Mechanically-Incorporated Controlled-Release Fertilizer Results in Greater Nitrogen and Salt Leaching Losses from Soilless Substrate in Containers

Round 1

Reviewer 1 Report

I found the study to be well conducted and the manuscript well-written, albeit it could be moderately reduced in length without affecting its contribution.  Suggestions, edits and comments were made directly on an annotated copy of the manuscript (attached).

Comments for author File: Comments.pdf

Author Response

Thank you for your edits. I corrected the text with your recommendations. I removed the sentence describing PAR in the greenhouse during the experiment because it was not pertinent to the text. I corrected typos and one incorrectly cited reference that your edits identified, thank you for taking the time to review.

Reviewer 2 Report

Uniform incorporation of controlled-release fertilizer (CRF) is a recommended best management practice to reduce nitrogen leaching losses from container-plant production. However, mechanically-incorporated into a soilless substrate often causes potential damage to the coating of controlled release fertilizer, and lots of nutrients can’t be stored for a long time and released quickly by leaching or runoff, leading to nutrients loss. In this paper, the author studied the impact of mechanically-incorporation on the leaching of inorganic nitrogen of coating controlled release nitrogen. Leachate volume, electrical conductivity (EC), pH and dynamic change of inorganic nitrogen content of the leachate were measured. Compared with manually-incorporation, more inorganic nitrogen was lost in the soilless matrix of slow and controlled release fertilizer with mechanically-incorporated, which indicated that mechanically-incorporation would damage the outer coating of controlled release fertilizer, resulting in loss of a large amount of nutrient elements. It provides a new scientific evidence for rational incorporation of controlled release fertilizer in agricultural production system. However, there are some problems in the present version of manuscript, thus, I would recommend a minor revision of the paper before published Horticulturae. The modification suggestions are as follows:

1、What is the logical relationship between the two experiments mentioned in this paper? What are the purposes of the two experiments? Why were the plants grown in Experiment 2? The above issues are not clearly explained in the introduction, methods and materials.

2、It is mentioned in the title that mechanically-incorporated will also cause salt leaching losses. But it is not clearly stated in the manuscript which salt-based ion leaching will be caused.

3、It is mentioned in the discussion part that mechanically-incorporation would cause damage to the envelope of controlled release fertilizer, but there is no substantial evidence in the paper to prove this point, and the discussion was not sufficient.

Author Response

1、What is the logical relationship between the two experiments mentioned in this paper? What are the purposes of the two experiments? Why were the plants grown in Experiment 2? The above issues are not clearly explained in the introduction, methods and materials.

      Experiment one was to measure total leaching losses from the manually- or mechanically-incorporated controlled release fertilizer. Experiment two existed to identify if plants exhibited toxicity symptoms in early stages of experiment and nutrient deficiency symptoms during later stages of the experiment in the mechanically-incorporated CRF treatment. However, we did not grow the plants long enough to see nutrient deficiency symptoms in those with mechanically-incorporated CRF because COVID-19 rules required us to end the experiment on day 76. This timeline did not allow us to grow the plants long enough to see symptoms. Additionally, container-plant producers and fertilizer manufacturers may question validity of results if plants were not planted in the soilless substrate which would diminish the efficacy of using these results in extension talks.

      We added ‘No plants were planted in this experiment to quantify total leaching losses from manually- or mechanically-incorporated CRF.” To lines 130-132. And we added “Plants were grown in this experiment to determine if nutrient toxicity symptoms occurred in beginning stages of experiment and if nutrient deficiency symptoms occurred in later stages of experiment. However, nutrient deficiency could not be evaluated because the experiment ended earlier than planned due to COVID-19 rules that required the conclusion of all “non-essential” research.” To lines 150-154

We added “Nutrient symptoms were not detected in any plants because COVID-19 restrictions required that the experiment end prematurely before symptoms developed.” To the discussion on line 345-347.

2、It is mentioned in the title that mechanically-incorporated will also cause salt leaching losses. But it is not clearly stated in the manuscript which salt-based ion leaching will be caused.

      We quantified salt leaching loss using electrical conductivity (EC) and leachate volume to calculate total milliequivalents of salt leached. The only fertilizer salts we quantified were inorganic nitrogen species, ammonium and nitrate. We utilized EC to quantify salt in leaching losses because we were not interested in the specific ions that were leached since majority of salt lost through leaching is from mineral fertilizer in the controlled-release fertilizer. Measuring EC is a simple and cost-effective method for determining salt content in solution. Identifying specific ions beyond inorganic nitrogen in leaching losses would not contribute additional information to the scientific literature because many reports and our knowledge of soilless substrate charge indicates that negatively charged ions leach more readily than positive ions.

3、It is mentioned in the discussion part that mechanically-incorporation would cause damage to the envelope of controlled release fertilizer, but there is no substantial evidence in the paper to prove this point, and the discussion was not sufficient.

      Lines 330-331 in revised text states “Previous research determined visibly damaged Osmocote prills leached more inorganic nitrogen than undamaged prills, especially in the first week of the experiment [16].” This statement plus the following lines (332-334) describing the method of controlled-release fertilizer incorporation are evidence enough that damage may have been caused to the prill coating. Furthermore, this comment is confusing because in the summary of our manuscript, the reviewer wrote this “Compared with manually-incorporation, more inorganic nitrogen was lost in the soilless matrix of slow and controlled release fertilizer with mechanically-incorporated, which indicated that mechanically-incorporation would damage the outer coating of controlled release fertilizer, resulting in loss of a large amount of nutrient elements. It provides a new scientific evidence for rational incorporation of controlled release fertilizer in agricultural production system.”  The quoted text indicates that the reviewer agrees with our assessment that the mechanically-incorporated CRF may have been damaged and results in more leaching losses.