Nutrient management

Nitrogen fertilisation strategies

White clover does not have the ability to fix N for the first 12 to 18 months post-sowing. Nitrogen fertiliser is important during this period to encourage growth and development. White clover needs a higher soil temperature for growth than grass. Grass starts growing at soil temperatures of 5-6°C while white clover needs soil temperature of 8°C. As a result, the contribution of white clover to the sward in early spring is low. As our systems have a requirement for pasture to feed animals in early spring, N fertiliser must be applied at similar rates to those used in grass-only swards. In the summer, when sward white clover content is increasing (≥ 25%), N fertiliser application can be reduced. Some N fertiliser is required in autumn so N should be applied in late-August/early September (before 15th September) to ensure pasture availability in the autumn. Suggested N application strategies for grass-white clover swards with average annual white clover content of 20% or greater are shown in the tables below.

Macro- and micro-nutrients

White clover has a poor competitive ability to absorb most soil nutrients compared to grass due to the different characteristics of the root systems of the two species. Perennial ryegrass has a denser longer, thinner and more finely branched root system compared to white clover. As a result, white clover is generally at a disadvantage when it comes to nutrient uptake from the soil.

Soil pH is critical for white clover development. White clover is more sensitive to lower soil pH than grass.

• Soil pH should be greater than 6.3.
• Low soil pH reduces soil nutrient availability for plant growth
• Low pH soils can be deficient in plant available calcium (Ca) and magnesium (Mg) which are necessary for rhizobia (N fixing bacteria) survival.
• In low pH soils manganese (Mn) and aluminium (Al) toxicity can have a major effect on white clover development.
• Formation of nodules for N fixation is reduced below soil pH 5.8.

White clover requires a minimum of soil Index 3 for Phosphorus (5.1 – 8 mg/l) and potassium (101 – 150 mg/l). Soil P and K availability are important for BNF which is reduced when there is a P and/or K deficiency. Grasses tend to dominate K deficient soils resulting in low sward white clover content.

In sulphur (S) deficient soils white clover growth and BNF will be reduced. Other soil nutrients such as calcium (Ca), sodium (Na) and molybdenum (Mo) are important for white clover growth. Cobalt (Co), boron (B), Ca Mo, iron (Fe), Ca and copper (Cu) are important for nodule development and function. A summary of the relevant importance of macro- and micro-nutrients is grass- white clover swards is presented in the table below.