization of the United Nations. Rome. 312 pp.

V. Content presentation

Part 1. Soil-water relations

1. Definition of soils

Soil is a collection of natural bodies occupying portion of the earth’s surface. It supports plants and have properties due to the integrated effect of climate and living matter acting upon parent material as conditioned by relief over period of time.

2. Functions of soil

Soil is a storehouse for plant nutrients. A medium for growing crops. It is a mixture of mineral and organic matter that is capable of supporting life. It provide(s) the crop(s) with essential plant nutrients in addition to water and oxygen for root respiration. Unless the supply of water and oxygen can be maintained, the rate of nutrient uptake is reduced. Other aspects of soils which have a bearing on plant growth are:

• Its temperature should be favorable to plant growth;• Its mechanical resistance to the movements of roots and shoots should not be

too high;• It should provide an environment free of chemical or biological conditions

detrimental to plant growth, (like extreme acidity, excess soluble salts, toxic substances, disease organisms, etc.).

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3. Major components of soils in relation to plant growth

The four major components of soils are mixed in complex patterns. However, the proportions of the soil volume occupied by each component are shown in Figure 1.

a. Solid (50%) - consisting of • Mineral matter (45%)• Organic matter (5%)

b. Voids (50%) – consisting of • Liquid (water) (25%)• Gaseous (air) (25%)

The relative proportions of these four components greatly influence the behavior and productivity of the soils.

3.1. The organic matter of the soil

The organic matter constitutes only between 1% and 6% of the weight of the topsoil in upland soils. However, the influence of the organic components on the soil is often greater than its small proportion would suggest. Organic matter gives the soils their dark color. An average ideal soil consists of only about 5% (by weight) organic matter, which is composed of a wide range of organic (carbonaceous) substances, including living organisms (the soil biomass), and carbonaceous remains of organisms.

For most soil, the average organic matter content ranges from 2 - 4 %. It holds and supplies Nitrogen and other nutrients, (like phosphorous and sulfur) for the plant and has also other important function in the soil such as chemical, physical and biological.

An organic matter content of less than 1% is considerably low and these are limited to desert areas. In contrast, organic matter content of more than 20% (by weight) are referred to as peat or peaty soils. Both cases reduce the productivity of the soil.

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Figure 1. Composition of an average soil at field capacity.

SolidsVoids

Influence of OM on the physical properties of soilsOrganic matter binds mineral particles into granular soil structure that is largely responsible for the loose, easily managed conditions of productive soil. Promotes the formation and the stability of soil aggregates. Organic matter also increases the amount of water a soil can hold for plant growth.

The decomposition of fresh organic matter, in particular, produces microbial and mycelia of organism, which are most effective in aggregating soils. Aggregation leads to increase porosity, which means increases aeration, infiltration and percolation of water, and a reduction in runoff and erosion hazards. The improved soil structure and the high water absorption capacity of humus increase the moisture retention capacity of the soil.

Influence of OM on the chemical properties of soilsThe decomposition of organic matter yields N, P, and S and promotes the extraction of plant nutrients through the formation of organic and inorganic acids. There can be a considerable fixation of nitrogen from the air by the non-symbiotic bacteria, which obtain their energy from the cell sap of leguminous crops. The humus components of organic matter significantly increase the Cation Exchange Capacity (CEC) of mineral colloids. The CEC of a soil is an important component of fertility, or at least the potential soil fertility (Thompson and Troeh, 1978).

Influence of OM on the biological properties of soils.Microorganism lives on dead plant materials in the soil, and in so doing, breaks it down to the black, finely divided organic matter known as humus. Organic matter, including plants and animal residues, is the main food that supplies the carbon and energy to soil organism.

3.2. The mineral composition of the soil

The mineral matter in the soil (which is about 45% by volume of the average soil) is a complex mixture of solid mineral materials. The soil inherits mineral matter from its parent rock and organic matter from its living organisms. These materials constitute the solid portion of the soil and form its skeleton. Some of the physical properties of the mineral soils are:

Soil texture This refers to the percentage by weight of each of the mineral fractions, sand, silt and clay (Figure 2). These fractions are defined in terms of the diameter (in millimeter) of the particles (nonspherical particles are considered to have an equivalent diameter between maximum and minimum dimensions). Loam soil refers to a mixture of sand, silt, and clay that exhibit the properties of each fraction about equally. It contains less clay than sand and silt because clay properties are strongly expressed relative to the amount of clay present.

