Validity of a Centrifuge-Based Method for Determining the Water Retention Curve of Growing Media

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Validity of centrifuge-based method for determining water retention curve of growing media, covers a topic that is quite intriguing relevant from a practical point. After a careful review, I think that your work has the potential to make a significant contribution to the field. Nevertheless, to enhance the quality and clarity of your manuscript, I request a major revision. Below, please find a list of the most important deficiencies of the paper that should be corrected and that require attention:

C1: Please consider being more specific in the abstract, such as under what conditions can a centrifuge rapid test be used as an alternative to a standard method? In addition, do the keywords focus on the topic, purpose, methodology, and important concepts of the paper's research?

C2: Is "extending the tension up to -30 kPa" reasonable, is there any basis, and does it have a significant impact on the final results of the experiment and the conclusions reached?

C3: I think your drawings need to be further adjusted and beautified, for example, some pictures are not clear enough or even the proportion is wrong, you can use other more professional drawing software.

C4: The authors consider the effect of centrifugation on material compression during experiments and make experimental comparisons, but can you tell us how this part of the experiment compares with the DIN method?

C5: The summary needs to be further refined to indicate where the proposed approach is applicable and, in addition, to consider whether the word "seems" should be used in the summary.

C6：Did the authors compare the results of other relevant papers with their own? More detailed results must be given.

Author Response

Reply to comments on

Validity of Centrifuge-based Method for Determining Water Retention Curve of Growing Media

 

The authors thank the three unknown reviewers for their very valuable comments. We considered their remarks as follows (the line numbers refer to the cleaned document:

 

Reviewer 1

Validity of centrifuge-based method for determining water retention curve of growing media, covers a topic that is quite intriguing relevant from a practical point. After a careful review, I think that your work has the potential to make a significant contribution to the field. Nevertheless, to enhance the quality and clarity of your manuscript, I request a major revision. Below, please find a list of the most important deficiencies of the paper that should be corrected and that require attention:

 

C1: Please consider being more specific in the abstract, such as under what conditions can a centrifuge rapid test be used as an alternative to a standard method? In addition, do the keywords focus on the topic, purpose, methodology, and important concepts of the paper's research?

To be more specific, we added “mean weighted diameter of the substrate particles is 5 mm or less”. (line 17 ff). We think that the keywords (including the keywords in the title of the paper, correctly reflect the main content!

 

C2: Is "extending the tension up to -30 kPa" reasonable, is there any basis, and does it have a significant impact on the final results of the experiment and the conclusions reached?

We changed all tension values to hPa to avoid confusion. Also, we added a sentence for clarification:

“The basis for extending the suction to -300 hPa is due to the unique texture of the growing media. Beyond such high suction, there will not be a significant change in the water content of the substrates“ (line 68 ff)

 

C3: I think your drawings need to be further adjusted and beautified, for example, some pictures are not clear enough or even the proportion is wrong, you can use other more professional drawing software.

We improved the resolution of the figures and replaced the previous figures with the improved figures

 

C4: The authors consider the effect of centrifugation on material compression during experiments and make experimental comparisons, but can you tell us how this part of the experiment compares with the DIN method?

We added a sentence on compaction in the standard method at the beginning of section 3.4:

“There is some small compaction in the course of the standard method (sand table or pressure plate systems) due to drainage and settling of the materials. This compaction is inherently taken into account in the standard method as the bulk density is calculated after the settling processes took place.” (line 366 ff)

 

C5: The summary needs to be further refined to indicate where the proposed approach is applicable and, in addition, to consider whether the word "seems" should be used in the summary.

We deleted the word “seems” and modified the text to: “… shows that the effect of compaction is not important for practical applications under the conditions investigated.” (line 453 ff)

 

C6：Did the authors compare the results of other relevant papers with their own? More detailed results must be given.

We have clearly explained at the very end of the problem statement in the introduction section that no studies have investigated the use of the centrifuge method for determining the water retention curve of growing media (line 92 ff). Therefore, we made a comparison between the DIN method and the Centrifuge method

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Abstract

1.      The authors do a good job of explaining the objectives of the study and presenting the comparison between the centrifuge-based method and the standard DIN method. Technical terms are used appropriately without overcomplicating the explanation. The phrase "notable compression effects at lower pF stages" could benefit from additional clarification. It might be too technical for a broad audience without context, so briefly explaining "pF stages and also the methods used in the study" in a simpler way might make this clearer.

