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Hoagland solution
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<h2 id="modifications">Modifications</h2> <p>The artificial solution described by <a href="/facts/Dennis_Robert_Hoagland/oERgG64s">Dennis Hoagland</a> in 1933,<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> known as Hoagland solution (0), has been modified several times, mainly to add <a href="/facts/Ferric_EDTA/h1CaD8aS">ferric chelates</a> to keep <a href="/facts/Iron/DkICCKIC">iron</a> effectively in solution,<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> and to optimize the composition and <a href="/facts/Concentration/wqBmz5od">concentration</a> of <a href="/facts/Trace_element/fcIekKdo">trace elements</a> other than iron, some of which are not generally credited with a function in plant nutrition.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> In Hoagland's nutrient recipes of 1938, referred to as Hoagland solution (1, 2), the number of trace elements was subsequently reduced to the generally accepted essential elements (<a href="/facts/Boron/Pmwgd7Mf">B</a>, <a href="/facts/Manganese/j9ELd1TL">Mn</a>, <a href="/facts/Zinc/80b1qgk3">Zn</a>, <a href="/facts/Copper/I8PEE8oX">Cu</a>, <a href="/facts/Molybdenum/RX6vp2bF">Mo</a>, <a href="/facts/Iron/DkICCKIC">Fe</a>, and <a href="/facts/Chlorine/y27UwYBM">Cl</a>).<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> Later research confirmed that their concentrations had been adjusted for optimal plant growth.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p><p>In Arnon's revision of 1950, only one concentration (Mo <i>0.011 <a href="/facts/Parts-per_notation/PozRuoLM">ppm</a></i>) was changed compared to 1938 (Mo <i>0.048 ppm</i>), while the concentration of <a href="/facts/Plant_nutrition/hXWLy7Sc">macronutrients</a> of the <a href="/facts/Dennis_Robert_Hoagland/oERgG64s">Hoagland solutions (0), (1), and (2)</a> remained the same since 1933, with the exception of <a href="/facts/Calcium/hf252CC0">calcium</a> (Ca <i>160 ppm</i>) in solution (2).<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> The main difference between solution (1) and solution (2) is the different use of <a href="/facts/Nitrate/5AnVlYnL">nitrate</a>-<a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a> and <a href="/facts/Ammonium/jYm8wc1n">ammonium</a>-nitrogen based <a href="/facts/Stock_solution/PePMLaJI">stock solutions</a> to prepare the respective Hoagland solution of interest. Accordingly, the original 1933 and the <i>modified</i> concentrations of 1938 and 1950 for each essential element and <a href="/facts/Sodium/eYuSPu2V">sodium</a> are shown below,<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> the calculation of the latter values being derived from Tables 1 and 2: </p> <ul><li>N 210 ppm</li> <li>P 31 ppm</li> <li>S 64 ppm</li> <li>Cl 0.14 ppm / <i>0.65 ppm</i></li> <li>B 0.11 ppm / <i>0.5 ppm</i></li> <li>Na 0 ppm / <i>0.023 ppm</i> / <i>1.2 ppm</i>*</li> <li>Mg 48.6 ppm</li> <li>K 235 ppm</li> <li>Ca 200 ppm / <i>160 ppm</i></li> <li>Mn 0.11 ppm / <i>0.5 ppm</i></li> <li>Zn 0.023 ppm / <i>0.05 ppm</i></li> <li>Cu 0.014 ppm / <i>0.02 ppm</i></li> <li>Mo 0.018 ppm / <i>0.048 ppm</i> / <i>0.011 ppm</i></li> <li>Fe 1 ppm / <i>2.9 ppm</i>* / <i>5 ppm</i>**<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a></li></ul> <h2 id="applications">Applications</h2> <p>Plant nutrients are usually <a href="/facts/Active_transport/7EAcsfb2">absorbed</a> from the <a href="/facts/Soil/5Ddpr3QM">soil solution</a>.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> The Hoagland solution, <a href="/facts/Dennis_Robert_Hoagland/oERgG64s">originally intended to imitate a (nutrient-) rich soil solution</a>,<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> has high concentrations of N and K so it is very well suited for the development of large plants like <a href="/facts/Tomato/3d5Xsqw7">tomato</a> and <a href="/facts/Bell_Pepper/UxPfTGfT">bell pepper</a>.