Abstract
Tillage pans in soils often restrict root access to subsoil moisture and nutrients. As part of research to reduce the nitrate load from upland fields to groundwater, we evaluated tillage pan breaking treatments in the Okhotsk region, one of the major onion (Allium cepa L.)-producing regions in Japan. For 4 years on a total of 7 sites on various soil types, we grew onions on plots subsoiled with a wide vibrator (W), subsoiled (S), or not subsoiled (NS). The W treatment reduced the penetration resistance to <1.5 MPa, reduced the bulk density, and increased the gaseous phase in the 20〜40-cm zone compared with the S and NS treatments. The rooting depth in the W treatment was almost twice that in the S and NS treatments. The W treatment increased N uptake, total dry matter, and yield of onion relative to the S and NS treatments. In addition, higher yields and N uptake in the W treatment following a dry period during the vegetative growth stage are attributable to the use of moisture below the broken tillage pan. The W treatment reduced the surplus in the N budget by an estimated 15 kg ha^<-1> owing to the higher N uptake, and reduced the residual inorganic N in the 0〜80-cm zone by 11 kg ha^<-1> at harvest. In 9 cases out of 10, the W treatment with less N fertilizer than recommended by Hokkaido Prefecture (150 kg ha^<-1>) reduced the surplus in the N budget to within the acceptable amount of residual N in the soil profile. We conclude that breaking the tillage pan in onion fields can reduce the nitrate load to groundwater by improving the N budget.
