Sulfur deficiency
Nitrogen (N)
Nitrogen is essential to the plant for protein synthesis, and is generally supplied by root nodule bacteria (Rhizobium). It is recommended that seed be inoculated at sowing to ensure that the correct, and most active, strain of nodule bacteria (group AL) will be present in the soil.
Nitrogen reserves can, at times, become depleted. This is generally a result of poor root nodulation or inactive root nodules. Factors commonly affecting nodulation or nodule activity are trace element deficiency, stress periods associated with disease, cold weather or very wet conditions.
Responses to nitrogen fertiliser can be obtained under these circumstances, especially from late autumn to early spring. Active root nodulation is the primary means of supplying nitrogen to lucerne plants. The application of nitrogen fertilisers will reduce the efficiency of atmospheric nitrogen fixation by rhizobia.
Root nodules are functioning effectively if their internal colour is pink to reddish. When the internal colour of nodules is creamy to white, or if they are rotten, they are not supplying nitrogen to the host plant.
Nitrogen fertiliser is not recommended at establishment because it tends to promote grass weeds at the expense of the lucerne.
Symptoms of nitrogen deficiency in lucerne plants include premature yellowing, especially in older leaves, which then die off. Plants are spindly and smaller than normal. In the early stages of N deficiency, a pink colour appears on the petioles of the oldest leaves and continues up the midrib on the underside of the leaflets.
Inadequate phosphorus
Phosphorus (P)
Adequate phosphorus is essential, particularly for lucerne root development.
Symptoms of phosphorus deficiency include light brown roots restricted in growth. Top growth is stunted, stiff and erect, with leaves small and abnormally dark or bluish green. Leaflets often fold together and the undersides and stems may be red or purplish.
Sulfur (S)
Sulfur is necessary for protein synthesis in the plant and deficiency symptoms of general yellowing of younger leaves should not be confused with those of N deficiency.
The fertiliser superphosphate is a source of both P and S to plants, and this fertiliser is often used to satisfy the needs for both elements to lucerne stands. Shortages of sulfur are more common than shortages of any other element. Where P is not limiting and only S is deficient, the cheapest form of S is gypsum.
Gypsum is used as a soil ‘conditioner’ at rates of 2.5 t/ha or more, to restore friability, internal drainage, moisture-holding capacity and ‘workability’. Additional sulfur will not be required in these circumstances. When used only as a source of sulfur, much lower rates, e.g. 200 kg/ha, are used. Not all grades of gypsum can be satisfactorily spread at these low rates. Consult a fertiliser agent for advice before ordering.
Symptoms of sulfur deficiency are similar to those described for nitrogen deficiency, although the yellowing is more evident on younger leaves.
Calcium (Ca)
In most neutral-to-alkaline soils, calcium is not a limiting element. However, in more acidic soils, the addition of this Ca is necessary to raise the pH. This is best achieved with lime, although dolomite can be used if it is cheaper or if soil magnesium levels are also low.
Symptoms of calcium deficiency include collapsing of the stalks of the youngest fully developed leaves. Leaves curl up and die.
Potassium
Potassium deficiency
Because of the large uptake of potassium by new lucerne growth, and the high tonnages of hay harvested each year, continuous cropping will eventually lead to shortages of potassium. This occurs most frequently in lucerne stands on lighter sandy soils that are naturally low in potassium or under irrigation where potassium removal rate is high (see Table 6.1 in the article Lucerne Nutrition and Fertiliser Management).
If in doubt about the requirement for potassium, apply test strips at rates of 50 kg/ha and 100 kg/ha. Use muriate of potash at 100 kg/ha and 200 kg/ha respectively as the potassium source, but only in test strips. Use other forms of potassium (e.g. sulfate of potash) when potassium deficiency has been confirmed by soil and tissue testing, and strip trials.
Recent research at Tamworth in potassium-deficient soils has demonstrated reduced leaf disease incidence in lucerne when it was adequately supplied with potassium. This demonstrates the inportance of adequate lucerne crop nutrition not only on yield, but also on leaf retention and, therefore, hay quality.
Symptoms of potassium deficiency in lucerne are small white spots or freckles at the margins of the lower leaflets. As the deficiency progresses the areas between the spots turn yellow and die and the spotting progresses up the plant.
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