Sugarcane Soil and Water Management
Ratoon crop emerging
Soils need to be well structured and fertile to maximize growth and produce high sugarcane yields.
Any soil is suitable as long as it is deep and well drained, with the water table below 1.5 to 2m depth, so as not to restrict rooting. However, the preference is for loamy soils with a good water holding capacity. Poor soil physical conditions such as compaction are much more difficult to ameliorate and need to be avoided within the crop or corrected prior to planting.
Drainage should be considered where salt accumulation could occur or where saturated soils can cause stool decline – sugarcane will not tolerate anaerobic conditions. In preparing land for the plant crop, old stools can be destroyed by mechanical methods or herbicide application and any compaction removed by deep ripping at a time when soils are dry.
Liming
The optimum soil pH is about 6.5, which maximizes nutrient availability. However, crops can tolerate a considerable degree of soil acidity or alkalinity. Production at extremes of pH around 5.0 and 8.5 is feasible particularly with today’s more pH tolerant modern varieties.
Under acidic conditions, the adverse effects are mainly due to aluminum, iron and manganese toxicity, which reduces root growth, water and nutrient uptake. As a result of poor rooting, tillering, shoot elongation and leaf area are severely affected leading to reduced sugarcane yield and juice quality.
When the soil pH drops below 5.5 phosphorus, magnesium, calcium, potassium and molybdenum availability declines. At pH levels above 7.5, zinc, manganese and iron can become deficient. Liming is essential for the production of high yielding crops on acid soils. Research shows that 10-20t/ha responses can be found from applications of lime made to the plant crop and also in the following two ratoon crops (Figure 5). This leads to a large cumulative yield response over the life of the crop.
Gypsum should be applied to reduce soil sodicity where this is required, or to increase the calcium saturation of soils, including acid soils. Sugarcane is moderately sensitive to soil salinity and yield reductions can be expected when the EC is above 1.7dS/m.
Water Requirement & Irrigation
There is a linear relationship between sugarcane yield and water availability. For every ton of sugarcane produced, the crop needs around 10mm of water. Sugarcane also transpires large volumes of water – at peak canopy this amounts to 5-6mm of water per day. Therefore a crop requires 1500- 2500 mm of available water per year with peak requirements during tillering and grand growth phase.
At establishment, water needs are low as buds simply need enough moisture to germinate and grow. Excessive water combined with cool conditions at this stage slows germination and encourages disease with the setts. Early vegetative growth requires a good water supply and the degree of tillering is often directly in proportion to the frequency of irrigation. However, over-irrigation at this stage can hinder nutrient uptake due to anaerobic conditions, particularly on heavy soils.
During the grand growth phase, lack of moisture affects the elongation of internodes, leaf production, girth, the production of sugar storage tissues and ultimately stalk weight. All significantly restrict sugarcane yield. During this phase, irrigation intervals can be extended, but amounts applied need to be increased.
At maturity, excessive water promotes additional vegetative growth which reduces sugar content of the cane sent to the mill.
Ideally, harvest should coincide with peak maturity but this is not always possible due to weather, harvesting schedules and labor or machinery limitations. Therefore sugarcane may be harvested before or after it has reached peak maturity in order to ensure a continuous supply of cane to the mill. Readiness for harvest is usually based on the farmer’s experience, the visual appearance of the crop and its age.
Others will assess the ratio between the brix of the bottom part of the cane compared to the upper part. Farmers also need to minimize levels of extraneous matter such as trash, soil and roots sent to the mill. Sugarcane should also be crushed as soon as possible after harvest as delays will accelerate the formation of reducing sugars and reduce overall sugar content.
