Many factors affect the frequency of watering needed for specific plants. What are the light and temperature where the plant is growing? How big is the plant in relation to the pot? What type of soil is it planted in?

The best single way to determine if a plant needs water is to feel the top of the soil with your finger. If the soil is dry on top, then it is probably time to water. After you have watered a plant several times you will find out about how long it takes to dry out. Wet soil is darker than dry soil, so after a while you can tell by color when a particular soil is starting to dry out. Not all potting soils are the same color, so you need to feel the soil a few times until you learn the relative color.

Since there are so many factors involved in how often a plant dries out, you should not try to water all plants at equal intervals. If you check plants every day or two for awhile you will find out which ones need frequent or infrequent watering.

Water quality is also important. Softened water has more sodium chloride than is good for plants. Sodium chloride or table salt damages plant roots and soil structure. Most cold water taps in the kitchen do not have softened water. However, cold water can slow plant growth, especially in the winter. I like to fill my watering can with cold water and let it sit until it reaches room temperature.

Older leaves turning yellow is a definite sign that plants are short of nitrogen fertilizer. Nitrogen is a key ingredient in chlorophyll, the compound which makes leaves green. Chlorophyll is necessary for the leaves to produce energy from photosynthesis. When nitrogen supply is limited, plants will remove nitrogen from older leaves and transfer it to new growth. This is a natural process which goes on all the time, but becomes accelerated when there is a shortage of nitrogen.

Other factors can also cause leaves to become lighter green. Overwatering can be a contributor.

I use a long lasting poly coated plant fertilizer such as Osmocote for my indoor plants. In most cases it lasts for 2 months or longer. The coated capsules release a little fertilizer every time plants are watered. However, capsules release a smaller amount of fertilizer toward the end of their life. I watch the leaf color to determine when I need to apply more fertilizer.

Since not all plants use fertilizer at the same rate, I apply additional liquid fertilizer to yellowing plants to give them a boost. When plants are overwatered, fertilizer is leached from the soil more quickly.

Another factor which can cause indoor plant yellowing is the development of an acid condition or low pH. The water in the Pacific Northwest has a naturally lower mineral content which gradually make the soil more acid. Small amounts of lime can counteract this condition.