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What is in a plant's diet? (2/4): The less-known macronutrients

According to the FAO's handbook Plant nutrition for food security: A guide for integrated nutrient management, published in 2006 and to this day one of the most solid and authoritative resources on plant nutrition, 80% of all the mass of mineral nutrients as present in most plants is taken up by nitrogen (N) and potassium (K). That is to say, basically, that eight out of ten nutrient molecules in an average plant are either molecules of nitrogen, or molecules of potassium. Next in abundance comes phosphorus (P), followed by sulphur (S), calcium (Ca) and magnesium (Mg), which between all four make up another 19% of all present nutrients. The remaining 1% is constituted by seven other nutrients, called micronutrients because of their reduced weight in the plant tissues. Only one in a hundred nutrient molecules is a micronutrient ― the other ninety-nine are divided among the six (and rightly called) macronutrients.Here’s the full list of all of them, as published by the FAO in their report:

It doesn't get more detailed than this!

Now, in our last article of this series we discussed the ‘three big ones’: nitrogen, phosphorus and potassium, the triad that form the so-often seen acronym ‘NPK’. In this article we will review the remaining three macronutrients, what they do and why they matter. Nutrient depletion is a major concern in most of the world’s cultivated land, and preventing it is a major benefit of organic agriculture, so it is important to know in simple, clear terms, what all nutrients do and how to spot their absence.

  • Calcium (Ca) is the first of all the less-common macronutrients, mainly because its role is to provide a material for plants to build the cellular walls and their membranes, effectively sustaining the ‘architecture’ of the cell. Its deficiency is, as such, a very severe issue. The handbook observes: “Calcium deficient leaves become small, distorted, cup-shaped, crinkled and dark green. They cease growing, become disorganized, twisted and, under severe deficiency, die. Although all growing points are sensitive to calcium deficiency, those of the roots are affected more severely.” A deficiency of calcium is especially hurtful for fruits that are developing. Entire harvests may rot away before maturing because the plant cannot produce enough cells to finish growth at the tip of the fruit (the opposite extreme to where the stem is), leaving exposed tissues that are infected by bacteria and fungi.

    The dread of the tomato farmer: a calcium deficiency in action.
  • Magnesium (Mg), in turn, does a ton of stuff (mainly related to the chemical workings of the cells, protein transfers and such), but its main role is in making up the center of the chlorophyll molecule. Without magnesium, plants cannot produce chlorophyll, cannot photosynthesize the light of the sun, and plainly die. As in the case of calcium, the handbook gives a list of symptoms of its deficiency: “A typical symptom… is the interveinal chlorosis of older leaves, in which the veins remain green but the area between them turns yellow. As the deficiency becomes more severe, the leaf tissue becomes uniformly pale, then brown and necrotic. Leaves are small and break easily (brittle). Twigs become weak and leaves drop early.”

    Here's a picture of a plant with magnesium deficiency. See how the older leaves are affected first?

  • Finally, there is sulphur (S, and which indeed does more than coming out of a volcano and killing everyone), which the plant uses almost as much as it uses phosphorus. Its role is in protein-making, so indirectly it affects the ability of the plants to create and maintain tissues; in the making of chlorophyll and, especially, in giving their onion-ness and their garlic-ness to onions and garlic. On its deficiency, the handbook says: “symptoms in most cases appear first on the younger leaves. Plants deficient in S are small and spindly with short and slender stalks. Their growth is retarded, and maturity in cereals is delayed. Nodulation in legumes is poor and N fixation is reduced. Fruits often do not mature fully and remain light green in colour”.

            A detailed series of shots detailing sulphur deficiency, courtesy of the folks at the Journal of Environmental and Experimental Botany.

These are the big ones, but the remaining 1% is just as essential even if not so widely distributed. So, what about micronutrients? In the next delivery of this series, we will move on to them, and after that to something even more interesting: how chemistry and biology intersect to make organic agriculture the best way of ensuring that your crops absorb and use adequately all of these nutrients. Until then… happy growing!

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