Hydroponic Lettuce Production II

Introduction:

Please refer to the previous article “Hydroponic Culture of Lettuce I” for details on varieties, starting the plants, nutrient formulation and nutritional & environmental disorders, pests and diseases. That article gives lots of specific information for the growing of lettuce under greenhouse hydroponic cultures. This article describes in detail the NFT narrow channel growing system.

Production systems:

A. Agri-Systems (narrow gutter) NFT:

This system differs from other NFT systems in that it uses long, narrow, aluminum gutters with a movable plastic tape cover (photo 1). The engineering behind this system was to create an automated system capable of increasing the percentage of greenhouse floor area used directly for production.

The gutters are extruded aluminum measuring 3 inches (7.6 cm) wide by 2 inches (5 cm) deep. Special ridges or “cooling sinks” are molded into the bottom of the extrusion to assist in cooling the channels through conduction as cold air passes underneath the channels. This design with heat sink ridges permits the solution to remain cool over more distance so that the channels may be up to 70 feet (21 meters). I do not recommend they exceed that length as nutrient and oxygen gradients along the gutters will cause nutrient imbalance and growth disorders.

The gutters are supported by a galvanized pipe bench frame at waist height (36 to 40 inches). The frame has at least a 2% slope from the inlet end to the catchment end of the channels. I would recommend with these longer channels to slope them from 3- to 5- percent. This greater inclination will permit the nutrient solution to flow faster below the root systems improving oxygenation and stability of the nutrient solution.

The channels are cooled with the assistance of the heat sinks on the bottom being exposed to cool air forced under the benches. The air is cooled with evaporative cooling pads at one end of the greenhouse and forced underneath the benching by large exhaust fans (photo 2). With polyethylene side curtains mounted on the benching (photo 3), and polyethylene convection tubes, cool air is directed past the bottoms of the NFT channels cooling them with the assistance of the heat-sink ridges on the bottom of the gutters.

A nutrient tank cistern at one end of the greenhouse stores 3000 to 4000 gallons (10,000 – 15,000 liters) of solution per acre (0.4 hectare) of production system. Generally, several smaller tanks of 1500 gallons (5500 liters), one for each section of NFT channels is better so that growing sections are independent of one another. The tanks may be as small as 500 gallons (2000 liters) provided that the greenhouse has an automatic electrical generator backup system in case of power failure. With a continuously re-circulating system more than half of the total volume of solution remains in the NFT gutters. If a pump stops the solution will soon overflow the nutrient tank.

The solution is pumped through a 2-inch diameter PVC inlet header at the higher end of the channels and enters each channel via a 1⁄4-inch drip line (photo 4). The nutrient solution is collected at the lower end of the channels by a catchment gutter and returned to the cistern by a main return pipe (photos 5-6).

A flexible, moveable, tape cover slides on top of two ridges of the inside face of the channel within 1⁄4-inch of the top edge of the channel as shown in photo 1. The cover is made of 1/8-inch thick plastic that is flexible enough to be rolled up. This flexibility permits it to easily enter the channel guides and be wound up on the other side when harvesting by a simple motorized drum. One-inch diameter holes are pre-punched in the moveable tape at 6-inch (15-cm) centers for the lettuce plants. Channels are spaced 6-inches (15-cm) apart giving the lettuce a 6” by 6” spacing as shown in photo 3. Using this spacing with 70-ft channels and a 6-ft aisle between sections, approximately 145,000 plants per acre may be grown on a 30- to 40-day cycle, depending upon the available sunlight.

Seeds are sown in special plastic 154-celled “cream cup” trays commonly used in restaurants (photo 7). Slits are cut in the bottom of the rows of cups with a multi-bladed table saw to permit drainage and the roots to emerge. Each cup (cell), about 1-inch deep by 3⁄4-inch in diameter, has a small lip about 1/8-inch wide along the top edge. The cell flats are filled with coarse vermiculite using a flat filler machine.

