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Fluid Journal : Fluid Journal 1996-1998
2 Fluid Journal Fall 1996 Figure 2. Chlorotic spots clustered near margins of older leaves. Figure 3. White chlorosis at the margin ofleaf and local- ized toward tip. Figure 4. Phosphorus deficiency in alfalfa. Note weeds invading deficient field due to thinner stand and less plant vigor. of small chlorotic spots (Figure 2) clustered near the margins of older leaves. As the severity of the deficiency intensifies, the number of chlorotic spots increases and individual spots become indistinguishable. The symptom first appears on older leaves. This classical symptom is widely recognized wherever alfalfa is grown. A second kind of K deficiency symptom observed in western states is a white chlorosis. It appears at the margin of the leaf and is localized toward the tip (Figure 3). As the severity increases, the chlorosis expands toward the leaf base. The boundary line between normal and affected leaf tissue is sharp and irregular. The two deficiency symptoms are distinctly different. They are not found on the same plant, but on individual plants growing in the same area. Plants showing no symptoms at all can also be found in this area. The soils are typically low in available K and do not have excessive salt or sodium (Na). Chlorotic margin symptom is also associated with a high tissue content of Na. Apparently when K is low, which results in less competition between K and other cations for plant uptake, some plants will take up large quantities of Na. The chlorotic margin symptom may be either an actual Na toxicity or simply salt burn. Genetic diversity of alfalfa may be the fundamental reason individual plants exhibit different symptoms, even though growing side by side. A carefully managed fertilizer pro-gram will avoid this problem. Proper applications of potassium will eliminate chlorosis. Plant tissue K contents will be increased and excess Na in the tissue decreased. The end result will be: • increased yield • more protein content • less disease • increased stand life. Soil testing is a useful tool in identifying potential deficiencies of K that can lead to chlorosis. Heavy initial K applications may be required to restore the K levels of a depleted soil. Eventually, lower rates to maintain desirable fertility levels should suffice. Hidden hunger Phosphorus-deficient alfalfa is difficult to spot. The plants do not show obvious symptoms. Yet loss of yield can be dramatic. A grower may not even be aware there is a problem. Plants deficient in P often appear water-stressed. Leaves tend toward dark green or even purplish and are smaller than normal. Leaflets are cupped upward. Plants are stunted, but appear more upright than normal with fewer stems per crown. Figure 4 shows P-deficient alfalfa. Note the weeds invading the deficient field due to thinner stand and less plant vigor. Leaves are smaller, although coloration appears near normal. Because phosphorus moves very little in the soil, it is important to maintain an available supply in the
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