RESPONSE OF CANOLA, WHEAT AND PEA TO FOLIAR PHOSPHORUS FERTILIZATION IN THREE SASKATCHEWAN SOIL ZONES

Abstract

As agricultural crop yields increase, greater amounts of phosphorus (P) are removed from soil in harvested plant material. As a result greater amounts of phosphorus fertilizers are required to maintain both crop yields and long-term soil fertility. However P fertilizer application practices must consider factors including high reactivity of P with soil constituents such as Ca which can render large proportions of soil applied P unavailable. As P is relatively immobile in the soil it must be placed near the seed for early crop access, but crops such as canola or pea are sensitive to injury from seed placed P. Foliar P fertilization can potentially address some of these limitations of soil applied P via the application of liquid P fertilizer to crop foliage, especially to address mid to late season P deficiency. This study evaluated the response (agronomic, nutritional, and environmental) to foliar mono-potassium phosphate (KH2PO4) fertilization of canola, pea and wheat grown in Brown, Dark Brown and Black soils in Saskatchewan. In a randomized complete block design (RCBD), each P fertilization treatment plot received equivalent P fertilizer rates of 20 kg P2O5 ha-1 with varying proportion of P applied as seed-placed mono-ammonium phosphate (MAP) versus foliar KH2PO4. The treatments were: 1) control with no added P; 2) 20 kg P2O5 ha-1 seed placed MAP; 3) 15 kg P2O5 ha-1 seed placed and 5 kg P2O5 ha-1 foliar applied; 4) 10 kg P2O5 ha-1 and 10 kg P2O5 ha-1 as seed placed and foliar applied P; 5) No seed-placed MAP with all 20 kg P2O5 ha-1 as foliar applied P. Foliar treatments were made prior to anthesis in controlled environment studies conducted with two soils (Echo and Krydor associations), and field studies with four soils (Echo, Krydor, Sutherland and Weyburn associations) in 2016 and 2017. Of the three crops, canola was the most responsive to foliar P fertilization in terms of yield and P uptake response, followed by wheat and pea. Pea showed little response to P fertilization in general, attributed to its ability to effectively scavenge soil reserves of P. Evidence of P uptake through canola and pea leaf material was observed, but foliar P application did not effectively balance off the yield lost by reduced rates of seed-placed MAP fertilizer. Foliar P fertilization at the rates applied in this study had limited effect on human nutritional value of the grain as assessed through effect on grain Zn, Fe and phytate concentrations. Furthermore, there were no large discernible impacts of proportion of P applied in foliar versus soil applied on the dissolved reactive inorganic P (DRP) measured in simulated snowmelt runoff from post-harvest soils in controlled environment and field studies. It is concluded that mid-season foliar P applications would be most suitable for a top up of P nutrition applied in small amounts under conditions of soil P deficiency rather than as a substitution for seed row applied P fertilizer. Foliar P fertilization may be most suitable for canola where P demands are high and amounts applied at seeding in the seed row may be limited by seed-row safety concerns.