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> For example, a half-strength <a href="/facts/Dennis_Robert_Hoagland/oERgG64s">macronutrient solution (1)</a> of Hoagland can be combined with a full <a href="/facts/Long_Ashton_Research_Station/Ekqhgjvl">micronutrient solution</a> of Long Ashton or <a href="/facts/Eric_John_Hewitt/AVNhkxhC">Hewitt</a> and a tenth-strength <a href="/facts/Ferric_EDTA/h1CaD8aS">ferric EDTA solution</a> of Jacobson to fertilize tomato <a href="/facts/Seedling/4v0rv1CT">seedlings</a>.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> Due to relatively high concentrations in the <a href="/facts/Water/0lkXnJPK">aqueous</a> stock solutions (cf. Tables 1 and 2) the Hoagland solution is very good for the growth of plants with lower nutrient demands as well, such as <a href="/facts/Lettuce/o68aNEJX">lettuce</a> and <a href="/facts/Aquatic_plant/Tr1rlK49">aquatic plants</a>, with the further dilution of the preparation to 1⁄4 or 1⁄5 of the <a href="/facts/Dennis_Robert_Hoagland/oERgG64s">original</a> or a <a href="/facts/Dennis_Robert_Hoagland/oERgG64s">modified</a> HS.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> </p> <h2 id="components">Components</h2> <p><a href="/facts/Salts/Fp0KhqyY">Salts</a>, <a href="/facts/Acid/FySU18n1">acids</a> and <a href="/facts/Coordination_complex/1tCyeQUm">complex ions</a> to make up the Hoagland hydroponic solution formulations (1) and (2):<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> </p> <ol><li>Potassium nitrate, KNO3</li> <li>Calcium nitrate tetrahydrate, Ca(NO3)2•4H2O</li> <li>Magnesium sulfate heptahydrate, MgSO4•7H2O</li> <li>Potassium dihydrogen phosphate, KH2PO4 or</li> <li>Ammonium dihydrogen phosphate, (NH4)H2PO4</li> <li>Boric acid, H3BO3</li> <li>Manganese chloride tetrahydrate, MnCl2•4H2O</li> <li>Zinc sulfate heptahydrate, ZnSO4•7H2O</li> <li>Copper sulfate pentahydrate, CuSO4•5H2O</li> <li>Molybdic acid monohydrate, H2MoO4•H2O or</li> <li><i>Sodium molybdate dihydrate</i>, Na2MoO4•2H2O</li> <li>Ferric tartrate or Iron chelate (<i>Fe-EDDHA−</i>)* or <i>Iron(III)-EDTA−</i>**<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a></li></ol> <h3>Components for Hoagland solution (1)</h3> <p>To prepare the stock solutions and a full <a href="/facts/Dennis_Robert_Hoagland/oERgG64s">Hoagland solution (1)</a><a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> </p> Table 1<table><tbody><tr><th rowspan="2">Component</th><th colspan="2">Quantities in solution</th></tr><tr><th>g/L</th><th>mL/L</th></tr><tr><th colspan="3">Macronutrients</th></tr><tr><td>2M KNO3</td><td>202</td><td>2.5</td></tr><tr><td>2M Ca(NO3)2•4H2O</td><td>472</td><td>2.5</td></tr><tr><td>2M MgSO4•7H2O</td><td>493</td><td>1</td></tr><tr><td>1M KH2PO4</td><td>136</td><td>1</td></tr><tr><th colspan="3">Micronutrients</th></tr><tr><td>H3BO3</td><td>2.86</td><td>1</td></tr><tr><td>MnCl2•4H2O</td><td>1.81</td><td>1</td></tr><tr><td>ZnSO4•7H2O</td><td>0.22</td><td>1</td></tr><tr><td>CuSO4•5H2O</td><td>0.08</td><td>1</td></tr><tr><td>H2MoO4•H2O, <i>or</i></td><td>0.09</td><td>1</td></tr><tr><td><i>Na2MoO4•2H2O</i></td><td>0.12</td><td>1</td></tr><tr><th colspan="3">Iron</th></tr><tr><td>C12H12Fe2O18, <i>or</i><i>Sprint 138 iron chelate</i>*</td><td>515</td><td>11.5</td></tr></tbody></table> <h3>Components for Hoagland solution (2)</h3> <p>To prepare the stock solutions and a full <a href="/facts/Dennis_Robert_Hoagland/oERgG64s">Hoagland solution (2)</a><a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> </p> Table 2<table><tbody><tr><th rowspan="2">Component</th><th colspan="2">Quantities in solution</th></tr><tr><th>g/L</th><th>mL/L</th></tr><tr><th colspan="3">Macronutrients</th></tr><tr><td>2M KNO3</td><td>202</td><td>3</td></tr><tr><td>2M Ca(NO3)2•4H2O</td><td>472</td><td>2</td></tr><tr><td>2M MgSO4•7H2O</td><td>493</td><td>1</td></tr><tr><td>1M NH4H2PO4</td><td>115</td><td>1</td></tr><tr><th