The mineral soil elements can be classified according to their sizes. The size distribution of the ultimate soil particles is referred to as “texture”. There are various textural classifications, but the most common used for agronomic purposes are given in Table 1. The relative proportion of sand, silt and clay in a soil determines its textural class.

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The soil triangle in Figure 2 shows the different number of possible textural classes. (The feel method of determining textural class is described in Annex 1).

Table 1. Particle Size Limits

Soil * Separate Diameter (mm) Limits (mm)

SAND ---------------- 2.00 – 0.050 --------------Very Coarse ---------------- 2.00 – 1.00Coarse ---------------- 1.00 – 0.500Medium ---------------- 0.500 – 0.250Fine ---------------- 0.250 – 0.100Very Fine ---------------- 0.100 – 0.050

SILT ---------------- 0.050 – 0.002 ----------------Coarse ---------------- 0.050 – 0.020Fine ---------------- 0.020 – 0.002

CLAY ---------------- < 0.002 -----------------

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* Materials larger than 7.5 cm in diameter is usually called a stone. Materials between 7.5 cm and 2 mm are termed gravel. Materials smaller than 2 mm is referred to as fine earth.

sand

Figure 2. Soil triangle for determining the textural classification of soils

Soil structure This refers to the 3-dimensional arrangement of the primary particles (sand, silt and clay) and/or secondary soil particles (micro-aggregates) into a certain structural pattern (macro-aggregates or peds) or larger unit. It results from the tendency of the fine soil particles, especially clay and humus to stick together. The aggregates of textural elements are held together by colloids (mineral or organic) and separated from one another by large pores and cracks.

Good structure depends on the desired speed with which the air and water move through the soil. Good structure for growing flooded rice is that attained by puddling (i.e., the destruction of aggregates) to eliminate the downward water movement. For other crops, good structure is that which maintains aggregate stability upon abrupt changes of moisture and intense rainfall.

Particle density (specific gravity) The particle density is the mass per unit volume of soil particle. It is usually expressed in grams per cubic centimeter of soil. The particle density is referred to as the true density. The specific gravity of soil components are:

Organic matter – 1.47 grams per cm3

Sand – 2.66 grams per cm3

Clay – 2.75 grams per cm3

The specific gravity of mineral soil varies from 2.6 to 2.9 (average – 2.65 grams per cm3).

Bulk density The bulk density or volume weight or apparent specific gravity is the dry weight of a unit volume of soil in its field condition. It is the mass of a dry soil per unit bulk volume, expressed in grams per cm3. It has the same numerical value as the apparent specific gravity, which is defined as the ratio of a unit bulk volume of soil to the weight of equal volume of water.

The bulk density of uncultivated soils usually varies between 1.0 and 1.6 grams per cm3. Compact layer may have a bulk density of 1.7 or 1.8. generally the finer the texture of the soil and higher the organic matter, the smaller is the bulk density.

Porosity (pore space)The pore space of a soil is that portion of a unit bulk volume which is occupied by air and/or water (volume of voids). Voids known as pore spaces occur between the solid particles (Figure 3). These pore space usually constitutes about half the volume of the A horizon and somewhat less than the volume for the B and C horizons. The volume of pore space depends largely on the arrangement of the solid particles. The porosity (η ) is the volume percentage of the unit bulk volume not occupied by the solid particles.

