How much sulfur is enough?

When it comes to sulfur, crops are not equal.

For some crops, soil supply can be sufficient while severe losses in quantity and yield are to be expected for others without appropriate sulfur fertilization. Sulfur fertilization is often quesswork. How much is enough?

Sulfur needs

Some crops need more sulfur than others. The table above summarizes crop needs as well as the amount of sulfur exported from the field and retained by crop recidues. The higher the sulfur demand, the higher the sensitivity to deficiency. Rapeseed has a very high uptake, but most of the sulfur remains in plant residues.

Uptake dynamics

Crop S uptake and content in harvested product and residues compared to fertilizer recommendations.

Cumulated uptake is only one aspect of sulfur needs. Uptake dynamics are the other important aspect. Crops with a short vegetative period need high amounts of sulfur in a short time. Plants with a longer vegetation cycle have more time to recover sulfur from the soil and are therefore less dependant on external supply. Rapeseed is specifically demanding with regard to sulfur, due to its short vegetation cycle and high uptake. Sulfur deficiency can therefore cause yield losses of up to 1 or 2 t/ha.

Soil sampling

As with nitrogen, soil sampling enables quantification of actually available sulfate in the soil. Results, however, change rapidly due to lower plant uptake, continues mineralization, capillary rise and leaching. Soil sampling is therefore rarely used except for rich soils and demanding crops such as rapeseed.

Plant analysis

The amount of sulfur in dry matter is a reliable indicator for sulfur deficiency. Sulfur concentrations shall exceed 0,3% of dry matter for most crops and 0,45% for rapeseed. Plant analysis in addition enables measuring N:S ratios. These are meaningful indicators for most crops. The Yara Megalab service offers fast and reliable analysis to precisely tailor fertilization rates.

Estimation charts

Sulfur requirements can also be estimated by means of observation. Charts and software tools have been developed to help farmers evaluate various parameters such as soil structure, weather conditions, crop systems and previous fertilization. These estimations have proven good reliability under real farming conditions.

During the growing season, sulfur needs for rapeseed and wheat exceed by far the S supplied by soil. In May, typically half of sulfur needs are missing for wheat and even two thirds for rapeseed.