1. AE 152 IRRIGATION & DRAINAGE
DR. GREGORIO S. ROA, JR.
Associate Professor V
ISPSC-PIA, Sta. Maria, Ilocos Sur

2. SOIL-WATER-PLANT RELATIONSHIPS
Learning Unit No. 1
SOIL-WATER-PLANT RELATIONSHIPS

3. Soil Major Components: Air pore spaces about 50% Water Solids minerals
organic matter
sand
about
45-49%
silt
mixed with
clay
about 1-5% OM

4. Water
Water is life!
Without water, there would have been no life on Earth.
Water is universal. It exists everywhere on Earth in a form of liquid, gas, and solid.
In plants, water continuously moves from the soil to the roots, from the roots into the various parts of the plant, then into the leaves where it is released into the atmosphere as water vapor through the stomata.

5. Water
transports soil nutrients through the roots to the plant tissues;
serves as principal medium for chemical and biochemical processes that support plant metabolism;
serves as cooling mechanism that allows plants to maintain the favorable temperatures necessary for metabolic processes; and
maintains turgor pressure in plant cells.

6. Water How does insufficient water affect yield of crops?
When there is too little water is available in the root zone, the plant will reduce the amount of water lost through transpiration by partial or total stomatal closure.
Stomatal closure decreases photosynthesis since the CO2 required for this process enters the plant through the stomata.

7. Water
Decreased photosynthesis reduces biomass production and results in decreased yields.
Research results show that…
the reduction in crop yield or quality resulting from water stress depends on the growth stage of the crop.
For example:
In corn, the most critical growth stage is the tasselling
to silking stage.

8. Soil-Water Relationship
Soil properties particularly soil texture directly affect the availability of water and nutrients to plants.
Soil water or soil moisture affects plant growth directly through its controlling effect on plant water status and indirectly through its effect on aeration, temperature, and nutrient uptake and transport.

9. Soil-Water Relationship
3 Kinds of Soil Water or Soil Moisture:
a. drainage water;
b. capillary water; and
c. hygroscopic water.
Different soil textures have different relative proportions of these 3 kinds of soil moisture.

10. Soil-Water Relationship
Saturation
In irrigation scheduling, soil moisture is expressed as:
a. Saturation;
Field capacity
b. Field capacity (FC);
Temporary wilting point
c. Temporary wilting point
(TWP); and
Permanent wilting point
d. Permanent wilting point
(PWP).
Oven-dry

11. Soil-Water Relationship
The soil moisture stored in the soil reservoir that can be used by plants is called plant-available water (PAW).
PAW = FC –
PWP = –

12. Plant Factors The daily plant water need is influenced by:
a. Evapotranspiration (ET);
b. growth stage;
c. plant root depth;
d. crop sensitivity to drought stress; and
e. soil and water quality.

13. Plant Factors
A. ET
It is the total amount of water lost to the atmosphere by evaporation (E) and plant transpiration (T) from the Earth's land surface.
It is a function of climatic conditions.

14. Plant Factors B. Growth Stage
A plant has different water needs at different growth stages. While a plant is young it requires less water than when it is in the reproductive stage. When the plant approaches maturity its water need drops.

15. Plant Factors
Plant Water Use Curve

16. Plant Factors C. Effective Root Depth
Different plants have different rooting depths.
Plant roots do not extract water uniformly from the entire root zone.
Water uptake is closely related to the root distribution in the soil.
Effective root depth is that portion of the root zone where the plant extracts the majority of its water.

17. Plant Factors
40%
30%
20%
10%

18. Plant Factors
The effective root depth is the depth that should be used to compute the volume of PAW in the soil reservoir.
The effective root depth for a mature crop is estimated to be one-half the maximum rooting depth reported in literatures.

19. Plant Factors D. Crop Sensitivity to Drought Stress
Different plants have different level of sensitivity to drought stress.
For a particular plant, sensitivity to drought stress is a function of its growth stage.

20. Plant Factors E. Soil and Water Quality
For optimum plant growth and development, the soil must have adequate room for air and water movement as well as for root growth.
Irrigation water with a high content of soluble salt is not as available to the plant, so a higher soil moisture content must be maintained in order to have water available to the plant.