ONION: Its Role in Soil and Water Conservation
Rani G. Kumar, Girish K. Panicker*, and Franklin O. Chukwuma Definition: Originated in Central Asia dating back to 3500 BC, onions (Allium cepa) are shallow rooted, biennial monocots grown as an annual vegetable when desired and as a two-yearly edible-plant when seed production is needed. Initially established in the West Indies by the Spaniards, this crop spread rapidly to other parts of Americas. Botanical historians believe that the first Pilgrims began cultivating bulb onions, from 1648. Rapidly, native Indians assented onion as one of their preferred vegetables, from the colonists. A cool season crop by nature, onion grows on nearly every type of soil, from sandy loams to heavy clays. Its non-perishable quality expands onion’s acceptance as a winter-favored vegetable. Purposes
To conserve soil and water through the use of vegetation To maintain and/or to improve soil availability, quality, and soil nutrients To suppress weeds, reduce insect pests and diseases, and increase crop yield To improve soil tilth, soil organic matter, and soil structure
Planting: Onion is adapted to a wide range of temperatures and is frost- hardy. Premium onions require cool temperatures throughout early growth and warmer temperatures during maturity. Best production is expressed at temperatures from 55 to 75 degrees F. Long day onions are grown in northern regions of US where 14-16 hours of light is accessible and short day varieties are raised in the southern areas where early summer get 12 hours of light. Also, in the south, onion thrives in the fall, winter, and spring, whereas onions are mainly a spring, summer and fall crop in the north. They can be started from seeds, transplants, sets (bulbs) or seedlings.
2 Soil: Fertile and well-drained soil, ample drainage, full sunlight, a pH of 5.5-6.8 and a soil temperature of 50o-95oF are requisites for onion production. They are sensitive to acid soils and superior onions grow best in productive, sandy-loam soils and in the highest state of tilth (muck soils are proper for production in the North). In accordance with soil test results, nitrogen, organic material, lime, fertilizer, etc. must be supplied periodically to the crop (1 pound of compost to each square foot and 4 or 5 lbs. of fertilizer to each 100 sq. ft., roughly 1½ inches deep, are recommended.) Regular watering is needed for enhanced bulbs, yield and flavor. Planting onions on raised beds enable sufficient space, fewer soil compactions, constant drainage, speedy soil warmth (in the spring), and easier maturity. Mechanical and chemical weed controls are crucial for high yields. Onion from sets (dormant bulbs/plants): Onion sets are generally planted in March or April. It is better to divide the sets into dual sizes before planting and use the prime sets in spring or fall for green table onions. The smaller sets are good for producing dry cooking onions. Mid-March is the right time for green bunching onions (scallions). Sets guarantee well-formed bulbs by the end of the season. Onion sets should be planted in rows between 12 and 24 inches apart and 1 to 2 inches deep. Maintain range between ½ inch and 7/8 inch in diameter and secure the soil around the bulbs. Sunlit location is recommended for hearty growth. For bulb production, plant sets in early spring, 1 to 2 inches apart and 1 to 2 inches deep in the row. Thin to 4 inches apart and advantageously the thinned plants are useful as green onions. Fall season yields perennial or multiplier styles of onions. Cold hardy onions must be planted 6 to 8 weeks before the killing frost. Side dressing occasionally with a nitrogen fertilizer will speed up development. Harvesting: Onions grow rapidly during early spring, in cool conditions. It is crucial to grow large plants initially in the season to support big bulb expansion. When bulbs grow bulky and the tops become weak and fall over, onions are ready for harvest. Stop watering them at this point and wait for onion tops and neck to dry totally. Next, cut the tops and roots an inch from the bulb to avert the entry of decay organisms. Handle the bulbs cautiously to prevent bruising and store them safely away from rain, snow or freezing temperatures. Crop rotation is a practice universally advocated and followed for centuries in vegetable production. It is the planting of different crops in the same space, in alignment. Grouping vegetables by family is the strategy to fruitful crop rotation. A well-planned crop rotation increases soil fertility, conditions soil structural stability, leads to farm diversity and helps growers preserve their fields under nonstop production. Rotation–based farming methods are vital for soil conservation, integrated weed management, reducing herbicides, and net profit advance. This approach strikingly aids the soil by not placing the same nutrient demand every season and manages pests, weeds, buildup of bacteria, fungi and viruses. Constructively, rotation reduces the loss of agricultural chemical off-field, the risk of weather damage, soil erosion and drought, and advances water and crop quality. Further, this scheme maintains good soil physical condition, organic matter, and distributes plant
3 nutrients in the soil by varying the feeding range of roots. Crop rotation certainly is useful in organic farming where pests are controlled without synthetic pesticides and fungicides. Onions can be preceded by potatoes, beans, carrots, cowpeas and corn. Leguminous green manure crop rotates well with onion and prevents the buildup of disease and insect pests. Mulching is a plant and soil protecting method regularly used by farmers. Customarily, organic (natural) or synthetic (plastic) materials are utilized as mulches. Natural mulches include leaf mould or raked tree leaves, dry straw, bark wood chips or well-rotted sawdust, grass clippings, pine needles, black-and-white newspaper, nut shells, compost, etc. Black plastic, landscape fabric, infrared transmitting mulch etc. are classified as synthetic mulches. The right time to mulch is at seed-planting but applying mulch too early affects the full growth adversely and increases the chance of cold damage. Mulching provides healthy growing conditions for plants and makes upkeep easy for the farmers. It retains soil moisture, improves nutrient and water retention, suppresses weed growth, eliminates soil crusting, regulates soil temperature, and safeguards the plant roots. Further, mulches minimize fertilizer leaching, soil erosion and plant damage by acting as a nutrient bank. They supply natural nutrients vital