2.      The logical order of objectives, methods, results, and conclusions is good. However, the "Results showed good agreement between the centrifuge and DIN methods..." could be refined with a stronger linking sentence.

3.      The content addresses the objectives, methodology, and findings without being overly detailed. The balance between too much and too little information is well-handled. While the key findings are mentioned, there could be more specific quantitative results to make the findings more tangible. For example, instead of just stating "good agreement," the authors could include the correlation coefficient or briefly mention how close the results were.

4.      The flow is decent, with the sentences connecting logically from one section to the next. More coherence between sentences, particularly transitioning from the methods to results, would improve the overall flow. For example, without simply writing the results of the DIN method, the methods section should describe the methods used in the study."

 

 

Introduction

1.      There are some instances where words are repeated or used unnecessarily, which makes the text slightly redundant. For example, the phrases "for best results" and "These porous media should be inert for best results" are repeated in the first paragraph.

2.      While most ideas flow logically, some sections benefit from smoother transitions between paragraphs or ideas. For instance, after discussing the physical properties of soilless growing media, there is a sudden shift to the water retention curve (WRC) without explicitly linking the importance of WRC to growing media properties.

3.      The introduction is slightly brief regarding the background of previous studies on water retention curves (WRC). Incorporating more recent studies on the centrifuge method and studies that have tested similar methods in soilless media could make the introduction more comprehensive.

4.      More clarity in the problem statement could help. State explicitly what gap this research is filling—whether it's a lack of quick methods or the difficulty of applying standard methods to certain substrates.

Materials and Methods

1.      It would be best if the authors decided how they wanted to write Materials and Methods. Present and past tenses are mixed throughout Materials and Methods. Moreover, the methodological descriptions are well done.

 

 

Results and discussion

1.      The authors should provide a more detailed analysis of how the findings could impact real-world applications. Compression is significant in practical applications, and discussing its effects in a broader context could strengthen this section.

2.      Including tables and figures is good, but some graphs, such as those showing water content over centrifugation duration, could benefit from more explicit labeling and interpretation. Ensure all visuals have clear titles and concise captions explaining what they demonstrate.

3.      The authors should provide more depth on the practical significance of the differences between the centrifuge and DIN methods. Explain why certain deviations might occur and what that means in practical applications.

4.      The discussion of bias (centrifuge results deviating from DIN results) could be more detailed. Explain the potential causes of the bias and its impact on practical applications or further research.

5.      The implications of using the centrifuge method need to be emphasized more. For example, what are the real-world applications of this method, especially in industrial settings? Can it reduce costs or improve efficiency?

6.      The authors should make some recommendations for further study, but they could be more specific. For example, indicate what substrates or conditions future researchers should focus on to address the limitations authors have identified.

Conclusion:

1.      While the conclusion summarizes the key points, the authors should frame it with a clearer takeaway message.

Some line correction suggestions

Line 17: Overall, the centrifuge method provided a valuable research and substrate development tool;

Line 31: in lesser-developed arid and semiarid regions of the world, and;

Line 35, 36: These porous media should be inert for best results to avoid chemical and biological reactions [1];

Line 42: The physical and chemical properties;

Line 46: compared to mineral soils [4];

Line 47: The hydraulic characteristics of growing media are critical because they have a significant;

Line 50: The analytical methods of assessing physical

Line 55: which is determined to be similar to mineral soils.;

Line 63, 64: Better water movement and redistribution knowledge would;

Line 71: water-absorbing process, although the dehydration process is commonly used in the laboratory [14].;

Line 72, 73: Several mathematical functions usually describe the functional relationship between suction and water content [15–18].;

Line 79, 80: This method is widely accepted to determine the WRC at a method-specific bulk density.;

Line 84: knowledge, scarce, if any, studies have;

Line 94: organic matter content, and raw materials, including peat-free or peat-reduced;

Line 105, 106: air capacity, and readily available water was determined according to DIN EN 13041 [21].;

Line 122: The double rings were comprised of lower and upper rings.;

Line 127: After each pressure head and equilibration,;

Line 130: lower ring material were determined;

Line 131: was calculated based on the dried;

Line 141: samples. As a result, an object;

Line 147: samples. As a result, an object;

Line 265, 266: The results of interlaboratory trials of the DIN method were used to evaluate this discrepancy.

