Salt Water Inumdation, Fresh Water Flooding

Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

Salt Water Inundation
Significant acreage across the state has been damaged by flooding, both with fresh rain water, and with salt water along the coast. In fields along the marsh next to the Delaware Bay, significant acreage was inundated with salt water from the tidal surge over the weekend. The tidal surge also brought large amounts of debris into these fields.

Salt water inundation occurred where fields were flooded with sea water, brackish water, or tidal surge water from the Delaware Bay. Salt contaminated soils will have several effects on crops. The first is osmotic where high salt levels in the soil solution will draw water out of germinating seedlings and the roots of plants, causing desiccation. In less severe cases, elevated salt levels will make it more difficult for plants to take up water, thus increasing water stress and reducing growth. The second concern is the toxic effect of salt water constituents. Excess sodium is toxic to crop plants. In addition, chloride from salt water can be toxic to many crops.

Soils that have had salt water leach into them will have high osmotic conditions (high dissolved solutes) and high levels of sodium. Levels of overall salts, sodium, and chloride will be reduced with leaching from rainfall, but this may take a considerable amount of time, depending on the amount of rainfall, soil type, water table, and the presence or absence of salt water intrusion in the ground water. On a sandy loam soil, salt levels may be reduced to tolerable levels within a year’s period of time. On heavier soils and soils with high water tables, it may take several years for salt levels to drop to acceptable levels. In areas where salt water ponded for long periods of time, also expect effects to last for several years. Other problems include salt water mixing with ground water contaminating shallow wells and tidal overwash into irrigation ponds, contaminating irrigation water sources.

Field crops vary in their sensitivity to high salt and high sodium levels. Soybeans are very sensitive and will not tolerate much salinity. Soybeans will not survive in any fields flooded with tidal surge waters if planted this year. Corn has more tolerance (rated as moderate salt tolerance), but again will likely not grow this year in salt water inundated soils. Sorghum and small grains have higher salt tolerance. These will be future options as salt levels drop (if they fit into your crop rotation). A number of millets also have salt tolerance.

A quick test for soluble salts is the electrical conductivity (EC) of the soil: the higher the conductivity, the higher the salts. Call your county extension office if you want to have your soils tested for EC. For sodium levels, a laboratory soil test will be needed. The Kent County extension office has an electrical conductivity meter equipped with soil probe sensors for direct soil EC measurements if you want to confirm soluble salt levels or monitor salt levels directly in fields during the year.

To reclaim a waterlogged, salt-affected soil, the water tables must be lowered, excess salts must be leached out, and where sodium is very high, it should be replaced with a more desirable cation such as calcium. The following are some strategies to manage salt affected soils:

● Moldboard plowing can help to dilute salts by mixing with the soil. Continued tillage can help keep salt that evaporates at the surface mixed with the soil.

● Irrigation, where available, will help to move salts out of the surface soil so that crops may be established. This requires significant amounts of water being applied over a long period of time so good drainage will be necessary.

● Plant salt tolerant crops once enough leaching from rainfall or irrigation has occurred. Sorghum species, including grain sorghum, sudangrass, sweet sorghum, and sorghum/sudangrass hybrids, have some salt tolerance. Many millets also are salt tolerant with Japanese millet being a good choice for salt contaminated soils. Small grains have relatively high salt tolerance. There are several perennial species such as coastal panic grass, tall fescue, and bermudagrass that have good salt tolerance. Salt tolerant alfalfa varieties are also commercially available.

● Add low salt containing organic materials to the soil such as leaf compost or yard waste compost (do not use manure, sewage sludge, or mushroom soil based compost).

Sometimes gypsum is recommended to remediate salt affected soils. Application of gypsum may or may not be beneficial. Gypsum (calcium sulfate) is a salt itself, and may actually increase the salt content in the short term. Application of gypsum to high sodium soils will provide calcium to displace the sodium. However, you still need rainfall or irrigation to leach the displaced sodium down and internal drainage to allow downward movement out of the root zone. If either of these conditions is lacking, then gypsum application alone will not help.

Fresh Water Flooding
Other areas throughout the state were flooded for a period of time with fresh water from recent heavy rains and there have been a number of questions on the effect of fresh water flooding on corn.

The extent to which flooding injures corn is determined by several factors including plant stage of development when flooding occurs, the duration of flooding, and air/soil temperatures. Prior to the 6-leaf stage (when the growing point is near or at the soil surface), corn can survive only 2-4 days of flooded conditions. Once corn has reached the silking stage shallow depths of flooding will not cause any noticeable amounts of damage. If temperatures are warm during flooding (greater than 77° F) plants may not survive 24 hours. Cooler temperatures prolong survival. Iowa studies found that flooding when corn is about 6 inches in height for 72, 48, and 24 hours reduced corn yields by 32, 22, and 18%, respectively, at a low N fertilizer level (50 lb N/acre). At a high level of N (350 lb N/acre) these yield reductions ranged from 19 to 14% in one year to less than 5% the following year.

Research indicates that the oxygen concentration approaches zero after 24-hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions – nutrient and water uptake is impaired, root growth is inhibited, etc. Even if flooding doesn’t kill plants outright it may have a long term negative impact on crop performance. If excess moisture in the early vegetative stages retards root development, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water.

If flooding in corn is less than 48 hours, crop injury should be limited. To confirm plant survival, check the color of the growing point (it should be white and cream colored, while a darkening or softening usually precedes plant death) and look for new leaf growth 3 to 5 days after water drains from the field.

Cold, wet weather conditions also favor development of seed rots and seedling blights. Seed treatments are usually effective but can provide protection only so long; if seedling development is slowed or delayed 2-3 weeks, soil-borne pathogens have a much greater opportunity to cause damage. Other disease problems which may become greater risks due to flooding and cool temperatures are corn smut and crazy top. The fungus that causes crazy top depends on saturated soil conditions to infect corn seedlings. There is limited hybrid resistance to these diseases and predicting damage is difficult because disease symptoms do not appear until later in the growing season.

Information on salt water flooding by Gordon Johnson, Extension Agriculture Agent, UD. Information on corn flooding from Peter R. Thomison, Extension Agronomist, the Ohio State University

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