for healthy soils and plant growth. A mixture of organic mulch types visibly improves the texture and fertility of the soil. Also, they help breakdown biological matter into humus, reduce evaporation by conserving water, and create a favorable soil environment for beneficial earth worms. Mulching around onions inspires the uniform development of bulbs. Start with a light layer of mulch and add more when seedlings emerge. A minimum of 2 inches thickness is recommended for onions. Mulched onions naturally are better in vigor and resist pests and diseases. For over wintered onions, mulches are mainly profitable. Avoid excessive amounts of mulch as it prevents moisture from reaching the roots and also leads to root rot. Weed control is necessary for successful onion production. Onions are slow growing and so the young plants are easily attacked by the weeds. For nutrients, water, light and space, weeds compete with onions. They create insect and disease problems that can easily harm the present or succeeding crop. Weed infestations in onions change as weed germination generally shift from winter annuals to summer annuals and biennials. Bulb rot and bacterial streak encourage the growth of weeds including spiny sow thistle, wild radish, purple cudweed, Virginia pepperweed, cut leaf evening primrose and dandelion. Bulb rot levels and bacterial streak can be stopped by destroying these weeds. A selected weed control program in alternate crops can suppress weed growth very well. Resistant weed biotypes will be a problem in areas where onions are cultivated frequently and related herbicides are applied each time. Annual and perennial grasses can be simply controlled in onion fields by using different herbicides. Nonchemical weed management is very effective with the usage of field surveys and field maps referring, fertile field selection, proper planting and harvesting times, suitable
4 and resistant onion varieties, certified plants, soil sanitization and solarization, green manure crop pre-planting, pre-germination and weed seedlings’ destruction, vegetable crop rotation and non-row crops, plant density maintenance, canopy closure style based row spacing, allelopathy treatment, intercropping upkeep, natural mulches (for small areas) and synthetic mulches (for large farms), hand weeding (when possible), flame weeding, cover cropping, etc. Chemical weed Management is crucial for effective onion production since this crop deals with warm and cool season weeds. Through the stale seed bed method, weeds are allowed to grow in the pre-prepared seed bed and then destroyed by the non-selective herbicide. Next, the crop production starts at the planned time with minimal soil disruption to prevent weed growth. Herbicides along with cultural practices suppress many weed pests of onion. Pre-plant treatments, metam sodium, glyphosate and paraquat are used to control perennial weeds. DCPA is applied for eliminating annual grasses and broadleaf weeds during the early stages of onion development. Other herbicides for onions include bensulide, bromoxynil, oxyfluofen sethoxydim, fluazifop-P-butyl, elethodim etc. Mixtures of urea and sulfuric acid (N-Phuric or Enquick) or nitrogenous fertilizers (e.g. ammonium nitrate) will destroy broadleaf weeds and promote the growth of onion seedlings by stimulating their competitiveness. Residue management is a system that harnesses the leftover crop residue after harvest. Crop residue management can be conducted throughout the year. It includes many field operations such as crop selection, use of cover crops, various tillage systems etc. Crop residue is needed for a good conservation plan. It is a vital source for soil sustainability and the environment. Onion crop residues left on the surface before or during planting reduce tillage. They shield soil particles from wind and rain until plants produce a protective canopy. Residue improves soil carbon, protects water quality, soil structure, soil tilth, and water holding capacity. Crop residue supplies more nutrients to the soil, intensely reduces soil compaction and prevents soil run-off. Further, it adds organic matter to the soil as it decomposes and saves time, energy and labor for the planter. Residue cover planning must begin at harvest and over the field, crop residues must be spread uniformly, ensuring that they do not conflict with the seeding process. Amount of tillage passes must be minimized since tillage pass buries crop residue. Cultivate onions at shallow levels with appropriate equipment (like straight pointed chisel plows), operated at low speed. Reduced till, ridge-till, mulch-till or no-till are practicable in crop residue management. Residue levels generally are estimated by using a camline that has 50 or 100 equally divided marks and amounts are expressed in percentage or in pounds. Poor residue management leads to low germination and low crop yield. Onion seeds will germinate at temperatures as low as 35o F (2o C). Mature bulbs stay dormant extensively depending on the variety and storage temperature. Most commercial bulb onions are hybrids but openpollinated varieties are still in demand. They are classified by day-length requirement (short, intermediate or long), market use (green fresh bulb or dehydration bulb), and bulb
5 color (yellow, brown, red or white). Onion consumption has steadily grown in the US, lately. The two nationally raised standard groups are Bermuda and Spanish varieties. Bulb onions are successfully produced in conservation tillage systems. Onion provides only 35-40% canopy cover during maturity. The maximum fresh residue cover after harvesting and disking recorded is 7%. Total fresh residue mass ranges from 7000 to 8000 pounds per acre (910-1040 pounds of dry residue). The maximum root depth recorded in Memphis silt loam (Typic Hapludalf, silty, mixed, thermic) is 8 inches. Onion residue contains 37% carbon (C) and 2.5% nitrogen (N). At this rate, this crop returns 370 pounds of carbon and 26 pounds of nitrogen per acre to the soil. The yield in Mississippi ranges from 5000 to 6000 pounds per acre.
_____________________________________________________________________ Published by Alcorn State University extension Program, School of Agriculture, Research, Extension, and Applied Sciences, Alcorn State University, Mississippi, in collaboration with the USDA/NRCS. Alcorn State University is deeply grateful to the USDA/NRCS for providing funds for this publication. *Corresponding author. Alcorn State University does not discriminate on the basis of race, color, religion, national origin, sex, age, or handicap