Functional interpretation of root densities in the field for nutrient and water uptake

Much field research on root systems has been performed in an agricultu-ral context. After the time-consuming and tedious research necessary to an-swer the question "how many roots does a plant have", always the next question is "how many does it need for maximum crop production". As soon as variation in root pattern and root density is noted, the question of practical implications follows. Is a higher root density always better for crop production Is the shoot/root ratio an important characteristic for cultivar selection Such questions are common, but real answers scarce so far. For water balance studies a correct definition of "root depth" is cri-tical; the "effective root depth" used so far for hydrological field stu-dies in the Netherlands (the depth above which 80% of the roots are concen-trated, RIJTEMA (1969)) is unsatisfactory as it is stated relative to the total root system instead of as an absolute value of a critical root densi-ty. A more intensely branched root system can extract the plant's needs from a poorer soil than a smaller root system. In any situation where the soil-root system is not supplying the crop's needs, the farmers' choice is ei-ther to improve the soil fertility or to improve root density (by culti-var selection, soil tillage, stimulating mycorrhiza, etc.). The need for clear guidelines on functional interpretation of root densities as they oc-cur in the field is apparent.

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