Bulk densityη = 100 1 − ----------------------

Particle density

Example: Given: Bulk Density = 1.40 gm/cc

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Particle Density = 2.65 gm/cc

Porosity (η ) = 100 [ 1 – (1.40/ 2.65)] = 47%

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Demonstration No. 1. Soil Physical Properties

Demonstration on the density of soil using a foam as a model

1. Size of foam (uncompressed) : 3.80 cm × 7.50 cm × 10.00 cm.

2. Size of foam (compressed) : 0.55 cm × 7.50 cm × 10.00 cm

3. Weight of (dry) foam : 7.45 grams

4. Volume of uncompressed foam : 85.00 cu cm

5. Volume of compressed foam : 1.25 cu cm

6. Density (Du) of uncompressed foam : 0.026 gram/cu.cm.

7. Density (Dc) of compressed foam : 0.181 gram/cu.cm.

8. Porosity (η ) = [1- (Du)/(Dc)] η = [ 1 − (0.026/0.181)] = 0.856 or 85.6%

9. Void Ratio (ε ) = [(Vv) / (1-Vv)]

ε = [0.856 / (1 – 0.856)]

ε = 5.94Remarks:

• The density of uncompressed foam would represent the bulk density of the foam.For soils, the average is 1.0 – 1.3 grams/ cu.cm.

• The density of the compressed foam would represent the particle density of the foam. For soils, the average particle density is 2.65. grams/ cu.cm

Note that the density increases as the material is compressed or compacted. Compacted soil will exhibit a greater resistance to root penetration. It also reduces the volume of voids and thus the free changes of water and air which is essential for crop growth.

Usually the porosity of mineral soils varies between 35 % for compacted soil and 60 % for loose topsoil.

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Demonstration No 2. Water holding (retention) capacity, wilting point, and available moisture.

Using a foam as a model:

1. Weigh the dry foam. Weight of dry foam = 7.45 grams.

2. Immerse the foam in water until it is fully saturated.

3. Raise the saturated foam and collect the water that drains from it.

4. Measure the volume and weigh the water collected until the drifting is completed.Volume = 89.6 ml; Wt = 90.41 grams

5. This amount of water (89.6 ml or 90.41 grams) is called the free or gravitational water.

6. Weigh the wet foam (weight of foam + water retained by the foam). Weight of wet foam = 93.08 grams.

7. Expel and collect the water in foam by squeezing it.

8. Weight of foam after expelling the water from the wet foam = 17.94 grams

9. Weight of water expelled from the wet foam = 93.08 – 17.94 = 75.14 grams.

10. Weight of water tightly held in the foam = Wt of wet foam-Wt of dry foam = 17.94 – 7.45 = 10.49 grams.

Summary:

1. The weight (volume) of free water = 90.41 grams (or 89.60 cu cm) [ 51.36% ]2. The weight of water expelled from the saturated foam = 75.14 grams [ 42.68% ]3. The weight of water tightly held in the wet foam = 10.49 grams [ 5.96% ]

Remarks:• Item 1 represents the amount of water that is lost due gravitational forces. This is

the amount of water lost through deep percolation. • Item 2 represent the amount of water that is retained in the foam after the free water

lost. This upper limit of water is available for plant use.• Item 3 represent the amount of water that is tightly held in the foam. This water is

not available for plant use. This is also collect the lowest limit of the available water because plants will show signs of wilting because they cannot extract sufficient moisture at the same rate in general, the moisture in the plant is lost by transpiration.

Void ratio (e) is defined as the ratio of the volume of voids to the volume of solids.

Vv Vve = ------------ or e = --------- Vs 1 – Vv

Example: A porosity of 35% correspond with a void ratio of: 0.35 0.35

e = --------- = ------------ = 0.54 1 – 0.35 0.65

A porosity of 60 % corresponds with a void ratio of:

0.60 0.60e = ----------- = ---------- = 1.5 1 – 0.60 0.40

3.3. The water and air composition of the soil

The water and air in the soil consist of about 50% (by volume of the average soil). At field capacity, the volume of air and water are on the average approximately 25% each. However, the proportion varies depending on the amount of moisture present in the soil.

Roots require oxygen for respiration and other metabolic activity. They absorb water and dissolve nutrients from the soil, and produce carbon dioxide, which has to be exchange with oxygen from the atmosphere. This aeration process, which takes place by diffusion and mass flow, requires open pore space in the soil. If roots are to be developed well, water plus nutrients and air must be available simultaneously.

