Balanced Plant Nutrition

Balanced plant nutrition is only one part of the soil balance relationship, but it can be quite difficult to achieve. The objective, of course is to provide plants with all the nutrients they need – in the right amounts and at the right times – throughout the growing season. This can be hard enough (even with a detailed soil test to go by), but what do you do when things go wrong and there are signs of a nutrient deficiency?

Diagnosing a nutrient disorder can be a tricky thing, but then we need to make use of the result to restore balanced plant nutrition. Visual symptoms alone are not enough to properly identify the problem; for example, pale color could be caused by any one of a number of nutrients (or a combination of them). However, visual symptoms do give a good warning sign that something is wrong – albeit well after the damage has been done and productivity lost.

The occurrence of such symptoms also needs to be taken in its proper context. For instance, orchardists who find a patch of trees with deficiency symptoms often make the mistake of believing that the problem is confined to those trees alone. Although visual symptoms are our first sign, they occur closer to the end of the tree’s response. That is, the problem existed long before any symptoms showed up and so many more trees may be "suffering in silence".

So there are two lessons to be learned before we can achieve balanced plant nutrition. The first is to avoid the approach of only topping up the poor patches and problem spots. The second is to use several sources of information, including soil, leaf and perhaps even irrigation water tests. In this way, you can diagnose and treat the real problem, not just the poor patch showing symptoms.

So let’s go back to the two basic principles in balanced plant nutrition:

1. There are at least 16 elements that are essential to plant growth and every species has its own particular requirements for each one. If we could ensure that, throughout each growing season, our trees had exactly the amount of each element they actually needed, then we would have achieved perfect Balanced Plant Nutrition.

This sounds fine, but it is here that people start to get off track, since the next statement in any talk on nutrition is generally something like: "The most important of these is..." In fact, if we are targeting balanced plant nutrition, we should accept that all nutrients are equally important, but needed in different amounts.
The elements needed in the largest quantities are Carbon, Hydrogen and Oxygen, but plants take these mainly from the water and air, so we can effectively forget about these three and only consider the ones we can actually manage by adding fertilizer. Or, at least that is how the theory goes! Unfortunately, problems like saline or sodic irrigation water can have effects that disturb proper nutrient uptake, as can other soil conditions like acidification or poor structure. This is why you need to find out the real cause of the problem – not just identify the deficiency - otherwise the fertilizer you use may be rendered ineffective and your money wasted.
2. There are only two reasons for doing a tissue test: To diagnose a problem or to monitor plant health and the extent to which you have achieved balanced plant nutrition.

For diagnostic tissue testing there are really two situations to consider and both require a soil test as well. The first is where there is a nutrient deficiency (or toxicity) in the plant, but adequate nutrient levels in the soil. In this case, treating plant symptoms will give only short-term relief. Unless the underlying problem can be identified (from the soil test) and treated, the problem will simply keep returning. The second situation is where there is a deficiency in both the plant and soil. This is a little more straightforward, but even so, treating the plant is still only a short-term ‘fix’. At some point the fertility level in the soil must also be addressed if balanced plant nutrition is to be achieved.

In assessing the results of tissue analysis, look for one or more elements that really stand out as being well outside their optimal ranges. These are your most likely culprits. Unfortunately, many times there will be a number of nutrients that are not quite right, but nothing that jumps up and says: "I’m deficient!" Again, this is where you need to have a soil test available as well.

In dealing with a possible deficiency, you should start by trying a foliar application of an appropriate product at its standard label rate. You cannot work out how much nutrient is needed from a tissue test, if one application is not enough (after say 2 or 3 weeks) then repeat it. But remember that the plants must be actively growing or it is quite likely that nothing will change. Also bear in mind that the only way to properly maintain balanced plant nutrition is to look for a problem in the soil and correct that.

One trick to try if the test levels don’t look to be all that bad is to use the foliar spray at half the recommended rate and mix in a small amount of a good quality Kelp extract. These are high in plant hormones that can help stimulate the plant’s metabolism.

When monitoring plant health, you also need to combine the results of soil and tissue tests so that you can manage soil fertility levels (and control problems that may affect nutrient uptake), in order to keep tissue levels within the optimum range for each nutrient. For example, the table below lists optimum levels in leaf tissue for several nut species:

The order of nutrients listed in this table goes from the one needed in the greatest amount (Nitrogen), to the one with the smallest requirement. The first thing to note is that some are in % and others in ppm. Remember that 1% = 10,000 ppm, so while the Sodium requirement for Chestnut is only 0.01%, this is still 100 ppm. Likewise, the upper limit for Manganese in Hazelnut is 800 ppm, which could also be written as 0.08%.

You will also notice some gaps in the table. This is not because there is no requirement for those nutrients, only that the research has yet to be done on what the requirements are.

Finally, there are other elements not included in our list, such as Nickel and Silicon. Recently, these elements have become more widely accepted as being essential to plant growth, but as yet the research on how much particular plants need is a bit ‘thin on the ground’.