Then why do plants yellow? Plants yellow from a number of reasons. It is poorly understood that yellowing is most likely caused by a number of nutrient deficiencies.
In fact one or more of 8 missing nutrients can cause plant yellowing.
The danger (or problem) when yellowing occurs
When yellowing is evident on plants, the farmer will often call on his adviser to do an on-the-spot sap test to verify whether nitrogen is low. Although nitrogen is extremely important to plants, unfortunately it often is the only product assessed by experts when yellowing occurs. Nitrogen is often thought to be the cause and it is the easiest and quickest product to buy and apply to remedy the situation.
However, apart from the fact that it may not be the only reason the plant is yellowing, applying 40 to 80 kg per hectare to cereal crops for example, creates future problems for the farmer. Should a frost occur, the damage can be more severe as the plant is weakened due to quick growth and the plant now contains extra watery sap. Extra screenings often result and gutless stubble is left behind that stock don’t like to graze as it far less palatable.
What is your option?
The more thorough method is to have a tissue test done (and also a soil test to compare if it has not already been done) and to send a sample to a laboratory that is capable of testing all the trace minerals. This will detect the missing nutrients.
Why plants yellow: eight main missing nutrients.
The lack of minerals and trace elements that cause plant leaves to yellow are:-
8. Phosphorus where firstly the plant leaves yellow and then change to purple making it easier to identify.
What to do?
Adding the missing nutrients in small quantities together with a biological food, such as fish or kelp, sprayed on the foliage often solves the problem.
Take care to trial your brew first as over concentration of nutrients in liquid will cause leaf burn. Usually the quantity of nutrients that can be applied as a foliar spray is a much smaller portion of what is required for soil correction.
However in addition to the above, the overuse of artificial nitrogen products can cause more future problems like setting back vital soil life causing compaction, increasing soil moisture needs, making plants susceptible to problems like fungal attacks and insect damage.