Yes, you can put seeds straight into the soil. A rule of thumb would be: if the seeds you are sowing are big, just sow them directly into the soil. Plants like zucchini, cucumbers, watermelons, corn, and chard can go straight into the soil. Furthermore, some plants do not like to be transplanted, so it would be better if you sow them directly into the soil. Carrots, for instance, are famous for being one of the smallest seeds, yet, it’s always better to direct sow them.