A new approach in defining the range of soil structure for optimal maize production is proposed. The effect of different degrees of compaction of ploughed sandy soils on soil aeration and mechanical impedance and on root and shoot growth of maize was tested in 5 model field experiments in the eastern Netherlands. In this first paper the effects on soil aeration and penetration resistance are described. The narrower the range of matric water potentials which allow unimpeded root growth, the greater the potential risks for plant growth and the greater the need for a controlled supply of water. Soil structures which substantially impede root growth even at the most favourable water potential are classified as dangerous. (Abstract retrieved from CAB Abstracts by CABI’s permission)