Slow-Release Urea Prills Developed Using Organic and Inorganic Blends in Fluidized Bed Coater and Their Effect on Spinach Productivity

Round 1

Reviewer 1 Report

I think this manuscript can be accepted for publication subject to minor revisions.

The authors have studied the slow-release coated urea prills. The research is very straightforward, and the conclusion is very clear. Several small issues need to be refined.

1. The writing needs to be improved. The language and the format.

Page 3, line 109: The abbreviation should be provide at the first time of their appearance. You should introduce ‘PVA’ and ‘POP’ before this table.

Page 3, line 111: change ‘make’ to ‘manufacture’.

2. The FTIR characterization is not necessary. I didn’t see any important information was provided by this characterization.
3. The scan angle of PXRD is ranging from 20 to 70 here. First, this is not scan angle, this is 2 theta angles. Second, the range should be from 5 to 40. Lower 2 theta angle gives more reliable information.

Author Response

We would like to thank the Reviewers for their comments on our manuscript. We took advantage of their remarks and changed the manuscript accordingly. We truly hope the revised manuscript answered all the comments of the reviewers. Below we report our detailed answers:

Reviewer # 1

I think this manuscript can be accepted for publication subject to minor revisions.

The authors have studied the slow-release coated urea prills. The research is very straightforward, and the conclusion is very clear. Several small issues need to be refined.

Q.1.The writing needs to be improved. The language and the format.

Response: The English language and format of the whole manuscript has now been improved.

Q.2.Page 3, line 109: The abbreviation should be provide at the first time of their appearance. You should introduce ‘PVA’ and ‘POP’ before this table.

Response: Abbreviations has been provided now.

Q.3.Page 3, line 111: change ‘make’ to ‘manufacture’.

Response: It has been changed.

Q.4.The FTIR characterization is not necessary. I didn’t see any important information was provided by this characterization.

Response: The FTIR characterization reveals bonding nature and chemical structure associated by the result of cross linking between different polymer material chains when polymeric solution is coated on urea. For example, 2100 cm^-1   Nitrile (C≡N) stretching  and  2010 cm^-1 Alkyne (C≡C) stretching vibrations were desired as observed for test batch formulations C-1 and C-3 due to reaction between the Nitrile and Alkyne of Gelatin and PVA with amino groups of urea during the polymerization reaction or coating [1]. The information has been incorporated in the manuscript.

Q.5.The scan angle of PXRD is ranging from 20 to 70 here. First, this is not scan angle, this is 2 theta angles. Second, the range should be from 5 to 40. Lower 2 theta angle gives more reliable information.

Response: The major characteristics peaks of coating material are observed at 2θ=22^o, 2θ=24.5^o, 2θ=29.5^o and 2θ=35^o[2]. The characteristics sulfur peaks are observed in the range of 22 to 30^O [3]. Addtionally, the more prominent peaks were observed in the range of 22^o to 25^o that is the reason we selected the range from 20 to 70. The information has been incorporated in the manuscript.

References Used in Response:

1. Fernandes, B.S., et al., Coating of urea granules by in situ polymerization in fluidized bed reactors. Polímeros, 2019. 29(1).
2. Jyothi, A.N., et al., Cassava starch‐graft‐poly (acrylonitrile)‐coated urea fertilizer with sustained release and water retention properties. Advances in Polymer Technology, 2018. 37(7): p. 2687-2694.
3. Rettig, S. and J. Trotter, Refinement of the structure of orthorhombic sulfur, α-S8. Acta Crystallographica Section C: Crystal Structure Communications, 1987. 43(12): p. 2260-2262.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

General comments:

The research is interesting as it tests several materials as a coating for urea, particularly ones that are likely to have less environmental impact than plastic coatings. The paper does a good job of describing physical characteristics of the coated ureas, the scanning electron microscope images were particularly intriguing. However, the paper is lacking in several areas. For example, statistical analyses are sometimes reported, though inconsistently, and methods are not described. Also, conclusions are not often supported by the data, particularly for the pot study. Specific comments are given below.

