Soil - Water - Plant Relations

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.
 

II. Learning Objectives
 

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

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.
 

IV. Soil Water
 

	One of the most important factors affecting crop production.
	Water must be available to replenish that lost by evaporation and transpiration.
	Soil water carries nutrients in solution to the growing crop.
	Has significant effect on aeration and temperature conditions of the soil.
	Seldom is the water content of soil at optimum value for maximum crop production.

 
Moisture Relations of Soils
 

	Field capacity - the surface layer is at or near the saturation point.
	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	CONCEPT   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).

Drainage  

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

CONCEPT   Too much water can be a problem as well as not enough water.

 
Plant-Water Relations
 

	Water comprises more than 80% of the living and growing cells of most plants.
	All actively growing plants have continuous liquid phase from soil to leaf.
	Growing plants need large amounts of water. a. Lose through leaves - transpiration
	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 plants’ growth on unfavorable moisture period.    CONCEPT   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

	The rate at which water if available would be removed from the soil and plant surface is potential evapotranspiration (PET).
	The ratio of evapotranspiration (ET) to (PET) gives figure called relative evapotranspiration or crop coefficient (Kco).
	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.

  

	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
	Removing a soil sample from appropriate depth/drying/weighing will give you a reasonable figure.
	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
	Measure rainfall/estimate ET on open pan.
	Computer using meteorological data to make recommendation to farmer.

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.
 

VI. Self Assessment

  How does water influence crop plant growth?

  What is the difference between evaporation and transpiration?

  How does water move in the soil?

  What is field capacity and wilting point?

  How would you measure water requirements of plants?

  How would you measure soil moisture?