Nutrient management for optimal sugar-beet growth

50% of the UK’s sugar comes from the sugar-beet crop, and each ‘beet’ can produce around 100 grams of sugar (also known as sucrose), which is is 1700 kJ (400 kcal) of energy. If this is calculated on an area basis, this represents 141,780 MJ / ha of energy harvested. This energy originates from sunlight that is falling onto the sugar beet plants leaf canopy. These leaves are natural solar panels and for them to operate efficiently they need looking after, just as solar panels are regularly maintained to achieve optimal performance.

From a grower’s point of view the target when growing sugar beet, as with other crops, is to produce a leaf canopy that is intercepting 90% of the incoming photosynthetically active radiation. The leaves convert this energy into sucrose via photosynthesis and then translocate this sucrose down to the root where it is stored.

During the early stages of growth, the carbohydrate (sugars) produced from photosynthesis will be utilised to grow more roots and leaves, as the canopy expands. Sucrose will only start to be stored in the root after approximately 30 days of growth. With all this in mind we can start to identify why there are some specific nutrients that play critical roles in sugar beet growth and development.

Nitrogen and potassium are the two nutrients in highest demand

First and foremost, the two nutrients in highest demand are potassium (450 – 500 kg K₂O/ha) and nitrogen (250 kg N/ha), however, the others, whilst not in such high demand, play very significant roles. Phosphate and magnesium are required in similar quantities but perform very different functions. Magnesium is the central element in the chlorophyll molecule, surrounded by the four nitrogen’s, that gives the leaves their green colour. It is therefore vital for the efficient capture of light coming onto the leaf canopy that the crop has sufficient magnesium.

The next step, having absorbed the light is to turn it into chemical energy, which involves the splitting of water as the first step. Research has identified that manganese and calcium play key roles in this process.

Phosphate’s role compliments the magnesium, manganese and calcium as it is required as part of the chemical energy storage pathway. This stored energy is then used to fuel many of the plant processes as it grows and goes through its development cycle. In sugar beet, as described above, one such process is the production of sugar (sucrose) in the leaves that is then transported to the roots for long term storage. This sugar is what is harvested in the beets and processed in the factory. The process of ‘pumping’ the sugar down to the root is an ‘active’ process, itself requiring the chemical energy generated during photosynthesis. The final piece of the nutrient jigsaw, when it comes to sugar translocation down to the root, is the role of potassium. Again, foliar applications of a relatively low concentration of potassium solution (lower than that achieved with YaraVita products containing potassium) has been associated with increasing the rate of loading of the sucrose from the leaves and into the plant phloem transport system.

Phosphate, magnesium, manganese, calcium and boron all have important roles

Consideration also needs to go into where all the sugar/sucrose is stored? This is actually stored in the vacuole of specialized cells (parenchyma) in the root. All the cells need to have a well-structured cell wall to enable sugar to be transported in and then held without leakage. Integral to a well-structured cell wall is the sufficient supply of two other nutrients – boron and calcium. These two nutrients are configured in an ‘egg box’ like structure that gives strength to all the developing and expanding cells.

Up to 90% of the cellular boron has been localised in the cell wall fraction, and again is associated with the ‘pectin’ binding the primary cell wall and stability of the middle lamella.

Having established some key roles of the various nutrients in the growth and development of sugar beet the issue then is are these at a sufficient level and available in your soil to ensure season-long supply? Analysing the big data set at the laboratory can give some insight into answering this question.

For samples recently processed then magnesium and boron appear to be of particular concern, with over 60% and 80% respectively of samples with deficiency.

With the leaf canopies well developed and now trapping that all-important solar energy, it is time to ensure none of these key nutrients are limiting the ability of the crop to produce a large harvestable yield of sugar.