 

Specific comments (line numbers refer to those seen in the PDF version downloaded from the MDPI reviewer system):

Lines 47-49 – This sentence is confusing because it only talks about excessive N during the nitrification process, which seems to indicate the authors are referring to nitrate. Nitrate is not the only problem in the environment. Also nitrate does not volatilize, ammonium does. Please rewrite to make it clear what is being said.

Line 49-51 – Is there a citation for this sentence? It seems to refer to specific research.

Line 58-60 – What recent global assessment has stated this? There is no reference included. Also, please clarify by what is meant by “four of nine planetary boundaries have now been crossed”. It is not clear what planetary boundaries are being spoken of.

Line 68-70 – Urea is not the cheapest N source in all regions of the world. Anhydrous ammonia for instance is cheaper in some areas and has a higher N content than urea. Please clarify what region the authors are discussing.

Lines 82-84 and 84-86 – Please add citations for this information.

Line 97 – “… all of analytical purity were obtained from (Daejung-Korea).” Is something missing in this line? Where were the materials obtained from? Or should the parentheses be removed to complete the sentence?

Line 130 – A treatment, C-0, is mentioned but is not listed in table 1. What is this treatment, uncoated? Please define at first mention.

Line 147 – ...“at leaves surface” is confusing. Do you mean “to stay on the surface of leaves”? Or something else?

Line 185-186 – What were the results of the soil test? Were P and K needs of the spinach met with the soil or did other fertilizers have to be added?

Line 187 – Why was 0.5 g of urea chosen? Is this a typical rate for greenhouse grown spinach in the size pot that was used? Please clarify.

Line 187 – “A total of 0.5 g urea was applied…”. Does this mean more N was applied to C-0 treatment since it didn’t have a coating? I assume the coating added weight to the urea prills. Please indicate the N application rate along with the fertilizer application rate. As an example, urea is 46% N. A popular polymer coated urea, called Environmentally Smart Nitrogen, is 44% N because the added polymer dilutes the N. I expect this would be true of the products in this study as well.

Line 172-193 – I do not see any mention of statistical analyses that were conducted for the pot study.

Line 188-189 – N concentration of plant tissue was only determined for the last cutting? N concentration of plants does not indicate effectiveness of N uptake. It would be better to multiply leaf N concentration by plant biomass to determine total plant N uptake. Since this was not measured for each cutting, it is difficult to know the amount of N that was taken up by the spinach from each treatment. Perhaps at least showing this for the last cutting would be helpful.

Line 193 – It says that N recovery efficiency was determined? This was not discussed in the paper, please remove.

Line 223 – Please explain that material is shown in figure 2-1(b)

Figure 2 – What are the differences between a and b photos? Please explain in the figure caption.

Line 285 – What statistical analysis was completed to analyze this data? It is difficult to know if there were significant differences between coating treatments in Figure 5.

Figure 6 – As mentioned, it would be nice to see a statistical analysis run on the efficiency data to tell whether treatments were actually different from one another.

Line 346-347 – Not sure treatments need to be defined again since they were defined in the methods (if the “blank treatment” was not defined in the methods, it should be).

Line 352 – “with non-significant differences” What statistical analyses were used for this analysis? It was not mentioned in the methods. Also, please indicate statistical results in the graphics. Usually people indicate significant differences with different letters above bars. There are examples of this in recent Sustainability articles.

Line 352-354 – The conclusion that these treatments met N requirements of the spinach is not substantiated by your data for two reasons. First, tissue testing for N concentration has been shown to vary drastically in many crops depending on type of tissue collect and even timing (early in the day versus late). If you can find a paper that discusses tissue testing in spinach, please add it here to your discussion. It will need to match the timing of when you collected tissue since that is important for establishing correlations with N sufficiency. Second, you did not apply varying rates of N, so the N still could have been deficient, even in the C-1 and C-2 treatments. Please rephrase.

