How do I manage nutrient balance? — Part 1

Question
Nutrition is the next aspect to be covered in this series about hydroponic management fundamentals. It is the aspect that especially new and intending growers often consider to be overwhelmingly the most important aspect of hydroponics. While it is certainly very important, failure to appreciate that it is only one of numerous important aspects of hydroponic management has often led to disaster.

There have been two classic scenarios: (a) “Hydroponics is all about nutrition— hence all my problems must be nutritional.” Wrong! and (b) “I have a proven ‘magic’ nutrient formula, therefore my problems can’t be nutritional.” Wrong again!

Answer
Root zone solution
Those who have followed this series will not be surprised to find me pushing the following critically important aspect. What is the most fundamental aspect of managing nutrition within a hydroponic system? It is the root zone solution which needs to be managed. Don’t get hung up on only the feed solution, which is not very important in itself, but rather in how it influences the root zone solution.

Nutrient balance change within a hydroponic system
An important concept to grasp is that the nutrient balance of the input (fertiliser, acid and water) into a system, will always be different to the nutrient balance of the root zone solution. While most growers come to terms with this, there are many who find the concept difficult.

The reason for the difference is that individual nutrients are taken up by the plant at different rates. The apparent relative rates of uptake by the individual nutrients are listed in Table 1.

Table 1a

Commonsense would suggest that a reasonable balance of nutrient to have in the root zone nutrient solution would be the same as the nutrient balance of the uptake into the plant. However, unless some compensation is made for these different uptake rates, such a solution would get out of balance.

The compensation made is to set up the nutrient balance of the root zone solution to optimise the uptake of nutrients by the plant. This is done by substantially raising the relative concentration of those nutrients with a slow rate of uptake, that is, calcium, magnesium and sulphur. This significantly increases the driving force to uptake those nutrients. Similarly, the relative concentration of those nutrients taken up easily is reduced.

In the special case of ammonium ion, the uptake is so rapid that it is gone soon after being added in the feed. In fact, analysis of root zone solutions usually shows very little ammonium remaining, if any at all. This rapid rate of uptake, especially relative to nitrate ion, can be used to control long term pH drift within the root zone solution.

(Nitrate is a negatively charged ion (NO3-), but ammonium is a positively charged ion (NH4+). With the rapid uptake of positive ammonium ions, to keep electrically neutral the plant roots exude positively charged hydrogen ions (H+), the acid ion. The addition of this natural acid lowers the pH and can be used to offset upward pH drift.)

Table 2 illustrates the typical difference in concentrations of the major nutrients between the input solution and the root zone solution for a recirculating system.

For cucumbers in rockwool—recirculating. Adapted from Sonneveld & Straver: “Nutrient solutions for vegetables & flowers grown in water or substrates”, Praktijkonderzoek Plant & Omgerving, Naaldwijk, the Netherlands.

For cucumbers in rockwool—recirculating. Adapted from Sonneveld & Straver: “Nutrient solutions for vegetables & flowers grown in water or substrates”, Praktijkonderzoek Plant & Omgerving, Naaldwijk, the Netherlands.

These figures are the Dutch recommendations for cucumbers in rockwool in a recirculating solution. Note that the input is the nutrients in the fertiliser, plus what comes in the water and any acid/alkali added. This is different to the feed into the medium, which also includes the recycle.
Normally the root zone solution has a higher EC than the input, due to relatively higher water uptake. In order to compare the two solutions, I have changed the input figures to the same strength as the root zone solution. The slower nutrients have a much higher concentration in the root zone, as shown in red.

It is important to recognise that the important guideline here is that for the root zone. The input guideline is just to give an indication what may be required, but it may need to be changed to keep the root zone solution steady throughout the life of the crop.

The classic case here is tomato, where the plant demand changes significantly throughout the life of the crop. This particularly shows up in the ratio of potassium to nitrogen (K/N ratio), which starts relatively low during the vegetative stage, then increases as the fruits grow with a high demand for potassium. The K/N ratio peaks at the start of harvest and then falls slightly as fruit is removed. These changes need to be followed in the fertiliser input in order to keep the root zone solution steady at its recommended optimum balance.

Part 2 – next month I’ll look closer at managing nutrition in recirculating and non-recirculating (free drainage) systems.   Ω

January 2014 / Issue 139


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