colspan="3">Micronutrients</th></tr><tr><td>H3BO3</td><td>2.86</td><td>1</td></tr><tr><td>MnCl2•4H2O</td><td>1.81</td><td>1</td></tr><tr><td>ZnSO4•7H2O</td><td>0.22</td><td>1</td></tr><tr><td>CuSO4•5H2O</td><td>0.08</td><td>1</td></tr><tr><td>H2MoO4•H2O</td><td>0.02</td><td>1</td></tr><tr><th colspan="3">Iron</th></tr><tr><td>C12H12Fe2O18, <i>or</i><i>Sprint 138 iron chelate</i>*</td><td>515</td><td>11.5</td></tr></tbody></table> <h3>Alternatives for some components</h3> <h4>Micronutrients</h4> <p><i>Sprint 138 iron chelate</i> is produced as <a href="/facts/Sodium/eYuSPu2V">Na</a>-Fe-<a href="/facts/EDDHA/JGN7NzQA">EDDHA</a> (<i>C18H16FeN2NaO6</i>), while Hoagland's original solution formulations contain <a href="/facts/Dennis_Robert_Hoagland/oERgG64s">ferric tartrate</a> (C12H12Fe2O18), but no sodium ions.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a><a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a><a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> Synthesizing a sodium-free <a href="/facts/Ferric_EDTA/h1CaD8aS">ferric EDTA</a> complex (<i>C10H12FeN2O8−</i>) in a <a href="/facts/Laboratory/kqR3Vtx9">laboratory</a> is sometimes preferred to buying ready-made <a href="/facts/Chemical_industry/3M6gfHer">products</a>.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a><a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> Variable <a href="/facts/Micronutrient/nwzKuSMQ">micronutrients</a> (e.g., Co, Ni) and rather non-essential elements (e.g., Pb, Hg) mentioned in Hoagland's 1933 publication<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> (known as "A-Z solutions a and b"<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a>) are no longer included in his later circulars.<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a><a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> Most of these <a href="/facts/Metal/pqQVs8QU">metallic elements</a>, as well as <a href="/facts/Organic_compound/gqCNaN45">organic compounds</a>, are not necessary for normal plant nutrition.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> As an exception, there is evidence that, for example, some <a href="/facts/Algae/U1ftVKU3">algae</a> require <a href="/facts/Cobalt/gqhScEw4">cobalt</a> for the synthesis of <a href="/facts/Vitamin_B12/vCk613gD">vitamin B12</a>.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li>Hoagland and Knop medium</li> <li><a href="/facts/Dennis_Robert_Hoagland/oERgG64s">Knop's solution</a></li> <li><a href="/facts/Long_Ashton_Research_Station/Ekqhgjvl">Long Ashton Nutrient Solution</a></li> <li><a href="/facts/Murashige_and_Skoog_medium/4og3pR52">Murashige and Skoog medium</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://web.archive.org/web/20120304025304/http://amadeus.biosci.arizona.edu/~jdhall/Protocols/PlantNutrientSolution.doc">Modified composition of the Hoagland solution</a></li> <li><a href="http://www.nasonline.org/member-directory/deceased-members/20001777.html">National Academy of Sciences: A Biographical Memoir of Dennis Robert Hoagland by Walter P. Kelley</a></li> <li><a href="http://www.bdsoft.de/demo/index.htm?/demo/biologie/lexikon/a/a-z-loesung.htm">"A-Z solution", Supplementary Nutrient Solution (Solution A), according to Hoagland and Snyder (1933)</a></li> <li><a href="http://scienceinhydroponics.com/2009/02/the-hoaglands-solution-for-hydroponic-cultivation.html">The Hoaglands Solution for Hydroponic Cultivation</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Hoagland, D.R.; Snyder, W.C. (1933). "Nutrition of strawberry plant under controlled conditions. (a) Effects of deficiencies of boron and certain other elements, (b) susceptibility to injury from sodium salts". Proceedings of the American Society for Horticultural Science. 