There is widespread concern about loss of sediments, NO‐3, and phosphate into surface water from irrigated farmlands. We studied the effects of 900 kg ha−1 wheat (Triticum spp.) straw, mechanically applied to irrigation furrows, on sediments and on N and P losses in the runoff. The field was on a Nyssa silt loam soil (coarse‐silty, mixed, mesic Xerollic Haplodurid) with 3% slope planted to onion (Allium cepa L.), and received 0, 50, or 200 kg P ha−1 incorporated to 0.1 m deep. Furrows were side‐dressed with 200 kg ha−1 N. Averaged across 17 irrigations, straw reduced runoff volume by 43%. Cumulative sediment lost after 17 irrigations was 17 Mg ha−1 for mulched and 333 Mg ha−1 for unmulched furrows. Straw also reduced NO‐3 and NH+4 losses in runoff solution and sediments. Total N losses during the first 6 irrigations were 33 kg ha−1 from mulched and 230 kg ha−1 from unmulched furrows. Straw effect on N loss was only through changes in the runoff volume and sediment loss. In contrast, as fertilizer P increased, PO4 concentrations in runoff solution and sediments also increased. Averaged across the first 6 irrigations, straw mulch reduced PO4 losses in the runoff solution by 59, 61, and 72%, for the 0, 50, and 200 kg P ha−1 treatments, respectively. Straw reduced PO4 losses in the sediment 15‐, 11‐, and 15‐fold for the 0, 50, and 200 kg P ha−1 treatments, respectively. Averaged across P fertilizer rates, total P lost after 6 irrigations was 18 kg ha−1 from mulched and 215 kg ha−1 from unmulched furrows. Mechanically applied straw mulch in irrigation furrows can substantially reduce soil erosion and N and P losses to surface water runoff.

Onion (Allium cepa var. cepa L.) is extensively grown under furrow irrigation in the western United States. Wheel compaction of furrows increases water runoff and erosion, and can lead to poor lateral water movement and reduced yields. We studied the effects of 560 to 800 lb/acre (630 to 900 kg·ha-1) wheat straw mechanically applied to the bottom of irrigation furrows on yield and bulb size of sweet Spanish onions in commercial onion fields in 1988, 1990, and 1991, and at an experiment station in 1991 and 1995. Furrows in commercial fields were either compacted with tractor wheels or not. In the commercial fields, straw application increased onion yield in plots with compacted furrows in 1988 and in all plots (with or without compacted furrows) in 1990. At the experiment station, straw mulch increased onion yield 64% in 1991, and 74% in 1995. Straw application primarily increased yields of jumbo (3 to 4 inches; 76 to 102 mm) and colossal (>4 inches; 10 cm) onions, whereas there was no effect on medium (2.25 to 3 inches; 57 to 76 mm) onions. We attributed yield improvements to decreased water runoff and increased lateral water movement and soil moisture.

There are concerns about residues of DCPA (dimethyl 2,3,5,6‐tetrachloro‐1,4‐benzenedicarboxylate) and its two acid metabolites in surface runoff from furrow‐irrigated croplands. We studied the effects of mulching irrigation furrows with wheat (Triticum aestivum L.) straw on DCPA movement in runoff solution, with sediment, and in the soil profile. The study site had a Nyssa silt loam (coarse‐silty, mixed, mesic Xerollic Haplodurid) with a 3% slope. The field was planted to onion (Allium cepa L.) and was furrow‐irrigated 17 times. Prior to mulching, DCPA was either broadcast at 10 kg a.i. ha−1 or banded at 3.4 kg a.i. ha−1 on the top of beds between furrows. DCPA (parent plus metabolites) was measured in the runoff sediment of the first 6 irrigations, and in the runoff solution of the 1st, 3rd, and the 5th irrigation. Regardless of herbicide application method, mulching reduced DCPA losses in sediment of all irrigations and in the runoff solution of the fifth irrigation. Averaged across the 6 irrigations and the 2 herbicide application methods, mulching reduced DCPA transfer in sediment by about 90%. Straw mulch effects on DCPA losses were primarily through reductions in soil loss and runoff volume, whereas DCPA concentration was not affected in runoff solution and was increased in runoff sediment of some irrigations. Mulching increased infiltration of broadcast DCPA but tended to decrease infiltration of banded DCPA. Also, compared with broadcast DCPA, banding the herbicide reduced DCPA losses in sediment of some irrigations. We concluded that the combination of straw mulching and banding DCPA reduces herbicide transfer in surface runoff and movement into thesoil profile.