Changes in soil available NPK in multiple cropping systems based on short duration sugarcane crops relative to a conventional sugarcane cropping system

Changes in soil available NPK were studied in four intensive crop rotations based on short duration (8 months) sugarcane crops (1. short duration plant cane/1st ratoon/2nd ratoon; 2. short duration plant cane/1st ratoon/finger millet/cotton; 3. finger millet/short duration plant cane/1st ratoon/wheat; and 4. finger millet/maize/short duration plant cane/1st ratoon). These rotations were compared with the conventional duration (12 months) sugarcane crop sequence (one plant + one ratoon) in a cycle of 24 months.Soil available nitrogen (SAN) declined when 100 or 150 kg N ha^–1 was applied in the short duration sugarcane based systems, but was either maintained or improved at a higher N application rates (200 or 250 kg ha^–1). The conventional system showed a sharp decline in SAN of about 14% from its original status at the end of the sequence. Close row spacing (60 cm) of sugarcane improved the soil N level over that in the conventionally spaced rows (90 cm) probably through greater rhizosphere biomass additions.Available P declined sharply from its original level in the soil in sequence 2, the decline being marked after cotton. In all the other short duration based sequences it was maintained. The conventional system also showed reduced soil available P at the end of the sequence. Soil available K declined in all crop sequences.Nitrogen uptake was far less than additions made by fertilizer. The actual soil N balance was much lower than the expected balance thus indicating large losses of N from the soil. Phosphorus removal was also less than the additions made and thus there were improvements in the soil available P status at the end of the crop cycle. In all the sequences, there was a negative potassium balance due to greater removal by the various crops when compared to K additions. However, in the system as a whole there were net gains of K as larger amounts were recovered than had been added.