A shortage of nutrients in soil is an easy problem to solve, while others such as lack of draingage, excessively compact soil and poor air circuulation are very harmful to plants and harder to solve effectively.

Kinds of fertiliser
Fertility is increased by adding the nutrients that soil needs or helping the soil to create it. Nutrients are added by using fertilisers that in general can be broken down into two groups: chemical and organic.

Chemical fertilisers work over the very short term, and give spectacular results in plants. Normally they are sold as compounds of nitrogen, phosphorus and potassium. The
amount depends on the kind of plant they are being used with. Although these products
are easy to use, be careful when applying them because using too much can be very harmful to the plant.

They are generally used to supply nutrients to a plant or group of them in particular, not to all the soil in a garden. Treatement should be repeated periodically because this kind of fertiliser does not allow the soil to develop its own fertiliser.

Organic fertilisers work over the middle and long term and in a much more complete fashion. Besides boosting soil's fertility, they improve its texture, making it spongier and letting more air in thanks to the action of millions of  helpful microorganisms. There are basically four kinds of organic fertilisers.
1.- Manure: It is the oldest known kind of fertiliser and is still useful, although with many restrictions. Its foul odor makes it hard to use in gardens, and it also is a vector for pathogenic agents that can affect plants. An excessive dosis can be very harmful. This fertiliser must also be used only in winter because its fermentation process produces a considerable rise in temperature.
2.- Humus: This is thoroughly fermented manure. The process takes more than a year.
The resulting product should be free of odor, and to the nake eye there should be no remains of straw or excrement. The texture should be loose and spongy. It is excellent fertiliser, but out on the market it is hard to find a well-made product that meets the conditions mentioned earlier.
3.- Earthworm humus: It is the same as humus but the process is accelerated by the action of red California earthworms which, under certain conditions of temperature and humidity, convert manure into humus, an odorless product that is free of weed seeds and rich in nutrients which, once in the ground, are released little by little. It improves the structure of soil greatly and helps retain moisture without causing puddles of water. 
4. - Compost: produced by the fermentation of plant remains and is very good for making soil spongy. But it adds much less in nutrients than the other kinds of fertiliser.

At the same time there are specially prepared substrates (such as for lawns) which blend a vegetable base such as peat, chemical fertiliser and humus. They feature what is good of the two kinds of fertiliser, and add the peat to make the soil spongy.

Tips for keeping soil fertile
- Choose plants that require little water because excessive watering washes away nutrients.
- Water with the trickle technique rather than with a sprinkler.
- Leave fallen leaves in pots and flowerbeds.
- Avoid species that deplete soil such as eucalyptus or pine, and choose ones that enrich it such as clover, which adds nitrogen to the soil.

Finally, there is a vicious cycle: where the soil is good, plants grow better. And where there is a large quantity and variety of plants, the soil generally improves on its own.