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Anno 2018 Numero 7-9

July - August - September 2018

Innovative technologies for lettuce cultivation

Whether in leafs or as a head of lettuce, in the open field or in protected scraps, lettuce production makes use of particularly advanced solutions, such as aeroponic cultivation and the application of artificial intelligence. In any case, with a high level of mechanization

The lettuce can be grown in almost all types of soil, apart from the very heavy ones and/or on the skeletal soil.  In fact, especially in the case of direct sowing, it is necessary to carefully prepare the soil by digging at 20-30 cm (which does not make a processing sole and avoids harmful standing water), followed by harrowing or milling, to chop the created plots.  It is important not to exceed the refinement, in order to avoid the risk of crust formation in case of heavy rains or rain irrigations, which would hinder the seedlings emergence in case of direct sowing.

To facilitate a smoother execution of all the subsequesnt operations, the skeleton can be brought in depth with an stone burier, which also provides a useful balance of the surface profile of the soil.  

It is possible to create some strips of land to facilitate the draining of excess water (with the appropriate width for the transit of vehicles), adequately leveled to favor a correct and constant seeding depth. In soils subjected to intensive cultivation, and therefore to considerable mechanized traffic, it is advisable to periodically subsoil to restore the correct soil structure.

Direct seeding for cutting lettuce is usually carried out at 5-7 mm depth, and 5 cm row width, through universal mechanical seeders.  The seeds go from the hopper to the distributor units and from there to the seed tubes, which deposit them on the bottom of the furrows created by the cutters. Everything is inserted between two rollers: the first one levels the the soil surface, while the other makes the ground stick to the seed.  With high temperatures it is necessary to intervene afterwards with a light irrigation.

Planting is the solution adopted for head lettuce, as it has clear advantages in terms of shortening of the production cycle, improvement of the health condition, and of the quality of production, as well as fewer weeds control issues. Usually the starting point are seedlings grown up to three real leaves in pressed peat cubes, deposited in the ground by dedicated transplanters, which are often combined with the mulching unit with a polyethylene film. Particularly in sandy soils, for water supply, a perforated hose is placed under the mulch to supply the water, sometimes integrated with nutrients. 

As regard fertilization, it is essential to ensure a suitable content of organic matter underground before the beginning of the crop cycles, with particular attention to nitrogen, to be added in the nitric, ammoniacal and ureic forms, given that each of these acts in different ways concerning the methods of absorption.

To get crisp leaves and little fibrous, it is necessary to irrigate the crop, ensuring a uniform distribution without covering the leaves in dirt. Static or dynamic sprayers are used with medium-low flow rates (70-120 l/h), and small ranges (3-5 m), so as not to cause excessive tenderness of the leaves (ie a higher fragility), whilst avoiding  the occurrence of fungal pathologies.

A thorough irrigation also contributes to an effective fight against weeds, especially important where the timing of germination, emergence and growth are quite long.  The most commonly used techniques are false-seed bed preparation or mulching.  It is certainly useful before planting to carry a soil disinfestation through steam, fumigation or solarization. 

Nevertheless, when direct sowing is used for the head product, there is a clear need of making a targeted selection, to allow a regular growth of the crops. Leveraging the solutions of the Artificial Intelligence (based on "computer vision" and "machine learning" technologies), the American startup Blue River Technology from Sunnyvale, California (recently acquired by John Deere) has developed a specific sprayer to thin the lettuce seedlings according to pre-set parameters. It works with an accuracy of 6 mm, and is able to treat up to 90 seedlings/s Obviously, the same technique is applied for selective weeding, with excellent ability to  distinguish between crop and weeds.

The harvest is done by cutting the leaves or picking the head.  The difficulty in identifying the best moment essentially consists in having to match the collection with the demands of the destination markets.

In any case, for both the cutting and the head lettuce there are pickers (often self-propelled) available equipped with bars cutting and interception carpets of the product, which is then conditioned and temporarily stored by workers on board.

