legumes for light and water. Under these conditions grasses dominate the pasture and legumes persist poorly.
To determine the effect of N on N fixation, research was conducted in New Zealand on a perennial ryegrass-white clover pasture. Nitrogen was applied at 0, 200 and 400 kg N/ha per annum. It was found that under 0, 200 and 400 kg N/ha per annum, N fixation decreased from 154 to 99 and 39 kg N/ha per annum, respectively. It was estimated that with every kilogram of N applied, N fixation decreased by 0.27 kg. The study concluded that N rates of up to 200 kg N/ha per annum can be beneficial for pasture production as well as N fixation. Even though it is not clear whether this is directly applicable under South African conditions, a similar reaction can be expected.
Legume persistence
When legumes are established as a component in a grass-legume pasture, stress factors will have a larger negative effect on the persistence of the legume than on the grass component. In other words, the grasses possess a greater competitive ability under adverse conditions than the legumes. Legume persistence is directly linked to N fixation. Therefore, stress factors like deficient soil moisture levels, soil acidity, insect pests and diseases will decrease legume persistence, as well as N fixation. Optimal environmental conditions and management practices should therefore be maintained to ensure optimum legume persistence and N fixation.
Common stress factors that can easily be avoided include soil moisture stress and soil acidity.
Soil moisture stress – Soil moisture stress influences the persistence of shallow rooting legumes like white clover more than deep rooting legumes such as lucerne and red clover. Nitrogen fixation of a perennial ryegrass-white clover pasture can decrease by 60% if subjected to a water stressed season. Therefore, optimal irrigation practises are critical to ensure N fixation and legume persistence.
Soil acidity – At a soil pH (KCl) below 5, Aluminium (Al) concentration increases and Al toxicity threatens legume growth. Aluminium inhibits root branching as well as root growth. The persistence and N fixation ability of most legumes decrease in acidic soils, therefore soil pH (KCl) must be maintained between 5.5 and 6.5.
Take home message
For the legume component to release a significant amount of N for grass uptake, pasture should be established for at least two years and managed so as to maintain legume persistence.
High N fertilisation rates will decrease legume persistence and N fixation. However strategic N applications can be beneficial.
Stress factors, such as poor soil conditions or disease, will decrease legume persistence as well as N fixation. Optimum management practices are therefore essential in ensuring beneficial effects of a legume component in pasture.
10
OPINION
Grassroots
August 2017
Vol. 17, No.3
Cli
Chair
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Figure 1: Kikuyu-perennial ryegrass and clover pasture
Figure 2: Kikuyu-perennial ryegrass-white clover pasture