NITROGEN
Recommendations are based on soil types and land systems (e.g. Coastal System), with four categories of N-mineralization capacity. These have been determined by yield responses from fertilizer field trials. The probability of a response to applied nitrogen increases as the organic matter content of soils declines from more than 4 percent to less than 2 percent. The organic matter content of soils, in turn, is related to soil types. Recommendations are further refined according to the rooting depth and differentiated between plant and ratoon cane. Rates for plant cane vary between 60 and 140 kg N/ha, those for irrigated cane being higher by about 20 kg N/ha. Rates for ratoon cane are higher than for plant crops and vary between 100 and 200 kg N/ha, again depending on the soil system, soil form and rooting depth.
PHOSPHOROUS
Recommendations are based on soil-P analysis, adjusted for the P-fixing capacity of soils occurring in the Natal Midlands. For plant cane, P-fertilizer is applied in the furrow to meet at least the P- requirements of the plant crop and the first ratoon. The recommendations aim to bring the soil P-level up to approximately 206 kg P2O5/ha, as in the following example:
|
Soil P level |
kg P2O5/ha |
160 |
115 |
68 |
28 |
|
Rate |
kg P2O5/ha |
46 |
92 |
137 |
183 |
For ratoon cane, soil P-levels are maintained at 70 kg P2O5/ha, which converts to application rates of 46 to 92 kg P2O5/ha for decreasing soil P-levels.
For the high P-fixing soils in the Natal Midlands, supplementary broadcast P-applications are recommended for plant cane. These vary between 240 and 366 kg P2O5/ha for soils with less than 92 kg P2O5/ha and which have moderate to strong P-fixing capacities.
No differentiation is made in recommended P-rates between rainfed and irrigated sugar cane.
No P-fertilizer is recommended when soil P-tests are above threshold values.
POTASSIUM
Sugar cane requires large quantities of potassium for sustainable production. Soils are quickly depleted of available K if K- fertilizer is under applied.
Soil threshold values for soil-K have been established through extensive SASRI-FAS research:
|
Percent clay |
kg K2O/ha |
ppm K |
|
30% or less |
300 |
112 |
|
30% or more |
498 |
150 |
|
40% or more* |
600 |
225 |
* For high base saturation soils in northern irrigation areas.
Potassium recommendations are influenced also by the winter or summer cycles on high base-saturated soils in the northern irrigation areas (Mpumalanga and Kwazulu-Natal).
The range of K-recommended rates can be summarized as follows:
|
Percent clay |
Crop |
kg K2O/ha |
|
Less than 30%1 |
Plant crop |
90 to 2103 |
| |
Subsequent ratoons |
150 to 2104 |
|
More than 30%, exc. high base saturation soils |
Plant crop |
120 to 2403 |
| |
Subsequent ratoons |
180 to 2404 |
|
More than 40% clay and high base saturation2 |
Plant crop |
120 to 3003 |
| |
Subsequent ratoons |
240 to 3004 |
1 Mostly Coastal and Kwazulu-Natal Midlands areas
2 Northern irrigation areas (winter cycle)
3 Depending on soil K- test below threshold
4 Whole cycle advice
Source: SASRI-FAS, 2002.
Rates for summer cycle cane are up to 25 per cent lower than for winter cycle, depending on the soil K-analysis
No K fertilizer is recommended if soil K-tests exceed threshold soil levels.