Figure 7 – As mentioned, please indicate results of statistical analyses in your figures where appropriate. Figures should be able to be interpreted without having to read through the text to find the results. Also, indicate that figure b is N concentration of leaves from the 4^th cutting. Otherwise readers may think this is the average across all cuttings.

Line 355-356 – Again, the conclusion does not make sense. Comparing N concentration at one leaf cutting is not the same as comparing biomass added together from all cuttings. Additionally, a high N concentration in a plant does not indicate efficient uptake, it may indicate that the plant is small and has less carbon in it (the plant is less mature). Showing N uptake (biomass multiplied by leaf N concentration) would be better. Since N concentration of tissue was only measured at the 4^th leaf cutting, total N uptake over the experiment cannot be calculated. Biomass varied so greatly across treatments and cuttings, doing N uptake for a single cutting may not be useful. Please change the conclusions so that they are supported by the data that is available.

Line 357 – Please change “(in leaves)” to “(in leaves at the fourth cutting)”. This makes it clear that N concentration was only measured at that time and that conclusions cannot be drawn for other cuttings.

Line 363 – If modeling curves that fit data, wouldn’t it be better to show figures rather than tables?

Line 386-387 – This is the first place that it was mentioned that there were not significant differences among coating types in regards to prill strength. The conclusion should not be the first place results are mentioned. In fact, this disagrees with what was said in line 287-288 that “C-1 showed lowest value due to poor integration of PVA and Plaster of Paris…”. If it was a lower value than the others, readers will assume it was significantly different. Also, please add the statistical analyses used in this study in the methods.

Line 388 – You experiment was not designed to determine if N needs were met by spinach. More N rates would be needed to determine that and this study only looked at one N rate. Please rephrase.

Line 388-389 – N use efficiency was not discussed in the paper and cannot be determined based on the methodology described. Please remove.

Line 389-390 – N concentration in plant tissue was only measured one time, so the conclusion that slower release allowed for maximized N concentration in tissues is not supported. Release of the N in the soil conditions was not measured either. Finally, N losses to the environment were not measured at all in this pot study. Please remove.

Line 392-395 – This is a strong assumption as many things remain to be tested, including the fertilizers in field studies with different crops. Release patterns of N in soils are not yet known. Please rephrase.

Author Response

Reviewer # 2
The research is interesting as it tests several materials as a coating for urea, particularly ones that are likely to have less environmental impact than plastic coatings. The paper does a good job of describing physical characteristics of the coated ureas, the scanning electron microscope images were particularly intriguing. However, the paper is lacking in several areas. For example, statistical analyses are sometimes reported, though inconsistently, and methods are not described. Also, conclusions are not often supported by the data, particularly for the pot study. Specific comments are given below.

Response: Statistical analysis has been done and information regarding these analysis are incorporated the manuscript. Methods are now described in details especially for pot experiment. Conclusion section has completely been rewritten.

Specific comments (line numbers refer to those seen in the PDF version downloaded from the MDPI reviewer system):

Q.1.Lines 47-49 – This sentence is confusing because it only talks about excessive N during the nitrification process, which seems to indicate the authors are referring to nitrate. Nitrate is not the only problem in the environment. Also nitrate does not volatilize, ammonium does. Please rewrite to make it clear what is being said.

Response: The manuscript is updated as per comment.

 

Q.2.Line 49-51 – Is there a citation for this sentence? It seems to refer to specific research.

Response: Reference has been incorporated now.

Q.3.Line 58-60 – What recent global assessment has stated this? There is no reference included. Also, please clarify by what is meant by “four of nine planetary boundaries have now been crossed”. It is not clear what planetary boundaries are being spoken of.

Response: The reference is attached which stated the global assessment of Planetary boundaries disorderness. A planetary boundary is a concept involving earth system processes that contain environmental boundaries[1]. These boundaries include climate change, bio-diversity loss, biogeochemical, ocean acidification, land use, fresh water, ozone depletion, atmospheric aerosols and chemical pollution. Manuscript is updated accordingly.