3.3.1. Water and air Water and air share the pore space in variable proportions. The smaller pores (micro-pores) generally contain water, and the larger ones (macro-pores) contain air. Figure 3 shows that the pore space of a coarse textured soils are larger than those on fine textured soils, but the total volume in the latter is generally greater than in the former. The shape

and continuity

of the large pores determine to large extent how well the soil is aerated. It is desirable for the water that enters the soil to continue moving downward through the profile until the pore space contain about two-thirds water and one-third air.

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Coarse Textured SoilFine Textured SoilMACRO -PORES

MICRO -PORES

Figure 3. Pore space of a fine textured and coarse textured soil.

Soil with high clay contents, especially those low in organic matter, may hold so much water but they are poorly aerated (Figure 2). Sandy soils may let water pass through too rapidly and not retain enough to support plant growth through a dry period.

The total pore space of a soil can be filled with air, water, or both. The last mentioned situation – that is for both air and water present – is the most desirable for soil of which agriculture is practiced. The air-filled porosity, then is the fraction of the bulk volume occupied by air. The water-filled porosity or fraction of the bulk volume occupied by water is often called soil water ratio (soil-water content on a volume basis). Soil water content is more commonly expressed as the “dry weight soil moisture fraction”, i.e., the ratio of the mass of water to the mass of dry soil. Hence, the soil water ratio multiplied by the ratio of water density over bulk density is equal to the dry weight soil moisture fraction.

Forms of soil water

• Gravitational or free water – This form of water is loosely held in the soil and could be easily lost by gravitational force.

• Capillary water – this form of soil water is held in the soil by capillary action (force) that is less than atmospheric pressure.

• Hygroscopic water – is a form of soil water that is present not only in the pores but also on the surface of the soil particle. It is tightly held in the soil and cannot be removed except by oven drying at 105 oC.

Graphical representations of the different forms of soil water are shown in (Figure 4).

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151/3 0

10,000

1,000

Figure 4. Soil moisture constants and their approximate equivalents in bars of suction as related to the relative availability of water to plants (courtesy Raymond W. Miller).

SUCTION IN BARS

INCREASING MOISTURE CONTENT

AVAILABLE WATER

GRAVITATIONAL WATER

(DRAINABLE)

UNAVAILABLE WATER

FIELD CAPACITY SATURATION

WILTING PERCENTAGE AIR DRY

OVEN DRY

FLOODED CONDITION

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Effect of OM on the water retention capacity of the soil.

Effect of OM on soil water content.

Water-retaining properties of the soilThe capacity of a soil to retain moisture that is readily available for plant growth is an important factor in irrigation and land use planning. This applies not only where there is adequate rainfall, but also in irrigation projects, where irrigation water has to be applied at the right time and at the right quantity.

Field Capacity (FC) – the moisture content of the soil when downward movement of water has vertically ceased. This condition usually exists in a well-drained soil about two or three days after rain or irrigation (Figure 5). Field capacity is closely related to soil texture and is influenced by the organic matter content, types of minerals present and soil structure.

Wilting Point (WP) – The wilting point, also called permanent wilting point, is the moisture content at which most plants roots are no longer capable of taking up water from the soil, and the plant suffer irreversible wilting. The moisture tension at wilting is about or often equal to 15 atmosphere. This represents the lower limit of available moisture.

Hygroscopic Coefficient – Is the percentage of water remaining in an air-dry soil. Evaporation can remove more water than can grow plants.

Available Moisture (AM) – Is the moisture holding capacity of a given undisturbed soil sample between field capacity (FC), the upper limit and wilting point (WP), the lower limit expressed in volume percentage.

The Total Available Moisture (TAM) – is the sum of available moisture values for each layer of the actual or potential rooting depth of the soil profile, i.e. the effective soil depth. The term effective soil depth refers to the depth of soil in which the plant roots can readily penetrate in search for water and plant nutrients. The characteristics of any layer limiting the effective depth will also affect the internal drainage of the soil. Limiting layer are compact or distinctly indurated layers, bedrock, gravel, coarse sand or any abrupt and pronounced discontinuity within the profile.