Author Response

Reply to comments on

Validity of Centrifuge-based Method for Determining Water Retention Curve of Growing Media

 

The authors thank the three unknown reviewers for their very valuable comments. We considered their remarks as follows (the line numbers refer to the cleaned document:

Reviewer 2

Abstract

1. The authors do a good job of explaining the objectives of the study and presenting the comparison between the centrifuge-based method and the standard DIN method. Technical terms are used appropriately without overcomplicating the explanation. The phrase "notable compression effects at lower pF stages" could benefit from additional clarification. It might be too technical for a broad audience without context, so briefly explaining "pF stages and also the methods used in the study" in a simpler way might make this clearer.

We modified the sentence to: “The Results showed good agreement (R² => 0.98) between the centrifuge and DIN methods, with an effect of material compression in the centrifuge method resulting in a statistically significant correlation between compression and bias at low pF stages. This, however, did not lead to significant deviations between the method’s results at low pF stages (< pF 2.0).” (line 20 ff)

 

2. The logical order of objectives, methods, results, and conclusions is good. However, the "Results showed good agreement between the centrifuge and DIN methods..." could be refined with a stronger linking sentence.

See remark above

 

3. The content addresses the objectives, methodology, and findings without being overly detailed. The balance between too much and too little information is well-handled. While the key findings are mentioned, there could be more specific quantitative results to make the findings more tangible. For example, instead of just stating "good agreement," the authors could include the correlation coefficient or briefly mention how close the results were.

We  added: “Results showed good agreement (R² => 0.98) between….” (line 21)

 

4. The flow is decent, with the sentences connecting logically from one section to the next. More coherence between sentences, particularly transitioning from the methods to results, would improve the overall flow. For example, without simply writing the results of the DIN method, the methods section should describe the methods used in the study."

To make this clearer, we modified the paragraph in the abstract to: “Water content vs. water suction values determined for the whole rage of the water retention curve with standard method and centrifugation were compared and evaluated by correlation analyses. The results showed good agreement (R² => 0.98) between the centrifuge and DIN methods….” (line 18 ff)

 

 

 

Introduction

1. There are some instances where words are repeated or used unnecessarily, which makes the text slightly redundant. For example, the phrases "for best results" and "These porous media should be inert for best results" are repeated in the first paragraph.

We deleted the phrase “For the best results”

 

2. While most ideas flow logically, some sections benefit from smoother transitions between paragraphs or ideas. For instance, after discussing the physical properties of soilless growing media, there is a sudden shift to the water retention curve (WRC) without explicitly linking the importance of WRC to growing media properties.

We added this simple clarification: “The water retention curve (WRC) is one of the most crucial physical properties of a substrate for managing root zone water” (line 60)

 

3. The introduction is slightly brief regarding the background of previous studies on water retention curves (WRC). Incorporating more recent studies on the centrifuge method and studies that have tested similar methods in soilless media could make the introduction more comprehensive.

To our knowledge, no studies have been carried out on the use of the centrifuge method in soilless media.

 

4. More clarity in the problem statement could help. State explicitly what gap this research is filling—whether it's a lack of quick methods or the difficulty of applying standard methods to certain substrates.

This issue has been explicitly addressed in the introduction section.  To make it clearer, we added

“… as a quick test to replace the time-consuming standard method….”  (line 97 ff)

 

Materials and Methods

1. It would be best if the authors decided how they wanted to write Materials and Methods. Present and past tenses are mixed throughout Materials and Methods. Moreover, the methodological descriptions are well done.

We changed the verbs related to the experimental process to simple past tense. However, the verbs related to the procedural and continuous facts should remain simple and present tense.

 

Results and discussion

1. The authors should provide a more detailed analysis of how the findings could impact real-world applications. Compression is significant in practical applications, and discussing its effects in a broader context could strengthen this section.

