Should I use nutrient solution analysis or plant tissue analysis?

I am a commercial tomato grower, experienced in soil growing, but have now gone into hydroponics, using bags of coco peat. I am used to having tissue analysis done, but I have been told that I would be better off getting nutrient solution analysis. What do you suggest?

Answer by RICK DONNAN

It is not a matter of one or the other, but both have their place. In simple terms, nutrient analysis of your root zone solution is ‘proactive’ by helping you keep an optimal nutrient balance around the plant roots, hence avoiding most nutritional problems. Tissue analysis is most useful for positively identifying the problem when you have a nutrient deficiency or toxicity in the crop, hence it is ‘reactive’.

Nutrient solution analysis

Firstly, a few definitions.

By nutrient analysis I mean the full chemical analysis of the solution. That is all the nutrient ions, plus other important ions such as sodium and silicon. This is separate to the simple daily analyses of EC (electrical conductivity) and pH. The typical ions analysed are listed further on.

The important solution to be analysed is that of the solution around the plant roots, because this is what the plant has available to it. Because it is impossible to withdraw a direct sample from most media, the sample universally used is the run-off solution collected over 24 hours.

The run-off (or ‘drain’) sample should be collected over 24 hours from as large a number of bags as possible and from an unbiased location in the greenhouse. The fewer the number of bags, the greater the possibility that the sample will not be representative.

Root zone recommendations for tomatoes

The following Dutch recommendation for root zone solution analysis for tomatoes comes from ‘Nutrient solutions and water culture for soilless cultures’ by de Kreij, Voogt and Baas, published by the Research Station for Floriculture and Glasshouse Vegetables, Naaldwijk, The Netherlands. I have converted their recommendations from milli-moles/litre to parts per million (ppm).

This is for a solution of strength EC 3.7 mS/cm and 33% run-off.

Nutrient elements

Nutrient elements

Note that these figures imply great accuracy. This is only because they have been converted from much smaller figures as millimoles/litre. For example, the recommendation for calcium is 10 milli-moles, multiplied by its atomic weight of 40.1 gives 401 ppm, which would be better rounded off to 400 ppm.

Take care as to how your laboratory reports the results. For example, nitrate content can be reported as ppm N or as ppm NO3. The difference is the allowance made for molecular weight (MWt). If quoted as N, this is based on a MWt of 14 for nitrogen N. If quoted as NO3 this is based on MWt of 14 for N plus 3 x 16 for oxygen O, a total MWT of 62. Hence, the ppm quoted as NO3 will be 62/14 = 4.43 times higher than the ppm quoted as N. Hence, for the recommendation of 322 ppm nitrate as N, would be 322 x 4.43 = 1426 ppm nitrate as NO3.

Comparison between analysis and standard

When you get a run-off analysis result you need to take care with its interpretation. Most important is that the analysis and the standard need to be at the same strength (that is, same EC). This usually means that the analysis numbers need to be adjusted to the same EC as the standard. That is, if your analysis EC was 3.4 compared to 3.7 for the standard, you multiple every nutrient’s ppm by 3.7/3.4 = 1.088. You can then reliably compare the modified analyses with the standard, to use as a basis for any corrective action you might make to your fertiliser formulation.

Tissue analysis

As mentioned above, tissue analysis is most useful in confirming any nutritional deficiencies or toxicities within the plant. Interpreting the result depends entirely upon the laboratory you use, in particular how large is their database and how much expertise do they have, especially for your crop. Selection of a suitable laboratory is essential.

Once you have a reputable lab, it is essential that you strictly follow their guidelines for sampling your crop. These clearly state what part of the plant you are to sample and how it is to be done. This is critically important because the correct sample is the basis for comparison with their database. How to then prepare and despatch the sample is also very important and many labs will supply kits for this purpose.

Accuracy

In both cases the accuracy of the analysis depends mainly upon how representative is the sample.

For solution sampling, the more plants from which the sample is collected the better. Care also needs to be taken that the sampling container is appropriate and located so that it is not exposed to direct sunlight.

For tissue analysis, sampling the correct part of the plant is critical. Because only a small number of plants (or even just a single plant) are sampled there is potentially a much bigger error here. Tissue analysis is inherently less accurate than solution analysis, but can still be very useful.

There is also some possibility that the analyses by the laboratory may have errors. However, this would be rare for an experienced reputable lab. However, just in case there are doubts, it is wise to keep part of the original solution sample or another tissue sample. b

Submit your inquiries to Rick Donnan

Int: +612 9905 9030   Email: info@hydroponics.com.au

November 2016 / Issue 173


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