The Effect of Nutrient Ratios on Plant Height

Height control is often achieved through the application of plant growth retardants (PGRs). However, environmental concerns surrounding the use of synthetic PGRs could potentially lead to restrictions on their use. Non-chemical methods, such as overhead irrigation with cold water and day and night temperature differential (DIF) treatments can be time consuming, labor intensive, and expensive. Some research suggests that plant height control may be achievable through modifications in plant nutrition. For instance, studies have suggested that limiting phosphorus supply limits plant height. Many growers also believe that nitrate (NO₃^-) nitrogen produces shorter plants than ammonium (NH₄⁺) nitrogen. The primary objective of this study was to evaluate an alternative means of plant height control through adjustments in macronutrients like N, P and K. Ratios of NO₃^-/NH₄⁺, NO₃^-/H₂PO₄^-, NO₃^- /K⁺, K⁺/H₂PO₄^-, NH₄⁺/H₂PO₄^- , and NH₄⁺/K⁺ were tested at about 8 different levels, and approximately 50 hydroponic aerated nutrient solutions were compared to quantify their effects on plant height in sunflowers (Helianthus annuus L. ‘Sunrich Orange’), marigolds (Tagetes erecta L. ‘Vanilla’) and tomatoes (Lycopersicon esculentum L. ‘Roma’). All solutions were variations of a modified Hoagland’s solution, which functioned as a control. Solutions were first used at an electrical conductivity (EC) of ~1.0mS/cm, which produced no significant differences in height and very few or inconsistent differences in plant biomass. Solutions were then evaluated at ECs of 0.5, 1.0, 2.0, and 4.0 mS/cm. Plant height and biomass were generally affected by varying EC levels but seldom by the ratios themselves at these different levels. Results from this study suggest that nutrient management, with the exception of EC modifications, may not be useful in plant height control.