We completely agree that compression has a strong effect on the physical properties particularly of growing media. However, in the context of this paper, we do not describe general effects of compression, but only the effect of compression during the centrifugation process compared to the absent compression during the standard procedure. To make this clear, we added a paragraph at the beginning of section 3.4 (lines …. - ….):

“Modifying the bulk density by compression generally has an important effect on the physical properties of any porous material. Therefore, any compression during the measurement process itself needs to be investigated. There is some small compaction in the course of the standard method (sand table or pressure plate systems) due to drainage and settling of the materials. This compaction is inherently taken into account in the standard method as the bulk density is calculated after the settling processes took place.” (line 354 ff)

 

2. Including tables and figures is good, but some graphs, such as those showing water content over centrifugation duration, could benefit from more explicit labeling and interpretation. Ensure all visuals have clear titles and concise captions explaining what they demonstrate.

 We  improved the graphs!

 

3. The authors should provide more depth on the practical significance of the differences between the centrifuge and DIN methods. Explain why certain deviations might occur and what that means in practical applications.

We added some sentences about possible reasons for the differences between centrifuge and DIN method toward the end of 3.4 (lines …. - ….):

“The reason for the differences (bias) between standard method and centrifuge method cannot be explained completely. Obviously, the absolute bias is decreasing from pF 1.0 (3.5 %vol)) to pF 3.5 (0.6 %vol) (Tab. 4), but the sign of the bias is not constant implying that there is a random component. A reason for possible errors might also be the accuracy of the revolution speed of the centrifuge. A setting error of 10 rpm would result in a tension error (hPa) of about 10% at pF 1.0, of about 3% at pF 2.0, and about 0.5% at pF 3.5 (eq. 6). Together with the much higher slope of the WRC at pF stages > 2.0 (Fig. 5), this would necessarily result in much smaller possible deviations at higher pF stages (as observed).” (line 411 ff)

Although compression modifies the pore system and may have some effect on the water storage properties, and possible inaccuracies in setting the centrifuge speed, the good correspondence between the centrifuge and the reference method also at lower pF stages shows that this is insignificant for practical purposes.

 

4. The discussion of bias (centrifuge results deviating from DIN results) could be more detailed. Explain the potential causes of the bias and its impact on practical applications or further research.

(see paragraph 3 above)

 

5. The implications of using the centrifuge method need to be emphasized more. For example, what are the real-world applications of this method, especially in industrial settings? Can it reduce costs or improve efficiency?

We added a paragraph at the end of the conclusions section (line 470 ff):

1. v) Use of the centrifuge method for practical applications: Researchers, but especially producers of growing media, are often faced with the problem of having to evaluate many different mixtures with different materials and proportions, especially when developing new substrates with certain substrate physical properties. Due to the complex mixing of materials of different sizes, a theoretical assessment of new physical properties is hardly possible; instead, the properties must be determined by laboratory analysis. The centrifuge quick test can contribute significantly to saving time and laboratory and personnel effort and thus to reducing costs.

 

6. The authors should make some recommendations for further study, but they could be more specific. For example, indicate what substrates or conditions future researchers should focus on to address the limitations authors have identified.

We added a paragraph in the conclusions section (line 461 ff):

2. iv) Need for further research: In this study, only growing media with mean weighted diameters up to about 5 cm were tested with thin and flexible fibres in standard centrifuge tubes with 2.35 cm diameter and 5 cm height. Coarse and rigid materials, such as pine bark, may react differently, which needs to be tested. Also, coarse and larger materials might need larger centrifuge tubes which can be accomplished without much effort. Coarser materials with larger centrifuge tubes need further investigation with respect to reliability of the method and necessary size of the centrifuge tubes. Also, the effect of inaccuracies in the centrifugation speed needs to be further investigated. In this context, simple optical methods for accurate measurement of rotational speed should be tested.

 

Conclusion:

1. While the conclusion summarizes the key points, the authors should frame it with a clearer takeaway message.

We slightly modified the last paragraph in the conclusions section (line 478 ff):

Because there is currently no DIN or recognized lab regulation, the centrifuge method cannot be used in place of the DIN process for legally compliant measurements. However, the centrifuge method turns out to be a very helpful, and time-saving and cost-effective procedure in research and substrate development.

