Fertilizer Potential Acidity/Basicity and Concentration Effects On Root

A root substrate consisting of 75% sphagnum peat moss and 25% perlite by volume was formulated with calcium carbonate powder to achieve substrate pH 6.5 without any residual lime effect during plant cultivation. Three commercial fertilizers were dissolved in deionized water at 100 and 200N mg·L^-1 N and consisted of an acidic fertilizer (AF), 20N-4.4P-16.6K (potential acidity: 193 kg/T), a neutral fertilizer (NF), 17N-2.2P-14.1K (0 potential acidity/basicity), and a basic fertilizer (BF), 13N-0.88P-10.8K (potential basicity: 150 kg/T). Liners of kalanchoe ‘Kerinci’ were grown in 16.5 cm diameter, 1.8-L volume pots (2 plants per pot). Plants were fertigated for 9 weeks. Fertilizers applied at the 100 mg∙L^-1 N rate resulted in sub-optimal substrate EC levels that declined to below 0.54 dS·m^-1. There was a decline in the BF and NF treatments to 6.0 and 5.4 at 35 days, with little change thereafter. Substrate pH in the AF treatment declined steadily to 4.4. The 200 mg·L^-1 N treatments resulted in substrate EC levels that rose to between 2.5 and 2.8 dS·m^-1, indicating luxuriant fertilization. Associated substrate pH levels declined steadily throughout crop time to 4.6, 3.9, and 3.9 in the BF, NF, and AF treatments, respectively. The pH levels for the actual fertilizer solutions for the 200 mg·L^-1 N BF, NF, and AF fertilizers did not relate to their potential acidity/basicity and were 5.8, 4.6, and 6.0, respectively. Results indicated that addition of fertilizer solution at sub-optimal concentration, regardless of potential acidity/basicity, caused an initial decrease in substrate pH due to the chemistry of the fertilizer (an abiotic effect). This was followed by a rise, leveling off, or fall in substrate pH, depending on potential acidity/basicity of the fertilizer, (a biotic effect of nutrient uptake).  When applied at a luxuriant rate, the biotic effect on pH was not expressed because the abiotic effect predominated and masked the biotic effect. Impact of fertilizer acidity/basicity rating on substrate pH is a function of the balance of the abiotic and biotic fertilizer effects which is determined in part by the quantity of fertilizer applied.