Foliar Applications of Phosphorus

In general P deficient soils require preplant broadcast-incorporated rates of 20 to 40 lb P₂O₅/ac or (10 to 20 kg P/ha) to correct the deficiency in either wheat or corn.  At a P use efficiency of 16%, this addition results in only 1.6 to 3.2 kg P taken up in the grain.  Although the literature does not provide information on relative efficiencies (soil applied versus foliar applied P), intuitively one would expect foliar applied P to be much higher.  We believe that the small amounts required to correct deficiencies can be easily introduced to the plant by a foliar P application.  This approach has been overlooked for decades because it was assumed that the amounts of fertilizer P required by the crop were too great to be satisfied by a single foliar application.  That assumption was easily accepted when P fertilizers were first used because soil deficiencies tended to be greater than today and solution fertilizers were uncommon.

Conventional P-soil test correlation utilizes knowledge that soil deficiencies may be represented as a percentage of the maximum yield when there is no P deficiency (Mitscherlich-Sufficiency Concept).  Consequently, soil test calibrations resulted that identified amounts of fertilizer-P (rates) required to correct the plant deficiency for a season, but which had little immediate affect on long-term available soil-P.  This is appropriate for soil-applied P as rates do not need to be adjusted for yield level.  However, rates of foliar P, need to address uptake deficiencies of the plant, which are influenced both by potential yield (biomass) and available soil-P.  Since our work to identify spatial needs for fertilizer-N includes estimates of potential yield, one aspect of addressing spatial variability of crop–P needs will be to apply some foliar-P in direct relation to potential yield.

Wheat Yield           Total N in grain          Total P in grain         P rate, P index=20

bu/ac                        lb/ac                               lb/ac                             lb P₂O₅/ac
30                             45                                   4.5                                40                  17.4 lb P/ac

45                             61                                   6.1                                40

60                             72                                   7.2                                40

2.50% N (30 bu/ac)
2.25% N (45 bu/ac)
2.00% N (60 bu/ac)

Wheat Yld          Total N, grain        N Rec       NUE,                Total P, grain    PUE

bu/ac                   lb/ac                         lb/ac          % (1/2 soil)      lb/ac                    % (1/5 soil)
30                        45                             60             38                     4.5                       20
45                        61                             90             34                     6.1                       28
60                        72                             120           30                     7.2                       33

Wheat Yld          Total P, grain        Increase Needed Foliar P, 50%                                                                      Foliar P, 40%

bu/ac                   lb P/ac                     lb P/ac               lb P/ac                  lb P/ac
30                        4.5                            -                          -                            -
45                        6.1                            +1.6                   3.2                        4.0
60                        7.2                            +2.7                   5.4                        6.8

Wheat Yld          Foliar P, 50%        Soil Applied    Savings

bu/ac                   lb P/ac                     lb P/ac               %  
30                        -                                -
45                        3.2                            17.4                   543%
60                        5.4                            17.4                   322%

RESPONSE Index

1. Fall N fertilization:  40-50 lb N/ac grain production only

2. Fall N fertilization STRIP:  80-100 lb N/ac (non-N-limiting)

Winter N fertilization:  Decision made based on the difference between 1 and 2

Average soil test P index (Winter Wheat, 2000 = 44)  = 22 ppm  at a P index of 40, recommend 20 lb P₂O₅/ac