Q.4. Line 68-70 – Urea is not the cheapest N source in all regions of the world. Anhydrous ammonia for instance is cheaper in some areas and has a higher N content than urea. Please clarify what region the authors are discussing.

Response: We agree with the reviewer and corrected the sentence.

Q.5.Lines 82-84 and 84-86 – Please add citations for this information.

Response: Citation has been added now.

Q.6.Line 97 – “… all of analytical purity were obtained from (Daejung-Korea).” Is something missing in this line? Where were the materials obtained from? Or should the parentheses be removed to complete the sentence?

Response: We agree with the reviewer and corrected the sentence accordingly.

Q.7.Line 130 – A treatment, C-0, is mentioned but is not listed in table 1. What is this treatment, uncoated? Please define at first mention.

Response: The C-0 is uncoated urea which is already mentioned in the abbreviation table at the starting of paper just after abstract.

Q.8. Line 147 – ...“at leaves surface” is confusing. Do you mean “to stay on the surface of leaves”? Or something else?

Response: Yes, we agree with the reviewer and corrected the sentence.

Q.9. Line 185-186 – What were the results of the soil test? Were P and K needs of the spinach met with the soil or did other fertilizers have to be added?

Response: Soil has initial pH of 8.1, EC 0.19 dS m^-1, total organic carbon 0.16%, dissolved organic carbon of 4.9 mg kg^-1 and mineral nitrogen of 4.1 mg kg^-1. Spinach P and K requirement was fulfilled by application of Phosphorous (P) and potassium (K) fertilizers as basal dose of 75:100 kg PK/ha through their source as triple super phosphate and muriate of potash, respectively. This information has now been added into the manuscript.

Q.10.Line 187 – Why was 0.5 g of urea chosen? Is this a typical rate for greenhouse grown spinach in the size pot that was used? Please clarify.

Response: Pot used in the study has internal diameter of 17.1 cm. Both coated and uncoated urea prills were applied at recommended rate of 100 kg N/ha in three splits, i.e. before sowing and immediately after 2^nd and 3^rd cuttings of spinach plants. Uncoated urea was applied at rate of 0.5 g/pot which equals to 100 kg N/ha. However, coated urea application was corrected for weight of coating materials on their surfaces. These information are now been added in the manuscript.

Q.11.Line 187 – “A total of 0.5 g urea was applied…”. Does this mean more N was applied to C-0 treatment since it didn’t have a coating? I assume the coating added weight to the urea prills. Please indicate the N application rate along with the fertilizer application rate. As an example, urea is 46% N. A popular polymer coated urea, called Environmentally Smart Nitrogen, is 44% N because the added polymer dilutes the N. I expect this would be true of the products in this study as well.

Response: Both coated and uncoated urea prills were applied at recommended rate of 100 kg N/ha in three splits, i.e. before sowing and immediately after 2^nd and 3^rd cuttings of spinach plants. Uncoated urea was applied at rate of 0.5 g/pot which equals to 100 kg N/ha. However, coated urea application was corrected for weight of coating materials on their surfaces. Equal amount of N was applied in each fertilizer treatment. These information are now been added in the manuscript.

Q.12.Line 172-193 – I do not see any mention of statistical analyses that were conducted for the pot study.

Response: Statistical analyses are now mentioned in the manuscript.

Q.13.Line 188-189 – N concentration of plant tissue was only determined for the last cutting? N concentration of plants does not indicate effectiveness of N uptake. It would be better to multiply leaf N concentration by plant biomass to determine total plant N uptake. Since this was not measured for each cutting, it is difficult to know the amount of N that was taken up by the spinach from each treatment. Perhaps at least showing this for the last cutting would be helpful.

Response: We agree with the reviewer and now presented foliage N uptake at last harvest rather than N content of leaves.

Q.14.Line 193 – It says that N recovery efficiency was determined? This was not discussed in the paper, please remove.

