Explanation:

Vegetation cover or grass-cover has a significant impact on the field capacity of soil. Field capacity is defined as the maximum amount of water that the soil can hold against the force of gravity. It is influenced by various factors, including the presence or absence of vegetation cover.

1. Increase in Field Capacity:
When vegetation cover is present, it can increase the field capacity of the soil. This is because the roots of plants help in the formation of soil aggregates, which create pore spaces in the soil. These pore spaces act as reservoirs for water, allowing the soil to hold more water. Additionally, the presence of vegetation cover reduces the impact of rainfall on the soil surface, preventing compaction and allowing more water to infiltrate into the soil.

Vegetation cover also helps in reducing evaporation from the soil surface by providing shade and reducing wind speed. This results in less water loss from the soil, allowing it to retain more moisture and increasing the field capacity.

2. Decrease in Field Capacity:
However, in certain cases, vegetation cover can also decrease the field capacity of soil. This can happen when the vegetation cover becomes dense and forms a thick mat on the soil surface. The dense vegetation cover can prevent the infiltration of water into the soil, leading to reduced field capacity.

3. Variation in Field Capacity:
It is important to note that the impact of vegetation cover on field capacity can vary depending on various factors such as the type of vegetation, soil type, climate, and management practices. In some cases, vegetation cover may increase the field capacity, while in others, it may decrease it. Therefore, the effect of vegetation cover on field capacity cannot be generalized, and it may vary from situation to situation.

Conclusion:
In conclusion, vegetation cover or grass-cover can increase the field capacity of soil by promoting the formation of soil aggregates, reducing surface compaction, and minimizing evaporation. However, the impact of vegetation cover on field capacity can vary depending on several factors, and it cannot be assumed that vegetation cover will always increase the field capacity.