Effect of Rice Hull Mulch on Nutrient Concentration of Fertilized Irrigation Water

Parboiled rice hulls are an effective mulch for controlling weeds in nursery containers. A layer of rice hulls between 1.25 and 2.5 cm deep has been shown to provide effective control of liverwort (Marchantia polymorpha), bittercress (Cardamine flexuosa), and creeping woodsorrel (Oxalis corniculata). There is some speculation that the rice hull layer may affect the nutrient content of fertilized irrigation water, either removing nutrients, such as nitrogen (N), or adding other nutrients, including phosphorus (P) and potassium (K). The objective of this study was to determine how increasing depth of rice hull mulch affected the nutrient concentration of fertilized irrigation water as it passed through the mulch later. Rice hulls were placed in Buchner funnels (13.1 cm i.d., 6.6 cm tall) at a depth of 0, 0.6, 1.2, or 2.5 cm. Each funnel was placed on a greenhouse bench equipped with an overhead irrigation system with fixed pattern nozzles. Each funnel was placed over a 400 mL glass jar so that all irrigation water passing through the funnel would collect in the jar beneath, and furthermore, only irrigation water passing through the funnel (and no other extraneous irrigation water) would enter the jar. A water soluble fertilizer (20N-4.4P-16.6K) was injected at 150 mg∙L^-1 N into the irrigation water. The irrigation system was run for 10 min per day for 9 days. Water samples from each jar were collected after each irrigation event and refrigerated until analysis with ion chromatography for concentration of NO₃^-, NH₄⁺, PO₄^-, and K⁺. All rice hull layers reduced NO₃^- concentration on day 1 by approximately 35%. Thereafter, only the 2.5 cm rice hull layer reduced NO₃^- concentration, but only for four additional days. A similar response was observed for NH₄⁺ concentration. All rice hull treatments resulted in higher PO₄^- and K⁺ concentrations than the non-mulched controls throughout the 9 day experiment. Parboiled rice hulls are a known source of PO₄^- and K⁺ when used as a substrate amendment. Additional PO₄^- and K⁺ provided by the rice hull treatments varied throughout the experiment, but generally ranged from a 20% to 30% increase compared to the non-mulched controls. These data demonstrate that fertilizer nutrient levels are affected by rice hull mulch layers, although nutrient concentrations are not reduced to the extent that rice hull mulch would reduce plant growth.