This earlier post lists the most important nutrients plants require to grow, flower and fruit. But barring a full laboratory soil or leaf analysis, most people only become aware of any deficiencies when their trees show poor growth and development.

By now all trees delivered last August to very cold winter climates (eg Goulburn NSW) should be coming out of bud and showing hints of green at the very least. The trees here in the milder Wollongong climate are always a month more advanced in their growth than their Goulburn cousins, and are now in full leaf and already producing flower buds. It’s thus timely to discuss the topic of nutrient deficiencies over the next few weeks, in case they make themselves apparent to anyone over the next few months.

The next few posts will detail the common nutrient deficiencies along with how to recognise their symptoms. Knowing that a plant isn’t receiving nutrients is just one step — knowing which specific nutrients are missing means problems can be addressed with targeted solutions. Not only can you correct a specific deficiency more efficiently this way before it gets out of hand, but you also reduce the risk of further harm from well-meant but unnecessary applications of already-abundant nutrients. (Some of which, if too abundant, can worsen the underlying deficiency even more!)

Before we get into the specifics of deficiencies, let’s firstly summarise previous posts that mention nutrient availability in soils and to plants.

Plant-friendly nutrients exist as cations [positively-charged ions, eg magnesium (Mg²⁺)] and anions [negatively-charged ions, eg phosphate (PO₄^3-)]. 

Nutrients may already be present on clay and humus colloids in the soil, or can enter the soil via decomposition of introduced organic matter such as compost and manures. Breakdown of soil minerals further adds some ions to soil. Liquid and solid commercial fertilisers are yet another way to introduce nutrients.

However, these nutritional cations and anions are only accessible to plant roots via the soil water they dissolve into. Those held by clay and humus particles must enter the soil water via exchange with other ions already in that water. Having a well-structured soil with good ped formation goes a long way in maximising the colloidal surface areas that hold nutrients and at which these exchanges take place.

Nutrients used by a plant must be replaced at at least the rate of uptake if that plant is to remain in good health, grow and develop, otherwise a deficiency will arise. A soil rich in clay, humus and mineral particles may hold large reserves of all the nutrients, and be able to meet a plant’s needs over more than one season. Breakdown by soil organisms of regularly added organic matter can extend that even further. Some nutrients may eventually drop below replacement levels, and adding ‘complete fertilisers’ (manufactured formulations) regularly is a good way to ensure all essential macro- and micronutrients become available to plants in sufficient quantities.

A nutrient doesn’t have to be missing in order to cause a deficiency — it simply has to be unavailable to a plant. The structures and properties of different soils come into play here, and the same amount of a particular nutrient in one soil type may result in healthy growth, but cause deficiencies in another.

The pH of a soil is a common cause of nutrient unavailability as the diagram on this page demonstrates. Phosphate and bicarbonate ions can interfere with iron uptake, by forming solid, slow to dissolve iron compounds. Similarly, phosphorus (the element from which phosphate ions are made) may exist in high amounts — but locked up in solid, insoluble minerals rather than available as soluble phosphate ions.

‘Nitrogen drawdown’ is a phenomenon whereby soil microbes draw on nitrogen ions in the soil in order to decompose high-carbon materials such as woodchips, sawdust, dried leaves, bark, and other ‘brown’ matter added as mulches. This nitrogen of course never becomes available to plants, and the problem is exacerbated should other organic matter in the soil not be broken down quickly enough to release replacement nitrogen.

Nutrient availability can be a complex subject! We’ll be covering the common deficiencies over the next few weeks — how to recognise them, how they harm a plant, and how to correct for them.