All plants have their own specific needs. What these needs are vary from one plant species to another, but all will mostly include varying amounts of nutrients, water and air.
The focus of this article is water, but this is intrinsically connected to air and nutrients.
Plants absorb light, carbon dioxide and oxygen (depending on light availabiity) through their leaves, but most activity actually takes place in their roots. Despite the fact that a plant's roots are usually well entrenched in the soil, one critical aspect to this soil (and is often overlooked) is that it contains air.
Much nutrient absorption in the roots requires energy and energy is made available in the roots by the process known as cellular respiration - this requires oxygen. This oxygen is NOT absorbed from the air by the leaves, but is in fact absorbed by the roots from the air spaces in the soil.
Therefore, if there are no airspaces in the soil, then there is limited oxygen available to be absorbed for cellular respration and therefore limited energy for the absorption of nutrients.
In fact, limited oxygen in the soil can actually lead to the death of the plant from the roots up - the plant can actually look healthy above the soil but may already be doomed from below the soil.
What can lead to poor oxygen availabiity in the soil?
Over watering your plants
Particularly in poor draining soils, overwatering plants can result in wet, sodden soils. Air spaces in soils that have been overwatered tend to become filled with water instead of air. This results in limited availability of oxygen to the roots and ultimately poor nutrient absorption.
When watering your plants, a good rule-of-thumb is to remember that it is best that the soil is moist, not wet.
Heavily compacted soils
Soils that are overly compacted tend to have the air spaces 'squeezed' out of them resulting in significantly reduced oxygen available to the roots. Again, this results in limited energy availability for nutrient uptake.
Poorly draining soils
Heavy clay rich soils are some of the most nutrient rich soils available in the garden, but ironically, they also tend to lock these nutrients up very tightly - while some plants grow well in clay soils, many don't. The reason for this again comes back to the lack of air spaces in the soil and therefore the relative absence of oxygen availble to the roots and the subsequent inability to absorb the necessary nutrients in quantites needed by the plant.
Therefore, to assist with healthy growth, most plants will benefit from ensuring that there is good oxygen availabilty in the soil - don't overwater your plants and keep your soil well aerated.