Role of Sulfur and Deficiency Symptoms
Plants absorb sulfur as the sulfate (SO4-2) ion. Sulfur is a part of several amino acids, and is therefore a component of most proteins. Sulfur has a role in initiating protein synthesis and is a part of flavor compounds in mustard, garlic and onions. It is also a part of some vitamins and many enzymes.
Plants tissues can accumulate sulfate when it is available in excess of needs, but plants cannot translocate extra sulfate to younger tissue should soil sulfate supplies run short. The ratio of nitrogen to sulfur in plant tissue is generally about 10:1, although the ratio can vary somewhat. Total sulfur in the plant contains both sulfate and organic sulfur compounds. Plant analysis of sulfur content generally includes the combination of forms.
Sulfur deficiency is characterized by a yellowing of younger leaves, although this symptom can be complicated by soil availability of sulfur at different times of the year. In North Dakota, plants may be deficient early, with yellow leaves, then roots will reach gypsum deposits deeper in the soil and younger leaves may recover, producing total plant symptoms similar to nitrogen for a period of time.
Mustard family crops, especially canola, are particularly sensitive to soil sulfur levels. Canola seed contains up to five times the sulfur concentration of wheat and barley, and twice the total soil removal of sulfur as small grains at similar relative yields. Canadian canola growers have reported that sulfur deficiencies were rare until canola was raised. Canola sulfur deficiency symptoms include poor seed development. Deficiency also delays maturity, and produces mature pods on green stems, with poor seed development within the pod.
Sulfur deficiencies can be verified with plant tissue analysis, however, the timing of testing is important to catch the plant tissue in the stage of deficiency and not. Sulfur soil tests are not entirely reliable when deficiency is diagnosed, but sulfur soil levels are extremely variable, and high composite soil samples need to be viewed with suspicion, especially on high sulfur requiring crops such as canola and yellow mustard.
For canola, sulfur is recommended regardless of soil test level, unless past history or knowledge of the existence of high sulfate containing water tables is available.