Crop response to fertilizer application depends not only on the level of available plant nutrients in the soil but is also related to crop physiology and morphology. For a well balanced nutrition the rate of nutrient supply to the roots must correspond with the rate of nutrient required for growth. Species or cultivars with a high growth rate generally respond more favourably to fertilizer application than those with low growth rates. An analogous relationship holds for the biomass produced per unit soil surface. Thus modern rice and wheat cultivars tolerate a more dense spacing than older ones. Due to the dense stand the yield and particularly the grain yield of the modern varieties may be several times higher than those of older cultivars, and therefore also the nutrient requirement, especially the demand for N and P, is higher for the modern cultivars. Modern cereal cultivars are characterized by a high crop index which means that after flowering a high proportion of grain filling material must be produced by photosynthesis. Assimilation and translocation of photosynthates are favoured by K+. Thus in particular modern cultivars require a high K+ content for optimum grain filling. Nutrient exploitation of soils by plant roots depends on root morphology and root physiology. Grasses generally have much longer roots than dicots. Thus the rate of K+ and phosphate uptake per unit root length is lower for grasses than for dicots. It is for this reason that dicots respond earlier to a K+ and phosphate dressing than grasses. Species living symbiotically with Rhizobium may depress the rhizosphere pH considerably and thus promote the dissolution of phosphate rock.