Herbage Testing Can Refine Fertiliser Choice

Herbage Testing Can Refine Fertiliser Choice

Farmers trying to work out their precise fertiliser needs for spring growth can increase the accuracy of their analysis by including herbage testing in the mix.

Aaron Stafford, Technical Adviser at Ballance Agri-Nutrients, says herbage testing is often overlooked yet it is a valuable additional tool to be used alongside soil testing in determining what nutrients should be applied.

‘Generally, urea is accepted as the most cost-effective general-purpose N-fertiliser available, but our research indicates that in some cases herbage testing can show you how to achieve a superior response,’ says Mr Stafford.

He says urea may not be the best choice if herbage testing indicates that a deficiency of other nutrients such as sulphur (S) may limit plant N-response.

‘In some cases herbage testing can help farmers make the right decision as to whether to apply sulphate of ammonia (SOA), n-rich ammo (a blend of SOA and urea) or n-rich urea.

‘The decision depends on whether plant S-availability is a factor limiting N-response, and if so, by how much. Short-term sulphate deficiencies can occur following leaching of sulphate over winter, before soils have warmed in spring for sufficient S mineralisation to occur. S-limited N responses are most likely to occur in spring, when soils with low organic matter and low anion storage capacity are subjected to high winter rainfall.’

Recent research conducted for Ballance suggests that where a marginal S deficiency is apparent (eg soil sulphate levels of 5-8 ppm, herbage S levels of 0.25-0.3%), n-rich ammo may give stronger, more cost-effective N responses than urea.

Mr Stafford advises, however, that under these conditions the high rate of S in SOA is unlikely to be necessary, and combined with the much higher cost per kg of N in SOA, would make it unlikely that SOA could be justified on a cost-efficiency basis.

‘Where a strong S deficiency is apparent (eg soil sulphate levels <5 ppm, herbage S levels <0.25%) and the opportunity for follow-up S-fertiliser applications is limited, SOA may be an option to consider – especially for pumice soils, where S deficiencies tend to be more pronounced and often need higher rates of fertiliser-S to correct.

‘One last factor to consider is what previous fertiliser has been applied, as this may affect plant S availability and therefore N-fertiliser options.’

For example, in the Ballance research no S response was observed at one site where there was a soil sulphate level of 1 ppm. Herbage tests revealed that the pasture contained about 0.3% S, therefore S was not limiting growth.

Further investigation with the farmer revealed sulphur gain 30S had been applied in the previous autumn. As a result, the elemental S component would have been contributing to plant production (but would not show up in the soil test). In addition, there may have been sulphate deeper in the soil profile that the ryegrass-dominant pasture could have accessed that was missed by the standard soil test

Mr Stafford says Ballance is happy to arrange for herbage tests to help farmers maximise their outcomes from fertiliser use. Its Technical Sales Representatives can be contacted for assistance on 0800 222 090.

ENDS

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