Lettuce, (Lactuca sativa L.), is an annual of the Asteraceae family and native to the Mediterranean. It is thought to have originated in Europe and Asia and has been cultivated for at least 2500 years. The cultivated lettuce Lactuca sativa is a popular salad vegetable and is grown on all continents throughout the world including Australia.
PRODUCTION
Approximately 6000 hectares are grown each year throughout Australia, producing 135,000 tonnes for both the fresh and processing markets. The Australian lettuce industry is estimated to be worth approximately $100 million with $8 million going to export. NSW is the third largest producing state with about 27,000 tonnes produced annually from around 1100 hectares.
Lettuce is grown in most districts of NSW with the bulk of field production centred in the three main growing areas of Sydney Basin, Murrumbidgee and Central West.
Sydney Basin Lettuce is sown and harvested all year round using both field and hydroponic systems. Production declines through summer due to the warm conditions.
About 520 hectares are sown each year by direct seeding and by transplants. Murrumbidgee Approximately 320 hectares is sown from early February through to late July for harvesting from April to the end of October. Production through summer is not possible due to high temperatures. Most crops are direct seeded with no hydroponic production. Central West Lettuce is only scheduled for harvesting during spring and autumn. Production outside these times is difficult due to harsh climatic conditions. Most crops are sown by transplants with no hydroponic production. Approximately 180 hectares are grown each year.
LETTUCE TYPES
There are four main types of lettuce.
Crisphead
More commonly known as iceberg which forms a tight firm head of crisp leaves. At maturity most varieties are mid green in colour and about 1 kg in weight. Crisphead lettuce is the major type grown in Australia and also the most difficult to grow.
Butterhead
Butterhead Lettuce
A heading type like the crisphead but with the leaves more loosely folded on top of one another. Head sizes range from 0.3–0.5 kg for hydroponically grown cultivars to 0.5–0.7 kg for field grown cultivars. The inside leaves, because of their lack of light, are cream or butter coloured. The outer leaves are brownish or a darker green than the crisphead and tend to bruise and tear more easily.
Romaine
Also known as Cos, this lettuce is upright in form and grows up to 30 cm tall. The tightly folded leaves are greenish white in the centre and medium green on the outside. Their flavour is sweeter than the other types of lettuce.
Looseleaf
Also known as leaf, speciality, fancy or European lettuce. Looseleaf lettuce is marketed as either a whole product or as an ingredient of ready-made salad mixes. This type does not form hearts and comes in a range of colours with various types of mottling or patterns. Looseleaf lettuce is generally grown in hydroponic systems and is considered the easiest type of lettuce to grow.
HYDROPONIC LETTUCE
Coastal areas of NSW have seen a large increase in production of hydroponically grown lettuce over the past few decades. Looseleaf and Butterhead are the main types of lettuce grown in these systems. Hydroponically grown looseleaf lettuce offers the advantage of producing a saleable product in half the time it takes to produce a Crisphead type. This Agfact does not cover issues of hydroponically grown lettuce but only the issues of field grown lettuce.
FIELD LETTUCE
Two different methods are used to produce lettuce under field conditions in NSW. One method uses transplants and overhead irrigation while the other is by direct seeding and furrow irrigation. In the Murrumbidgee district most growers prefer to direct seed and furrow irrigate due to a heavier soil type. In the Sydney Basin and the Central West where the soil type is lighter, most growers prefer to use transplants and overhead irrigation.
SEEDBED PREPARATION
Bed size width varies with soil type and irrigation method. In the Murrumbidgee district where furrow irrigation is used the bed width ranges from 1.4 to 1.6 m from centre to centre. In other districts where overhead irrigation is used the bed width ranges from 1.6 to 1.8 m from centre to centre. Field lettuce needs to be sown on raised beds to aid water infiltration and retention and allow adequate soil aeration and drainage. Beds should be cultivated and formed to produce a fine soil tilth in which the seed can germinate readily without being affected by soil crusting. Lettuce is a shallow-rooted crop, and a fine seedbed is needed to allow the roots to establish and grow.
SOWING
Lettuce crops are seeded or transplanted with 2 to 4 rows per bed. The most common method for direct seeding is establishing 2 rows per bed under furrow irrigation. When direct sowing, the seed can be pelleted or unpelleted depending on the grower’s preference and type of seeding equipment used. Unpelleted seed is sown at high rates of about 0.3 kg/ha resulting in seed intervals of about 60 mm. The crop is later thinned to about 300 mm per row when the plants reach about the 6 leaf stage. Where bed centers are 1.5 m apart with two rows per bed you can expect a maximum plant population of about 45,000 per hectare. Pelleting individual lettuce seeds permits the use of precision planters when seeding. This allows for lower seeding rates, more evenly spaced plants and less labour costs to thin the crop. Lettuce germinates best at low temperatures between 0ºC to 10ºC, and depending on variety, germination is inhibited when temperatures reach 25ºC to 35ºC. This is known as thermo-inhibition, with the seed germinating when returned to more suitable temperatures. Unfortunately, continued exposure to high temperatures may induce a secondary dormancy, called thermo-dormancy which means the seed will not germinate even when returned to non-inhibiting temperatures. Thermo-inhibition and thermodormancy is a common cause of germination failure during summer. Germination of lettuce is also promoted by light and inhibited by dark; which is known as photo-dormancy. As light is essential, the seeds need to be planted very shallow to allow good germination. Transplanting overcomes many establishment problems including germination failure and the cost of thinning. The cost of transplants varies from $20 to $40 per 1000 with some growers producing their own. Transplants are planted on beds, usually with 3 or 4 rows per bed, depending on bed width. The plants are spaced about 300 mm apart along the rows, giving a plant population of about 60,000/ha
NUTRITION
Lettuce is a rapid growing plant that needs a wellbalanced nutritional program to produce a high quality, high yielding crop. Field lettuce grows best on well-drained fertile soils with a pH between 6.0 and 7.0. Where soil pH is too low (too acid), the application of lime or dolomite is necessary to allow better uptake of nutrients. For optimum results incorporate the lime or dolomite into the soil at least 6 weeks before planting. A soil test will help determine the soil nutrient status and should be the basis of any fertiliser program. A full soil test costs around $100 per sample and analysis results will be received a few weeks after sending. The following recommendations are based on the nutrient requirements to produce a 50 t/ha lettuce crop.
