How can I improve the soil in my planter without putting all new soil?

You will have to add at least half new soil for your plants to get the necessary nutrients.

Try adding some good compost to the top layer.

It's a routine now every spring I take advantage sales on planter soil an replace soil every spring

Soil looses the nutritional value to feed your plants it needs to be changed at least once a year, I have had house plants for years that have turned into trees 🙄 An didn't even know they could get that big

Grow a cover crop and turn it in to add organic matter to the soil. Mulch the growing crops deeply to help prevent soil from becoming dry, and as it breaks down the added mass will help raise the soil, and grow crops in fast succession always mulching deeply four to six inches always. Mulch with compost, shredded paper, coffee grounds, grass clippings, most leaves, weeds with NO seed heads. You can add bags of potting soil or planting mix whenever but it disappears in a big bed, you need alot. If you have a deep layer of ongoing healthy mulch, you can add local site soil whenever you have other projects that result in spare soil.

I have added a new layer of soil.Both compost and natural soil mixed.At present I am in a very very hot place in India.Less water can spoil the seeds.I have covered them with plastic gunny.Just to keep the soil damp.I have planted seeds of aster.

Is it OK?

I dump the soil from all my pots into the wheelbarrow and add compost and coir ( or you can add some moisture retaining granular to it) and compost then I refill my pots and plant. I water my pots w a liquid fertilizer at planting and again at bloom. Nature doesn't throw out the soil and pay for new...why should you?

Liquid compost....apparently the people who show the flower baskets add a few drops in every days watering to have them looking at their best at all times

add in some leaf mold and chopped up sea weed.

Geoff