Performance Story: Crop Response to Foliar Applied Phosphorus Fertilizer
Dr. Jeff Schoenau, University of Saskatchewan
As agricultural crop yield potentials 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 that 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. There has been little recent research on the response of crops and soils to foliar applications of phosphorus. 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 controlled environment (growth chamber) and field trials, P fertilization treatments received equivalent P fertilizer rates of 20 kg P2O5 ha-1, with varying proportions 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 at canopy closure prior to anthesis in controlled environment studies conducted with two soils (Echo and Krydor soil associations), and field studies with four soils (Echo, Krydor, Sutherland and Weyburn soil associations) in 2016 and 2017. Measurements were made of crop grain and straw yield, P uptake, grain zinc, iron and phytate content (human nutritional value), and potential export of P from the soil surface in simulated snowmelt run-off treatments imposed after harvest.
Of the three crops, canola was the most responsive to 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 leaf material was observed when P was applied in foliar form, but foliar P application did not effectively balance off the yield lost by significantly reduced rates of seed-placed MAP fertilizer. These findings point to the importance of having P available in the soil for uptake by the young seedlings. 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 zinc, iron 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 was 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 a substitution for seed row applied P fertilizer. It may be most suitable for crops like canola where P demands are high and amounts applied at seeding in the seed row may be limited by seed-row safety concerns. Sequential applications of equal, low rates of foliar P at tillering, boot and anthesis stages to supplement seed placed P may be considered for future testing along with consideration of different foliar P formulations.