Response: We have not calculated N recovery efficiency. However, we have now presented foliage N uptake at last harvest. This has now been corrected throughout the manuscript.

Q.15.Line 223 – Please explain that material is shown in figure 2-1(b)

Response: Figure 2-1(b) is the SEM image of uncoated urea prill. The surface of urea at 2000 magnification is shown in figure 2-1(b).

Q.16.Figure 2 – What are the differences between a and b photos? Please explain in the figure caption.

Response: The figure a and b are the SEM images at two different magnifications. The manuscript is updated as per comments. 

Q.17.Line 285 – What statistical analysis was completed to analyze this data? It is difficult to know if there were significant differences between coating treatments in Figure 5.

Response: Univariate analysis was done to check treatment effect by using SPSS statistics version 20 (IBM, NY, USA). Main effect of fertilizers formulations was analyzed through analysis of variance (ANOVA) at 5% probability level. The multiple comparisons was done by Tukey-HSD test. Statistical difference among treatments is shown by different small letter on value bars.

Q.18.Figure 6 – As mentioned, it would be nice to see a statistical analysis run on the efficiency data to tell whether treatments were actually different from one another.

Response: Needful has been done now.

Q.19.Line 346-347 – Not sure treatments need to be defined again since they were defined in the methods (if the “blank treatment” was not defined in the methods, it should be).

Response: We agree with respected reviewer and deleted the sentences. The blank (untreated control) treatment is mentioned in material and method section under the heading of Pot Experiment.

Q.20.Line 352 – “with non-significant differences” What statistical analyses were used for this analysis? It was not mentioned in the methods. Also, please indicate statistical results in the graphics. Usually people indicate significant differences with different letters above bars. There are examples of this in recent Sustainability articles.

Response: Because N content are now replaced with N uptake. The statistical analyses are mentioned in the Methodology section. Now different lettering has been done above bars to show significant differences among treatments.

Q.21.Line 352-354 – The conclusion that these treatments met N requirements of the spinach is not substantiated by your data for two reasons. First, tissue testing for N concentration has been shown to vary drastically in many crops depending on type of tissue collect and even timing (early in the day versus late). If you can find a paper that discusses tissue testing in spinach, please add it here to your discussion. It will need to match the timing of when you collected tissue since that is important for establishing correlations with N sufficiency. Second, you did not apply varying rates of N, so the N still could have been deficient, even in the C-1 and C-2 treatments. Please rephrase.

Response: We agree with the respected reviewer. Because plant N content data is now replaced with N uptake. Therefore, conclusion is completely rewritten now. 

Q.22.Figure 7 – As mentioned, please indicate results of statistical analyses in your figures where appropriate. Figures should be able to be interpreted without having to read through the text to find the results. Also, indicate that figure b is N concentration of leaves from the 4th cutting. Otherwise readers may think this is the average across all cuttings.

Response: Needful has been done according to the advice of the respected reviewer.

Q.23.Line 355-356 – Again, the conclusion does not make sense. Comparing N concentration at one leaf cutting is not the same as comparing biomass added together from all cuttings. Additionally, a high N concentration in a plant does not indicate efficient uptake, it may indicate that the plant is small and has less carbon in it (the plant is less mature). Showing N uptake (biomass multiplied by leaf N concentration) would be better. Since N concentration of tissue was only measured at the 4th leaf cutting, total N uptake over the experiment cannot be calculated. Biomass varied so greatly across treatments and cuttings, doing N uptake for a single cutting may not be useful. Please change the conclusions so that they are supported by the data that is available.

Response: We agree with the respected reviewer and replaced N content data with N uptake at last harvest.

Q.24.Line 357 – Please change “(in leaves)” to “(in leaves at the fourth cutting)”. This makes it clear that N concentration was only measured at that time and that conclusions cannot be drawn for other cuttings.

Response: The manuscript is updated and this sentence has been deleted now.

Q.25.Line 363 – If modeling curves that fit data, wouldn’t it be better to show figures rather than tables?

