The crop residue of a crop having a wide CN ratio takes a long time to decompose. During the process of decomposition, part of soil nitrogen is temporarily immobilized. (When CN ratio is more than 30 immobilization occurs.)
Such an effect is pronounced in sorghum. It is a fast-growing crop that exhausts the nutrient status of the soil. Also, stubbles of sorghum have a wide CN ratio hence take a long time to decompose.
Temporary immobilization of Nitrogen affects the growth of succeeding crops.
Early nitrogen deficiency symptoms appear in succeeding crops due to the wide CN ratio of sorghum residue. This is called the sorghum effect or sorghum sickness.
To reduce the sorghum effect, 25% more nitrogen is applied at the time of the first fertilizer dose of a succeeding crop. It increases the rate of decomposition and overcomes the immobilized nitrogen.