Nitrogen (N)
A soil test measures nitrate nitrogen and where levels are very low a maximum fertiliser rate of 200 kg/ha of N is recommended. As soil nitrate levels increase the fertiliser rate should be reduced. When soil nitrate levels are high (50 ppm or more), fertiliser applications should be no more than 50 kg/ha of N. Nitrogen is easily leached from the soil by irrigation and rain and is not recommended for all nitrogen to be applied as a base application. As a general guide, apply one third as a base application and apply the remainder in even ammounts during the crop’s life. Monitoring sap nitrate throughout the season may help with this decision making. Side dressing of nitrogen should be drilled in when cultivating for weeds or spun over the top and irrigated. The number of side dressings required varies from one in soils with high nitrate levels to three or four evenly split over regular intervals where a high amount of nitrogen is required. When high inputs of nitrogen are required it is recommended that the fertiliser be in the nitrate form rather than the ammonium form to reduce the chance of ammonium toxicity (jelly butt). Care needs to be taken not to apply excess nitrogen to minimise the incidence of jelly butt, excessive growth and outer leaf burn.
Phosphorus (P)
Where a soil test shows very low phosphorus levels, a maximum fertiliser rate of 100 kg/ha of P is recommended. As soil phosphorus levels increase, the fertiliser rate should be reduced. A rate of only 20 kg/ha of P is required when soil tests show high levels of phosphorus. Phosphorus is stable in the soil and does not move much after it has been applied; therefore it can be applied weeks before the crop is planted. The entire recommended rate should be applied as a base dressing. Broadcasting phosphorus fertilisers is common but banding it below the seed line is preferable, allowing roots to quickly find it. There is of little value in applying phosphorus fertilisers as a side dressing after a crop has already been established.
Potassium (K)
Where a soil test shows very low potassium levels, a maximum fertiliser rate of 150 kg/ha of K is recommended. As soil potassium levels increase, the fertiliser rate should be reduced. A rate of only 20 kg/ha is required when soil tests indicate high levels of potassium. Potassium is more stable in the soil than nitrogen but can be leached over the season. Application recommendations are the same as nitrogen with one third as a base application and the remainder applied evenly over the crop’s life.
Other
Calcium, magnesium and molybdenum become less available to lettuce as soil pH decreases. The amount of lime needed to correct pH varies with soil type. Soil testing will help with this decision making. Heavier soil types will need higher rates of lime to increase pH than lighter soils.
Animal Manure
Animal manure can be used for growing lettuce and should be incorporated into the soil at least 4 to 6 weeks before sowing. This is important, especially with fowl manure to avoid ammonium toxicity (jelly butt). It is recommended that fresh manure be cured (dried) before use to avoid plant burn and Bottom rot (Rhizoctonia) problems. Animal manures help increase the soil’s organic matter, improving soil structure and water and nutrient holding capacity. Animal manures are generally low in N:P:K content, with fowl manure generally the highest at approximately 2.1% N, 1.6% P and 1.0% K. An application rate of about 7 t/ha (approximately 20 m³) makes a useful base dressing for lettuce crops. Nutritional Disorders
Tipburn
Fernando Soto make a few adjustments to the tractor before fertilizing a crop of romaine lettuce Monday morning at Felice Farms.
Tipburn is considered a problem of calcium deficiency. Most vegetable growing soils have enough calcium with the problem usually being associated with water stress. Tipburn is the breakdown of leaf margins, particularly on the inner heart leaves and is difficult to see during harvest. Where this disorder is a problem, it is common practice to apply weekly foliar sprays of calcium nitrate. However, using less susceptible varieties and good soil moisture management will give better tipburn control than using foliar supplements later on.
Ammonium toxicity (Jelly Butt)
Jelly Butt can be a problem when high inputs of nitrogen are used in the ammonium form. The ammonia released in the soil becomes toxic and causes poor emergence of seedlings, followed by wilting and death. Symptoms in older lettuce are seen in the central root tissue with browning and tissue breakdown. Jelly Butt is common where heavy applications of fowl manure are applied on wet soils. Fowl manure needs to be incorporated into the soil at least 4 to 6 weeks before sowing.
Suggested Fertiliser Program
The base fertiliser application will depend on soil fertility with between 300 and 900 kg/ha of 8:11:10 compound (or similar) usually being required. If applying fowl manure at 20 m³/ha the base fertiliser application rate can be reduced by one third. Ensure that the total phosphorus requirements are applied at this stage. Banding the base application under the seed or transplant line will give the best results. If broadcasting prior to hilling-up, you will need to increase the base application by 50 to 75%. Side dressing of nitrogen should start at about the 6 leaf stage and can be timed with weed cultivation. A rate of about 100 kg/ha of ammonium nitrate (nitram) should be applied at each application with the number of applications depending on total nitrogen requirements. Intervals of at least 2 weeks are needed between each application. Yellowing of the older leaves is a good indication of nitrogen deficiency. If extra potassium is required then the first side dressing of nitram can be substituted with nitrate of potash. Muriate of potash is also used, as it is one of the cheapest forms of potassium available.