Seeds are sown into each cell using a vacuum-seeding wand or seeder as described in the earlier article on lettuce culture (Vandana Seeder). The flats are soaked with raw water after sowing and then kept in a cooler for 1 to 2 days until germination occurs. Be careful that the seedlings are removed from the cooler as soon as their shoots start to emerge from the seed, otherwise, they will rapidly extend and develop into weak, spindly, seedlings, which will bolt (form a shoot to flower) before the lettuce is fully mature. Such bolted lettuce is not marketable. The seedlings upon germinating are then transferred to an ebb-and-flow benching system in a special seedling greenhouse (photo 8). In the ebb-and-flow benches nutrient solution is periodically pumped into the bed to flood the trays sitting in the bed. The solution then rapidly drains back to a cistern until the next irrigation cycle. On the return the solution is oxygenated as it falls back into the cistern. The seedlings remain in the seedling house for 10 to 14 days until they are ready to transplant at the 2- to 3-true leaf stage (photo 9).

In preparation for transplanting each small cup with its seedling is punched out of the trays and placed in flats lying on their sides. A worker then transplants these seedlings directly into the moveable tape cover of the NFT gutter (photo10). A planting-harvesting machine feeds the tape cover into the top of the NFT channel while a worker places the seedlings into the holes of the cover as it is moving into the channel. This is done using two of these machines. One at the end where the worker is transplanting has a roll of the tape cover on a drum and another at the far end of the gutters is pulling the tape into the gutter. While the tape is moving, a worker is placing the seedlings into the holes of the cover.

The lip of the cup sits on top of the moveable tape with the rest of the cell positioned inside the gutter so that the bottom is just above the bottom of the gutter. This small distance between the bottom of the cup and the gutter permits good oxygenation and rapid flow of the solution past the suspended roots in the gutter. The plant roots lying in the bottom of the gutter permit the nutrient solution to flow over them. Eventually, the roots will form a complete root mat on the bottom of the gutter. For this reason, it is important that the gutters have adequate slope of 2% to 3% or greater so that the solution will flow past the roots carrying oxygen to the roots and not backing up behind the root mass.

A unique challenge exists in using this NFT system in that the channels are spaced only 6 inches apart. This close spacing, while adequate for lettuce, permits the plants to growth tightly together as they mature. In doing so, the moving of an individual row during harvesting creates abrasion of the plants in the moving gutter against the neighboring ones. This can damage the lower leaves of the lettuce and break them off the plant. To overcome this problem and at the same time reduce light competition among the rows of the plants, every third row is transplanted until the entire section is completed (photo 11). Next, every second row is transplanted and finally every first row. In this way, each adjacent row has plants of 10 days difference in age. Having smaller plants next to older ones gives better light to the crop and reduces the abrasion of one plant against another during harvesting when the tape is pulled from the channels.

During harvesting the moveable tape cover is attached to the motorized drum of the planting-harvesting machine and the tape is pulled along the NFT channel. As the tape winds onto the machine, several workers cut the lettuce heads from the tape and remove the roots (photo 12). The harvested lettuce is stored in a mobile cooler that transports the product to the packing room where the lettuce is packaged in heat-sealed polypropylene bags and placed into cardboard boxes for cold storage until shipped (photo 13).

The moveable tape covers are sterilized between crops in a vat of 10% bleach solution (photo 14). They are rinsed and dried before re-using them in transplanting. The NFT gutters are not sterilized between each crop, but only every 3 to 4 months or as needed if a disease problem arises. Pythium is the most serious disease with lettuce. It gets into the roots and kills them causing wilting of the plant and stunting to finally death due to the inability of the plant roots to take up sufficient water. It can be controlled by keeping the nutrient solution chilled to under 70 F and by using a Ridomil fungicide in the solution of the cistern for the section being cleaned prior to changing the nutrient solution.

A one-acre greenhouse can grow in excess of 140,000 plants. The greenhouse shown here is 20,000 square feet and grows 70,000 plants at any time. Based upon a 32-day average cropping period, 2200 head of lettuce can be harvested daily (photo 15).

This is a very efficient system for growing lettuce, bok choy and some herbs such as basil. The crops must be a one-time harvest. Basil is best cut as one harvest, so could be grown in this way. The system, however, is very expensive. Another disadvantage is that you have no access to the sections of crops. This can make any spot spraying difficult, but with the use of fogging sprayers the grower does not need access as the spray will carry sufficient distance to fully cover the crop.