All the nutrient are present in soil. However, their presence in soil, doesn't mean such soil is fertile. Some nutrient are absorbed in cationic , anionic and some are in the reduced form.There must be sufficient amount of nutrient enough to meet the minimum requirement of crop and also must be in a available form.

Thus, soil fertility can be also defined as the ability of soil to supply plant nutrient in adequate quantity and in available form. The main cause of soil fertility declines can be summarized by the following points..

1.  Acidification of soil: Mostly middle mountain and eastern Nepal soil are acidic due to parent material like sandstone, slitstone and use of acidyfying fertilizer.

2. Soil erosion: Mostly high hill upland and marginal land has high rate of soil erosion.

3. Siltation problem: Seti and Narayani irrigation sector command area are more prone to siltation.

4. Non agricultural degradation: Degradation of Forest and Marginal land.

5. low quality fertilizer

6. low fertilizer use

7. Environmental Portion

8. Specific problems to red color soil