Phosphogypsum (PG, CaSO42H2O) is a by-product resulting from the manufacture of phosphate fertilizers, composed mainly of calcium and sulfur. Furthermore, since it contains silicon, phosphorus, magnesium, etc, it has high potential value recycled in various fields. However, PG containing small amounts of heavy metals and radionuclide. For these reasons, a large amount of PG cannot be recycled and is currently managed in open storage. However, depending on the environment phosphorite are generated in, the content of toxic substances varies. Results of stability evaluation of PG generated in Korea came out as below standard values. Therefore, domestic soil and crop-specific studies should be conducted to utilize PG as a soil amendment and fertilizer. The objective of this study is to investigate the physicochemical properties of soil and the effect on the quality and yield of onion according to the application of PG. Also, to optimize the amount of PG required for onion cultivation. The reason onion crop was selected is its high sulfur demand compared to other crops.
This study involved four different treatments of PG application: 0% (Control), 50% (PG50; 1,043 kg ha-1), 100% (PG100; 2,086 kg ha-1), and 150% (PG150; 3,129 kg ha-1). For comparison purposes, a standard application of oyster shell treatment (OS; 2,000 kg ha-1) was added and chemical fertilizers (N-P2O5-K2O = 84-379-80 kg ha-1) were applied for all treatments above.
During 7 months of onion cultivation, the pH of soil treated with PG decreased significantly compared to Control and OS. For soil EC, the higher PG concentration in soil, the more EC increases, with PG150 having the highest value (2.7 dS m-1) compared to optimal soil EC (2.0 dS m-1). Exchangeable calcium increased by 5% and 10% in PG100 and PG150, respectively, compared to soil pre-onion cultivation. In contrast, exchangeable magnesium and PG concentration showed a negative correlation. Based on soil depth (0~70cm), both water-soluble calcium and sulfur increased with PG concentration in soil.
According to PG concentration, the yield positively correlated with higher levels of PG compared to Control, yet showed lower yield than Control at excessive amounts of PG (PG150). The reason behind low yields at PG150 is predicted to be a response to salt damage during onion’s early growth.
Nutrient content in onions N, P, Ca and Mg were measured to be the highest at PG100, S and K were the highest at PG150 or had no significant difference with PG100. As a result, the use of PG increase the absorption rate of onions for nutrients like N, P, Ca, K, S and Mg, which affect crop quality such as plant metabolism, pest resistance, onion taste and aroma.
Total amino acid content in onion tissues increased as the concentration of PG increased in soil due to sulfur supply promoting amino acid synthesis. The maximum value for total amino acid (4,848 mg 100g-1) was measured at PG150. The concentration of cysteine and methionine was observed to be higher at PG100 than PG150.
In this study, we designated the optimum application amount needed for improving onion quality and productivity to be PG100 (2,086 kg ha-1), which satisfied 65% of calcium saturation. Additionally, we demonstrated that calcium and sulfur-based PG has a positive effect on quality and quantity of onions as well as soil properties especially in enrichment of subsoil (70cm) in calcium and sulfur due to its high solubility characteristics. Additionally, the improved soil environment helped increase onion yield and its nutrients content compared to both Control and OS treatments. Therefore, in order to safely utilize PG in the agricultural environment, further research is required to find out the application effect for different kinds of soils and crops and to define the optimal application amount of PG.