Total Readily Available Moisture (TRAM) – Not all the total available water (AM x Effective Root Depth) may be considered available because the crops (or most crops could

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Figure 5. The speed with which two soils of different texture drain to field capacity. Field capacity is reached when the curve levels off to a constant moisture percentage. Drainage will be faster in coarse textured soils than in fine-textured ones.

not extract the moisture near wilting point. A rule of thumb is that the TRAM-value, the total readily available moisture, is about two-thirds of the TAM-value.

Illustrative example:

Given a sandy clay loam soil with the following properties:

Effective depth of soil (De) is 0.50 m, it consists of two layers: A=0.30 m and B=0.20 m.For layer A the Field Capacity (FC) = 30% (by weight); wilting point (WP) = 10% (by weight) and bulk density (BD) of 1.20 grams per cubic centimeter. For layer B the field capacity (FC) is 25% (by weight), wilting point (WP) is = 8% (by weight), and bulk density (BD) is 1.4 grams per cubic centimeter.

Question: Determine the total available moisture in the soil.

SOLUTION: d = FC –WC BD De100

where, d = amount of water in soil (mm),De = depth of soil (mm),BD = bulk density (gram per cu cm)FC = field capacity (%)WC= wilting coefficient (%)

.. . d1= 30-10 1.2 0.30 × 1000 mm/m 100

= ( 0.2 ) ( 1.2 ) ( 300 )

= 72 mm (TAM at Layer A)

d2 = 25 − 8 1.4 0.20 M × 1000100

= ( 0.17 ) ( 1.4 ) ( 200 )

= 47.6 mm (TAM at Layer B)

.. . TAM = d1 + d2 = 72 + 47.6 = 119.6 mm (for layers A and B)

The Total Readily Available Moisture (TRAM) as mentioned in the above section,

TRAM = ( 2 / 3 ) ( TAM )

This 2/3 value may be referred to as the factor, which defines the allowable moisture depletion (AMD), for a given crop. Therefore, this value may be modified depending on

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the crop requirement and your decision as a manager. For example, vegetable crops the AMD would be about 0.20 – 0.25, but for corn (maize) = 0.60 – 0.70.

So, for grain (maize) crops, the allowable moisture depletion (AMD) would be 0.70. Therefore, the TRAM would be

TRAM = (0.70) (TAM)= ( 0.70 ) ( 119.6 ) = 83.7 mm

For vegetable crops (onion), the rooting depth is about 0.30 m and the AMD would be 0.20. But in this case, the TAM should be taken at the 0.30 m soil depth because the rooting depth of vegetable is within layer an only. Therefore for onion use only the TAM for the upper Layer A

TRAM = ( 0.20 ) ( 72 ) = 14.4 mm.

In this example, the TRAM for corn (maize) would be 83.7 or 84 mm while that of onion is 14 mm. Therefore, corn (maize) should be irrigated when about 84 mm equivalent depth of soil moisture shall have been depleted and for onion, irrigate when about 14 mm of soil moisture is depleted.

Water-transmitting properties of the soil The rate of water movement is governed by gravity or capillary forces, or both are referred to as soil permeability. The term soil permeability (as used in general) is a quantitative sense and mean as the readiness with which a soil conducts or transmits water. To define soil permeability more precisely, a distinction is made between (a) the surface intake rate which determines the relation between water absorption and runoff, and (b) the subsurface percolation rate which determines the internal profile drainage. The aspects of soil permeability are shown in Figure 6.

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ZoneTopsoil

Subsoil

Fig 4. Three aspects of soil permeability

Sub stratumDense Layer

Ground- water

Infiltration Rate

Percolation Rate

Hydraulic Conductivity

Impervious Layer

Water Flow condition

Unsaturated

Nearly saturated

Saturated

Soil Permeability

////////\\\\///\\

Figure 6. Three aspects of soil permeability.

