Why do earthworms die when salt is sprinkled on them?
Earthworms are grouped under the animal phylum Annedlida, which was derived from the Latin word, anellus meaning “little” rings. As this name implies, the body of an earth worm seems to be made up of several rings which is known as segmented body.
The skin of the earth worm secretes mucous. This mucous keeps the skin moist always, which is vital for the survival of earth worm. Earthworms have no respiratory organ and hence the respiration takes place through the moist skin.
The oxygen from the environment diffuses passively across the moist skin and it is carried by the circulatory system to the cells. Similarly Carbon Dioxide from the cells will also be carried by circulatory system and diffuses out through the moist skin. So the earthworms will die if their skin dries out.
Saltiness of the soil in which the earthworm lives is an important factor which decides it survival because earthworms are highly sensitive to salt stress. The salinity may reduce their growth at low salt concentrations or cause mortality at high salt concentrations.
They are unable to tolerate high ionic strength, because high salt concentration destroy their sensitive skin, the earthworms cannot have the control over the osmotic regulation. In addition, the neurosecretory cells in earthworms play a vital role in water balance as well as ionic and osmotic regulation.
The salt stress and desiccation significantly interfere with the functions of these neurosecretory cells. Hence salt like sodium chloride are extremely toxic to most of the earthworm species
Sodium Chloride could cause mortality at a concentration of 0.5 per cent itself, where as other alt such as potassium chloride are toxic at a concentration of more than 1 per cent. However, the salts at a lower concentrations may not cause mortality in earthworms, although they would substantially reduce the reproduction, especially production of cocoons
THE LIVING SOIL: EARTHWORMS
Of all the members of the soil food web, earthworms need the least introduction. Most people become familiar with these soft, slimy, invertebrates at a young age. Earthworms are hermaphrodites, meaning that they exhibit both male and female characteristics.
They are major decomposers of dead and decomposing organic matter, and derive their nutrition from the bacteria and fungi that grow upon these materials. They fragment organic matter and make major contributions to recycling the nutrients it contains.
Earthworms occur in most temperate soils and many tropical soils. They are divided into 23 families, more than 700 genera, and more than 7,000 species. They range from an inch to two yards in length and are found seasonally at all depths in the soil.
In ter ms of biomass and overall activity, earthworms dominate the world of soil invertebrates, including arthropods.
WHAT DO EARTHWORMS DO?
Earthworms dramatically alter soil structure, water movement, nutrient dynamics, and plant growth. They are not essential to all healthy soil systems, but their presence is usually an indicator of a healthy system. Earthworms perform several beneficial functions.
Stimulate microbial activity. Although earthworms derive their nutrition from microorganisms, many more microorganisms are present in their feces or casts than in the organic matter that they consume. As organic matter passes through their intestines, it is fragmented and inoculated with microorganisms. Increased microbial activity facilitates the cycling of nutrients from organic matter and their conversion into forms readily taken up by plants.
Mix and aggregate soil. As they consume organic matter and mineral particles, earthworms excrete wastes in the form of casts, a type of soil aggregate. Charles Darwin calculated that earthworms can move large amounts of soil from the lower strata to the surface and also carry organic matter down into deeper soil layers. A large proportion of soil passes through the guts of earthworms, and they can turn over the top six inches (15 cm) of soil in ten to twenty years.
Increase infiltration. Earthworms enhance porosity as they move through the soil. Some species make permanent burrows deep into the soil. These burrows can persist long after the inhabitant has died, and can be a major conduit for soil drainage, particularly under heavy rainfall. At the same time, the burrows minimize surface water erosion. The horizontal burrowing of other species in the top several inches of soil increases overall porosity and drainage.
Improve water-holding capacity. By fragmenting organic matter, and increasing soil porosity and aggregation, earthworms can significantly increase the water-holding capacity of soils.
Provide channels for root growth. The channels made by deep-burrowing earthworms are lined with readily available nutrients and make it easier for roots to penetrate deep into the soil.
Bury and shred plant residue. Plant and crop residue are gradually buried by cast material deposited on the surface and as earthworms pull surface residue into their burrows
WHERE ARE EARTHWORMS?
Different species of earthworms inhabit different parts of the soil and have distinct feeding strategies. They can be separated into three major ecological groups based on their feeding and burrowing habits. All three groups are common and important to soil structure.
Surface soil and litter species – Epigeic species. These species live in or near surface plant litter. They are typically small and are adapted to the highly variable moisture and temperature conditions at the soil surface. The worms found in compost piles are epigeic and are unlikely to survive in the low organic matter environment of soil.
Upper soil species – Endogeic species. Some species move and live in the upper soil strata and feed primarily on soil and associated organic matter (geophages). They do not have permanent burrows, and their temporary channels become filled with cast material as they move through the soil, progressively passing it through their intestines.
Deep-burrowing species – Anecic species. These earthworms, which are typified by the “night crawler,” Lumbricus terrestris, inhabit more or less permanent burrow systems that may extend several meters into the soil. They feed mainly on surface litter that they pull into their burrows. They may leave plugs, organic matter, or cast (excreted soil and mineral particles) blocking the mouth of their burrows.
LOOKING FOR EARTHWORMS?
It is easy to determine whether you have an adequate population of earthworms in your soil. Look for their casts in the forms of little piles of soil, mineral particles, or organic matter at the soil surface. They can be seen moving over the soil surface or even breeding, particularly on warm, damp nights. Dump a spadeful of moist soil into a bucket or onto a sheet of plastic, and sort through for earthworms. Can you identify different species? To find the deep burrowing species, pour a dilute mustard solution onto the soil. Many will quickly come to the soil surface in response to this irritant.
ABUNDANCE AND DISTRIBUTION OF EARTHWORMS
The majority of temperate and many tropical soils support significant earthworm populations. A square yard of cropland in the United States can contain from 50-300 earthworms, or even larger populations in highly organic soils. A similar area of grassland or temperate woodlands will have from 100-500 earthworms. Based on their total biomass, earthworms are the predominant group of soil invertebrates in most soils.
The family of earthworms that is most important in enhancing agricultural soil is Lumbricidae, which includes the genuses Lumbricus, Aporrectodea, and several others. Lumbricids originated in Europe and have been transported by human activities to many parts of the world. The United States has only one or two known native species of lumbricids. Others were brought to this country by settlers (probably in potted plants from Europe), and were distributed down the waterways.
Generally, lumbricids are much more common in the north and east than in the drier south and west of the United States. They tend to be more abundant in loam and clay loam and even in silty soil, than in sandy soil and heavy clay. Populations also build up in irrigated soil. Earthworm populations tend to increase with soil organic matter levels and decrease with soil disturbances, such as tillage and potentially harmful chemicals.