Response: Fractional urea release is now being presented in Figure rather than a table. However, the release kinetics was tested at different time intervals so the number of curves on the figures is very large therefore it is better to present the data in the tabular form instead of figures [2].

Q.26.Line 386-387 – This is the first place that it was mentioned that there were not significant differences among coating types in regards to prill strength. The conclusion should not be the first place results are mentioned. In fact, this disagrees with what was said in line 287-288 that “C-1 showed lowest value due to poor integration of PVA and Plaster of Paris…”. If it was a lower value than the others, readers will assume it was significantly different. Also, please add the statistical analyses used in this study in the methods.

Response: Statistical analysis has now been done. The conclusion section is now completely rewritten. Therefore, these sentences are now been removed.

Q.27.Line 388 – You experiment was not designed to determine if N needs were met by spinach. More N rates would be needed to determine that and this study only looked at one N rate. Please rephrase.

Response: We agree with reviewer and rewritten the sentences.

Q.28.Line 388-389 – N use efficiency was not discussed in the paper and cannot be determined based on the methodology described. Please remove.

Response: We have not calculated N use efficiency instead we only presented foliage N uptake at last harvest of spinach. The whole manuscript has now been updated.

Q.29.Line 389-390 – N concentration in plant tissue was only measured one time, so the conclusion that slower release allowed for maximized N concentration in tissues is not supported. Release of the N in the soil conditions was not measured either. Finally, N losses to the environment were not measured at all in this pot study. Please remove.

Response: Needful has been done now and conclusion section is completely rewritten.

Q.30.Line 392-395 – This is a strong assumption as many things remain to be tested, including the fertilizers in field studies with different crops. Release patterns of N in soils are not yet known. Please rephrase

Response: Needful has been done now and conclusion section is completely rewritten.

References Used in Response:

1. Mohajan, H., Planetary Boundaries Must not be Crossed for the Survival of Humanity. 2015.
2. Al-Zahrani, S., Utilization of polyethylene and paraffin waxes as controlled delivery systems for different fertilizers. Industrial & engineering chemistry research, 2000. 39(2): p. 367-371.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

General comments:

The added information makes the paper stronger, thank you to the authors for considering earlier comments. One thing I noticed in this second revision is that there is a lack of discussion in regards to comparing these fertilizers to other studies on growth of spinach. There are also still a few minor issues, see my specific comments below.

Additionally, it is interesting that all N fertilizers were split applied to the spinach. Isn’t the point of a coated-urea to be able to apply it once so that the urea-N could slowly release over time? I would expect that applying it late into the growing stages of spinach would allow too much N to release late after peak N uptake, causing more nutrient losses later. This does not need to be addressed in this paper, but the authors could consider this for future experiments.

 

Specific comments (line numbers refer to those seen in the PDF version downloaded from the MDPI reviewer system):

Figure 7: It is unclear what the letters in Figure 7a represent. Were “all cuts” analyzed separately from each other? This needs to be labeled differently if that is the case, or described differently in the figure caption. Otherwise it looks like in the “no N treatment” that all cuttings (first through fourth) were statistically higher than the “all cuts” since b and c are higher than d. Perhaps changing to "Different letters (small or capital) on bars show significant difference among treatments within cuttings at 5% probability level."

Lines 437: This sentence seems to indicate that Treatment C-1 had that much more N uptake throughout the experiment, but it did not. It only had that much at the 4^th leaf cutting. Since N uptake seems to follow biomass production (Figure 7b) you would assume it would do the same for “all cuts” biomass production in Figure 7a. I would suspect that C-2 and C-4 had the highest amount of N uptake. Unfortunately, this was not measured so it is hard to understand N uptake dynamics here. I would likely leave this out of the conclusions since not much information can be drawn from this. The information is interesting to see, but nothing can be extrapolated from it. It is unlikely that in other experiments where people would want to compare their results to yours, that they would only consider N uptake in one cutting out of four.

