Introduction
Phosphorus pollution of the Willamette River and its tributaries is a problem affecting agriculture in western Oregon. Many of our soils are high in available (soluble) P, as measured by the tests commonly used to determine plant-availability of P. Consequently, many streams have background levels of P that are conducive to algal blooms and poor water quality for fisheries and recreational use. Agriculture may also contribute to P pollution of streams through the use of large amounts of phosphate fertilizers and is under pressure to reduce P applications.
Growers of vegetables and certain other crops tend to use high levels of P fertilizer even on soils that test very high for available P, especially in early-season plantings. This use is supported by past research at OSU which indicates that stand establishment and early growth of seedlings is stimulated by banded applications of phosphate on soils which do not benefit from broadcast applications of P. Planting typically takes place when soils are cold and wet and P uptake is limited. Growers and water quality would both benefit greatly if a product were available which would provide soluble, mobile P but at rates greatly reduced from those commonly used in vegetable production. Biagro Western Sales, Inc. distributes a line of "Nutri-Phite" fertilizers based on phosphite rather than phosphate sources of P. The company has data indicating that these products provide a more plant-available source of P.
The purpose of this research was to evaluate the response of bulb onions and broccoli to a Nutri-Phite P fertilizer on a soil which tests very high in available P but where banded phosphate fertilizers have resulted in crop responses and are routinely applied beneath the seed row of vegetable crops. This was a challenging test of the ability of the Nutri-Phite P Foliar (4-30-8) formulation to enhance yield in a situation where soil P levels were high, banded phosphate was applied in combination with the Nutri-Phite P, and the crops have a waxy cuticle resistant to foliar uptake of nutrients.
Methods
Onions. The test site was a Willamette silt loam typical of soils commonly used for vegetable production in the area and tested high for both available P and K (Table 23). 'New York Early' onion was seeded on 29 May, following plowing, disking, and cultimulching to form a seedbed. The stand was thinned three weeks after emergence. No pre-plant pesticides were used. Each plots size was 10 x 20 feet, with four rows/plot. All rows were treated but yield data were taken from the centermost 10 feet of the two center rows of each plot. Treatments were replicated four times in completely random design. Weeds were controlled by three applications of oxyfluorfen at 0.1 pound/acre and by hand-hoeing. Nitrogen applications included 50 pounds/acre broadcast just after emergence (3 June) and another 50 pounds/acre broadcast on 9 July. All N was in the form of ammonium nitrate. Irrigation was by ovehead sprinkler, as needed.
Treatments are presented in Tables 24 and 25. The check treatment received no P. The banded P treatment received concentrated superphosphate (0-45-0) at a rate of 90 pounds/acre, banded two inches beneath and to the side of the seedline. The Nutri-Phite applications were in addition to the 90 pounds phosphate-P/acre and included DyneAmic spray adjuvant at 1 ml/liter. The initial application of Nutri-Phite P was on 19 July, 50 days post-emergence, with a second and third application on 5 and 19 August, respectively. Applications were made approximately two feet over the plot rows with a two-nozzle boom attached to a backpack sprayer operated at 40 psi pressure. Total amounts of phosphite-P applied in the Nutri-Phite treatments were 1.13 and 2.27pounds/acre for the 2 and 4 pint rates, respectively. Corresponding rates of K applied were 0.58 and 1.16 pounds/acre, respectively.
Onions were lifted on 10 October, removed to a greenhouse bench on 16 October, topped on 23 October, and weighed and graded on 25 October. Bulbs were graded as large (over 2-inch diameter) or boiler size. Leaf samples were collected for P and K analysis on 10 October and submitted to the Central Analytical Laboratory, Oregon State University.
Broccoli. Methods were as above, for onions, except as follows. 'Gem' broccoli was direct-seeded on 16 July following a broadcast, incorporated application of trifluralin herbicide at 0.75 pound/acre and chlorpyrifos insecticide at 1.3 pounds/acre. Nitrogen, as ammonium nitrate, was applied at 50 pounds/acre just after emergence, 100 pounds/acre on 22 August, and 100 pounds/acre on 19 September. Seedlings were thinned starting on 23 July and some hand-hoeing was necessary for good weed control. Two applications of diazinon (1.0 pound/acre) were made for control of cabbage root maggot. Nutri-Phite applications were made on 19 August (30 days after emergence), and 3 and 17 September. Maturity was concentrated and a single harvest was made on 14 October from the center two rows. Leaf samples were collected at the same time.
Results
Given the combination of high soil test for P and K, the application of Nutri-Phite P Foliar in combination with banded phosphate, the low rates of phosphite-P and K applied, and the resistance of the crops to foliar uptake of nutrients, it is not suprising that there was little broccoli or onion response to the Nutri-Phite phosphite-P fertilizer. Visual observations of the plots did not indicate improved plant growth in response to either the banded phosphate treatment or to Nutri-Phite.
The number and weight of large onion bulbs appeared to increase with application of P in any form (Table 24) as did the total weight harvested. This would be in agreement with the long history of response of vegetable crops to banded P applications. However, as can be seen from the probability levels for statistical significance given in Table 24, these differences were not near the normally accepted probability level of 0.05 for statistical significance. Onion leaf P and K concentrations also failed to respond to treatment. Broccoli yields showed no trend in reponse to P application (Table 25). Given the late planting date on warm soil, the lack of response is not surprising. Broccoli leaf P levels appeared to increase with application of Nutri-Phite but the results were not significant at p=0.05.
Table 23. Nutrient levels in Willamette silt loam soil, NWREC, on May 29, 1996 pH P K Ca Mg NH4-N NO3-N (ppm) (ppm) (meq/100g) (meq/100g) (ppm) (ppm) 5.7 228 367 5.5 0.80 23.1 5.3 Table 24. Yield and nutrient content of onion as affected by banded P fertilizer and Nutri-Phite P Foliar, NWREC, 1996 Treatment Large bulbs Total bulbs Leaf Leaf No./ kg/ bulb No./ kg/ bulb P K plot plot wt.(g) plot plot wt.(g) (%) (%) Check 8.5 0.86 101 133 3.36 25 0.42 2.17 Banded P 17.0 1.75 103 146 5.13 35 0.40 2.30 Nutri, 2 pt 15.0 1.30 87 149 5.43 36 0.42 2.25 Nutri, 4 pt 20.5 2.24 109 132 5.27 40 0.46 2.31 P level 0.65 0.64 0.60 0.70 0.50 0.47 0.17 0.56 Table 25. Yield and nutrient content of broccoli as affected by banded P fertilizer and Nutriphite-P Foliar, NWREC, 1996 Treatment kg/plot Leaf P (%) Leaf K (%) Check 4.5 0.35 2.36 Banded P, 90 lb/A 3.8 0.36 2.46 Nutri-Phite, 2 pt/A 5.1 0.38 2.45 Nutri-Phite, 4 pt/A 4.0 0.38 2.38 Probability level 0.93 0.15 0.69