Tomato problems #2 yellow older leaves

If only your Tomato’s older leaves are going yellow, it’s a sign that there is a deficiency in one or more mobile elements. It is usually Nitrogen or Magnesium but could also be Potassium or Phosphorus.

What are ‘mobile’ elements?

While all nutrients are important to plant health and a deficiency of an element will cause problems, some are far more important to plant growth and health. They are so important that a plant will withdraw them from old growth so that they can be used by the new if it feels that there is a deficiency.

Why are these elements so important?

A little look at those elements will show us why plants consider them so vital to their growth that they are willing so sacrifice old growth to favour new.

The elements mentioned above are vital to a plant’s growth. Nitrogen and Phosphorus are  important for the production of proteins. They are  important for the formation of Amino Acids, especially DNA and RNA which are crucial for all living things.

Phosphorus is the building block of Adenosine tri-phosphate which is the energy source for most cells. The breakdown of ATP to ADP (Adenosine di-phosphate) releases the energy that all cellular functions need to operate.

Magnesium is needed for the manufacture of chlorophyll, which is the source of energy for plants. Plants convert sunlight and carbon dioxide into augars through chlorophyll – without it a plant suffers and dies.

Potassium is vital for the movement of water within a plant and the balance of water inside an outside of cells. It is particularly important to, rapidly growing, new cells

Possible causes of nutrient deficiency

If you add the missing nutrient(s) to a plant suffering from yellow older leaves, it may not make those leaves become green again. It depends on the plant and the source of the problem. For example,  I pinch off the yellow leaves from Tomatoes but leave them on my Citruses in Winter because there is a different cause in both cases. I like to hink also that Tomatoes are so fast growing that they are always in a panic and are likely to withdraw nutrients at the drop of a hat.

When looking at a nutrient deficiency and before you start throwing on the fertilizer, check the conditions in and arounfd the affected plant.

Nutrient deficiences in plants may not be due to a lack of that nutrient in the soil. Waterlogging of soil in Winter reduces the uptake of Magnesium from waterlogged soils in the cold weather. Soil pH also has a great affect on a plant’s ability to uptake nutrients when it is outside of a favourable range for that species of plant.

Probably the best way to resolve nutrient deficiences is to ensure that the microbes and other soil flora and fauna are healthy and that the soil moisture is kept constant. The best way to do both is through the practice of good mulching.