Understanding Soil Salinity: Impact on Plant Growth

1. Unit Plant Science

2. Problem Area Initiating Plant Growth

3. Lesson Salinity and Seed Germination

4. Student Learning Objectives • 1. Explain the cause of excessive buildup of soluble salts in soils. • 2. Describe the effect excessive concentrations of salts in the soil have on plants. • 3. Describe the benefits and dangers irrigation has in relation to soil salinity levels. • 4. Explain how saline soils effect seed germination.

5. Terms • Capillary action • Evaporation • Ion • Irrigation • Leaching • Osmosis • Percolate • Salt • Soluble salts

6. How does excessive buildup of soluble salts in soils occur? • Saltsare compounds that have no carbon, hydrogen or oxygen. • A. The formation of salts in soil is a natural geologic phenomenon. As weathering occurs to minerals salts are often formed. As water percolates, moves down through the soil, it carries the soluble salts, those that will dissolve in water, lower. When a saturation zone is reached, the water will move toward the surface through capillary action. Through evaporation, the changing of liquid water into vapor, pure water is removed, leaving concentrations of salt in the root zone.

7. B. Irrigation water is the primary source of excessive soluble salts in the soil. As irrigationwater is applied to agronomic fields it often contains fertilizers. The fertilizers used are primarily ammonium nitrate and potassium sulfate, both of which are salts. As the fertilizers are applied in the irrigation water, they tend to buildup in soils. • C. Improper watering of container grown plants can also result in the buildup of soluble salts in their growing medium. If these plants are fertilized often the salts may buildup. One method of reducing this action is to water plants until water is seen over-flowing the container or running from drain holes in the container. This allows the salt compounds to be removed by the soil through leaching.

9. What effect do excessive concentrations of salt in soil have on plants? • Salt disrupts the flow of nutrients into plant roots. • A. When salt is dissolved in water ions, electrically charged atoms, are formed. This causes the water to bind more tightly with soil particles effectively reducing water availability to the plants. In saline soils only 10% of water at field capacity is available to plants, compared to 50% in nonsaline soils.

10. B. High salt levels can also reduce water uptake by plants. The process of osmosis is the driving force of plant roots absorbing water from the soil solution. • Osmosisis the movement of a solution from high concentration to low concentration across a semi-permeable membrane. Dissolved salt in the soil solution may cause an instance when the water concentration in the root cells is higher than in the soil solution. Water intake by the roots will cease, and may flow from the cells causing the plant to wilt.

13. How is irrigation both beneficial and dangerous to soil salinity levels? • A. Irrigation can be beneficial to saline soils by bringing more water into the soil solution. In doing so, the soluble salts can be leached deeper into the soil profile and out of the root zone. • B. The negative implication of irrigation water is that soluble salts are in all irrigation water. By adding the irrigation water only exacerbates the problem on saline soils or where water drainage is a problem.

15. How is seed germination effected by saline soils? • Salt interferes with water intake. • A. As with plant roots, seeds must take in water. The process of germination is initiated by the presence of water. In saline soils the water concentration of the soil may be lower than the water concentration in the seed, therefore osmosis will not occur. Without osmosis the water cannot enter the seed to begin the germination process.

17. Review/Summary • How does excessive buildup of soluble salts in soils occur? • What effect do excessive concentrations of salt in soil have on plants? • How is irrigation both beneficial and dangerous to soil salinity levels? • How is seed germination effected by saline soils?