The intake and percolation rates, both refer to vertical permeability under unsaturated conditions. These two terms however are not synonymous. The intake (or infiltration) rate refers specifically to the entry of water into the soil surface (transmission plus storage), whereas percolation rate refers to the water movement through the soil and maybe defined as the quantity of water passing through a unit area of cross section per unit time at a given depth in the soil mass.

Infiltration and redistribution of soil water in the soil profile.Figure 7 shows the redistribution of soil water in a fine textured and coarse textured soil after the infiltration of a gentle flooding of the (soil) surface with a head of water not exceeding 1 cm. The redistribution of water in the profile after infiltration has stopped is considerably faster in the fine textured soil than in the coarse-textured sand. The wetting front penetrates into the soil as long as there is water infiltration into the soil, but soon after infiltration ceases, the movement of the wetting front will stop. The movements of the wetting front in a fine-texture-soil continue until it stabilizes at greater depth.

Figure 7. Infiltration and redistribution of soil water in a clay loam and sandy soil.

Often soil water profiles, for both vertical and horizontal flow, have sharp increases in water content near the infiltration boundary (x = 0) or at the surface soil. As a result, the water content in the saturated zone near the surface is higher than in the transmission zone, which is the zone of near constant water content lower in the profile. The value of the water content in this transmission zone decreases as the rate of water entering the soil decreases, and it is highest in the case of flood infiltration as shown in Figure 8. This figure shows where the water content profiles are shown after 8 cm of water has infiltrated into a clay loam soil by three different methods of wetting: that is – gentle flooding, and raining with two intensities, 1 cm/hr and 0.1 cm/hr. The rate of flows in the rain treatments is approximately equal to the capillary conductivity at the water content of the constant (transmission) region.

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Figure 8. Infiltration into clay loam with three different wetting methods.

4. Depth of root zone

The importance of having an adequate depth of soil (Figure 9) in which to store satisfactory amounts of irrigation water at each irrigation should be emphasized. Shallow soil root zone require frequent irrigation to keep crops growing. Deep soil of medium texture and loose structure permit plants to root deeply, provide for storage of large volumes of irrigation water in the soil, and consequently sustain satisfactory plant growth during relatively long periods between irrigation.

Soil retains water against the gravitational force because soil porosities exhibit capillary or matric forces. The root zone water storage capacity depends strongly on the profile texture distribution and to the total soil depth proliferated by plant roots. Soil structure affects soil water storage by influencing pore volume and size distribution, resistance to root extension, and profile hydraulic properties.

The volume of water actually absorbed by some plant roots and consumed to produce a crop may be practically the same for shallow and deep soils. Yet nearly all practical irrigators recognize that more water is required during the crop-growing season to irrigate a given crop on a shallow soil that is required for the same crop on a deep soil.

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DEPTHROOT ZONE

Figure 9. Typical root zone depth of a rice plant.

End of filtration

1 hr after end of infiltration

24 hr after end of infiltration

Excessive deep percolation losses usually occur when shallow soils overlying coarse textured, highly permeable sands and gravels are irrigated.

Assuming a favorable unrestricted root zone, the depth or rooting increases during the entire growing period. The hotter the climate or the longer the growing period, the deeper the roots will penetrate. A rough guide, depth of rooting varies from 1 to 1-1/2 ft (0.30 m - 0.45 m) depth per month of active growth, depending upon the crop and climate.

Part 2: Water-plant relations

5. Function of water on plants

Water is one of the most important factors in rice production. It affects the • Physical characters of the rice plant,• Nutrient status of the soil, • Nature and extent of weed growth, and • The various cultural properties are done

5.1. Effect of water application rates on the growth and yield of rice

Generally, research has shown that rice grows best on saturated or flooded soils and that maximum potential yield can be achieved when the soil are maintained under such moisture conditions.

Studies at IRRI during the dry seasons showed that optimum water application intensity (for IR 8) ranged from 6 to 7 mm per day (Figure 10). Shows that increasing water application above this optimum level did not materially increase the yield, but below it, yield sharply declined. The low yields attained were closely associated with the length of time during the crop growth that the soil moisture was seriously depleted. In other words, yield declined because of water deficit in some parts of the growth stage during the season. Hence to ensure optimum growth and yield, adequate supply of water has to be provided relative to the various stages of crop growth, particularly rice crop.

