Phosphate fertilization strategies for soybean production after conversion of a degraded pastureland to a no-till cropping system

- The low soil-P availability is a concern when using degraded land for agriculture.
- P fertilization strategies were examined to a black oat-soybean cropping sequence.
- Soil-P status improved by establishing no-till and P fertilization in crop furrows.
- Source with high soluble P had the best effect on soybean yield in weathered soil.
- P application to the cover crop is a strategy that should be encouraged.

A considerable area of previously degraded pastureland in Brazil is appropriate for agricultural use, mostly for soybean cultivation. These degraded soils can be restored by establishing no-till systems which should protect the soil, enhance nutrient cycling, and be economically feasible. One of the main challenges when performing agricultural operations in these soils is soil phosphorus (P) status, due to the low levels and difficult management of this nutrient in highly weathered soils. This is mainly related to a lack of information regarding the best management strategy with respect to fertilizer sources and their placement and time of application. We hypothesized that applying P to the cover crop, rather than to the soybean furrow, is a better strategy for the production system, because it would improve P recycling and soil cover and guarantee better timing and operational efficiency in soybean sowing. A 4-year field trial examined the effect of P application (44 kg P ha− 1 year− 1) using various P sources (RPR - Reactive Phosphate Rock, TP - Thermophosphate and TSP - Triple Superphosphate) at different times (black oat furrow or soybean furrow), on a sandy loam Oxisol of a degraded pastureland in Southern Brazil. A black oat (Avena strigosa Schreb) - soybean (Glycine max L. Merrill) cropping sequence was used for 4 years after no-till establishment. Soil P content, plant nutrient uptake, black oat dry biomass yield, and soybean grain yield were assessed. Soil P content increased from 2.5 to 13.3 mg dm− 3 (0-0.20 m depth) on average across all times and sources of P application after 4 years. Different application times resulted in a residual P for black oat (when P was applied to soybean furrow) and soybean (when P was applied to black oat furrow). Soybean exhibited better use of residual P (based on nutrient uptake and grain yield) than black oat (based on effects on nutrient uptake and biomass yield). Black oat P uptake was positively correlated with soybean P uptake (r = 0.71, p < 0.01) and soybean yield (0.69, p < 0.01). The use of the TSP source led to a greater P-use efficiency, which was about 67 kg of black oat dry biomass for each kg of P applied to the black oat sowing furrow, and about 40 kg of soybean grain for each kg of P applied to the sowing furrow of black oat or soybean. Compared to TSP, less soluble phosphate sources (RPR and TP) led to a two to three times lower P-use efficiency. Results suggest that, on a previously degraded sandy loam Oxisol, P application to cover crop boosts P recycling and improves soybean yield by improving soil cover, and increases operational efficiency at sowing, even at low soil P levels. The use of water-soluble P sources should be preferred over less-soluble sources for the application on cover crop furrow.