IR1. Soil Water Plant Relationships
Instructional objectives
After finishing this lesson, the student should understand: 1. How important soil and water are for plant growth.
2. Soil classification in relation to agriculture
3. Identifying the type of water trapped in soil pores
4. The importance of soil-water constants
5. Crop watering intervals
6. Water’s significance in plant growth
Introduction
Soil and water are crucial for plant growth, providing a structural base and allowing the root system to spread. Soil pores hold moisture and nutrients, essential for plant growth. Most plants require oxygen for respiration, so full soil pores can restrict root growth. Irrigation practice requires an adequate and timely water supply to the plant root zone for optimal crop yield. Studying the relationship between soil pores, water-holding capacity, and plant water absorption rate is essential for water resources engineers.
1 Soil-water system
Soil is a complex mixture of solid, liquid, and gas, with mineral matter and organic matter being the main components. Its pore space contains degraded organic matter. In dry regions, water molecules surround soil particles, causing pressure lower than atmospheric due to surface tension capillarity. This results in osmotic pressure, which plants need to exert to extract water for growth. Understanding soil-water relations is crucial for understanding soil properties and types of soils.
2 Soil properties
A diverse mixture of mineral and organic particles make up soil. The following significant characteristics categorize soil in accordance with its applicability to crop production (which in turn influences the decision-making process of irrigation engineering):
• Soil texture
• Soil structure
Soil texture:
Soil texture is determined by the proportions of sand, silt, and clay in the soil, with 12 main classes identified by the US Department of Agriculture.
A soil can generally be categorized as follows using textural gradations:
• Soils with an open or light texture are often coarse or sandy and contain little silt or clay.
• Soils with a medium texture have significant amounts of sand, silt, and clay, much like loamy soil.
• Soils with a tight or heavy texture and a significant concentration of clay.
Soil structure:
Soil structure, characterized by the arrangement of soil particles and aggregates, is a crucial property affecting aeration, permeability, and water holding capacity, and can be classified using three indicators:
• Type: Primary structures come in four different varieties: platy, prism-like, block-like, and spheroidal.
• Class: Each of the main categories has five recognized classifications. extremely fine, fine, medium, coarse, and extremely coarse are the available options.
• Grade: This is a representation of the aggradation level, or the ratio of aggregate to unaggregated material, that occurs when the aggregates are moved around or lightly crushed. Structure-less, weak, moderate, strong, and very strong grades are used to describe aggregates according to how stable they are under disturbance.
3 Soil classification
The qualities and characteristics of soils vary greatly. They must be categorised in order to establish the relationships between their qualities. The following types of soils can be found in India:
• Alluvial soils: Alluvial soils are formed by silt deposition from rivers during floods, flood plains, and coastal belts. These soils are the largest and most important in India, contributing to its agricultural wealth. They vary in texture from clayey loam to sandy loam, and have good water holding capacity for irrigation.
• Black soils: Black soils, derived from the Deccan trap, are heavy-textured, clay-content, and found in Maharashtra, Madhya Pradesh, Andhra Pradesh, Gujarat, and Tamilnadu, with high water holding capacity but poor drainage.
• Red soils: Red soils, formed from weathered igneous and metamorphic rock, are found in Tamil Nadu, Karnataka, Goa, and other regions, with low water holding capacity and well-drained nature.
• Laterites and Lateritic soils: Laterite, a unique formation in India and tropical countries, forms on hills in Karnataka, Kerala, Madhya Pradesh, Orissa, Maharashtra, West Bengal, Tamilnadu, and Assam, with low clay content and good drainage.
• Desert soils: The arid region in western Rajasthan, Haryana, and Punjab, between the Indus and Aravalli ranges, is characterized by recent desert conditions, poor soil development, and good irrigation water application.
• Problem soils: Problem soils, such as highly eroded, ravine, and steeply sloping areas, require proper reclamation measures for crop cultivation due to their unique characteristics.
The following table includes a list of some of the primary soil types in the nation:
4 Classification of soil water
As previously mentioned, different amounts of water may be present in the pores of the soil. The following definitions pertain to the water that is held in the soil pores:
• Gravitational water: Gravitational water, a portion of soil that can easily drain off due to gravity, is not available for plant use as it quickly leaves the root zone.
• Capillary water: Capillary water, retained in soil after gravitational water drains, is held by surface tension and gradually absorbs by plant roots, serving as the primary source of water for plant growth.
• Hygroscopic water: Hygroscopic water, a thin film of soil under negative pressure, is not available to plants due to its absorption by moist air.
Soil water’s definitions are based on physical factors, but some properties are not directly related to plant growth, which will be discussed further.
5 Soil water constants
Soil water constants are defined proportions of soil water that determine the availability of water for plant growth in a specific soil.
• Saturation capacity: Soil saturation capacity refers to the maximum water holding capacity of the soil, where the soil’s moisture tension is nearly zero at all pore filling.
• Field capacity: Soil moisture tension at field capacity varies between 1/10 to 1/3 atmospheres, indicating that water is retained in micropores of soil, despite gravity draining off macro-pores and drained off micropores.
• Permanent wilting point: Plant roots extract water from saturated soil, but as extraction progresses, moisture content decreases and negative pressure increases, leading to plant wilting.
There are two stages of wilting points, and they are as follows:
• Temporary wilting point: The amount of water in the soil that causes a plant to wilt during the day but recover during the night or when water is given to the soil is indicated by this.
• Ultimate wilting point: Even after adding water to the soil, the plant wilts at such a soil water content and dies.
