The sugarcane area expansion is promoting peanut production where the peanut is planted during the sugarcane reform in soils with low boron (B) content. This study aimed to monitor: (i) the efficiency of B application via soil and foliar on peanut yield parameters; (ii) the right B rate and source to increase peanut production; and (iii) the B leaching in the tropical soil. Methods: Two experiments were run in an area of sugarcane reform in São Paulo, Brazil. Study 1 applied foliar B (Bm, source: monoethanolamine) using rates from 250 to 1500 g ha−1, while study 2 applied soil B (Boct, source: sodium octaborate; rates) using rates of 0.5 (Boct0.5) to 1.0 kg ha−1 (Boct1.0), associated with foliar B (Bba, boric acid) with rates from 250 to 1500 g ha−1. Peanut yield parameters were measured, and an incubation study monitored soil B leaching. In both studies, isolated foliar Bba and Bm rates improved linearly the 100-grains yield of peanuts with an average of 74 g, while foliar Bba rates increased the grain yield. When there was an association between soil and foliar B, the optimal rates of Bba ranged from 700 to 900 g ha−1 at Boct0.5, while the optimal rates ranged from 400 to 700 g ha−1 at Boct1.0. Soil B leaching was lower in Boct < Bm < Bba with 19%, 24%, and 22% of leached-B, respectively. All sources increased leaf B indicating the B was absorbed by plants with a positive effect on leaf contents of calcium, potassium, nitrogen, and phosphorus. Based on the results, we conclude that the association of Bba (foliar) + Boct (soil) was shown to be the better alternative for increasing the peanut yield parameters and nutrient balance in leaves.
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