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Optimising nutrients for healthy spring growth

Spring is just around the corner. Here in SESL’s neighbourhood spring flowers are already out.

Nutrient availability in spring can determine a crop’s or sward’s growth for the entire growing season. If nutrients are lacking or unbalanced when the plants start to grow, yields will be compromised. Now is your best opportunity to optimise nutrients for best growth. (You can always add or adjust nutrients later, of course, but by then you will have lost the part of the growing season.)

So now is the time to get your soil tested and to act on the results.

Why do we harp on soil testing? (Apart from the fact that it’s our core business, that is.) Because testing is the only way to know what’s present in your soil. It’s not visible to the eye, and even apparently healthy plant growth is not an indicator of best plant growth.

More than NPK

Everyone tends to focus on nitrogen, phosphorus and potassium – NPK – but there are 17 elements known to be essential to plant growth. (In case you’re interested: N, P, K, Mg, Ca, S, Zn, Mn, Fe, Cu, Na, F, B, Si, Mo, Co, Cl. In addition, animals need I and Se.) Most of these are needed in only tiny quantities, but their lack can compromise growth.

In some cases, an excess of one element can inhibit plant uptake of another nutrient; for example, too much iron can inhibit the uptake of manganese. Only soil testing can tell you whether a particular deficiency is due to not enough of a nutrient in the soil or the inability of the plant to take up what’s there. Simply adding more might not fix the problem.

More commonly, one or more nutrients will be deficient, perhaps because they were never present in the underlying geology, or more likely because continued cropping has removed them from the system. One or two nutrients will then become limiting factors in plant and animal growth.

Liebig’s barrel

This concept of a limiting nutrient or resource was popularised by a German chemist called Justus von Liebig (1803–1873), who coincidentally invented the Oxo stock cube. What is now called Liebig’s law of the minimum explains that growth is controlled by the scarcest resources (the limiting factors), not by the total of all resources. Von Liebig explained the concept in terms of a barrel with staves of different lengths. The shortest stave (the limiting resource) determines the maximum amount of water the barrel can hold (the maximum growth that can be achieved).

Optimising plant growth thus relies on increasing the quantities of the limiting nutrients.

The SESL pantry

SESL views the soil as a pantry with essential supplies stored in it. You have two options: you can shut the pantry and go to the supermarket for your daily supplies, or you can look in the pantry to see what you’ve got, take what you need and top up the missing supplies as needed. If you ask a fertiliser company to analyse your soil, you should not be surprised if the answer comes back that you need to add fertiliser (that is, keep going to the supermarket). That, of course, is the fertiliser company’s core business. If you ask a soil lab instead, you are likely to get a more comprehensive answer that takes into account what you already have in your soil (your pantry).

Which approach you take is up to you. A common approach among growers is to “mine” the soil; that is, keep removing the pantry contents until they reach a critical minimum. This is, of course, cheaper than the alternative. But the alternative will become essential at some point when one or more nutrients have been lost and thus become limiting.

The laboratory analysis

When you send your soil samples in for testing, SESL will always start with pH. This measure of soil acidity determines the availability of both nutrients and toxic metals. When pH drops significantly below 5.2 (remember that 7 is neutral), the acidity starts to dissolve aluminium, which becomes toxic to roots. Simultaneously, molybdenum can become unavailable, thus compromising the growth of legumes.

Often, simply correcting the pH of your soil (usually raising it with lime) is enough to resolve nutrient imbalances.

During soil testing, SESL also analyses the nutrients, using tests that reflect the plant’s ability to take up available nutrients but not chemically locked-up nutrients.

This holistic approach (looking at the whole, not just the parts) is essential to a proper balance of nutrients in the soil.

The laboratory report

When you get your soil test results back, they will show you in coloured diagrams how much of each nutrient your soil has in relation to deficient and excess ranges. A recommendation for how to correct any imbalances (e.g. how much lime to apply, how much fertiliser to apply, soil inversion) interprets the results for you. All you need to do is decide whether or not to implement our recommendations.

Further reading

Wikipedia: Liebig’s law of the minimum

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