 

Some line correction suggestions (line numbers refer to the original reviewed manuscript)

Line 17: Overall, the centrifuge method provided a valuable research and substrate development tool;

“the correction was done”

Line 31: in lesser-developed arid and semiarid regions of the world, and;

“the correction was done”

Line 35, 36: These porous media should be inert for best results to avoid chemical and biological reactions [1];

“the correction was done”

Line 42: The physical and chemical properties;

“the correction was done”

Line 46: compared to mineral soils [4];

“we left it like that “[4] (p.5)” because it is a book!”

Line 47: The hydraulic characteristics of growing media are critical because they have a significant;

“the correction was done”

Line 50: The analytical methods of assessing physical

“the correction was done”

Line 55: which is determined to be similar to mineral soils.;

“the correction was done”

Line 63, 64: Better water movement and redistribution knowledge would;

“the correction was done”

Line 71: water-absorbing process, although the dehydration process is commonly used in the laboratory [14].;

“the correction was done”

Line 72, 73: Several mathematical functions usually describe the functional relationship between suction and water content [15–18].;

“the correction was done”

Line 79, 80: This method is widely accepted to determine the WRC at a method-specific bulk density.;

“the correction was done”

Line 84: knowledge, scarce, if any, studies have;

“the correction was done”

Line 94: organic matter content, and raw materials, including peat-free or peat-reduced;

“the correction was done”

Line 105, 106: air capacity, and readily available water was determined according to DIN EN 13041 [21].;

“we corrected punctuation and grammar, but left the technical term “easily available water” because this is more commonly used in growing media literature, particularly in the cited literature! Also, the generally used abbreviation is “EAW””

Line 122: The double rings were comprised of lower and upper rings.;

“the correction was done”

Line 127: After each pressure head and equilibration,;

“the correction was done”

Line 130: lower ring material were determined;

“the correction was done”

Line 131: was calculated based on the dried;

“the correction was done”

Line 141: samples. As a result, an object;

“the correction was done”

Line 265, 266: The results of interlaboratory trials of the DIN method were used to evaluate this discrepancy.

“the correction was done”

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Review of horticulturae manuscript 3274354

Validity of centrifuge-based method for determining water retention curve of growing media, by H.H.A. Muhammed, L. Schumm, R. Anlauf, T. Reineke, and D. Daum

The purpose of the study was to evaluate a centrifuge method for determining the water dynamics of plant potting material that is quicker to conduct and is comparable in precision with the standard method.

My overall impression of the study and manuscript is a product of high quality that is concise, well prepared, and easily understood.

The Abstract is informative and provides essential information about the study and results.

The Introduction concise and descriptive and provides several references to early work leading up to work by the authors on their method, which I found most helpful to understand likely importance of their method for commercial application.

The Materials and Methods was the longest and most detailed section of the manuscript. This section is well done. The control treatment in the study were samples prepared by the published DIN procedure. I consider the pairwise comparison correlations between the water retention curves of the standard (DIN) and centrifuge method as appropriate.

The Results and Discussion section is well done, and the figures are useful for understanding the results. I missed seeing mention of replication of the tests. However, it appears some amount of replication was used because six of the plotting points in Fig. 2 and several other figures (as mentioned at line 269) had error bars for the DIN method. I understand that potting material is likely very uniform in composition and density, which reduces the need for replication and that is fine with me for this study. However, the assumption of uniformity of commercial potting material could be useful information about your tests for other researchers evaluating the centrifuge method. My comment above about replication may be presented in the manuscript, but I failed to recognize it in the text.

The Conclusions section provides a good summary of the study and results.

Author Response

Reply to comments on

Validity of Centrifuge-based Method for Determining Water Retention Curve of Growing Media

 

The authors thank the three unknown reviewers for their very valuable comments. We considered their remarks as follows (the line numbers refer to the cleaned document:

Reviewer 3

The purpose of the study was to evaluate a centrifuge method for determining the water dynamics of plant potting material that is quicker to conduct and is comparable in precision with the standard method.

 

My overall impression of the study and manuscript is a product of high quality that is concise, well prepared, and easily understood.

