The current guidelines and recommendations for P fertilization in Uruguay can be improved by adjusting the sampling depth and analytical methods of testing soil P in pastures with different fertilization history. A set of field trials was conducted between 2008 and 2012 on 14 sites of Uruguay for the purpose of comparing two sampling depths and three methods for assessing P availability by their correlation with annual dry matter yield response. The trials had a randomized complete block design and were sown with Trifolium repens or Lotus corniculatus. Soil P availability was determined by sampling 0-7.5 and 0-15 cm depth using three analytical methods: Bray I, cationic resins, and citric acid. Rock phosphate and triple superphosphate were applied at five P rates. (0-240 kg kg P2O5 ha-1). Annual forage yield was recorded. Relative yield was calculated as measured yield/maximum yield observed within a block. The correlation between relative yield and soil P availability was studied using the modified arcsine-logarithm calibration curve (ALCC) and analyzing the resulting correlation coefficient, root-mean-square error (RMSE) of the soil P test, and RMSE of the relative yield. The 0-7.5 cm soil sampling depth did not show a better fit than the deeper sampling (0-15 cm), with the latter having less variation in soil P test values. When rock phosphate was used, the correlation coefficients were 0.50 for cationic resins, 0.53 for citric acid, and 0.38 for Bray I. When triple superphosphate was used, the correlation coefficients were 0.37 for cationic resins, 0.44 for citric acid and 0.43 for Bray I. For both P sources, the citric acid method with the sampling of 0-15 cm soil depth is the preferred soil P test method for management of P in leguminous pastures for the soils studied.

The intensification of rice–livestock systems has encouraged the integration of forage species into rice cultivation. However, the use of imidazolinone-tolerant rice cultivars (imazapyr + imazapic, IMIs), essential for weedy rice control, may hinder the establishment of sensitive forages due to herbicide residues. This study evaluated the emergence and early growth of six forage species through bioassays in soils with different IMI use histories. Over three years (2021–2023), soils were collected from three rice-growing regions in Uruguay with contrasting soil properties, at depths of 0–15 and 16–30 cm, under three IMI conditions: Control (no prior use), IMI-1 (applied one season before sampling), and IMI-2 (applied in the season before sampling). Emergence, plant height, and shoot and root biomass were analyzed using linear mixed models and principal component analysis. Shoot biomass was reduced by up to 60% in IMI-2 soils. Poaceae species and T. pratense were the most affected, while T. repens and L. corniculatus exhibited higher tolerance. Multivariate analysis indicated stronger residual effects on sandy loam soils. These findings highlight that imidazolinone persistence varies with forage species and soil properties, underscoring the need for careful planning in rice–pasture rotations to prevent adverse effects on forage establishment and system sustainability.

Phosphorus (P) deficiency limits plant growth. Suboptimal or excessive P fertilization of crops and pastures implies unwanted economic and environmental impacts. Critical P concentration (Pc) dilution curves can be used to diagnose P status. However, P and nitrogen (N) metabolisms are closely interrelated, and it is unknown whether Pc is affected by N deficiency. This study evaluated the effect of N deficiency on Pc dilution curves and then assessed the usefulness of the N:P ratio to distinguish between N vs. P deficiency vs. N and P co-limitation. For this purpose, a factorial combination of several N and P fertilization rates was applied in nine field experiments conducted during winter and spring, on annual ryegrass (Lolium multiflorum) and tall fescue (L. arundinaceum (Schreb.) Darbysh) pastures, at different sites in Uruguay and Argentina. Across all sites, N deficiency did not alter Pc. However, Pc values did differ among sites. This was due to variations in the scale parameter, not the dilution parameter. Combining Pc and Nc curves revealed that the critical N:P ratio decreased as shoot biomass increased, given that N was diluted more rapidly than P. In general, the N:P ratio had low accuracy to discriminate between N and P deficiency. Instead, at low shoot biomass, using individual [N] and [P] values predicted better plant nutritional status. These results support the use of reference Pc dilution curves under contrasting N nutritional conditions and indicate that [N] and [P] (not N:P ratios) could yield useful inferences whenever shoot biomass data is unavailable.