Effect of phosphorus on dynamic growth and nutrient absorption of hydroponic lettuce

Phosphorus is one of the main nutrient elements needed by plant growth. Because phosphate fertilizer is difficult to move under traditional soil culture, there is a significant gap between applied phosphorus fertilize and available phosphorus of plant. Under hydroponic conditions, however, phosphate fertilizer is evenly dissolved in nutrient solutions. This is not only beneficial for nutrient uptake of plant, but also critical for accurately and quantitatively analyzing the effect of phosphorus on plant. To determine the effect of phosphorus level on lettuce growth, quality and dynamic absorption of mineral elements, 0.2 mmol·L^-1, 0.4 mmol·L^-1, 0.6 mmol·L^-1, 0.8 mmol·L^-1 and 1.0 mmol·L^-1 of NaH₂PO₄ were separately added to a nutrient solution. The growth, yield, quality and dynamic mineral absorption of lettuce were measured and quantitatively analyzed. The results showed that the quality of lettuce was best under 0.4 mmol·L^-1 NaH2PO4. In 0.6 mmol·L^-1 of NaH2PO4 solution, the yield of lettuce hit peak amount, and the total absorption of N, P, K, Ca and Mg during the whole growth period reached highest point (691.26 mg·plant^-1) with the corresponding proportions of absorption of 1︰0.20︰1.34︰0.80︰0.24. The differences in nitrogen absorption were not significant under different phosphorus levels. However, the uptake of phosphorus increased with increasing level of phosphorus. In other words, there was luxury absorption of phosphorus with increasing levels of phosphorus dosage. The absorption of nitrogen and phosphorus was mainly concentrated in 2030 days after planting, the absorption amounts at this period, respectively, accounted for 79.86% and 59.25% of the total absorptions during the whole growth period. The absorption of potassium apparently increased with time, reached its maximum in the last 10 days, with absorption amount of 47.68% of total absorption during the whole growth period. The absorption of calcium and magnesium did not change significantly during the whole growth period, while absorption of calcium was mainly concentrated in 1030 days after planting, that of magnesium was mainly concentrated in the last 30 days of lettuce growth. During full growth period, potassium absorption proportion was the largest among nutrient elements, followed by nitrogen and calcium. Under higher phosphorus level, the proportion of phosphorus absorption was higher than that of magnesium, while under low phosphorus level, it was lower than that of magnesium. Phosphorus use efficiency decreased with increasing phosphorus level applied. Low phosphorus level enhanced the use efficiency of mineral elements.