The Ortomec of Cona (VE) has recently proposed the 9700 Lettuce, a self-propelled harvester with oscillating blade that cuts the heads from the root and transfer them to the same collecting position via a conveyor belt on the main platform, where they are processed for the first sorting, cleaning and packaging. Powered by a 74 Hp 4-cylinder Kohler turbo-diesel engine, the 9700 has a hot-dip galvanized steel frame, stainless steel bonnets, 300 mm wide tracks (steel or, on request also rubber tracks), with a work space up to 23 m² for a capacity of up to 3,000 kg of product. 

All the secrets of lettuce

The kind Lactuce includes more than 100 species, although the most popular are the head varieties (with a dense white heart with a round shape), the Batavia (with thicker leaves and good resistance to drought), those with “oak leaf” (particularly soft), the Roman (with long leaves and wide and curved main rib) and the asparagus or celery varieties (which recall the flavor of these vegetables, and are consumed especially in the East).

China is the world’s largest lettuce producer, followed by the United States, India and Europe (especially Spain, Italy and France). Lettuce consists for 95-96% of water, and obviously has a very low caloric content of only 19 kCal/100 g, but it is a very useful source of fiber, vitamins (A, B9, C) and other trace elements (potassium, manganese, iron, magnesium and antioxidants, such as lutein and zeaxanthin).

It is especially well known for its beta-carotene content, which is transformed into vitamin A in the body, that is useful to absorb iron, for the skin and the mucous membranes health, for vision, the immune system and for growth.  Consider that a portion of lettuce is enough to meet half of the daily requirement. 

The lettuce is mainly consumed raw, often properly seasoned with the vinaigrette: apart from the classic combination with the “slice” of grilled meat (where, thanks to its fibers, the salad makes the meat more digestible), it can accompany ham, seafood, goat cheese, camembert, tomatoes, carrots, red beet, corn, avocado, apple, dried fruit, etc. It can however also be cooked, with peas and carrots, as a side dish of lamb or in soup, or with other vegetables (leeks, broccoli, spinach, zucchini...).

 In our climates, it should be consumed preferably when it’s in season, that is to say from May to September; it must be kept away from light (otherwise it quickly loses its vitamin C and B9 content), in the refrigerator, at most for 3-4 days in a perforated bag in the vegetable compartment or, for a few days more, cut and placed in an airtight container.

Lettuce in aeroponic cultivation

This is an evolution of the well-known hydroponic method, where plants do not grow on a solid substrate, but with the root system immersed in water.  Conversely, the aeroponic cultivation is characterized by the absence of any solid or liquid substrate, natural or artificial, such as earth, peat, rock wool, coconut fiber, water, etc.  Plants are onlyfed by an aqueous  nutritious solution sprayed on the roots, containing 2% of minerals and trace elements. All this is complemented by an insufflation of CO2 (in the measure of 600-1000 g/m²) to obtain a productive plus of 35-40% depending on the species, and a certain bacteriostatic action and inhibition of mold sporulation.

It is a “terrestrial” application of a NASA research for food production in the absence of gravity and without soil, already started almost 60 years ago and resumed in the ‘90s of the last century in the many space missions, where logically it is not possible to grow fresh vegetables in the traditional way, and the water is a very precious commodity.

After the necessary passage in the germination cell, the seedlings are placed in greenhouses on appropriate supports, often arranged in overlapping layers to make the best available confined volume, feeding them with a nutritive solution nebulized on their roots, maintained at a constant temperature in a closed cycle, since the unabsorbed part is re-introduced into the circulation, with a consequent considerable saving of water and fertilizers.

With aeroponics it is possible to get high quality vegetable products: not only salads (including lettuce), but also medicinal plants, bulbous plants, etc. without using pesticides, with a high mechanization index. In the specific case of lettuces, 25-50 plants/m² are placed, with a fresh material production rate that reaches 12-25 kg/month.

by Domenico Pessina

Keywords: Lettuce Harvesting
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