A decrease in the use of fertilizers and a decrease in soil fertility direct the research focus towards the ability of the crops to adapt to low potassium (K) conditions and to their ability to utilize sparingly soluble K sources. Pea (Pisum sativum), red clover (Trifolium pratense), lucerne (Medicago sativa L.), barley (Hordium vulgare), rye (Secale cerale), perennial ryegrass (Lolium perenne L.), and oilseed rape (Brassica napus oliefera) were grown in thin layers of soils to study their root morphological plasticity and ability to utilize different K pools at 26 and 60 mg exchangeable K kg(-1) soil. The legumes (pea, red clover, lucerne) accumulated larger amounts of nitrogen but lower amounts of K than rye, ryegrass, barley, and oilseed rape. The differences in K accumulation correlated with root hair length (R-2 = 0.50). Rye had an outstanding root surface that in total as well as per unit root dry matter was twice that of the other crops. The ranking in decreasing order was rye, ryegrass, oilseed rape, lucerne, barley, pea, and red clover. This ranking was unaffected (P > 0.05) by the initial soil K content although the root surface in all cases increased when grown in soil low on K compared to soils with a better K supply. This increase was achieved by increasing root hair length as root hair density was similar (P > 0.05) for all crops and soils. Efficient use of sparingly soluble K pools was important for all investigated crops. In conclusion, crops modify their root hair length as response to low K conditions and thereby maintain the uptake from sparingly soluble K sources.
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