Author Response

Dear Editor,

We would like to thank the Reviewer for his comments on our manuscript. We took advantage of his remarks and changed the manuscript accordingly. We truly hope the revised manuscript answered all the comments of the reviewer. Below we report our detailed answers:

Reviewer # 1

The added information makes the paper stronger, thank you to the authors for considering earlier comments. One thing I noticed in this second revision is that there is a lack of discussion in regards to comparing these fertilizers to other studies on growth of spinach.

Response:

Following lines have been added (lines 393-402 MS WORD file) to compare our findings with other studies that focused on slow release of nitrogen on spinach growth:

“Similar findings, i.e., increase in spinach biomass with the use of coated fertilizers have also been reported by other researchers (Tachibana, 2007 reported by Trenkel, 2010; Zhaohui Li et al., 2013). Zhaohui Li et al (2013) concluded that compared to the conventional fertilizer, with the same amount of nutrients applied, higher spinach yields could be achieved when zeolite coated/blended fertilizer was used even though, the difference was statistically not significant. Similarly, results from three-year trials carried out by Tachibana (2007) indicate that using a controlled release N fertilizer (Dd-Meister) allowed a substantial reduction (30%) in the amount of N fertilizer applied, while obtaining high-quality leaves, equal spinach yields, and reducing adverse environmental effects (Trenkel, 2010). In leafy vegetables, particularly spinach, the use of Dd-Meister had considerably reduced the nitrate-N content of leaves”.

Zhaohui Li , Yingpeng Zhang & Yan Li (2013) ZEOLITE AS SLOW RELEASE FERTILIZER ON SPINACH YIELDS AND QUALITY IN A GREENHOUSE TEST, Journal of Plant Nutrition, 36:10, 1496-1505, DOI: 10.1080/01904167.2013.790429.

Trenkel, M.E (2010). Slow- and Controlled-Release and Stabilized Fertilizers: An Option for Enhancing Nutrient Efficiency in Agriculture. Second edition, IFA, Paris, France, ISBN 978-2-9523139-7-1.

 

 

 

 

There are also still a few minor issues, see my specific comments below.

Additionally, it is interesting that all N fertilizers were split applied to the spinach. Isn’t the point of a coated-urea to be able to apply it once so that the urea-N could slowly release over time? I would expect that applying it late into the growing stages of spinach would allow too much N to release late after peak N uptake, causing more nutrient losses later. This does not need to be addressed in this paper, but the authors could consider this for future experiments.

 

Specific comments (line numbers refer to those seen in the PDF version downloaded from the MDPI reviewer system):

2. Figure 7: It is unclear what the letters in Figure 7a represent. Were “all cuts” analyzed separately from each other? This needs to be labeled differently if that is the case, or described differently in the figure caption. Otherwise it looks like in the “no N treatment” that all cuttings (first through fourth) were statistically higher than the “all cuts” since b and c are higher than d. Perhaps changing to "Different letters (small or capital) on bars show significant difference among treatments within cuttings at 5% probability level."

Response:

Following clarification has been added under Figure 7 caption:

“Under Figure 7a, different letters on bars for each separate/specific cut depict significant difference(s) among treatments at 5% probability level.”

3. Lines 437: This sentence seems to indicate that Treatment C-1 had that much more N uptake throughout the experiment, but it did not. It only had that much at the 4th leaf cutting. Since N uptake seems to follow biomass production (Figure 7b) you would assume it would do the same for “all cuts” biomass production in Figure 7a. I would suspect that C-2 and C-4 had the highest amount of N uptake. Unfortunately, this was not measured so it is hard to understand N uptake dynamics here. I would likely leave this out of the conclusions since not much information can be drawn from this. The information is interesting to see, but nothing can be extrapolated from it. It is unlikely that in other experiments where people would want to compare their results to yours, that they would only consider N uptake in one cutting out of four.

 

 

Response:

As per suggestion of the reviewer, following statement has been deleted from the conclusion

“Treatment C-1 has 53 and 87% higher N uptake than control and uncoated urea, respectively.”

Author Response File: Author Response.pdf