Understanding plant nutrition. How to decide on the right NPK mix?

The fact that all plants need NPK should not lead to a mistaken prognosis that all plants need the same amount of NPK at all the times. The relative amount of NPK needed by different plants at any specific point in time may vary due to a number of factors like the growing medium, the relative age of the plant, growth stage of the plant, specific plant species, external temperature, humidity, and climatic conditions etc.  Measuring all of these and adjusting the nutritional inputs accordingly can bring in a fundamental improvement in the crop yield. This is generally followed in large-scale commercial farms that use various precision farming techniques.  For the scope of this article, we will focus only on the first three viz the growing medium, the relative age of plant and the growth stage of the plant.

Growth Stage of Plant:
In layman’s terms, a plants’ growth stage can be broadly divided into three phases.

  1. Seedling & Sapling – root development and initial growth
  2. Vegetative Growth – major leaf and root development
  3. Flowering & Fruiting – blooming and ripening

Taking cues from our last Tuesday’s post, in the initial few weeks, plants will need a higher proportion of Phosphorus. During the vegetative stage of their lifecycle, plants will need a lot of Nitrogen and relatively less amount of Phosphorus and Potassium. Once flowering sets in, the focus moves on to maximizing the yield and having a healthy and nutritious crop. At this stage, plants need a lesser amount of Nitrogen but an increased extent of Phosphorus and Potassium.

The above looks like a pretty simple and straightforward approach to plant nutrition. Yet,  this simple approach may not work for all the cases. An equally important factor that one must consider, before deciding on the right mix of plant nutrition for any kind of plants, is the growing medium.

The Growing Medium:
Always remember, you add nutrients to the growing medium and not to the plants (unless you are using foliar spray or growing in an inert medium like distilled water or air). Thus, the mix of nutrients that you are adding should consider what is lacking in the growing medium. If a growing medium is already rich in Nitrogen, adding additional Nitrogen during vegetative stage may not help. Similarly, if your growing medium is poor in Potassium, you should not cut down on it during sapling and vegetative stage.

Also remember, if your growing medium was rich in nutrients last year, this would not mean that it is still rich in all those nutrients. The last years’ crops and yields will have absorbed the necessary nutrients from the growing medium and it will need replenishment. The correct way of deciding on the nutrition mix for plants is to first conduct an analysis of the growing medium to determine which nutrients should be added to the mix and in how much quantity. However, conducting this analysis is not always easy, economical and not even required for home growers (unless you are managing huge farms and are looking for supernormal yields).

A safe approach one can take is to decide on nutrient mix based either on plant growth phase or to go for an equal mix of NPK which can be a safe all-purpose solution. One should start with adding small quantities of nutrients and slowly increase it to find the optimal requirement for one’s specific case. This brings us to the third and last aspect that we will cover today.

The relative age of the plant:
Consider the analogy of humans, or animals or any other living form. Most of the members of any species will need a similar mix of nutrients but not in the same amount. A cub will not eat as much as a lion. Same is true for plants. A 2-week old sapling will not need as much nutrition as a 2-month old plant. Also, unlike the animals, once ingested, a plant cannot remove extra food from their bodies. This will start showing as signs of tip/leaf burns – brownish edges on the leaves. This means that plants are being overfed and the amount of nutrition should be reduced. Tip burn, though an irreversible condition, is largely harmless to plants and do not impact other growing leaves or the greenish part of the impacted ones.

We may cover Tip burn in a separate post in future. For now, do remember the following golden rules of plant nutrition:

  1. Plants’ nutritional needs vary by their growth stage.
  2. Nutritional composition of the growing media may need you to change your nutritional mix plan.
  3. While adding external nutrition to plant, always start with small quantity and gradually increase to find the right mix for your specific case.

Happy Farming!

Gaurav is an entrepreneur whose first Start-up was focused on providing services to patients suffering from Chronic Diseases. While researching about the causes of lifestyle diseases and the ways to reduce their incidence, he chanced upon the idea of growing healthy and nutritious food using advanced farming techniques.

He founded CityGreens with a mission to enable City Dwellers to access Safe, Healthy and Fresh food.

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