The position of localized soil compaction determines root and subsequent shoot growth responses

Plants growing in soils typically experience a mixture of loose and compact soil. The hypothesis that the proportion of a root system exposed to compa ct soil and/or the timing at which this exposure occurs determines shoot gr owth responses was tested. Broccoli (Brassica oleracea var. italica cv. Gre enbelt) seedlings were grown in pot experiments with compact, loose and loc alized soil compaction created by either horizontal (compact subsoils 75 or 150 mm below loose topsoil) or vertical (adjacent compact and loose column s of soil) configurations of loose (1.2 Mg m(-3)) and compact (1.8 Mg m(-3) ) soil. Entirely compact soil reduced leaf area by up to 54%, relative to l oose soil. When compaction was localized, only the vertical columns of comp act and loose soil reduced leaf area (by 30%). Neither the proportion of ro ots in compact soil nor the timing of exposure could explain the differing shoot growth responses to localized soil compaction. Instead, the strong re lationship between total root length and leaf area (r(2)=0.92) indicated th at localized soil compaction reduced shoot growth only when it suppressed t otal root length. This occurred when isolated root axes of the same plant w ere exposed to vertical columns of compact and loose soil. When a single ro ot axis grew through loose soil into either a shallow or deep compact subso il, compensatory root growth in the loose soil maintained total root length and thus shoot growth was unaffected. These contrasting root systems respo nses to localized soil compaction may explain the variable shoot growth res ponses observed under heterogeneous conditions.