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
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.|
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.|
|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).|
||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.
|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.
||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.
effects of excess water are:
|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.
||Plant suction in the day might be so high that little growth takes
||Number of units of water/unit of D.M. produced is called transpiration
||Crop characteristics important in determining the ability of plant
to adjust to moisture stress.
||Water moves into the plant whenever suction in the water in the plant
is greater than that in the water in the soil.
|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.
||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.
|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.
|Measure rainfall/estimate ET on open pan.|
|Computer using meteorological data to make recommendation to farmer.|
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?