SUBSURFACE COMPACTION REDUCES THE ROOT AND SHOOT GROWTH AND GRAIN-YIELD OF WHEAT

The recent use of heavy machinery combined with frequent disk tillage has created a subsurface compacted horizon in many of the irrigated soils of Morocco. The objective of this study was to evaluate the effect of subsurface compaction on the root and shoot growth, grain yield, and grain yield components of wheat (Triticum aestivum L.). Field experiments were conducted in 1982 and 1983 on a Moroccan clay loam soil (typic Calcixerolls). Soil compaction was artificially created. The 0.10-m surface layer was removed from all plots with a road scraper. The exposed subsurface layer was then packed by making four passes over the plots with a 7.5-ton tractor. The removed soil was then replaced and leveled. Control plots were tilled with a disk plow followed by two passes of a disk harrow. Both compacted and control plots were then roto-tilled for final seedbed preparation. The result of this compaction was a 12 to 23% decrease in grain yield and 9 to 20% decrease in straw yield. The decrease in yield was accompanied by a consistent reduction in the number of shoots per unit area. Number of kernels per spike and kernel weight were unaffected. Both root growth and distribution were markedly changed as a result of subsurface compaction. Wheat plants in compacted plots had a denser, finer, and shallower root system than wheat plants in control plots. Plant height, and leaf area and dry matter per shoot were also unaffected. The decrease in shoot number might be attributed to a limitation in the amount of available soil N to the roots.