After fertilizing my houseplant, how often do I need to water it?
For most houseplants, you should be watering on the plants needs rather than a schedule - regardless of when you fertilized. What your plants needs are can vary. Some plants, like cactus and succulents need the soil to be nearly completely dry before you should water them. Others, like rubber trees or umbrella plants, need the top of the soil to be dry before you water. And still others, like African violets and many carnivorous plants, need to be moist all the time.
If you are using a water based fertilizer, that counts as a watering and then check the soil based on your plant's watering needs after that. If you are using a granular, solid or powdered fertilizer, water right after you fertilize so that the fertilizer does not burn the roots.
I'm doing a project for school and need to know if a plant would drink more salt water or more tap water?
Not too many plants would be particularly fond of the salt water, though they would still take it up. Generally, any type of water, with exception to salt water, is beneficial to plants; however, the use of distilled water can cut down on salt buildup resulting from fluoride in regular tap water. Rainwater can also benefit plants, as it is filled with nutrients, and typically this type of water is the best choice for plants whenever possible. The following article should be of some help to you:
How does water affect plant growth?
This article will help you with that: https://www.gardeningknowhow.com/special/children/how-does-water-affect-plant-growth.htm
How much water do you need to grow a green plant? Can you also include how much sunlight it needs?
This depends on the type of plant you have as all of them require their own growing conditions. This article may help with watering: https://www.gardeningknowhow.com/houseplants/hpgen/how-to-water-a-plant.htm
Most plants do require at least 4-6 hours of sunlight; however, this too depends on the type of plant, as some will actually grow quite well under low light or shady conditions.
Is softened water ok for plants?
We do not recommend it for watering plants because the sodium level is too high.
We live in the country with very hard, rusty water so we have to have a water softener. Is soft water bad to use for plant water? If so and I have to buy water, is it best to buy just plain drinking water or should I get spring water?
Normally, using distilled water or rainwater is better. Softened water contains sodium (salt), which can have adverse effects on plants. However, there are steips you can take to improve their chances if soft water is your only alternative. This article will provide more info: https://www.gardeningknowhow.com/plant-problems/environmental/softened-water-and-plants.htm
My plants have not grown yet, but I'm afraid they won't. I think I over watered them. How do I recover my ungrown plants if they are overflowed with water?
Give them a little time and let them dry out a bit. They may still grow. You may need to replant if you do not see seedlings in two weeks. But when you replant, try watering from below. This will reduce the chances that the seeds will be overwatered.