Effects of Nitrogen and Boron Fertilization on Lettuce Mineral Nutrition in a Calcareous Soil

The effect of nitrogen (N) on the boron (B) nutrition of plants is of great significance because N may promote or reduce B absorption. Therefore, a fertilizer factorial experiment of randomized complete block design was conducted in pots under greenhouse conditions using lettuce (Lactuca sativa L.) as test plant. Plants of lettuce were grown in a calcareous sandy clay loam soil with different N and B fertilizer levels to investigate the effect of applied N and B as well as their interactions on nutrient distribution in leaves and roots of lettuce plants. Treatments consisted of four levels of N (0, 200, 400, and 600 mg N kg−1 soil) and two levels of B (0 and 5 mg B kg−1 soil). The results showed that increasing N level up to 600 mg N kg−1 significantly increased soil nitrate (NO3 −) N and potassium (K) but had no effect on soil available phosphorus (P) and B. Application of 5 mg B kg−1 soil significantly decreased available K, whereas P concentrations increased. Nitrogen application increased leaf and root N and reduced leaf P and K. By increasing N concentration in the soil, leaf B concentration was reduced in all N treatments. However, B had no effect on the N, P, and K contents of leaves. In the case of roots, B was affected by N treatments and it was reduced significantly in the treatment of 600 mg N kg−1 soil + 5 mg B kg−1 soil. These results suggest that N fertilization potentially may prevent B toxicity and keep leaf B concentration at normal levels in lettuce plants. Therefore, the appropriate management of N fertilizers in soils with excess B is very important.