Water contents are expressed as percentage of water held in soil pores compared to fully saturated soil. Available water for plants is defined as the difference in moisture content between field capacity and permanent wilting point. Soil constants are expressed as a percentage by weight of moisture available at that point.
6 Soil water constants expressed in depth units:
Soil water constants can be expressed as weight percentages of moisture content in dried soil samples or as volume of water stored in root zones per unit area, converting them to units of depths:
Assume the following:
• Root zone depth = D (m)
• Specific weight of soil = γs (kg/m3)
• Specific weight of water = γw (kg/m3)
• Area of plot considered = 1m x 1m
It may be noted that plants cannot extract the full available water with the same efficiency. About 75 percent of the amount is rather easily extracted, and it is called the readily available water. The available water holding capacity for a few typical soil types are given as in the following:
Soil Texture
Field Capacity (FC) percent Permanent Wilting Point (PWP)
percent Bulk Density(γs) Kg/m3 Available water per meter depth of soil profile(m)
Sandy
5 to 10
2 to 6
1500 to 1800
0.05 to 0.1
Sandy loam
10 to 18
4 to 10
1400 to 1600
0.09 to 0.16
Loam
18 to 25
8 to 14
1300 to 1500
0.14 to 0.22
Clay loam
24 to 32
11 to 16
1300 to 1400
0.17 to 0.29
Clay
32 to 40
15 to 22
1200 to 1400
0.20 to 0.21
7 Water absorption by plants
Water absorption in plants is primarily through the roots, with a higher concentration of roots near the soil surface. In normal soil with good aeration, most plants’ roots remain within 0.45m to 0.60m of surface soil layers, meeting most water needs. As water availability decreases, plants extract more water from lower depths. When top layers of the root zone are kept moist through irrigation, plants extract about 40% of their water. The remaining 10% is extracted from the lower quarter of the root zone. Water extracted from the soil moves upwards through transpiration, losing 95% of it.
8 Watering interval for crops
Water is essential for crop growth, either naturally from rainfall or artificially through irrigation. Irrigation intervals depend on soil moisture depletion rate, with shorter intervals during summer and shorter intervals for sandy soils. If water supply is limited, intervals may be prolonged, allowing soil moisture to deplete below 50% before irrigation. Optimal rates of soil moisture for typical crops are provided.
• Maize : Field capacity to 60 percent of availability
• Wheat : Field capacity to 50 percent of availability
• Sugarcane: Field capacity to 50 percent of availability
• Barley : Field capacity to 40 percent of availability
• Cotton : Field capacity to 20 percent of availability
Rice requires a constant standing depth of 5cm water throughout its growing period, resulting in 50-70% water loss. Most crops, except rice, apply water to raise soil moisture content to its field capacity. Soil moisture depletes gradually due to evaporation and transpiration, with evapo-transpiration (ET) determining the soil water depletion rate. Operational soil moisture ranges for common crops are provided:
Rice:
The rice crop, grown in lowland and upland conditions, requires a constant submergence of 50mm soil for maximum yields. Water resources are limited, so land must be submerged during critical growth stages. 50-75% of water applied to the crop is lost through deep percolation, with soil pores filled with water. The total water requirement varies with soil texture.
Wheat:
Tall wheats require optimum soil moisture between field capacity and 50% availability, while dwarf wheats require more wetness. The active root zone varies, with water requirements varying in northern and central India.
Barley:
Barley, a similar crop to wheat, has a deeper, well-spread root system that can withstand droughts, with an active root zone ranging from 0.6m to 0.75m on various soil types.
Maize:
The crop is grown across India, with optimal soil moisture ranges of 100-60% and root zone depths of 0.4-0.6. Irrigation requirements vary based on rainfall, with north India requiring 0.1m and 0.15m before monsoon, and south requiring 0.3m.
Cotton:
Cotton crop requires optimal soil moisture between field capacity and 20% available water, with root zone ranging up to 0.75m, and total water requirement 0.4m to 0.5m.
Sugarcane:
Sugarcane requires 100-50% of water availability in the maximum root zone, with water depth requirements varying across Bihar, Karnataka, and Madhya Pradesh.
9 Importance of water in plant growth
Water plays a crucial role in a plant’s life cycle, maintaining cell turgidity, providing oxygen and hydrogen for photosynthesis, acting as a solvent for nutrients, and distributing food throughout the plant body. Transpiration, a vital process, occurs at the maximum rate when water is available, and plants help dissipate heat from solar radiation through transpiration, which uses plant water. Insufficient soil moisture can limit transpiration rate.
10 Irrigation water quality
The quality of irrigation water is crucial in agriculture, as it contains soluble salts that affect crop growth. The suitability of water is determined by factors like electrical conductivity, sodium percentage, boron content, PH, and the presence of cations and anions.
Salt-effected rivers and deltaic regions produce water with high concentrations, affecting tank and lake water quality. Soil salinity and aridity influence water quality, while groundwater resources from shallow or deep wells are generally poor.
• high aridity
• high water table and water logged conditions
• in the vicinity of sea water
11 Important Definitions
1. Root Zone: The region of soil surrounding a plant where its roots come into contact with the soil is known as the soil root zone (Figure 4).
2. Soil Moisture tension: Moisture tension, which is equivalent in size to soil water pressure but the opposite in sign, exists in soils that are only partially saturated with water. Moisture tension is the amount of pressure needed to force soil water through a porous, permeable wall or membrane into hydraulic equilibrium with a body of water with a similar composition.
3. Wilts: Plant wilting results in drooping. Due to exhaustion or weakness brought on by a lack of water, plants bend or droop downward.
Bibliography
• Majumdar, D K (2000) Irrigation Water Management by, Prentice Hall of India.