The Abstract is informative and provides essential information about the study and results.

The Introduction concise and descriptive and provides several references to early work leading up to work by the authors on their method, which I found most helpful to understand likely importance of their method for commercial application.

The Materials and Methods was the longest and most detailed section of the manuscript. This section is well done. The control treatment in the study were samples prepared by the published DIN procedure. I consider the pairwise comparison correlations between the water retention curves of the standard (DIN) and centrifuge method as appropriate.

 

The Results and Discussion section is well done, and the figures are useful for understanding the results. I missed seeing mention of replication of the tests. However, it appears some amount of replication was used because six of the plotting points in Fig. 2 and several other figures (as mentioned at line 269) had error bars for the DIN method. I understand that potting material is likely very uniform in composition and density, which reduces the need for replication and that is fine with me for this study. However, the assumption of uniformity of commercial potting material could be useful information about your tests for other researchers evaluating the centrifuge method. My comment above about replication may be presented in the manuscript, but I failed to recognize it in the text.

We mentioned four replications in the 2.2. section (materials and methods: standard method, line 131), and in 2.3.3 section (materials and methods: measurement procedure centrifuge method, line 235). However, the main objective of the study was not an experimental design, rather it was four replications to take the average”

The Conclusions section provides a good summary of the study and results.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The revised document incorporates corrections and additions that effectively address the reviewers' comments, as indicated in the cover letter and tracked in the revised document. Here’s a review based on the primary areas of revision and improvements made:

1. Materials and Methods:

This article is about systematic comparisons, so the authors should include a formal inclusion of uncertainty models to express fine-grained uncertainty in the methods. Otherwise, authors should include the Statistical Analysis sub-heading in the Materials and Methods chapter.

2. Results and Discussion:

The visual state of Figures 4, 5, and 6 can be modified using any data analysis and graphical software. Adding the RMSE can enhance the validity of the comparison.

3. Conclusion:

The iv and v numbers are not part of the objectives, I suggest improving the conclusion.

Author Response

Reply to reviewer’s comments – Round 2

 

Comments from Editor

Please delete the abstract's first sentence and write an introductory sentence instead.

We deleted the first sentence and added instead:

“The water retention curve (WRC) determines important properties of growing media with respect to water dynamics. However, the standard laboratory determination is time-consuming and laborious. The objective …”

What does (p 5), after [4] at manuscript line 51? Please delete it.

We deleted “p. 5” (as a reference to the page number in the referenced book).

Please refer to an article in Scientia Horticulturae, 2004, regarding the “Physical properties of wood fiber substrates” and similar papers when discussing water retention curves and pore size distribution.

We included a few research papers related to the physical properties of wood fibre substrates to the results and discussion ranging from lines 369 to 378.

Comments from reviewer 2

The revised document incorporates corrections and additions that effectively address the reviewers' comments, as indicated in the cover letter and tracked in the revised document. Here’s a review based on the primary areas of revision and improvements made:

1. Materials and Methods:

This article is about systematic comparisons, so the authors should include a formal inclusion of uncertainty models to express fine-grained uncertainty in the methods. Otherwise, authors should include the Statistical Analysis sub-heading in the Materials and Methods chapter.

We added a sub-section (2.4 Statistical indicators) to the end of the materials and methods.

2. Results and Discussion:

The visual state of Figures 4, 5, and 6 can be modified using any data analysis and graphical software. Adding the RMSE can enhance the validity of the comparison.

We modified the figures 4, 5, and 6 adding the standard deviation bars to the curves (see fig. 5). The standard deviations in 4 and 6 are very small so that they do not appear clearly.

3. Conclusion:

The iv and v numbers are not part of the objectives, I suggest improving the conclusion.

We added a few words in the “objectives” paragraph to comply with the conclusions section:

“Therefore, the objective of this study was i) to compare a simple substitution method to describe the bulk density of growing media with results of the standard method, ii) to test the practical applicability of the centrifuge method as a quick test to replace the time-consuming standard method for a wide range of growing media, and iii) to give a recommendation under which conditions the centrifuge quick test could serve as a replacement for the standard method (including its advantage for practical applications and identifying needs for further research).”

Author Response File: Author Response.pdf