Soil properties, lime and phosphorus fertiliser management affecting grass production in Ireland

Abstract

Phosphorus (P) is a macro nutrient and is an essential element required by grasslands for optimum production and herbage P concentration for animal dietary requirements. However phosphorus is a finite resource and efforts must be made to reduce P needs in grassland to prolong its remaining lifespan. In Ireland current P fertiliser recommendations do not account for soil type differences, but large differences in fertiliser P responses can exist across differing soil types. The use of this one size fits all P fertiliser recommendation system can result in the depletion or over supply of P fertilisers compared to grassland P requirements.

Lime applications to grassland soils have been found to have both significant positive and negative effects on soil P availability over shorter and longer term studies. Given the finite nature of rock P and potential for P fixation with acid cations in the soil there is need for a greater understanding of the effects of lime and soil P interactions under Irish grassland soil conditions. When lime applications strategies are combined with applied P fertilisers this could result in decreased requirement for fertiliser P for grass production by significantly improving P assimilation by the grass plant.

Developing soil specific P fertiliser recommendations is one way of elongating P reserves. The aim of this project was to i) investigate the soil physical and chemical characteristics effecting soil P availability, ii) to determine the fate of chemical and organic P source fertilisers across differing grassland soils and iii) to aid in the development of new soil specific P fertiliser recommendations for Irish grassland farms.

To achieve these aims a series of experiments were designed and undertaken to evaluate the fate of P fertilisers across a range of the most important agricultural grassland soils in Ireland. A controlled soil - only incubation study was undertaken across 22 grassland soils to investigate the fate of applied P fertilisers (chemical and organic P) with and without lime applications. A simulated grass silage (3 cut) pot study was then conducted across the same range of soils under semi – natural environmental conditions for the development of a model on the effect of P source fertilisers also with and without lime incorporations on soil P and herbage dry matter yield. Finally an intensive simulated grazed grass (9 cut) field plot study across contrasting soil types was undertaken to determine the response to fertiliser P on soil P, herbage dry matter yield and herbage P concentration where trial plots received lime treatments based on previous lime applications.

The incubation study identified a number of soil physical and chemical properties effecting the change in soil test P from applied P fertilisers. In these mineral grassland soils clay and organic matter had significant negative correlations (P<0.0001) with the change in soil P levels (-0.46 & -0.42). While soil chemical properties aluminium and calcium displayed similar negative correlations with the change in soil test P (r = -0.33 & -0.37). Soil pH correction through lime addition was found to significantly increase soil test P (P><0.0001) (r = 0.48), while applications of organic P (dairy slurry) to low P and low pH soils showed greater returns in soil test P with 124% fertiliser P replacement value, when compared to chemical P additions in these incubated soils.><0.0001) with the change in soil P levels (-0.46 & -0.42). While soil chemical properties aluminium and calcium displayed similar negative correlations with the change in soil test P (r = -0.33 & -0.37). Soil pH correction through lime addition was found to significantly increase soil test P (P<0.0001) (r = 0.48), while applications of organic P (dairy slurry) to low P and low pH soils showed greater returns in soil test P with 124% fertiliser P replacement value, when compared to chemical P additions in these incubated soils.

Following on from this the grassland pot study showed a wide range of dry matter yield responses across a range of differing soil types. Predictive models for soil P and herbage dry matter yield were developed from the results of this trial to determine the factors effecting future changes in soil P and in the prediction of herbage dry matter yield. The models developed had a good fit to the data generated for the soil and grassland prediction models (R 2 = 0.76 & 0.71 respectively). Both models identified soil pH as the single biggest factor effecting soil P and herbage dry matter yield (DMY) across these grassland soils. These results indicate that an additional 20kg P ha-1 above current recommendations would be required to maintain soil P levels in these recently reseeded intensively managed mineral grassland soils.

Finally the results of the contrasting grassland field study found that soil pH management is a key determinant for increasing and maintaining soil P availability for optimum grassland production and in meeting herbage P concentrations. This trial found a significant increase in soil P (2.23 mg P L-1 ) from lime applications in the first year after lime application. It also found that P fertiliser applications ≥40 kg P ha-1 were required to meet herbage P concentration targets (3.5 mg P kg-1 ) and that lime applications in conjunction with adequate P fertiliser was necessary to sustain high DMY in the longer term.

This project and its findings will further develop our knowledge on soil P, herbage P and herbage DMY for the future development of soil specific P fertiliser recommendations in Irish grassland soils.