SUBSURFACE COMPACTION EFFECTS ON TILLERING AND NITROGEN ACCUMULATION IN WHEAT

Compaction in the 0.05- to 0.30-m depth may limit tillering and N accu mulation in wheat (Triticum aestivum L.); these effects may be yield r educing. Before imposing compaction treatments in a field study in Min nesota, perforated small-diameter tubing was buried in a Waukegan silt loam (sandy skeletal, mixed, mesic Typic Hapludoll) at a depth of 0.2 m in all plots. Compacted plots each received four passes with a heav y vibrating roller, while control plots were disk plowed and then disk harrowed. All plots were then surface roto-tilled and seeded to wheat . At early tillering a (KNO3)-N-15 solution was applied to the subsurf ace via the tubing. Plants sampled for N-15/N recovery 24 h after N-15 application, at anthesis, and at maturity showed no significantly dif ferent N-15 or N concentration or accumulation per culm/individual org an in response to compaction. Because subsurface compaction reduced ti llering, N uptake in the grain and straw was reduced by 21 and 30%, re spectively. After the 0.10-m surface layer was removed from all plots in a field study on a clay loam soil (Typic Calcixerolls) in Morocco, the subsurface was compacted by four passes with a 7.5-ton tractor. So il was then replaced, leveled, and only the control plots were disk pl owed and disk harrowed twice. Both compacted and control plots were ro to-tilled and seeded. Nitrogen concentration and accumulation per culm /individual organ immediately after tillering was reduced less than 5% by the compacted treatment in both 1982 and 1983. Reduced tillering i n the compacted treatment reduced average N uptake 11 and 28% in the g rain and siraw, respectively. In all three tests N concentration in th e shoot at the end of tillering was high; yet it was higher in the com pacted treatment. Then N concentration and uptake was reduced by compa ction on a per culm basis at maturity, and earlier in one of the field studies. Root length density was reduced in the compacted zone in res ponse to greater penetrometer load and higher bulk density. Tiller bud development was suppressed by the compaction treatment as much as 20% even though concentrations of N were high in the shoot. Tiller losses and declines of N concentration in the shoot in the post tillering st ages were nearly the same in both treatments. Additional N before till ering therefore cannot overcome the adverse influence of subsoil compa ction.