Why are my chile plants turning yellow?

Courtesy of Byron

Spider Mites - Usually pin point yellowing
Insects
Nematodes
Root Rot Diseasses
Viral Disease
Overwatering
Nutrient deficiencies - lack of nitrogen or magnesium
Soil Chemistry Excessive salt, pH
Old Age.

Remedies: Check for insects.

This part by Dwalls

If there is no insect damage, give a small quantity of nitrogen - low N value fertilizer. You can also give epsom salt for magnesium. 1-2 tbsp. per gallon of water or sprinkle epsom salt on ground around base of plant.

This part by Byron

Yellowing of leaves could also be caused by excessive nitrogen. Bottom leaves turn light yellow, veins are dark green/brown.
Odds are the leaf tip and or margins will turn brown and become very brittle.

Fix is to over water for several occassions. If the roots are not salt burned (brown areas), the plants have a chance to recover.