Effect of phosphorus-coupled nitrogen fertigation on clogging in drip emitters when applying saline water

Emitter clogging risk might be enhanced by the specific fertigation practice when applying saline water. Two stages of on-site drip irrigation experiments applying saline water were conducted to investigate the effects of the phosphorus (P) fertigation and combined P and nitrogen (N) fertigation and the water salinity on emitter clogging when using a plain channel emitter and a labyrinth emitter. During stage one, the water-soluble phosphate fertilizers monoammonium phosphate (MAP), diammonium phosphate (DAP), and urea phosphate (UP) were tested in comparison with a control treatment without fertigation (CK). During stage two, four combined P and N fertigation practices were tested by mixing one phosphate fertilizer (MAP or DAP) with one N fertilizer (urea (U) or ammonium sulphate (AS)). Four levels of water electrical conductivity measuring about 0.5 (groundwater, G), 2 (S2), 4 (S4), and 6 (S6) dS m⁻¹ were tested at both experimental stages. The discharge rates of the emitters were measured following every other two fertigation events during stage one and following each fertigation event for stage two. At the end of the experiments, several representative emitters were sampled to measure the dry weight and the chemical constituents of the clogging substances in the emitters. The results demonstrated that the P fertigation with a weak or a slightly stronger acidic fertilizer such as MAP or UP reduced the emitter clogging effectively during groundwater application. For saline water application, the emitter clogging clearly increased with the water electrical conductivity. Phosphorus-coupled nitrogen fertigation accelerated the precipitation of phosphate sediments greatly when either groundwater or saline water was applied. In fact, the precipitation of phosphate sediments exceeded 80% of the dry weight in the emitters under saline water irrigation. Rapid development of emitter clogging occurred for both types of emitters when saline water with electrical conductivity higher than 4 dS m⁻¹ was applied under phosphorus-coupled nitrogen fertigation. The phosphorus-coupled nitrogen drip fertigation should be applied with cautious when applying saline water with electrical conductivity higher than 4 dS m⁻¹. Fertigating with nitrogen fertilizers along with acid phosphate fertilizers together is a promising way to alleviate the emitter clogging caused by the ion components in saline water.