Dear Luz,

The explanation of this phenomenon on a very basic level is that plants do not take up what there is but what they need. Indeed, if plants would not accumulate certain elements and exclude others there would no such thing as a ‘plant’, take the most obvious example C, that is much more concentrated in plants then in the atmosphere.

On the next level, for mineral nutrients there is a certain range of concentration in the tissues desirable for plants. Tissue concentration below this level may enhance a certain nutrient uptake through activation of different uptake pathways, or modification of physico-chemical parameters in the rhizosphere. Tissue concentration above that level, the particular nutrient will not be actively taken up or even exuded from the plant. For main nutrients, luxury consumption might be an issue. Also, in case of lack or overflow, elements might be transferred to different plant organs. However, these mechanisms of homeostasis are powerful for some elements and plants and less so for others so that there might be some variation of mineral content in plant tissue due to its availability in the soil. Also, for particularly for N, P, microbial activity and symbioses might be an issue for uptake, that I am less familiar with.

Finally, on the soil level, nutrients usually occur in different chemical species, some of which are accessible to the plant, for example because they are in an adsorption equilibrium with the soil solution. Other chemical species are not. Maybe these species can be mobilized by physico-chemical modification of the rhizosphere, maybe not. Soil tests are empirically calibrated to evaluate the part of nutrients accessible to plants, but in a particular soil and for a particular plant this might not be accurate.

So indeed the issue of nutrient uptake, and tissue concentration is complex and in some points controversial.

Luz G. Asio It just means that there's no direct relationship, interdependence between soil test and plant tissue test values for N, P, and K. You could think of it as meaning that soil test and plant tissue test have nothing to do with each other.

If the availability of the nutrients in soil are optimum, then usually the correlation between the soil test and plant tissue concentration shows no correlation. In case of graded application of any one of the nutrient, the correlation shows positive or negative depending upon the type of soil, nutrient etc.

Dear Luz,

The explanation of this phenomenon on a very basic level is that plants do not take up what there is but what they need. Indeed, if plants would not accumulate certain elements and exclude others there would no such thing as a ‘plant’, take the most obvious example C, that is much more concentrated in plants then in the atmosphere.

On the next level, for mineral nutrients there is a certain range of concentration in the tissues desirable for plants. Tissue concentration below this level may enhance a certain nutrient uptake through activation of different uptake pathways, or modification of physico-chemical parameters in the rhizosphere. Tissue concentration above that level, the particular nutrient will not be actively taken up or even exuded from the plant. For main nutrients, luxury consumption might be an issue. Also, in case of lack or overflow, elements might be transferred to different plant organs. However, these mechanisms of homeostasis are powerful for some elements and plants and less so for others so that there might be some variation of mineral content in plant tissue due to its availability in the soil. Also, for particularly for N, P, microbial activity and symbioses might be an issue for uptake, that I am less familiar with.

Finally, on the soil level, nutrients usually occur in different chemical species, some of which are accessible to the plant, for example because they are in an adsorption equilibrium with the soil solution. Other chemical species are not. Maybe these species can be mobilized by physico-chemical modification of the rhizosphere, maybe not. Soil tests are empirically calibrated to evaluate the part of nutrients accessible to plants, but in a particular soil and for a particular plant this might not be accurate.

So indeed the issue of nutrient uptake, and tissue concentration is complex and in some points controversial.

Under such situation, it means there is either nothing in the soil or some hungry plants depleted the nutrients. I would have a look at the control and compare what happened. I would assume, if I do not find any in the soil than would in the tissues.

Simon Blotevogel

Hello Simon. Thank you very much for sharing your knowledge.

Luz

Mean that the methodology adopted don't it is useful to characterize the nutritional state of the soil concerning the adopted species like the reference (nutrient concentration in test-plant). This effect can be caused by some no identified interaction of the nutrient with the soil (pH, clay, organic matter...), or plant (organic acid liberation, root development, symbiotic associations). In these cases, multiple regression with one of these factors can be useful. In tropical soils, the good correlation between the methodology of N determination in soil and your levels in the plant is difficult because of the specific dynamic of the nitrogen. Phosphorus shows an elevated and sometimes complicated interaction with the soil mineralogy.

First of all, what were the soil test methods used to determine soil nutrient status? For example, There is a difference between a test like total N (Kjeldahl ) N versus a test for soil nitrate or soil nitrate + ammonium. The first gives a much higher value but it measures primarily nitrogen in the organic matter which needs to mineralize before it is available to the crop - and it will take perhaps many seasons to mineralize. The latter are measures of mineral or plant available N that is immediately available to the crop.

