Sugar and Potassium in Sugarcane

Sugarcane stores sugar in the stalk, and the amount it accumulates late in the season depends heavily on potassium supply. A well-nourished crop has more sugar per unit of cane, while a potassium-short crop can grow tall and produce a lot of fibre but yield less sugar. The grower is paid for sugar, so potassium is one of the most valuable nutrients in the programme, even though the crop removes large amounts of it.

The crop is long-season and often ratooned, meaning that the stubble from one harvest grows into the next crop without replanting. This makes the nutrition plan a multi-year cycle. Nutrients removed in the cane must be replaced for the ratoon, and the soil reserve must be kept in balance so that each cycle starts from a good base.

Potassium and sugar accumulation

Potassium helps move sugars from the leaves into the stalk and supports the growth of the storage tissue. It also regulates the water balance of the plant and the activity of the enzymes involved in sucrose synthesis. Crops short of potassium often show weaker stalks, reduced sucrose content at harvest and more lodging, which reduces both the sugar and the ease of harvesting.

The timing of potassium supply matters as much as the amount. The stalk accumulates most of its sugar in the ripening period, so the potassium supply should be sufficient through the grand growth stage and into ripening. A potassium shortage at this time reduces the sugar that the crop can store, even if the earlier stages were well fed.

Ratoon crops

Each harvest removes nutrients in the cane, and ratoon crops grow from the stubble of the previous cycle. The root system of the old crop is partly retained, which helps the ratoon establish quickly, but the nutrients in the removed cane are gone. Replacing the nutrients removed, and applying nitrogen at the right time after harvest, supports a strong ratoon yield.

Ratoon management starts before harvest. The stubble should be left in good condition, the field should be drained and prepared as needed, and the nutrition plan for the ratoon should be ready so that the first dose can be applied soon after harvest. A delay of several weeks in applying nitrogen to the ratoon can reduce its growth, because the young shoots take up nutrients only once they are established.

• Apply nitrogen in split doses across the growth cycle, with the first dose soon after harvest for the ratoon.
• Maintain potassium at a level that matches removal, checked by soil test and by the cane yield record.
• Keep phosphorus and micronutrients at adequate levels on the basis of a soil test and a leaf test.
• Use the harvest record to adjust the next cycle, because the cane yield and the sugar content show how well the programme worked.

Nitrogen and sugar

Nitrogen supports the leaf area that feeds the stalk, and it is needed in the early and middle stages of growth. Too much nitrogen late in the season delays ripening and reduces sugar content, because the crop continues to grow leaves and fibre instead of storing sucrose. The nitrogen plan should therefore finish well before the ripening period, so that the crop can mature and store sugar.

The balance between nitrogen and potassium is important. A high nitrogen supply with a low potassium supply produces tall, leafy cane with low sugar, while a balanced supply gives a well-filled stalk with high sugar. The leaf tissue test shows whether the balance is on track, and it is the best guide for the late-season adjustment.

Monitoring the crop through the season

Check the cane at the tillering, grand growth and ripening stages. Tissue tests taken at the same stage each season show whether potassium and nitrogen are on track. Keep the sampling method consistent so the trends can be compared, and sample the same leaf position in each field.

Record the cane yield, the sugar content and the stalk quality at harvest, and link them to the fertilizer records. Over several cycles, the record shows which potassium rate and timing gave the best sugar, and it can justify changes to the programme for each block.

Soil and water

Sugarcane is grown on a wide range of soils, from sandy to heavy clay. Sandy soils lose potassium quickly through leaching, so the potassium is best applied in more than one dose. Heavy soils hold more potassium but may be waterlogged, which reduces root activity and uptake. Drainage and irrigation scheduling should be part of the nutrition plan.

Water stress during the grand growth stage reduces both tonnage and sugar. A crop that is short of water cannot use the nutrients applied, and fertilizer on dry soil is exposed to loss. Irrigation planning and the fertilizer plan should be made together for each block.

Economics of the programme

The cost of potassium is usually a large part of the nutrition budget for sugarcane. The return comes from the sugar paid for at the mill, so the value of each dose should be judged against the expected sugar response. Where the soil reserve is high, the potassium rate may be reduced for a few seasons, provided the soil test and the tissue results show that the crop is adequate.

Harvest timing and the sugar record

Harvest timing decides how much of the sugar potential is realised. Cane that is cut before it is ripe carries less sucrose and more water, while cane left too long loses sugar through respiration and flowering. The mill's sugar measurements, collected block by block, show how well each block ripened, and they should be read against the potassium and nitrogen records of the season.

Keep a harvest log for each block that shows the cane yield, the sugar content, the age of the crop and the harvest date. Over several cycles, the log identifies the blocks that ripen well and the blocks that need a change in nutrition, irrigation or variety.

Flowering in sugarcane is undesirable for sugar production, because flowering diverts sugar to the seed head. Some varieties flower more than others, and the nutrition plan should avoid a late nitrogen surge that encourages vegetative growth at the expense of ripening.

Common questions

Does potassium affect cane yield too? Yes. Potassium supports both the tonnage of cane and the sugar it contains. A balanced programme keeps both on track, rather than increasing one at the expense of the other.

What about nitrogen late in the season? Late nitrogen can delay ripening and reduce sugar content. Plan the nitrogen to finish well before the ripening period, and use the tissue test to confirm the crop is not short of it.

Can I rely on the soil test alone? The soil test gives the reserve, but the crop's removal and the sugar response are better guides to the rate. Combine the soil test with the harvest record and the tissue test for the best decisions.

Recommendations depend on local soil, climate and crop conditions. Follow the product label and local regulations, and consult a qualified agronomist for field decisions.