Soil, Water & Plant Relationships

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SOIL, WATER AND PLANT RELATIONSHIPS

Next THE SOIL'S AVAILABLE

INTRODUCTION

Also see this link to a United Nations soil and water training manual that covers this same material, but describes things in metric units. Another link to material on this topic is the NRCS National Engineering Handbook, part 652 .

This chapter will introduce you to the basics of what scientists call "soil-water-plant" relationships. The ideas form a model system of how water enters the soil, moves through the soil, into the plant root system, and back to the atmosphere. More important they identify the important components of the system and provide standards of measurement so that we can control this movement.

Major ideas presented are . . .

Soil "holds" water against the pull of gravity, retaining it for crop use. There are limits to this ability. The upper limit is call FIELD CAPACITY. The lower is called PERMANENT WILTING POINT.
You can add water to soil above FIELD CAPACITY. It will not hold this additional water. It will drain below the effective root zone to become deep percolation.
Between field capacity and permanent wilting point is the soil's AVAILABLE WATER HOLDING CAPACITY (AWHC). This is the amount of water that the soil will hold that is available for the crop to use.
As the crop pulls water from the soil, the soil holds onto the remaining water harder and harder, putting more and more stress on the crop. You will see the crop wilt during the hottest parts of the day. Sooner or later, if the stress gets big enough, the crop will permanently wilt. The soil moisture is at PERMANENT WILTING POINT.
Soil water can be measured in two ways . . .
a. Volumetrically, the actual amount of water in the soil.
b. Tension, a measure of the water-holding forces in the soil.
The crop doesn't care how much water is physically in the soil, only how hard it is to get out. Thus, although a volumetric measurement will tell us how much water is in the soil and, therefore, how much to irrigate, the tension measurement is more important in terms of preventing crop stress.
The volumetric standard of measurement is inches of water held per foot of soil or just inches/foot. Available water holding capacities vary from 1 to 2.5 inch/foot.
The tension standard of measurement is pressure, usually centibars.
The rate at which crops extract water from the soil is called EVAPO-TRANSPIRATION, ETc. ETc is the combination of soil surface evaporation and plant transpiration and is measured in terms of inches of water per day. Normal ETc rates for cotton are around .05 in/day as seedlings to .35 in/day as a full-grown plant.
ETc varies with the plant, the climate, the level of soil moisture, and plant condition (fertilizer/pest/disease stress). ETc can be measured and predicted.
The INFILTRATION RATE, measured in inches/hour, is a measure of how fast water is soaking into the ground. Infiltration rates will decrease during an irrigation.
The APPLICATION RATE, also measured in inches/hour, is a measure of fast we are applying water. Knowing the application rate of sprinkler systems is especially important. They usually run from .1 to .5 inches/hour.
If the application rate is higher than the infiltration rate, runoff occurs. There should not be excessive runoff with a trickle or sprinkler irrigation system.
There are several methods available for measurement of soil moisture both for volumetric (the neutron probe, gravimetric, "feel") and tension (tensiometers, gypsum blocks, leaf pressure chambers). They all have their strengths and weaknesses.
Very high or out-of-balance salts will modify many of the measurements and results of different measurements (high or low). Refer to the chaper on salinity for further information.

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WATER HOLDING CAPACITY

Soil "holds" water available for crop use, retaining it against the pull of gravity. This is one of the most important physical facts for agriculture. If the soil did not hold water, if water was free to flow downward with the pull of gravity as in a river or canal, we would have to constantly irrigate, or hope that it rained every two or three days. There would be no reason to preirrigate. And there would be no such thing as dryland farming.

The soil's ability to hold water depends on both the soil texture and structure. Texture describes the relative percentages of sand, silt, and clay particles. The finer the soil texture (higher percentage of silt and clay), the more water soil can hold.

Gravity is always working to pull water downwards below the plant's root zone. To counteract the pull of gravity, soil is able to generate its own forces, commonly called "matric forces" ("matric" because of the soil "matrix" structure that forms the basis for the forces).

An important fact about the soil's water-holding forces is that as the level of soil moisture goes down, the soil generates more force. This is the reason that some water will move up into the root zone from a shallow ground water table. As the plant extracts water in the root zone, the soil pulls water up from the area with more water to the area with less.

As you would expect, the rate at which the water-holding forces go up with decreasing soil moisture is different for different soils. In a coarse soil, they will go up slowly. This means that plants can extract a great amount of water from coarse soils before they stress. In contrast, these forces rise quickly in finer soils.

Graphically, the relationship can be described by the Figure SWP-1. Looking at the lowest line for a coarse soil. You can see that at A, the soil moisture level is very high and the water-holding forces are low. This means that the plant can extract water easily from the soil. At B, the soil moisture level is lower but the water-holding forces haven't gone up that much. The plant can still extract water easily. However at C, the soil moisture level is very low and the water-holding forces have increased greatly. The plant cannot extract water easily and will be stressed.

FIGURE SWP-1: Soil Moisture Level (Depletion, %) vs. Soil Moisture Tension (Bars).

Looking at the top line for a finer soil. At A, as with the coarse soil, the water-holding forces are low when the soil moisture level is high. However, at B, the soil moisture level has dropped somewhat but the water-holding forces have gone up greatly. And at C, where the soil moisture level is low, the water-holding forces have gone up very high.

We will be coming back to this idea of increasing soil water-holding forces with decreasing soil moisture many times.

