Sorting out the causes for any symptoms of poor health in your plants is a technical challenge that most busy parents don't have time for. Yellow leaves can be a sign of any number of health issues, but there are a few quick guidelines that can help you pinpoint the problem and choose the correct course of action. Nutrient deficiencies, improper soil pH and any number of pests and disease may be involved.
A soil's pH is a measure of its relative acidity or alkalinity, represented on a scale from 1 to 14. Most plants prefer to grow in soils with a neutral or mildly acidic pH between 5.5 and 7, though a fair number of common garden plants grow well with a more acidic soil between 4 and 5.5. Very few plants like high-pH soils, except for desert species where alkaline soils are common. Because soil pH determines how well plants can absorb nutrients from the soil, symptoms of nutrient deficiency -- such as yellowing leaves -- can be caused by improper pH, even when nutrient levels are abundant.
Iron deficiency is a classic cause for yellowing leaves and is very easy to identify. If your plant has yellow leaves but the veins remain green, insufficient iron uptake is almost surely the cause. More than likely, there is plenty of iron in your soil, but the pH is too high and the iron is locked up in a form that your plants cannot assimilate. Applying iron chelate is the foolproof remedy, whether the deficiency is due to high pH or an actual shortage of iron. Products vary, so read the label to determine the suitable application rate and any necessary precautions.
Nitrogen deficiency also causes yellow leaves, but this is not usually the result of a pH problem. Nitrogen is the primary nutrient responsible for green, vegetative growth in plants and is available in limited quantities in the soil. If your leaves are yellow, including the veins, nitrogen deficiency is probably the cause. If you plant summer vegetables too early in spring when the soil is still cold, they may be unable to absorb nitrogen and show symptoms of deficiency. This will be corrected as soon as the soil warms, however. Otherwise, the simple solution is to apply some form of nitrogen fertilizer. The first of the three numbers on a bag of fertilizer indicates the percentage of nitrogen -- use a product containing at least 7 percent nitrogen for best results.
Other Causes of Yellow Leaves
Poor drainage can cause leaves to yellow. Check the soil around the base of your plants regularly -- if it is constantly soggy, poor drainage is the most likely cause. Diseased plants often exhibit yellow foliage, as do plants that are under attack by insect pests. If insects are the cause, the leaves may suddenly turn yellow, as opposed to the gradual process that is typical with nutrient deficiencies. Plus, if you look carefully you may see the insects themselves or other signs of damage, such as oozing sap or deformed foliage.