Fertilisation of clover swards

The use of clover in grassland to replace fertilizer N can substantially lower greenhouse gas and ammonia emissions from pasture-based production. James Humphreys, Researcher and Dan Barrett, Farm Manager, outline Clover sward fertiliser management from 12 years research at Teagasc Solohead Farm.

Using Clover to replace Fertiliser N

The use of clover in grassland to replace fertilizer N can substantially lower greenhouse gas and ammonia emissions from pasture-based production. Averaged over 12 years at Solohead Research Farm we have found similar pasture production from grass-clover swards receiving of 95 kg/ha of fertilizer N applied in spring compared with swards receiving 245 kg/ha applied across the growing season (Figure 1). Clover has a higher temperature requirement for growth than grass and therefore we apply fertilizer N in February, March and April to compensate for this. Most of the N fixation by clover takes place during the summer and autumn and we have found little or no benefit from applying fertilizer N to well-managed grass-clover swards from May onwards.

Don't neglect P and K

Stopping applying fertilizer N can mean that P and K can get neglected during the summer and autumn. Clover is a shallow rooted species with around 15% of the root density of perennial ryegrass making it much less competitive for soil nutrients. Regular applications of a P & K compound fertilizer throughout the growing season are necessary for high levels of clover productivity and biological N fixation. This can be achieved by applying an N-containing compound fertilizer in spring; such as 24:2.5:10 or 18:6:12. Sulphur should be included in these compounds in sulphur deficient areas, particularly in April. Small regular applications of a non-N compound fertilizer are beneficial from May onwards; for example, a half bag of 0:7:30 per acre after every second grazing is very cost-effective.

Achieving Nitrogen fixation

Nitrogen fixation is a biological process that is regulated by soil pH (lime status). Optimum soil pH for biological N fixation is in the range 6.5 to 7.0, which is higher than is typically recommended for grassland (6.2 to 6.5). We aim to keep soil pH levels at around 6.5 at Solohead. This is achieved by regular soil testing and application of ground limestone. It is important to be aware that soil pH levels >6.2 are not recommended for high molybdenum soils. This does not mean that clover should not be grown on such soils; it means being careful about applying lime and ensuring livestock are adequately supplemented with copper. Your soil test results should indicate whether or not you are in a high molybdenum area.

Figure 1. Average daily pasture growth rates over 12 years on grass-clover swards receiving 95 kg/ha of fertilizer N in spring (green line) and swards receiving annual fertilizer N input of 245 kg/ha (blue line) at Solohead Research Farm

Find out more about the research work at Solohead Farm here