30: 288–294. <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Hoagland & Arnon (1938). The water-culture method for growing plants without soil (Circular (California Agricultural Experiment Station), 347. ed.). Berkeley, Calif. : University of California, College of Agriculture, Agricultural Experiment Station. OCLC 12406778. <a href="/wiki/OCLC_(identifier)" target="_blank">/wiki/OCLC_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Hoagland & Arnon (1950). The water-culture method for growing plants without soil. (Circular (California Agricultural Experiment Station), 347. ed.). Berkeley, Calif. : University of California, College of Agriculture, Agricultural Experiment Station. (Revision). Retrieved 1 October 2014. <a href="https://archive.org/details/watercultureme3450hoag" target="_blank">https://archive.org/details/watercultureme3450hoag</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"The water-culture method for growing plants without soil". Google Scholar. Retrieved 3 February 2020. <a href="https://scholar.google.com/scholar_lookup?title=The%20water-culture%20method%20for%20growing%20plants%20without%20soil&author=Hoagland&publication_year=1950" target="_blank">https://scholar.google.com/scholar_lookup?title=The%20water-culture%20method%20for%20growing%20plants%20without%20soil&author=Hoagland&publication_year=1950</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Smith, G. S.; Johnston, C. M.; Cornforth, I. S. (1983). "Comparison of nutrient solutions for growth of plants in sand culture". The New Phytologist. 94 (4): 537–548. doi:10.1111/j.1469-8137.1983.tb04863.x. ISSN 1469-8137. <a href="https://doi.org/10.1111%2Fj.1469-8137.1983.tb04863.x" target="_blank">https://doi.org/10.1111%2Fj.1469-8137.1983.tb04863.x</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Hoagland, D.R.; Snyder, W.C. (1933). "Nutrition of strawberry plant under controlled conditions. (a) Effects of deficiencies of boron and certain other elements, (b) susceptibility to injury from sodium salts". Proceedings of the American Society for Horticultural Science. 30: 288–294. <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Jacobson, L. (1951). "Maintenance of Iron Supply in Nutrient Solutions by a Single Addition of Ferric Potassium Ethylenediamine Tetra-Acetate". Plant Physiology. 26 (2): 411–413. doi:10.1104/pp.26.2.411. PMC 437509. PMID 16654380. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC437509" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC437509</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Arnon, D.I. (1938). "Microelements in culture-solution experiments with higher plants". American Journal of Botany. 25 (5): 322–325. doi:10.2307/2436754. JSTOR 2436754. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Hoagland & Arnon (1938). The water-culture method for growing plants without soil (Circular (California Agricultural Experiment Station), 347. ed.). Berkeley, Calif. : University of California, College of Agriculture, Agricultural Experiment Station. OCLC 12406778. <a href="/wiki/OCLC_(identifier)" target="_blank">/wiki/OCLC_(identifier)</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>van Delden, S.H.; Nazarideljou, M.J.; Marcelis, L.F.M. (2020). "Nutrient solutions for Arabidopsis thaliana: a study on nutrient solution composition in hydroponics systems". Plant Methods. 16 (72): 1–14. doi:10.1186/s13007-020-00606-4. PMC 7324969. PMID 32612669. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7324969" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7324969</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Hoagland & Arnon (1950). The water-culture method for growing plants without soil. (Circular (California Agricultural Experiment Station), 347. ed.). Berkeley, Calif. : University of California, College of Agriculture, Agricultural Experiment Station. (Revision). Retrieved 1 October 2014. <a href="https://archive.org/details/watercultureme3450hoag" target="_blank">https://archive.org/details/watercultureme3450hoag</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Nagel, K.A.; Lenz, H.; Kastenholz, B.; Gilmer, F.; Averesch, A.; Putz, A.; Heinz, K.; Fischbach, A.; Scharr, H.; Fiorani, F.; Walter, A.; Schurr, U. (2020). "The platform GrowScreen-Agar enables identification of phenotypic diversity in root and shoot growth traits of agar grown plants". Plant Methods. 16 (89): 1–17. doi:10.1186/s13007-020-00631-3. PMC 7310412. PMID 32582364. