Effect of Sulphur on Wheat Plant Growth
Sulphur is a building block of protein and a key ingredient in the formation of chlorophyll. Without adequate sulphur, crops cannot possibly reach their full potential in terms of yield or protein content. Nor can they make efficient use of nitrogen, phosphorus and other vital elements.
Sulphur Content and Requirement of Wheat Plants
Wheat plants have a lesser requirement for sulphur than oilseed crops such as canola. A 2690 kg/ha (40 bu/ac) wheat crop will contain in the seed and straw 12 kg/ha (10 lb/ac) of sulphur, of which only 40% is in the seed (Table 1).
Table 1. Sulphur requirements of wheat
Adapted from Simplot, 1986
Sulphur Deficiency Symptoms
Sulphur deficiency symptoms in cereal grains include yellowing of new leaves, in contrast to nitrogen deficiency which affects old leaves first. Plants will be spindly and growth will be slow. Deficiencies can cause a delay in maturity of3 to 10 days.
When crops are severely deficient these effects will be quite clear and large yield responses to sulphur fertilizer will usually occur. Visual symptoms may not be very clear when the crop is suffering from slight to moderate sulphur deficiencies and when the crop is young. Therefore, soil testing combined with tissue testing may be used to further confirm the deficiency.
Many soils in Alberta contain adequate sulphur for plant growth, however, a number of specific soil types are deficient in sulphur. The brown and dark brown soils in southern Alberta are generally not sulphur deficient. These soils tend to release sulphur from the breakdown of soil organic matter and have a higher content of inorganic sulphate particularly calcium sulphates in the subsoil. As a result wheat grown in these soil areas normally do not suffer from sulphur deficiency.
Sulphur deficiency can occur particularly on well drained, coarse to medium textured thin black and black soils. Sulphur deficiency is very common in Gray Wooded soils in both central Alberta and in the Peace River region.
The ratio of nitrogen to sulphur usually ranges from 10:1 to 7:1. Sulphur is slowly released from organic matter by microbial decomposition, and converted in the soil to a plant available form called sulphate. Sulphate is similar to nitrate-nitrogen in that both are soluble and will move readily with soil water. The plant available sulphur is not held firmly by soil clay and organic matter particles thereby making it subject to leaching below the root zone or loss in surface runoff waters. Soils low in organic matter or cold, wet soil with slow organic matter decomposition are prone to sulphur deficiency.
Another source of sulphur is in the form of inorganic sulphate such as gypsum (calcium sulphate) which accumulates in the subsoil at depths of 30-90 cm (12-36 in). The sulphur in gypsum is available to plants but this sulphur may not be reached by plant roots when the crop is young and sulphur deficiency may occur. However, if this sulphur is reached early in the growing season, which is generally the case for wheat in the brown and dark brown soils, the crop need for sulphur will be met.
Soil testing for sulphur will help identify soils which have insufficient available sulphur. It is important to note that plant available sulphur levels can be highly variable in some fields. Therefore, areas of fields that are well drained, course textured or low in organic matter should be soil sampled separately to ensure that sulphur or other nutrients are not deficient.
Sulphur Fertilizer Recommendations
General sulphur fertilizer recommendations are provided for wheat in Table 2.
Table 2. Sulphur fertilizer recommendations for wheat
* D=Dry, M=Moist, W=Wet
Seedbed soil moisture conditions at seeding D=25%; M=50%; W=75% of field capacity.
Wheat Yield Response to Applied Sulphur
The need for sulphur fertilizers is closely related to the amounts of nitrogen being applied to crops. The magnitude of response to sulphur increases with the rate of nitrogen added.
Sulphur is an essential component of amino acids which are the building blocks of protein. Sulphur deficient plants accumulate nitrogen as other compounds, (nitrate, ammonium, amides) therefore the quality of protein is reduced in such plants. Research has shown that flour milled from sulphur deficient wheat has poor milling and baking qualities.
The sulphate form of sulphur is immediately available to crops. Generally, only small amounts of sulphur, in the range of 10 to 15 kg/ha (9 to 14 lb/ac), are required for adequate wheat production. The best placement is either with the seed or side banded near the seed. Sulphate-sulphur is quite mobile within the soil, therefore broadcast or broadcast-incorporated application will likely be satisfactory under adequate moisture conditions. However, when dry conditions occur within the soil, even the soluble fertilizer can become stranded above the actively absorbing roots. Therefore, seed-placed or deep banded sulphur is generally superior.
Elemental sulphur is not plant available. Therefore, elemental sulphur must be oxidized to the sulphate form, by soil micro-organisms before it becomes available to plants. The conversion is most rapid when the elemental sulphur is broadcast and well incorporated. The conversion process is slow and may take three or more years for the majority of the elemental sulphur to convert to sulphate-sulphur. Therefore, the use of elemental sulphur should not be used to correct a sulphur deficiency in the year of application. The use of elemental sulphur should be used as a building program to increase sulphur levels over the longer term. Broadcast or various band applications of elemental sulphur are likely to produce poor results in the year of application due to the limited time for microbial oxidation to occur.
Time of application
If sulphate fertilizers are applied in the fall, a portion of the sulphate could leach downward as much as 50 cm (20in), depending on the amount of fall and spring precipitation and the texture of the soil. Therefore, sulphate fertilizer is usually most efficient when applied in the spring.
Elemental sulphur fertilizer applied in the spring of the crop year may not oxidize to sulphate sufficiently or early enough to supply the crop needs. Farmers who anticipate sulphur deficiencies and plan to utilize elemental S, should apply 10-20 lb/ac of elemental sulphur on an annual to biannual basis to avoid deficiencies.