The daily rate of evaporation is closely related to the daily potential evapotranspiration requirement of the crop. At the later stages of plant growth, transpiration represents a major portion of the potential evapotranspiration rate. The water requirement of the rice crop at flowering is not greater than at the other growth stages.

This type of relationships however, did not account for the timing effects of water stress during the seasons. Studies have shown that stress during the vegetative growth stage was significantly less damaging to yields than stress at the reproductive period. The crop appearance recovered quite well from lack of water during the vegetative stage, but it did not recover from stress that lasted the reproductive period.

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Yield (t ha

Y

–1)

Water application intensity (mm/day)

Maximumyield

Incremental yield increase (∆ Y2)

Additional amount of water applied to attain maximum yield (∆ W2)

Incrementalyield increase (∆ Y1)

Additionalamount of water applied(∆ W1)

Figure 10. Functional relationships between rice yield and water application intensity for a rice crop with 100 kg N ha−1 (IRRI).

Optimum yield

Rice is especially sensitive to declining water availability since it requires much water and has relatively low water use efficiency.

Soil drying up to field capacity for 2 weeks at maximum tillering and panicle initiation caused highest yield reduction on Maahas clay (IRRI, 1987). Extending the period to 4 weeks depressed yields further. However, drying periods on Luisiana clay resulted in increases. The same trends were found with saturated water regimes when drainage effects were investigated.

• High yields of rice do not require continuous standing water on the field. High yield could be obtained with saturated soil regimes maintained continuously in the field to allow ET to take place at the potential rate. However, if weed growth is a problem, continuous flooding up to panicle initiation and then continuous saturation after that seems the most appropriate technique of water efficient irrigation without significant yield reduction.

• In some situations, yields have been increased by permitting occasionally slight drying of the soil provided the soil is flooded or saturated from the panicle initiation until the crops nears maturity or 10 days (for sandy soils) to 15 days (for clayey soils) before harvest to hasten maturity.

• The height of the rice plant seems to be directly related to the depth of water in the paddy, i.e., and the height increases with increasing depth. Tiller number, on the other hand, appears inversely related over a relatively wide range of moisture conditions.

• Maintaining continuous saturated conditions could significantly reduce percolation rate. But if soil cracks are developed, percolation rate will be increased, which may even surpass the rate for shallow flooded field conditions. The tiller number increases as the depth of water decreases as the soil dries. But when the soil drying has reached a relatively extreme level, the till number reduces sharply.

5.2. Effect of water deficit on rice yield

Water stress or deficit in irrigated rice areas is measured and expressed in various ways. In this module, we will express as water deficit index (WDI) which is defined as;

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n de.du 2

WDI =∑

where, de = maximum depth of water table below the ground surface; du = number of days the water table remained below the ground surface in one

deficit period; n = number of water deficit periods during a season.

Figure 11 shows the relationship between water stress and rice yield. Note that water stress is much more detrimental to rice crops in saline than in non-saline (normal) soils. For example, a WDI of 200 caused about 0.5 t ha−1 yield reduction in non-saline soil, but the reduction was about 3 times more in saline soil. The higher yield loss in saline soil was the accentuated effect on the plant of soil salinity created deficit. At WDI = 0, where there is no water deficit, the yield levels were nearly the same, but with increasing WDI, the yield reductions in saline soils were more drastic.

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3. Summary

The lecture notes describe some of the basic soil properties, which are very essential in irrigation water management. It discussed the soil composition and functions of soils in crop production, the importance of organic, matter in the physical and chemical properties of soils. The soil moisture relation, such as moisture retention and the basic water movement concepts, such as infiltration and percolation were described. It also described the effects of withdrawing water on the growth and field of the rice plant and other crops.

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Figure 11. Water deficit index (WDI) and rice yield relationship Inarihan River Irrigation system (IRIS), Camarines Sur, Philippines, 1986 and 1987 DS.

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Annex 1.

