Get the most from early spring nitrogen

Getting the best response from every kilogram of nitrogen (N) fertiliser applied is more important than ever, and sulphur (S) is part of the solution.

Nitrogen is the main nutrient applied in creating additional early spring pasture growth, but this response may be impacted by a shortage of plant available sulphate S which is often low at that time of year.

For pastures to take up N, they need adequate sulphate S, so a pasture deficient in S will limit the response to the applied N fertiliser.

Why is sulphate S low in spring?

Sulphur is often lacking in New Zealand soils, especially the sulphate S in early spring when it is most needed.

The winter's wet, cold conditions affect the soil supply of sulphate S when:

• it leaches, the degree will depend on amount of rainfall (how long the soil is saturated) and the soil type (more leaching in coarse soils such as sand and pumice than ash or sedimentary soils).
• cold temperatures slow down the activity of bacteria responsible for converting non-plant available elemental S (provided by fertiliser) and organic S (found in soil) into plant available sulphate S.

By late winter and early spring, leaching and a slowdown in conversion take their toll, often resulting in a short supply of sulphate S for pasture.

Identifying and overcoming deficiencies

Pastures deficient in S appear pale green or yellow, with clovers affected first. Clover in particular needs S to efficiently fix N, and having less clover in a sward means the supply of N to other pasture species is also reduced.

Sulphur is relatively cheap and straightforward to apply with other nutrients, and overcoming early spring S deficiencies ensures spring pasture can take up N and flourish.

Spring herbage testing can be used to complement soil testing when deciding if sulphate S is required for a good pasture response to N application (see table 1).

Nitrogen and sulphur options?

When choosing products, keep in mind why and when N and the type of S is needed and applied.

For spring, if a N and S combination is required, two options are the SustaiN Ammo range and PhaSedN Quick Start (see table 2).

SustaiN Ammo products provide the ratio of N and S pasture requires. All the S content is sulphate S, readily available for plant uptake. The inclusion of SustaiN reduces the amount of N lost as ammonia gas and increases N efficiency. The SustaiN also allows more flexibility in timing of application, as it does not need at least 10 mm of rain or irrigation water within 8 hours to reduce losses as ammonia gas.

On soils prone to S leaching, such as pumice, podzols, organic and South Island Recent, PhaSedN Quick Start combines PhaSedN with sulphate of ammonia, providing both elemental and sulphate S and N.

Table 1: S content of spring mixed pasture samples and impact on N response

Table 2: Sulphur and nitrogen content and forms for SustaiN Ammo and PhaSedN Quick Start

Note: For an effective N response applied in late winter, early spring, the soil temperature needs to be 6 degrees and rising.

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Article supplied by Ballance Agri-Nutrients.