Often something happens between the time of the soil test and the growth period of the crop that causes the N status of the soil to change. An example of this may be exceptionally heavy rainfall or heavy irrigation that causes the N to leach our of the crop rooting zone,so that the crop experiences a totally different soil nutrient environment than the soil tests indicated.

Finally, if the soil is sampled to a depth pf 15-20 cm and the crop is a deep rooted crop, again the soil test may not accurately describe the nutrient status of the soil in the rooting zone. In this case, the nutrient, if mobile, may have leached into a deeper zone, say 30-40 cm, and the soil test will not identify the much higher nutrient availability. However, the crop will access it and there will me no correlation to the soil test. In this case, deeper soil sampling and testing is needed.

Your question did not identify which soil tests were used and how deep the soils were sampled. My answers are directed mostly at nitrogen which is mobile. Answers for P and K are pretty well covered by the previous answers above.

some more information regarding the experiment is required for answering this querry. Plant tissue study will reveal the status of nutrients in the tissue at the time of sampling. Soil test value will also reflect on the soil nutrient content at the time of soil sample collection. If the experiment was conducted to know the effect of some fertiliser application on the crop nutrient content, then time of sampling is a crucial factor. The fixation of the nutrient in soil, its further release, absorption by crop all will affect the time at which the effect be reflected in the plant tissue. Generally plant tissue test can give an indication of the nutrient deficiency experienced by the plant that needs to be corrected by soil application. The correlation between the two is always depend on the time of sampling

Larry Cihacek

Dear Professor Cihacek,

Thank you very much for your very enlightening and very informative explanations for the non-correlation between soil test and plant tissue test values we have observed in our research on sweetpotato. I fully agree with you that test methods (we determined soil total N, available P and exchangeable bases), depth of sampling, time of testing and growth period, and effect of heavy rainfall can explain this observation. Keep safe. ---- Luz

This can ocur frequently due to i) each chemical methods can extract different amounts of nutrient element, ii) each plant has the ability to diferantiate the rhozosphere at different degree, iii) some plants have special way of overcoming nutrient deficiency, iv) plants have different compositions at changing growth stage, v) the leaf nutrients are to translocate to seed and fruits in generative stage, vi) agricultural practices have some impact on the nutrient availability e.i. irrigation is very critical in P and redox elements availability, vi) time of soil sampling: the results can change winter and late summer samplings (Fe, Mn, Zn, Cu and P)… and there may be included so many other reason. You can try total nutrient uptake if you have biomas data. This may give higher correlation.

I am not surprised about the lack of correlation. For Nitrogen in particular, there is really no sound soil test for crops in general because of the ubiquitous sources from atmospheric deposition, soil organic matter mineralization, symbiotic and asymbiotic N-fixation in addition to inorganic N fertilizer addition or residue in the soil. On the basis of this, blanket rates of N fertilizer is often recommended for non-legumes based on empirical field data collection for specific crops in specific locations. In order to truly assess the correlation between soil test and tissue test for N, tracer technique involving N-15 or isotopically-labelled N fertilizer trial must be conducted and soil and tissues analyzed with Mass spectrophotometer.

Veli Uygur

Dear Professor Veli Uygur,

Thank you for sharing your knowledge about the non-correlation between soil test and plant tissue test values. I agree with you on the possible explanations that you have mentioned. In fact we also observed in some of our previous researches the role of extraction methods as well as the ability of some crops to overcome nutrient deficiencies. ----Luz

The tested soil may be highly fertile and the tested plants may be requiring small quantity of nutrients. However, it may depends on the type of nutrients tested.

Jothimani Subbiah

Thank you for your answer. ---Luz

I consider that the type of plant and soils should be taken into account in matters related to the mechanisms and transformations of minerals at the nanometric level that takes place in the soil. Within this, consider the interaction of organisms, the genetic responses of plants to changes in the contents of nanomaterials deposited by irrigation from rivers, lakes, etc.

Hermes Pérez-Hernández

Thank you for your important contribution to the discussion on the relationship between plant and soil analysis. --Luz

Generally correlation coefficient values ranges between -1 to +1. If r = > 0, it denotes positive correlation and r = < 0, it denotes negative correlation between two variables (X and Y). If r=0, then two variables are not correlated.