A method for determining texture by feel

The first, and most critical, step in the texture-by-feel method is to knead a walnut-sized sample of moist soil into a uniform puttylike consistency, slowly adding water if necessary. This step may take a few minutes, but a premature determination is likely to be in error as hard clumps of clay and silt may feel like sand grains. The soil should be moist, but not quite glistening. Try to do this only one hand so as to keep your other hand clean for writing in a field notebook (and shaking hands with your client).

While squeezing and kneading the sample, notes its malleability, stickiness, and stiffness, all properties associated with the clay content. A high silt content makes a sample feel smooth and silky, with little stickiness or resistance to deformation. A soil with a significant content of sands feels rough and gritty, and makes a grinding noise when rubbed near one’s ear.

Get a feel for the amount of clay attempting to squeeze a ball of property moistened soil between your thumb and the side of your forefinger, making a ribbon of soil. Make the ribbon as long as possible until it breaks from its own weight (see Figure A.1).

Interpret your observations as follows:

1. Sand: Gritty, loose, without cohesion whether moist or dry.2. Sandy Loam: Very gritty, some cohesion because of colloidal material and

grinding noise.3. Loam: Characteristic of grittiness predominant but particles stick.4. Silt Loam: Smooth and floury feel prominent, no grinding audible.5. Clay Loam: Slightly gritty, plastic, some tendency in shine of rubbed when moist

or cut when dry. Dry lumps can be crushed with some difficultly.6. Silty Clay Loam: Smooth and floury with little grit, very plastic, noticeable shine

if rubbed when moist or cut when dry. Dry lumps can be crushed between fingers, but with difficulty.

7. Clay: Stiff, plastic without grit, even when a small sample is bitten between the teeth. Tendency to shine strongly when rubbed. Dry lumps have a polished surface when cut, cannot be crushed between fingers.

8. Sandy Clay Loam: Soil exhibits moderate stickiness and firmness, forms ribbons 2.5 to 5 cm long, and grinding noise is audible; grittiness is prominent feel.

9. Loamy Sand: Soils form a ball, but will not form a ribbon.10. Sandy Clay: Soil exhibits dominant stickiness and firmness, forms shiny ribbons

longer than 5 cm, and grinding noise is audible, grittiness is dominant feel11. Silty Clay: Smooth, floury feels prominent no grinding audible.

A more precise estimate of sand content (and hence more accurate placements in the horizontal dimension of the textural class triangle) can be made by wetting a pea-sized clump of soil in the palm of your hand and smearing it around with your finger until it your palm becomes coated with a soup like suspension of soil. The sand grains will stand out visibly and their volume as compared to the original “pea” can be estimated, as can their relative size (line, medium, coarse etc.).

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It is best to learn the method using samples of known textural class. With practice, accurate textural class determinations can be made on the spot.

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Annex 2.

General relationship between the soil moisture and feel and appearance of the soil

Moisture between Wilting

Feel or Appearance

Point and field Capacity: Coarse Soil Light Soil Medium SoilHeavy & Very Heavy Soils

0 ( wilting point) Dry, loose, single grained, flows through fingers

Dry, loose, flows through fingers

Dry, sometimes crusted but breaks down easily to powder condition

Hard, baked, cracked, sometimes has loose crumbs on surface

50% or less Appears dry, will not form a ball

Appears dry, will not form a ball

Somewhat crumbly, holds together under pressure

Somewhat pliable balls under pressure

50%-75% Same as above Tends to ball under pressure, but seldom holds

Forms ball, somewhat plastic, will sometimes stick with pressure

Forms ball, ribbons out between thumb and forefinger

75% to field capacity Tends to stick together, sometimes forms very weak ball under pressure

Forms weak ball, breaks easily, will not stick

Forms ball, very-pliable, sticks readily if high in clay

Easily ribbons out between fingers, has stick feeling

Field capacity Wet outline of ball

Same as coarse soil

Same as coarse soil

Same as coarse soil

Above field capacity Appearance of free water when soil is balled in hand

Free water released with kneading

Can squeeze out water

Puddles and free water forms on surface

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