Organic Gardening Magazine

Your complete guide to gardening - naturally!

No minor matter

In the second part of her back-to-basics guide to soil fertility, Gaby Bartai uncovers the major importance of the minor nutrients

Think of plant nutrients, and what springs to mind is the ‘big three’ – nitrogen, phosphorus and potassium. Indeed, the chemical shorthand for this trio, NPK, is often used as a synonym for plant nutrients. But it’s very far from being the whole story.

As well as needing oxygen, carbon and hydrogen, which they obtain and utilise via access to air, water and sunlight, plants also need to obtain a range of minerals from the soil. There are three macronutrients besides nitrogen, phosphorus and potassium, which I discussed last month (July 2008, pages 42-44): the others are magnesium, calcium and sulphur. There are also a range of micronutrients or trace elements, the core list comprising iron, manganese, copper, zinc, boron and molybdenum. This is not a complete list, though it’s as far as most gardening books will take you. Scientists have now added cobalt, chlorine, sodium, silicon and nickel to the micronutrient list, at least for some plants in some circumstances. The role of some of these micronutrients is not fully understood – and the list may still be incomplete.

Plants need these nutrients in only tiny quantities. Of plants’ total nutrient needs, 96 per cent is accounted for by oxygen, carbon and hydrogen. Nitrogen and potassium account for a further 1.5 per cent apiece, the rest is made up by the remaining four macronutrients and all the micronutrients. Iron accounts for only 0.01 per cent, and molybdenum for a mere 0.00001 per cent. But because the different nutrients are needed for different aspects of growth, it is vital that plants obtain the complete range.

Availability and uptake
It’s important to realise that, in the vast majority of British soils, all of these nutrients are present in more than adequate quantities, released either through the weathering of rocks or through the breakdown of soil organic matter (or both). Plants, remember, require them only in tiny amounts. While, to achieve the accelerated growth, colourful displays and high yields that we demand, it is necessary to supplement levels of nitrogen, phosphorus and potassium in the more productive parts of our gardens, any soil with a reasonable organic matter content should contain sufficient quantities of the remaining nutrients.

A specific minor nutrient ‘deficiency’ is most likely owing to insufficient nutrient uptake, probably as a result of too acid or alkaline a soil (see OG June 2008, pages 32-35). The most regular instance of this is the yellowing foliage that indicates an iron (and possibly also manganese) deficiency. Iron is never actually deficient in soil, being the second most abundant metal on earth. The condition is called lime-induced chlorosis, and it occurs when acid-loving plants are grown on alkaline soils, from which they cannot obtain iron.

• For the full story, see this month's issue, available to buy online!

Plan crop rotation

Copyright information

All content is © Mortons Media Group 2008 • Kitchen Garden magazine | Subscribe to OG | Buy current issue