Effects of Soil Acidity and N Fertilizers on Vegetable Stands

Previous experiments at the North Willamette Station (1979, 1980) have established that stands of carrot, cauliflower, and lettuce on Willamette silt loam are inhibited at pH less than 5.8 compared to higher pH. In addition, broadcast applications of N fertilizers have reduced stands when compared with unfertilized soil. In the 1979 experiments, ammonium sulfate at 112 kg N/ha depressed stands by approximately 15% to 45%, depending on the crop; at 224 kg N/ha, ammonium sulfate depressed stands by 30% to 55%. On the other hand, calcium nitrate at 112 or 224 kg N/ha did not reduce stands. It was postulated that the effect of ammonium sulfate was caused by lowered soil pH or ammonium-toxicity.

In 1980, a wide range of fertilizers was applied at 224 kg N/ha to attempt to separate the salt injury, ammonium-toxicity, and pH altering effects of the fertilizers. As in 1979, raising the soil pH increased stands. However, unlike 1979, all fertilizers except urea depressed initial stands of carrots and lettuce. The effects of calcium nitrate and ammonium sulfate were about equal. Other fertilizers such as potassium nitrate, ammonium nitrate, and potassium chloride also reduced stands. These fertilizers represent a broad range of soil acidifying potential and salt injury potential. Urea, at 224 kg N/ha, had a lower salt damage potential than the other fertilizers.

In 1981 and 1982, the experiments were designed to shed further light on the stand depressing effects of various N sources and to determine whether incorporation of the fertilizer into the soil would reduce fertilizer effects on stand.

Methods

In 1981, cultivars used were 'Salad Bowl' lettuce and 'Scarlet Nantes' carrot. Main plots [four levels of soil pH, (Table 1)] were in randomized block design, split by three methods of fertilizer application, and these subplots split by application of calcium nitrate, ammonium sulfate or potassium nitrate at 112 kg N/ha and an unfertilized check. Each treatment combination was replicated four times. The application methods were 1) broadcast entire amount before planting, 2) band half the fertilizer (5 cm to the side, 5 cm below the seed row) at planting, broadcast remainder before planting, and 3) band half the fertilizer at planting, broadcast remainder before planting and incorporate into top 5 to 8 cm of soil. All plots were seeded at 30 seeds/m. Stand counts were made 10 days after seeding.

In 1982, treatments were in split-plot factorial design with pH of 5.6, 5.9, 6.2, and 6.6 as main plots, broadcast vs. broadcast and incorporated fertilizer as subplots, and broadcast application of the following fertilizers at 168 kg/ha as sub-subplots: ammonium sulfate, ammonium nitrate, calcium nitrate, nitroform, and unfertilized check. Plots were seeded to 'Salad Bowl' lettuce. Stand counts were made one week after seeding.

Results

In 1981, as in previous years, increasing the soil pH from 5.0 to 6.0 tended to increase stands, but the difference was statistically significant only for lettuce (Table 1). For both lettuce (Table 1) and carrots (Table 2), incorporating the broadcast portion of the fertilizer into the soil clearly prevented the stand-reducing effects of fertilizer application. All fertilizers tended to reduce stands of both crops. Stand reduction was most severe with calcium nitrate. This is in contrast to results in 1979, when calcium nitrate did not depress stands, and also contrasts with results in 1980 when calcium nitrate depressed stands, but no more than ammonium sulfate or potassium nitrate did.

In contrast to 1979, but in agreement with 1980, there was no trend for a significant pH x fertilizer material interaction: raising soil pH did not ameliorate stand reduction effects of one fertilizer more than another fertilizer. There was a significant interaction of application method and fertilizer material in the case of carrots (Table 3). When fertilizer was broadcast or banded and broadcast, fertilizers significantly reduced stands when compared with unfertilized plots. However, when the broadcast portion was incorporated into the soil, fertilizers did not significantly reduce stands.

  Table 1. Main Effects of Soil Acidity, Method of Fertilizer Application, and   Type of Fertilizer on Seedling Stands of Lettuce, 1981                              Soil   Lettuce stand  Application       Lettuce stand  Fertilizer    Lettuce stand   pH     (seedlings/m)     method         (seedlings/m)  material      (seedlings/m)  5.1         5.5       Broadcast              6.5       None               16.5  5.7         8.5       Band and broadcast     7.0       Ammonium sulfate    9.5  6.2        19.0       Band and incorporate  19.5       Calcium nitrate     5.8   6.6        10.8                LSD (0.05)    4.1       Potassium nitrate  12.0   LSD (0.05) 3.3                                             LSD(0.05)      5.0          Table 2. Main Effects of Soil Acidity, Method of Fertilizer Application, and   Type of Fertilizer on Seedling Stands of Carrots, 1981                         Soil   Carrot stand  Application      Carrot stand  Fertilizer   Carrot stand  pH     (seedlings/m) method          (seedlings/m)  material    (seedlings/m)  5.1       11.0       Broadcast             9.5      None              14.5  5.7        9.8       Band and broadcast    9.5      Ammonium sulfate  11.3  6.2       12.3       Band and incorporate 15.3	    Calcium nitrate   10.8   6.6       12.8                LSD (0.05)   3.6      Potassium nitrate  9.5   LSD(0.05) NS                                             LSD(0.05)    2.1         Table 3. Interaction of Fertilizer Application Method and Fertilizer   Material on Seedling StandsZ of Carrots, 1981  ____________________________________________________________________  Application                      Fertilizer material                method                None     Ammonium      Calcium     Potassium                                 sulfate       nitrate     nitrate                          -----------------seedlings/m ----------------  Broadcast             12.8       10.0           8.3         6.8   Band and broadcast    14.3        7.8          10.5         6.8   Band and incorporate  16.3       15.5          14.5        14.8     	  ZAveraged over four levels of soil pH. LSD(0.05) = 2.8.  

In 1982, the effect of soil pH on lettuce stand was highly significant (Table 4). A soil pH of 6.2 produced the best stands and there were no significant pH x fertilizer or pH x application method interactions. Stands were reduced slightly at pH 6.6 and were lowest at pH 5.6. Incorporation of fertilizer tended to increase stands, but the increase was not significant. Form of fertilizer did not have a significant effect in this trial.

The most useful general conclusion that can be drawn from the four years of this study is that for best stands, soil pH should be near 6.0, pre-emergence fertilizer applications should be moderate, and broadcast fertilizers should be soil incorporated.

  Table 4. Main Effects of Soil Acidity and Fertilizer Application   Method on Lettuce Stands, 1982                                    Soil           Lettuce stand    Application    Lettuce stand  pH             (seedlings/m)    method         (seedlings/m)      5.6                 5.1         Broadcast            8.3   5.9                 9.1         Incorporated        10.1   6.2                12.8            LSD(0.05)         1.5   6.6                10.4      LSD(0.05)       1.5                                           

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