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Fertilizing sulphur sufficient soils

Apply a blanket application of S.


November 23, 2007
By Bruce Barker

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38Canola requires high levels of nitrogen (N) and sulphur (S) fertility to produce optimum yields.

And with today’s hybrids yielding even more, greater attention is required when deriving fertilizer recommendations. Previously, S fertilization was focussed on correcting S-deficient soils and was usually applied in a specific N:S ratio such as a 5:1 or 7:1. However, these recommendations do not take into account the ‘starting’ soil residual levels of either N or S, leaving the impression that a recommended ratio should be adhered to independent of the original soil test N and S values.

“Our research results indicate that the practice of balancing N and S to a fixed ratio is unnecessary and wasteful on canola grown on soils containing sufficient S,” says Rigas Karamanos, manager, agronomy, with Westco Fertilizers in Calgary, Alberta. Earlier Westco research had also found that it was not necessary on S-deficient soils.

A 17 site-year Westco trial from 1999 to 2001 compared yields between hybrid and conventional canola cultivars. All experiments were carried out on soils with very ‘low to moderate’ soil S levels. This trial found that hybrids had a yield advantage over conventional canola cultivars of 14 percent when the optimum N fertility regime for the conventional canola was reached (120lb/ac N). Karamanos says presumably this increase can be attributed to hybrid yield superiority due to genetics. However, application of an additional 40 pounds per acre N increased that advantage to 24 percent.

From an agronomic point of view, this trial showed that hybrid canola requires a much higher N regime, but it appears that it utilizes S soil reserves more efficiently than conventional canola, since both hybrids and conventional canola had access to the same levels of S fertility.

Subsequently, a 10 site-year Westco study was conducted between 2002 and 2004 to determine whether applying a specific N:S ratio is necessary for soils containing ‘sufficient’ S levels. Generally, a soil test level above 20 pounds per acre in the top 12 inches, or 40 pounds per acre in the top 24 inches, is considered sufficient. The practice was tested for both hybrid and open pollinated canola varieties by applying six rates of N (0, 36, 72, 108, 144 and 180 pounds per acre) and supplementing N rates with S, so that three N:S ratios (1.5, 6 and 12 to 1) were achieved.

Aim to satisfy individual N and S requirements
In the trial, Karamanos explains that seed yield, protein and oil content responses of both hybrid and open-pollinated canola varieties were obtained primarily with N, with no evidence for a need of balancing this nutrient with S additions in a specific ratio.  Application of fertilizer N and S in three distinct ratios (1.5:1, 6:1 and 12:1) overall had no impact on the seed yield, protein or oil content of either hybrid or open-pollinated varieties. However, conventional canola yields were restricted at the narrow N:S ratio (1.5:1) when N rate exceeded 108 pounds per acre N, suggesting that S was the limiting nutrient.

All soils in the second experiment contained S levels that are considered to be adequate, so application of fertilizer N and S in three distinct ratios (1.5:1, 6:1 and 12:1) had virtually no impact on the yield of hybrid canola, explains Karamanos.  Not surprisingly and similar to many other studies, hybrid canola yielded 23.7 percent more than open-pollinated varieties at the same N application rates (108lb/ac) where yields were maximized for the conventional canola. Application of an additional 27 pounds per acre N to obtain maximum hybrid yields though, only resulted in an additional 1.5 percent yield advantage for the open-pollinated variety.

Karamanos cautions that the increased hybrid yield also reduces N residual fertility, so that assessing residual soil fertility with a soil test on the canola stubble would be a highly beneficial practice.

38bBeware false soil test readings
Soil test S recommendations are often based on sampling to either 12 or 24 inches. Even when the soil test comes back sufficient, that does not mean the field is uniformly sufficient because the soil test reports an average (mean) of the sample. Many studies have shown that the level of crop available S can vary dramatically across the field.

For example, information from Alberta Agriculture and Food shows that sampling (zero to 24 inches) within a field near Stettler, Alberta, found sulphate levels varied from 18 to 12,660 pounds per acre with the average being 1176 pounds per acre. Based on a random composite sample, no fertilizer recommendation for S would be made for this field. However, the mode (the value of most frequent occurrence) was only 22 pounds per acre, indicating the field was deficient (40lb/ac for a 24 inch sample depth is generally regarded as sufficient). If the soil test results from the composite sample were followed in developing a fertilizer program, no S fertilizer would have been recommended and the majority of this field would be lacking adequate S.  Karamanos also cites a trial on a field near Rosetown where a grid soil sampling was conducted, and with S variability shown graphically. Despite appearing relatively flat and uniform,
S fertility varied tremendously across the field.

The other difficulty with soil tests is that if there is even a small amount of gypsum in the soil sample, the gypsum will overwhelm the results. However, gypsum will not significantly contribute to S-availability in the soil, so a false reading would be given.

Given these difficulties with predicting soil-S fertility, Karamanos recommends a blanket application of 10 pounds per acre of ammonium sulphate fertilizer for canola, even on soils testing sufficient.

Manitoba Agriculture’s Soil Fertility Guide supports that recommendation.   It says that “Sampling a variable field as a whole would typically result in a recommendation that no S fertilizer is needed, yet crops in some areas may be highly S-deficient. For this reason an ‘insurance application’ of S fertilizer may be advisable on variable soils or where high value, high S-demanding crops, such as canola, are to be grown.”  “Unless the soil test is off the scale, canola should be grown with S as part of the fertility package,” says Karamanos.

Local farmer and agronomist knowledge also becomes important when looking at soil test results. If a field is testing sufficient, but is showing signs of S-deficiency in parts of the field, then an S fertilizer trial could be tried to see if there is a response. However, based on the Westco trials, S fertilizer need not be applied in a specific ratio to N fertilizer.