An understanding of the right way to irrigate lettuce is critical for:

• quality
• shelf life
• taste
• yield
• saving of water
• nutrition balance
• environmental awareness

A 'classic' case of 'tip burn' in a crop of Iceberg lettuce.

Is it simply a case of too little water to allow the plant to take up the required calcium?

When irrigating a crop of lettuce a grower needs to know:

• Varietal differences
• Soil variability
• Likely rooting depth and rate
• nutrient availability

Lettuce use very little water in early development. At about 2/3 through their life, water use takes off and roots penetrate to 30cm and sometimes to 50cm depth.

The graph below shows early season overhead irrigations/rain for establishment, followed by dry conditions from 3rd June through to maturity and beyond.

The following points are of interest:

1. Water use started to take off at 10 & 20cm depths on 4th June, followed at 30cm depth on 8th June.
2. Water use at 10cm slowed down about 12th June when about half the soil moisture had been used: full was about 25% moisture and the roots were starting to struggle when it reached 12%.
3. At 20cm, full was higher at 41% and water use slowed down around 15th June when values reached 24%.
4. At 30cm, full was 46% and water use started 4 days later and finished on about 20th June when the value was down to 19%.
5 . At 50cm, water use started after the top 10cm and 20cm had run out on about 14th June.

If irrigation had been applied before 12th June to bring total soil moisture back to 170mm, head weight would have been greater or harvesting date sooner.

What would the effect have been on quality and profit?

This is just a description of the water use in a crop of a particular variety allowed to mature without irrigation in a specific soil type and particular weather conditions. For the grower to maximise profit under his own conditions, he needs to trial different combinations and monitor the results.

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