Chemicals in Cacti ... 3/3

Why?

The plant goes to a lot of trouble, using energy that could be used for growth or reproduction, to produce chemicals that in some cases have no ascertainable beneficial use.

One reason to produce chemicals would be as a defence mechanism, alkaloids themselves are usually sour or unpleasant tasting and certainly put an animal off chewing on that particular plant (plus the hallucinary effects may discourage the animal, or at least make it forget where it is so it can't find the plant again!!!!).
Colours in flowers are very important in terms of attracting pollinators to the pollen receptors, insects see the colours differently to us humans and the colours in the flowers form a pattern to direct the insect to the pollination site, rather like lights on an airport runway.

Nutrients

There are two types of Nutrients: Macro (present in large quantities) and Micro (present in smaller quantities).

Macro nutrients include Nitrogen, Phosphorous, Potassium, Calcium, Magnesium and Sulphur and they are usually important building blocks e.g. DNA is made up of Nitrogen and Phosphorous; Potassium is important for ionic exchange and controlling opening and closing of stomata; Proteins are made up of Nitrogen and Sulphur; Calcium is like the cement that holds the whole plant together; and Magnesium makes up Chlorophyll and involved in enzymes in photosynthesis.

Micro nutrients, although in less quantities, are still very important as they are involved in enzymes and the chemical reactions they initiate, without which the plant couldn't keep on growing and surviving.

Most good fertilisers contain the macro and micro chemicals needed for the plant to grow, for cacti the fertiliser should be low in Nitrogen so as to not force the plant and make it prone to disease or rot, and to grow naturally.
The pH (acid/alkaline balance, pH7 is neutral) also plays a big part in the availability of nutrients to the plant determining what are soluble and therefore absorbable to the plant in it's water uptake. Not many plants can grow in High Alkalinity or Acidity, and those than do are specially adapted to do so. Some cacti (e.g. Ariocarpus) grow better in alkaline conditions (pH8) while others (e.g. Gymnocalycium??) prefer acid (pH6), this is the result of evolution in response the growing conditions of it's environment.

References: The Cactus Primer, A. Gibson & P. Noble, 1986.