Overfertilization combined with flooding irrigation contributes to soil nutrient accumulation and pH decline in greenhouse vegetable production systems

Conventional overfertilization combined with flooding irrigation is common practice in solar greenhouse vegetable production systems in Eastern China. However, how these practices may affect soil nutrient concentrations remains understudied. Here we report on a regional soil sampling study in Shouguang county, which is the major and pioneer area of greenhouse vegetable production in China. In our study, we measured soil nutrient concentrations (mineral N, Olsen-P, and available K) as well as soil physico-chemical parameters (e.g. pH, SOC) down to 3 m soil depth in 60 greenhouses differing in years since establishment (5, 10, and 20 years) and compared those to nutrient concentrations in 20 adjacent arable fields. We found that in greenhouse soils compared to soils of adjacent arable fields: (1) soil nutrients were significantly elevated in the topsoil (0-30 cm), while nutrient concentrations in the subsoil were much lower and differences between greenhouse and arable soils diminished; (2) N:P:K ratios were imbalanced towards P and K as these elements are not easily leached as N; (3) the soil pH is decreased. After about 20 years of continuous greenhouse vegetable production, the topsoil pH was lowered by 0.92 ± 0.07 units. Our study shows that overfertilization combined with flooding irrigation has serious impacts on soil chemistry and soil nutrient concentrations. The observed nutrient imbalances are dramatic and indicate that e.g. P and K fertilization may not be needed for years to come. Also the observed decline of the significant decrease in the soil pH of calcareous soil indicates, that the soil pH carbonate buffer system is at the edge to break down, with unknown consequences for soil fertility. These facts strongly suggest that unnecessary excessive fertilizer and irrigation water applications for producing vegetables in solar greenhouse systems should be stopped and replaced with more sustainable production methods, e.g. drip fertigation with reduced fertilizer application.