Most solution cultures require maintenance of relatively constant concentrations of nutrients in the root environment. This requirement is most relevant for the macronutrient phosphorus (P), the plant demand for which is high but whose concentration in nutrient solution should be kept low to mimic the concentration in soil solution. This study examined suitability of iron phosphate (FePO4) and hydroxyapatite (HAP) to supply P to N-2-fixing plants of Lupinus albus (cv. Kiev Mutant) grown in nutrient solution. Five plants were grown in each 5-L pot containing dilute nutrient solution, with five dialysis pouches containing either HAP or FePO4 at rates of 0.02, 0.1, 0.5 or 2 g L-1. Solution pH was maintained at 5.6 using 2 mM MES. The solutions and pouches were renewed weekly.During 3 weeks of plant growth, concentrations of P in solution supplied with HAP fluctuated in ranges of 0.6-1.5, 0.7-2, 0.9-2.7 and 1.1-2.8 muM for rates of 0.02, 0.1, 0.5 and 2 g L-1, respectively. Concentrations of P in solution with FePO4 ranged from 0 to 0.7 muM and did not significantly increase with application rate. Plants produced significantly more dry matter when grown in the presence of HAP than FePO4. Dry weights of shoot and root increased as the application rate of HAP increased from 0.02 to 0.1 g L-1 but did not increase further as application rates increased from 0.1 to 2 g L-1. Plants grown with HAP had 76-157% higher P concentration in shoots and 94-185% higher in roots than those grown with FePO4. Increasing application rates of HAP and FePO4 increased P concentration in plants, the increase being much greater in plants supplied with HAP than with FePO4. Irrespective of P source and application rate, there was a good relationship between shoot P concentration and shoot dry weight, with a critical level of about 1.6 mg P g(-1) shoot dry matter. The results suggest that the HAP/dialysis pouch system may be used as a P source in solution culture.
Sas, L., Tang, C., & Rengel, Z. (2001). Suitability of hydroxyapatite and iron phosphate as P sources for Lupinus albus grown in nutrient solution. Plant and Soil, 235, 159-166. https://doi.org/10.1023/A:1011989922343