2003 IRISH SCIENTIST YEAR BOOK

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University of Limerick
Cecily Leonard
Does farm grassland fertility depend on fertiliser inputs?

Agriculture relies heavily on fertilisers: however, because fertilisers may contaminate drinking water, Irish farmers apply less nitrogen nowadays. The research described here was carried out on fifteen grassland farms in Counties Limerick and Clare, and focuses on how grassland soil works in the context of reduced fertiliser inputs.



Factors that influenced grassland production, graphed together. This 'principal components' diagram took the jumble of data describing fifteen farms and their environments, and flattened it to expose the most influential factors. An arrow points towards where each named influence was strongest amongst the farms studied. Together, influences separated productive, average and unproductive farm groups.

Mineral nitrogen (N) is not applied to organic grassland. Instead, organic N inputs arise from biological mineralisation of nitrogen from legumes, and application of manures and slurries. Organically managed soil should, therefore, be more biologically active than conventional. Instead, we found that soils without fertiliser inputs, organic or mineral, were more biologically active and mineralised nitrogen more rapidly than fertilised soils. Furthermore, nitrogen mineralisation was most rapid in the least productive farms studied.

So, what drives grassland production? In the study, soil pH, pasture renewal, fertiliser N additions, soil depth, density of rooting (root mass), and nitrogen mineralisation activity were shown to influence production, but none explained outputs fully. Instead, when all the influential factors were graphed together (see Figure), farms became separated into productive (green), average (yellow) and less productive (red) groups, showing that grassland production is accounted for by all these factors, working together.

While N-fertilised farms were generally more productive, the organically-managed farm L produced more grass than the conventionally-managed farm Q receiving 230 kg N/ha/yr. So, although nitrogen inputs help, they are not the sole production determinant. Soil pH is important, as is the age of the pasture, and changing these without necessarily adding more N alone may improve soil fertility. Findings suggest that moderate N inputs, and moderate N mineralisation rates, serve both production and environmental purposes.

However, to optimise production while reducing N inputs, we need to understand how to manage the biological supply of N to grasses in the sward. Grassland management effects on soil biological activities are being investigated at the University of Limerick with Teagasc and EPA (Environmental Protection Agency) assistance.

Contact: Dr. Cecily Leonard, Dept. of Life Sciences, University of Limerick; E-mail: [email protected]