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7310412" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7310412</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Jacobson, L. (1951). "Maintenance of Iron Supply in Nutrient Solutions by a Single Addition of Ferric Potassium Ethylenediamine Tetra-Acetate". Plant Physiology. 26 (2): 411–413. doi:10.1104/pp.26.2.411. PMC 437509. PMID 16654380. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC437509" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC437509</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>"Importance of soil solution". Plantlet. Retrieved Aug 26, 2020. <a href="https://plantlet.org/importance-of-soil-solution/" target="_blank">https://plantlet.org/importance-of-soil-solution/</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Arrhenius, O. (1922). "Absorption of nutrients and plant growth in relation to hydrogen ion concentration". Journal of General Physiology. 5 (1): 81–88. doi:10.1085/jgp.5.1.81. PMC 2140552. PMID 19871980. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2140552" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2140552</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Genzel, F.; Dicke, M. D.; Junker-Frohn, L. V.; Neuwohner, A.; Thiele, B.; Putz, A.; Usadel, B.; Wormit, A.; Wiese-Klinkenberg, A. (2021). "Impact of moderate cold and salt stress on the accumulation of antioxidant flavonoids in the leaves of two Capsicum cultivars". Journal of Agricultural and Food Chemistry. 69 (23): 6431–6443. doi:10.1021/acs.jafc.1c00908. PMID 34081868. S2CID 235335939. <a href="https://doi.org/10.1021%2Facs.jafc.1c00908" target="_blank">https://doi.org/10.1021%2Facs.jafc.1c00908</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>He, F.; Thiele, B.; Watt, M.; Kraska, T.; Ulbrich, A.; Kuhn, A. J. (2019). "Effects of root cooling on plant growth and fruit quality of cocktail tomato during two consecutive seasons". Journal of Food Quality. Article ID 3598172: 1–15. doi:10.1155/2019/3598172. <a href="https://doi.org/10.1155%2F2019%2F3598172" target="_blank">https://doi.org/10.1155%2F2019%2F3598172</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>"The Hoaglands Solution for Hydroponic Cultivation". Science in Hydroponics. Retrieved 1 October 2014. <a href="http://scienceinhydroponics.com/2009/02/the-hoaglands-solution-for-hydroponic-cultivation.html" target="_blank">http://scienceinhydroponics.com/2009/02/the-hoaglands-solution-for-hydroponic-cultivation.html</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>Epstein E. (1972). Mineral Nutrition of Plants: Principles and Perspectives. John Wiley & Sons, New York, pp. 412. <a href="/wiki/Mineral_Nutrition_of_Plants:_Principles_and_Perspectives" target="_blank">/wiki/Mineral_Nutrition_of_Plants:_Principles_and_Perspectives</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>Jacobson, L. (1951). "Maintenance of Iron Supply in Nutrient Solutions by a Single Addition of Ferric Potassium Ethylenediamine Tetra-Acetate". Plant Physiology. 26 (2): 411–413. doi:10.1104/pp.26.2.411. PMC 437509. PMID 16654380. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC437509" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC437509</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>Hoagland & Arnon (1938). The water-culture method for growing plants without soil (Circular (California Agricultural Experiment Station), 347. ed.). Berkeley, Calif. : University of California, College of Agriculture, Agricultural Experiment Station. OCLC 12406778. <a href="/wiki/OCLC_(identifier)" target="_blank">/wiki/OCLC_(identifier)</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> <li id="fn:22"><p>Hoagland & Arnon (1950). The water-culture method for growing plants without soil. (Circular (California Agricultural Experiment Station), 347. ed.). Berkeley, Calif. : University of California, College of Agriculture, Agricultural Experiment Station. (Revision). 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