Soil fertility is the ability of the soil to supply essential and adequate nutrients required for plant growth.
A soil is said to be fertile if there is adequate amount of nutrients and at the required proportions. A soil can be naturally fertile but due to some activities on it, losses its fertility, these activities include:
• Crop removal.
• Soil erosion.
• Leaching
Crop removal
A crop feeds directly from the soil through absorbing the nutrients therein with its roots, these nutrients are what the crop uses to support its vegetative and reproductive activities.
The amount of yield obtained from the crop  is directly proportional to the soil nutrients the crop absorbed or removed from the soil. Grain crops like maize, absorb more nitrogen than other crops, forage crops like vegetables, coffee, sugarcane, root crops and fibre crops like cotton, kenaf etc. absorb a large amount of potassium.
If any grain crop is planted, the amount of nitrogen present in the soil will be reduced by the virtue of the affinity grain crops have for nitrogen. So also forage crops reduce the potassium content of the soil. This is one of the ways soil losses its fertility.

Soil erosion
Soil erosion is the washing away of the most fertile part of the soil (the top soil). It houses more nutrients than the other parts of the soil, if it is not protected by crop cover, it may be slowly washed away by surface run off. The loss of the top soil represents high losses of organic matter, minerals solids and plant nutrients.
Ways to avoid this include: 
• Planting of cover crops, like cowpea, pumpkin leaf(Ugwu).
• Practicing of crop rotations. 
• Terracing.
Leaching is the process by which soil nutrients are washed down by water from the root zone to the underground water. This is common in the tropics where rainfall is common, it also makes the soil acidic. Leaching can be minimized through:
• Timely addition of organic manure.
• Proper timing and placement of fertilizer.
However, the soil fertility can be maintained through the following ways:
• Fallowing: This means that that the soil is allowed to rest for sometime before planting is resumed on it.
• By using a well planned system of crop rotation.
• By regular application of manure and fertilizer.
• By intercropping the crops with leguminous crops like cowpea, groundnut etc.

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