Effect of solution electrical conductivity (ec) and pre-plant nutrient form on the ph of a peat-perlite substrate
2014
Fisher, P.R. | Dickson, R.W. | Mohammad-Pour, G.S. | Huang, J.
Substrate-pH in a peat-perlite substrate tends to drop as substrate-EC increases because of displacement of protons by cations in fertilizer and water. Conversely, if the substrate is leached of pre-plant nutrients (for example, during mist propagation), it is common for substrate-pH to increase. This trend is important for crop management, because of the pH effect on micronutrient availability. A 70% peat-30% perlite substrate was prepared, with dolomitic hydrated limestone applied at 2.2 g/L. Application of limestone increased base saturation from 16 to 93 percent, indicating a high level of cation exchange by the Ca and Mg ions provided by the lime. Combinations of four cations (ammonium, calcium, magnesium, and potassium) and four anions (chloride, nitrate, phosphate and sulfate) were applied to the limed substrate in all 16 combinations at an applied solution-EC of 3.0 dS/m for the soluble salts, and an equivalent molarity of Ca and Mg for the low-solubility salts calcium sulfate, calcium phosphate, and magnesium phosphate. Soluble Ca and Mg salts tended to result in the lowest substrate-pH after 7 days. The pH of the applied salt solutions did not affect the substrate-pH, indicating the substrate-pH effect was the result of cation exchange. In a second experiment, calcium chloride and calcium nitrate were added to peat-perlite at solution-EC levels from 0 to 4.5 dS/m. As sub-strate-EC increased, substrate-pH decreased with each salt, with a diminishing returns relationship. Results emphasize that when selecting a limestone incorporation rate, it is desirable to include a pre-plant nutrient charge in order to measure the equilibrium substrate-pH that is likely to occur following fertilization.
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