I am a hobby hydroponic tomato grower. My plants are producing well, but recently I noticed what appears to be a nutrient deficiency. The youngest leaves are turning yellow between the veins. Can you suggest what it is?
Answer by RICK DONNAN
Photos of plant nutrient deficiencies (and toxicities) are readily available in books and on the Internet.
Even with these, however, interpretation of plant symptoms is difficult, even for experienced experts. There are several reasons for this: the photographed deficient plants have usually been fed a balanced nutrient solution with just the one nutrient missing – in the real world there will also be other nutrients in the root zone solution higher or lower than standard and influencing the symptoms; there are significant differences in symptoms between different species, or even different varieties within a species; the growing conditions can influence the symptoms, and many symptoms look similar for different nutrients. Consequently, commercial growers who suspect a specific nutrient deficiency would send a tissue sample to a reputable lab for tissue nutrient analysis.
The symptom you describe is most likely to be iron deficiency. Most other nutrient deficiencies are more difficult to pick visually, but iron is a classic recognisable case. The thin green veins with yellow interveinal tissue in the youngest leaves are the giveaway. However, there also needs to be a recognisable reason why this has occurred. Apart from the obvious one of leaving the iron chelate out of a fertiliser batch, the most common reason is prolonged high pH in the root zone solution. If you know or suspect that this has happened then you can be confident that iron deficiency is the problem.
Accurately identifying other nutrient disorders is usually not so easy.
Fixed and mobile nutrients
The priority demand by a plant for nutrients is to supply the plant’s growing point and young leaves. How a nutrient can move within a plant can help identify the symptoms.
The property of some nutrients is that once they lock into the plant structure, that is where they stay. They are known as ‘fixed’ nutrients. Consequently, if a shortage of that nutrient arises, it cannot be satisfied by taking it from the older parts of the plant. This means that the nutrient deficiency symptoms will show up in the youngest parts of the plant, that is, the growing point and/or youngest leaves.
Nutrients, which are fixed are: Calcium (Ca), Iron (Fe), Boron (B), and Manganese (Mn).
Other nutrients are known as ‘mobile’ and can move from where they have located in the plant structure. Consequently, if a shortage of that nutrient arises, it can be satisfied by taking it from the older parts of the plant to the point of greatest demand. This means that the nutrient deficiency symptoms will show up in the older parts of the plant, usually the older leaves.
Nutrients, which are mobile are: Nitrogen (N), Phosphorus (P), Potassium (K), Magnesium (Mg), and Chlorine (Cl).
There are other nutrients some nutritionists refer to as ‘variably mobile’, namely: Sulphur (S), Copper (Cu), Zinc (Zn), and Molybdenum (Mo).
These may show up over the whole plant (S and Mo) or are different for different species (Cu and Zn).
The mobility of nutrients and specific deficiency characteristics allow you to go through a sequence, which may identify possible nutrients as deficient, helped by comparative photos. Do not expect this process to give a guaranteed result. The term chlorosis is used here, meaning a yellowing of plant tissue.
Follow through each of the stated steps as you answer each question:
A. Are symptoms at the growing point/ youngest leaves? Yes – go to B (fixed); No – go to J (mobile).
B. Is growing point distorted or dying? Yes – go to C. No – go to E.
C. Are young leaves light green at base, growing point dead? Yes = Boron deficiency; No – go to D.
D. Are young leaves hooked at first? Yes = Calcium deficiency. (Other classic symptoms are blossom end rot in vine vegetables and tip burn in lettuce)
E. Are young leaves chlorotic between veins? Yes – go to F. No – go to H.
F. Is there a sharp division between chlorosis and veins? Yes = Iron deficiency. No – go to G. (note – if middle leaves have interveinal chlorosis, plants stunted = Zinc deficiency.)
G. No sharp division between veins and chlorosis, spotty? Yes = Manganese deficiency.
H. Leaves light green all over? Yes = Sulphur deficiency. No – go to I.
I. Is there chlorosis of young leaves, then tips wither and die? Yes = Copper deficiency.
J. (Symptoms on older leaves). Is the whole plant dark or light green? Yes – go to K. No – go to N.
K. Are old leaves dark green with red/purple patches? Yes = Phosphorus deficiency.
L. Are old leaves light green or yellow with no spots? Yes = Nitrogen deficiency. No – go to M.
M. Are old leaves light green with spots and sometimes rolled? Yes = Molybdenum deficiency.
N. Is there localised chlorosis with red or dead spots? Yes = Magnesium deficiency. No – go to O.
O. Are old leaf edges burnt and no interveinal chlorosis? Yes = Potassium deficiency. No – go to P.
P. Are there spots with sharp edges and no interveinal chlorosis? Yes = Chlorine deficiency.
There are many different versions of this sequence, reflecting the variability in real plant symptoms for different species. For more detail refer to photos and detailed symptom descriptions. Most importantly, always be aware that your interpretation may be wrong. So, don’t rush in and take some drastic corrective action on your whole crop.
(I have seen this done, leading to total crop loss.)
For any significant change, test it on just a few plants first. Often, the simplest action would be to renew the nutrient in your system with a new start-up solution. RD Ω
December 2016 / Issue 174