When should I start to fertilize seedlings?
The best time to start fertilizing seedlings depends on the type of fertilizer you are using. Follow the instructions on your fertilizer package and err on the side of caution, using less rather than more.

If you are using a slow-release fertilizer, you can mix it into the potting mix or soil before planting your seeds. This will provide nutrients to the seedlings as they grow.
If you are using a liquid fertilizer, you should wait until the seedlings have developed their first set of true leaves before applying it. At this stage, the seedlings will have used up most of the nutrients stored in the seed and will benefit from additional nutrients.

In general, it's important not to over-fertilize seedlings, as this can cause them to become leggy and weak.
Thank you for the answer, what is the relationship between the fertiliser and water amount that is given to plants and the frequency of providing water?
The relationship between the amount of fertilizer and water given to plants and the frequency of providing water is complex and dependent on several factors, including the type of plant, the type of fertilizer, the growing medium, and environmental conditions.

In general, plants require water to uptake nutrients from the soil and to transport those nutrients to various parts of the plant. Fertilizer provides those essential nutrients to plants that are not present in sufficient quantities in the soil. However, giving too much fertilizer or water can be harmful to plants, as it can cause root burn or root rot.

The frequency of providing water depends on the moisture needs of the plant and the rate at which water evaporates from the growing medium. Some plants require more water than others, and the amount of water required can vary depending on environmental conditions such as temperature, humidity, and wind. The type of soil or growing medium also plays a role in determining how often to water plants, as some soils hold moisture better than others.

When fertilizing plants, it is important to follow the recommended application rate and timing for the specific fertilizer being used. Over-fertilizing can cause excess nutrients to build up in the soil, which can harm the plant's roots and reduce its ability to take up water. Therefore, the frequency of providing water may need to be adjusted when fertilizing to avoid over-saturating the soil and causing nutrient leaching.

Overall, the relationship between fertilizer and water amount and frequency of providing water is complex and requires careful consideration of the specific needs of each plant. It is important to monitor plants regularly and adjust fertilizer and watering practices as needed to ensure healthy growth and development.