The importance of balanced nutrition in winter crops

Phosphorus application in the 1960s was mainly by super phosphate, which supplied sulphur in excess to crop needs, building soil sulphur levels. The 1980’s saw the introduction of high analysis fertilisers, containing little amounts of sulphur and the switch to canola production. Soils were mined of sulphur and therefore S deficiencies appeared.

Blanket recommendations of 20-30 Kg/ha S were implemented for canola production and this has been followed for many years.

However, this has been questioned in recent years as high application rates of sulphur have built up the adequacy levels of our soils. Applications of sulphur and nitrogen need to be balanced in sound crop nutritional programs.

Nitrogen commands a lot of attention when fertiliser programs are being developed. Targeted application rates are calculated by taking into account available soil nitrogen from soil testing, yield and protein goals and estimated mineralisable nitrogen. Using this approach for sulphur to develop balanced crop nutritional programs would help appropriate rates of sulphur to be selected.

Sulphur is an essential nutrient for all plants and together with nitrogen is a vital building block in proteins and there is a quantitative relationship between the two. The ratio of nitrogen to sulphur is particularly important in bread and durum wheat because low grain sulphur or nitrogen can significantly reduce dough quality (elasticity).

The ratio of nitrogen to sulphur in wheat/ durum grain is approximately 15 to 1, with the relationship for canola nearer to 7 to 1. If crops are deficient, they cannot make full use of applied nitrogen; resulting in both yield and quality suffering.

Also, crops with nitrogen/ sulphur imbalances appear to be more susceptible to diseases, because the nitrate and sugar levels in the plant are raised by the inability of the plant to synthesis some amino acids, providing ideal conditions for spreading fungal diseases.

Based on the ratios found in proteins, if a cereal crop has a total nitrogen requirement of 200-250 kg/ha there will be a total requirement for 16-20 kg of sulphur or in the case of canola, a total nitrogen requirement of 150-200 kg/ha will need 30-40kg of sulphur from soil and fertiliser.

As with nitrogen, crop sulphur requirements can be supplied by available sulphur in the soil profile, mineralisation of organic matter in the soil and from fertilisers applications. The quantity of sulphur released from mineralisation can vary greatly and depends on; seasonal rainfall, crop rotation, tillage type and organic matter levels in soils. The remainder of a crop's sulphur requirement must be supplied by fertilisers to satisfy crop requirements.

Sulphate sulphur is mobile and leaching can be a problem in lighter textured soils. Sulphate leaching causes soil acidification and loss of magnesium, calcium or potassium. Soil samples should be taken from appropriate depths to measure the plants available sulphur and allow targeted fertiliser rates to be calculated. Application rates in excess of crop requirements can increase sulphur leaching.