Soil - Water - Plant Relations
I. Topic

This lecture builds on the preceding three lectures. Soil physical properties influence the amount and availability of water for plants. Therefore, this chapter places the crop plant into that soil/water continuum and discusses the dynamic relationship between soil/water/and plants.

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II. Learning Objectives
tree To understand the role of water in crop plant growth.
tree To become familiar with moisture relations in soils and relate them to crop production. 
tree To understand the movement of water in soil systems. 
tree To become aware of plant water requirements and methods to measure and control soil moisture.

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III. Overview

Soil water is the most limiting factor for crop production in the world. Only 45% of the earth's arable land receives adequate moisture for crop growth. Water is lost from both the soil surface (evaporation) and the plant surface (transpiration), and is seldom optimal for maximum crop production in dry land (non-irrigated) agriculture. Soil water carries nutrients to a growing crop and has a significant effect on aeration and temperature of the soil.

There are certain limits for soil water. Field capacity is when the soil pores are so full of water that the next drop will leach downward out of the rooting zone. The opposite extreme is wilting point, the level at which plant roots can no longer take in water and turgor is lost (wilting).

The goal of a soil, water, plant continuum is to maintain the soil water between these extremes, allowing nutrient movement, aeration, and supplying water in excess of evaporation and transpiration (evapotranspiration). Measuring plant available water and adjusting water levels with irrigation is another way mankind has tried to modify the environment to maximize food and fiber production.

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IV. Soil Water
tree One of the most important factors affecting crop production.
tree Water must be available to replenish that lost by evaporation and transpiration.
tree Soil water carries nutrients in solution to the growing crop.
tree Has significant effect on aeration and temperature conditions of the soil.
tree Seldom is the water content of soil at optimum value for maximum crop production.
Moisture Relations of Soils
tree Field capacity - the surface layer is at or near the saturation point.
tree Wilting point - moisture drops so low that roots can no longer obtain water (wilting).
Energy Concept of Soil Moisture
Expression in terms of energy makes it more easy to compare availability of the moisture in soils of different textures.


Most commonly accepted unit at present is bars of suction. 
  • Suction is negative pressure, the higher the numerical value the lower the energy status of the water. 
  • Soils at field capacity - 0.1-0.3 bars of suction 
  • When soils at wilting point - 15 bars of suction
Managing soil moisture is the critical management component of irrigated agriculture.
 Water Movement in Soils  

Water will move in a soil from one point to another if the water at the first point has higher energy status than the water at the second.
Water entering a dry soil is held at higher suction in the zone below the wetting front and water moves down. 
  • The rapidity of movement depends on the size of the energy difference and soil characteristics.


If water is applied to the surface by rain or irrigation much faster than it can enter soil and be transmitted downward, the excess water accumulates on the surface. 
  • If the slope is great, erosion will likely result (unless surface stable or protected by plant residues).
tree  Detrimental effects of excess water are: 
  • Water moving laterally across the surface 
    • Water loss and erosion
  • A layer with limited water permeability 
    • Can be treated with subsoiler or tile/plastic drain
Too much water can be a problem as well as not enough water.
Plant-Water Relations
tree Water comprises more than 80% of the living and growing cells of most plants.
tree All actively growing plants have continuous liquid phase from soil to leaf.
tree Growing plants need large amounts of water. a. Lose through leaves - transpiration
tree In dry climates, weight of water lost may be 100s or 1000s of time dry weight of plants.
Water loss through stomates. 
  • If these partially close to shut down some transpiration, it inhibits CO2 intake and slows photosynthesis.
Plant suction in the day might be so high that little growth takes place. 
  • Crop may make large portion of its growth at night.
Number of units of water/unit of D.M. produced is called transpiration ratio. 
  • Below 200-over 1000, variance from crops 
  • Inverse of this ratio called water use efficiency 
  • Much of differences more related to amount of ground covered/weather conditions when measurements made
Crop characteristics important in determining the ability of plant to adjust to moisture stress. 
  • Stage of development important 
  • Can alter planting date to avoid unfavorable climatic conditions 
    Can use variety of hybrid differences to avoid critical period in plantsí growth on unfavorable moisture period.
Water amount and availability is critical to crop production.
Availability of Soil Water to Plants  
Water moves into the plant whenever suction in the water in the plant is greater than that in the water in the soil. 
  • Most plants withdraw water from soils until soil moisture reaches about 15 bars. 
  • Fine textured soils hold more water than sands at field capacity 
    • Fine textured soils are less droughty
Water Requirements of Crop Plants
tree  The rate at which water if available would be removed from the soil and plant surface is potential evapotranspiration (PET).
tree  The ratio of evapotranspiration (ET) to (PET) gives figure called relative evapotranspiration or crop coefficient (Kco).
tree Energy is required to evaporate water from soil and to cause plants to transpire.


Crops utilize only 1 to 2% of energy received. 
  • Utilization of energy may become the next limiting factor when moisture is adequate and good cropping practices are followed.
Many crops have critical stages of growth when a water deficient will cause unusually large reduction in yield.

Plants must me matched to water regime.
  Controlling and Measuring Soil Moisture
Maximum crop production would be attained if soil moisture suction could be held at a value low enough that the energy exerted by the plant would be minimal.

Instruments of many types marketed for measuring soil moisture. 
  • Resistance measurements 
    • Blocks buried in soil, electrical meter used to measure resistance
  • Neutron probe 
    • More sophisticated and expensive, used in research 
  • Tensiometer 
    • More useful in sands
tree  Removing a soil sample from appropriate depth/drying/weighing will give you a reasonable figure.
tree Some soils change color as they go from wet to dry. You "feel" it.
Observe both crop and soil closely for signs of moisture stress. 
  • Leaf reeling in corn
tree Measure rainfall/estimate ET on open pan.
tree Computer using meteorological data to make recommendation to farmer.
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V. Summary

In dryland (non-irrigated) agriculture, the soil, water, and plant continuum is controlled by climate and rainfall. In areas with low rainfall and high evapotranspiration, water is quickly lost from the soil and plant surface. Only natural rainfall can replenish the soil moisture. Plants in dryland systems are at the mercy of climate. However, when water can be delivered to the crop field, the farmer can regulate the soil/water/plant system. Water can be applied when the available water reached a certain level, and the land can be filled back to capacity with water. By doing this, the plant is not subject to water stress. If all other factors (fertility, temperature, pest control, etc.) are at optimum levels, there is little to no restriction on maximum crop yield. This is why the early cities of Mesopotamia, China, and Egypt were so productive (Lecture 1) in the flood plains of rivers with irrigation.

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VI. Self Assessment

tree  How does water influence crop plant growth?

tree  What is the difference between evaporation and transpiration?

tree  How does water move in the soil?

tree  What is field capacity and wilting point?

tree  How would you measure water requirements of plants?

tree  How would you measure soil moisture?


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Last updated January 23, 2008