Issue 4 2020
Let us learn from lettuce
Quick to grow, lovely to eat – the humble lettuce has become popular at mealtimes right around the world. All luscious leaf, it can be hard for growers to get the field growing conditions and nutrition exactly right for optimum quality and productivity. A recent experiment showed the potential for Polysulphate to play a key part in effectively nourishing lettuce in systems where conditions are controlled both above and below each growing lettuce plant.

Establishing control

Many growers invest in protected cropping where environmental factors such as light and temperature can be carefully controlled. After all, too warm and lettuce will bolt, or too much temperature variance and lettuce will discolor. Nutrition can be completely fine-tuned when the crop is grown without soil. Soilless production is an attractive option, particularly using perlite. However, the adequate provision of macro- and micro-nutrients is essential.

So, the researchers in a recently reported trial testing how well Polysulphate would meet lettuces’ needs knew they had their work cut out and the lettuce leaves themselves would soon tell them whether they could flourish or fail.
Romaine lettuce trial grown on perlite mixed with standard Polysulphate in a greenhouse in Israel. Photo by N. Cohen Kadosh.
Not all nitrogen

As a leafy crop, it’s often assumed that the top nutritional priority for lettuce has to be nitrogen. True. However, potassium (better biomass) and calcium (better postharvest condition) must not be overlooked and are why multi-nutrient Polysulphate could be what lettuce plants need.

An experiment conducted at Eshel HaNassi, located in the Western Negev district of Israel (11/11/19-14/01/20) tested standard Polysulphate at 0, 12.5, 25, and 37.5 g L–1 perlite, while nitrogen (N), phosphorus (P) and micronutrients were provided via fertigation throughout the experiment, with no Ca or Mg donor other than Polysulphate.

The rising Polysulphate rate increased the lettuce biomass. The increment in the lettuce fresh weight was due to both greater numbers of leaves per plant and leaf size, indicated by plant height. Polysulphate also significantly promoted the development of the lettuce root system.
Effects of standard Polysulphate concentration in the growth medium (perlite) on plant height at harvest. Similar letters indicate no significant differences between treatments at p <0.05.
Reading the leaves

The research team reports significant response to Polysulphate with better fresh weight of lettuce and leaf size. In their useful discussion of the results they factor in the variables that might affect the use of Polysulphate on a commercial basis in soilless production systems.
Effect of standard Polysulphate concentration in the growth medium (perlite) on the root overall appearance at harvest. Photo by N. Cohen Kadosh.
Food for thought

Lettuce is often part of the meal on our plate. This information from looking at optimizing the nutrition of lettuce is part of the bigger picture when it comes to understanding how best to supply crops of any kind with balanced nutrition.

Just like delicious lettuce itself, the information crunched from this trial is welcome food for thought.

See the full research findings in the March 2020 edition of the e-ifc from the International Potash Institute.
Effect of standard Polysulphate concentration in the growth medium (perlite) on lettuce plant size and appearance before harvest. Photo by N. Cohen Kadosh.
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Mined in the UK, ICL is the first – and only - producer in the world to mine polyhalite, marketed as Polysulphate