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REMOVING SOIL CONSTRAINTS TO CROP PRODUCTION 7the supply of nutrients. Some decrease in root extension can occur when the soilwater potential is still above -1 bar; nutrient uptake and yield can be rapidlyaffected as the potential falls and death can occur if the entire root systemexperiences -10 to -15 bar. However the dessication of the older parts of roots,which are nearest to the soil surface may have little effect, provided thatsufficient of their apical growing regions receive adequate water. This characteristicis often of prime survival value during drought because water is lostmuch more rapidly from the surface soil than from the deeper layers; the waterpotential at the depth of 20 - 25 cm may fall from close to zero to below -5 barwithin a week, but at the depth of about 1 m the water potential decreases muchmore slowly. Continuing growth then depends on the downward extension of rootsbeing sufficiently rapid for an adequate part of the root system to be in soilwhere water remains available. Compensatory growth can much assist this;decreasing water potential in the surface can cause roots to be more numerous,sometimes by a factor of more than four, in the deeper soil (Klepper et al.,1973).The main characteristics of soil which facilitate survival of drought are:(1) the conservation of water in the rooting zone; this depends on there beingsufficient stable and continuous transit pores for rain from periodic storms toenter, and adequate storage pores to retain it, and (2) conditions which permitdeep root penetration. Major requirements for this are an adequate depth of soiland the absence of stresses which restrict root penetration; mechanical impedancecan be among the most important constraints which soil management can mitigate.Mechanical Impedance: Modern understanding of the effects of this stress in fieldconditions can be considered to date from the demonstration, by Wiersum (1957),that roots cannot decrease in diameter to enter pores smaller than themselves;indeed mechanical constraint commonly causes their diameter to increase. If theyhave to resist pressures as low as 0.2 bar to expand pores their rate of elongationcan be much reduced (Figure 2) though it can continue at a slow rate whenpressures are considerably greater. Species differ but present evidence suggeststhat root growth is always retarded if roots have to resist external pressures of0.5 bar or more. Reports that a few species have a special ability to grow inheavily compacted soil may be due to them being able to survive with a very slowrate of root extension. The sensitivity of roots to very small external pressuresis due to physiological mechanisms in the apical meristem which are presumablyeither hormonal, or the result of changes in fine structure (Russell and Goss,1974; Goss and Russell, 1980).If the extension of root axes is impeded, but there are pores large enoughfor the finer laterals to enter freely, these latter show compensatory growth;laterals proliferate immediately above the zone where compaction occurs. Whenadequate nutrients and water are present in this restricted zone the yield ofplants may be little affected, but, as explained above, shallow rooted plants arevery susceptible to injury from even slight water stress. Low water potential inthe soil also reduces root elongation and increases soil strength, i.e. itsresistance to deformation, so that the forces roots must resist in enlarging poresare increased. Thus the effects of water stress and mechanical impedance caninteract positively in several ways.Roots can penetrate the soil by two strategies and increasing interest inreduced cultivation or "no till" systems makes it important to distinguish betweenthem. If the soil is relatively homogeneous, with few continuous pores larger indiameter than roots, the extension of roots depends primarily on their ability toexpand pores. Because pore size and the bulk density of soil vary inversely, themeasurement of this property may then indicate when it is desirable to increaseporosity by soil disturbance. The use of penetrometers can also help by showing