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Last updated September 2000


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Cost-Share Programs Irrigation Guide USBR Water Manage- ment Plan Water Management Handbook Water Quality Data	SOIL, WATER AND PLANT RELATIONSHIPS Next THE SOIL'S AVAILABLE
	INTRODUCTION Also see this link to a United Nations soil and water training manual that covers this same material, but describes things in metric units. Another link to material on this topic is the NRCS National Engineering Handbook, part 652 . This chapter will introduce you to the basics of what scientists call "soil-water-plant" relationships. The ideas form a model system of how water enters the soil, moves through the soil, into the plant root system, and back to the atmosphere. More important they identify the important components of the system and provide standards of measurement so that we can control this movement. Major ideas presented are . . . Soil "holds" water against the pull of gravity, retaining it for crop use. There are limits to this ability. The upper limit is call FIELD CAPACITY. The lower is called PERMANENT WILTING POINT. You can add water to soil above FIELD CAPACITY. It will not hold this additional water. It will drain below the effective root zone to become deep percolation. Between field capacity and permanent wilting point is the soil's AVAILABLE WATER HOLDING CAPACITY (AWHC). This is the amount of water that the soil will hold that is available for the crop to use. As the crop pulls water from the soil, the soil holds onto the remaining water harder and harder, putting more and more stress on the crop. You will see the crop wilt during the hottest parts of the day. Sooner or later, if the stress gets big enough, the crop will permanently wilt. The soil moisture is at PERMANENT WILTING POINT. Soil water can be measured in two ways . . . a. Volumetrically, the actual amount of water in the soil. b. Tension, a measure of the water-holding forces in the soil. The crop doesn't care how much water is physically in the soil, only how hard it is to get out. Thus, although a volumetric measurement will tell us how much water is in the soil and, therefore, how much to irrigate, the tension measurement is more important in terms of preventing crop stress. The volumetric standard of measurement is inches of water held per foot of soil or just inches/foot. Available water holding capacities vary from 1 to 2.5 inch/foot. The tension standard of measurement is pressure, usually centibars. The rate at which crops extract water from the soil is called EVAPO-TRANSPIRATION, ETc. ETc is the combination of soil surface evaporation and plant transpiration and is measured in terms of inches of water per day. Normal ETc rates for cotton are around .05 in/day as seedlings to .35 in/day as a full-grown plant. ETc varies with the plant, the climate, the level of soil moisture, and plant condition (fertilizer/pest/disease stress). ETc can be measured and predicted. The INFILTRATION RATE, measured in inches/hour, is a measure of how fast water is soaking into the ground. Infiltration rates will decrease during an irrigation. The APPLICATION RATE, also measured in inches/hour, is a measure of fast we are applying water. Knowing the application rate of sprinkler systems is especially important. They usually run from .1 to .5 inches/hour. If the application rate is higher than the infiltration rate, runoff occurs. There should not be excessive runoff with a trickle or sprinkler irrigation system. There are several methods available for measurement of soil moisture both for volumetric (the neutron probe, gravimetric, "feel") and tension (tensiometers, gypsum blocks, leaf pressure chambers). They all have their strengths and weaknesses. Very high or out-of-balance salts will modify many of the measurements and results of different measurements (high or low). Refer to the chaper on salinity for further information. Top of Page	WATER HOLDING CAPACITY Soil "holds" water available for crop use, retaining it against the pull of gravity. This is one of the most important physical facts for agriculture. If the soil did not hold water, if water was free to flow downward with the pull of gravity as in a river or canal, we would have to constantly irrigate, or hope that it rained every two or three days. There would be no reason to preirrigate. And there would be no such thing as dryland farming. The soil's ability to hold water depends on both the soil texture and structure. Texture describes the relative percentages of sand, silt, and clay particles. The finer the soil texture (higher percentage of silt and clay), the more water soil can hold. Gravity is always working to pull water downwards below the plant's root zone. To counteract the pull of gravity, soil is able to generate its own forces, commonly called "matric forces" ("matric" because of the soil "matrix" structure that forms the basis for the forces). An important fact about the soil's water-holding forces is that as the level of soil moisture goes down, the soil generates more force. This is the reason that some water will move up into the root zone from a shallow ground water table. As the plant extracts water in the root zone, the soil pulls water up from the area with more water to the area with less. As you would expect, the rate at which the water-holding forces go up with decreasing soil moisture is different for different soils. In a coarse soil, they will go up slowly. This means that plants can extract a great amount of water from coarse soils before they stress. In contrast, these forces rise quickly in finer soils. Graphically, the relationship can be described by the Figure SWP-1. Looking at the lowest line for a coarse soil. You can see that at A, the soil moisture level is very high and the water-holding forces are low. This means that the plant can extract water easily from the soil. At B, the soil moisture level is lower but the water-holding forces haven't gone up that much. The plant can still extract water easily. However at C, the soil moisture level is very low and the water-holding forces have increased greatly. The plant cannot extract water easily and will be stressed. FIGURE SWP-1: Soil Moisture Level (Depletion, %) vs. Soil Moisture Tension (Bars). Looking at the top line for a finer soil. At A, as with the coarse soil, the water-holding forces are low when the soil moisture level is high. However, at B, the soil moisture level has dropped somewhat but the water-holding forces have gone up greatly. And at C, where the soil moisture level is low, the water-holding forces have gone up very high. We will be coming back to this idea of increasing soil water-holding forces with decreasing soil moisture many times. Next Page Top of Page
	Last updated September 2000