Use of geophysical survey as a predictor of the edaphic properties variability in soils used for livestock production

Mapping of the apparent soil electrical conductivity (ECa) can be used to estimate the variability of forage yield within a plot. However, forage production can vary according to the growing season and to soil properties that do not affect the ECa (e.g. nitrogen (N) content). The aim of this study was to assess the relationship between ECa and forage yield of tall fescue (Lolium arundinaceum (Schreb.) Darbysh.) during different regrowth periods and contrasting levels of N availability and then use this information to determine potential management zones. The ECa was measured and geo-referenced in a 5.75-ha paddock that sustained a permanent pasture dominated by tall fescue. In addition, a 30 m by 30 m grid cell size was chosen and 43 sampling areas, each 4 m2 in size, were geo-referenced and divided into two experimental units of 1 m by 2 m, one of which was fertilised with 250 kg N ha–1 (N250) at the beginning of four regrowth periods (spring 2015, spring 2016, autumn 2016 and autumn 2017) and the other was not fertilised with N (N0). At the end of each regrowth period, we estimated the accumulated biomass. During the spring growing season, accumulated biomass was positively associated with ECa in both N0 and N250 treatments (R2 = 47% and 54%, respectively). By contrast, in autumn, accumulated biomass and ECa were poorly associated (R2 = 10% and 27% for N0 and N250). This may be due to seasonal interactions that alter soil–yield relationships. To assess whether ECa can be used to determine management zones, the differences in accumulated biomass were compared through analysis of variance. Results showed that ECa is associated with the spatial distribution of tall fescue forage yield variability in spring at different N availabilities. Thus, ECa can be reliably used for defining management zones in marginal soils under permanent pastures.

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.