Fire has fascinating effects on ecosystems. After a fire, there are lots of physical and chemical changes. Exactly what happens depends on what things were like before the fire, how hot the fire was, and what happened after the fire, but there are some general things we can predict.
Soil nutrients are very important for plant growth. Several of these nutrients have a positive charge. We call these cations. They are found in living plants and dead plant material. Normally they slowly return to the soil when the dead plant decomposes, but the fire releases them immediately. They usually stay behind in the ashes, so right after a fire, levels of phosphorus, magnesium, potassium, and calcium increase. Sulphur and carbon tend to blow away after a fire.
The air is 80% nitrogen, but its not in a form plants can use. We call turning the nitrogen in the air into molecules the plants can use fixing. Some nitrogen blows away after a fire, but some will actually be fixed by the fire or by soil microbes after the fire, so even though the total amount of N may be lower, more might actually be usable.
When it rains, ash filters down into the soil, making it less acidic and more basic (alkaline). Another way to say this is that the ashes raise the pH of the soil. Opposite charges attracts, so the less acidic soil (which has more negatively-charged ions) holds onto some cations better, increasing soil nutrient levels.
Some chemical changes make the soil water-repellent, so that the water runs off instead of soaking into the soil. This can last for about a year. After a fire, there are fewer plants holding the soil in place, and less dead vegetation to absorb the water, so erosion can be a big problem. When the soil washes away, it takes nutrients with it.
After a fire, the ground is dark, so it absorbs more light and reflects less, making it hotter. The removal of plants and dead plant material also means that more light hits the soil, making it even hotter. This can actually help some plants sprout and grow.
Soil is usually full of microscopic life. If a fire isnt too hot, they can usually survive if they are deep enough in the soil.
Theres a great reference on Google Books called Fire in Californias Ecosystems Its pretty technical, but your teacher may be able to help you understand it.