The purpose of this study was to investigate the effect of several combinations of soil pH and nitrogen fertilizer rates on yield and mineral uptake of bush beans, carrots, and crisphead lettuce. Of particular concern are the nutrient elements potassium, calcium, phosphorus, magnesium, zinc, and copper, and heavy metals such as manganese which may be toxic to plants if present in sufficient quantity. Optimal soil pH levels are not well known for many vegetable crops and probably vary with soil type, cation exchange capacity, amount of organic matter etc. As in 1977, this study was designed to determine pH optima for the three crops listed above. The design is essentially a repeat of the 1977 plan except that the number of nitrogen rate variables was decreased to two, 50, and 150 pound/acre.
Methods
Cultivars used were Spartan Arrow bean, Ithaca lettuce, and Chantenay carrot. Two hundred pounds/acre each of 0-0-52.5 and 0-45-0 were broadcast and incorporated. All plots were seeded on May 25. Nitrogen rates were applied on May 28 as 150 and 450 pounds/acre of 34-0-0. Soil pH varied from 4.9 to 6.7. Seeding and herbicide applications took place during very wet weather with resulting problems of herbicide run and soil compaction. Row spacing for all crops was 30 inches. Acceptable stands were achieved for all crops. Weed control was adequate for protection through the seedling stage, but cultivation of all crops was necessary by July. Harvest dates were August 3 for beans and lettuce, and August 22 for carrots.
Results and Discussion
I. Beans
Bean yields in general were much lower than in 1977, caused in part by lower density planting and earlier harvest, but also by poorer plant performance. Plant vegetative growth and pod yield were inhibited severely in certain parts of the field, and apparently in a pattern not related to treatment. Soil compaction or herbicide injury may have been a problem. Bean plant vegetative yield was not significantly affected by treatment. Maximum bean pod yield of 4.1 tons/acre occurred at pH 6.4 and 50 pounds nitrogen. Leaf tissue concentrations of phosphorus, potassium, and magnesium were unaffected by nitrogen or pH while calcium levels increased with increasing pH up to pH 6.4. Zinc levels decreased with increasing pH and some values for individual plots were in the deficient range of less than 20 parts per million (ppm) at pH 6.4 and above. The foliage also showed possible zinc deficiency symptoms. The most striking effect of pH was the inverse relationship between soil pt and leaf manganese levels. Manganese concentration of 200 ppm was obtained at pH 5.0, but only 103 ppm at pH 6.6. In contrast, in 1977, leaf manganese varied from about 65 ppm at pH 5.0 to 30 ppm at pH 6.6.
II. Carrots
Total carrot yield responded significantly to both pH and nitrogen rate. At pH 5.6, carrots yielded 60 percent more than at pH 5.0 but liming to higher pH did not result in any further yield increase. Application of 150 pounds/acre of nitrogen reduced yield at each pH level below that obtained with 50 pounds/acre of nitrogen. This was due at least in part to inhibition of stand establishment by the high nitrogen rate. The cause of this nitrogen effect might be ammonium toxicity, salt injury, or a pH lowering effect.
Yield of Grade 1 plus Grade 2 (all but culls) carrots responded in the same fashion as total yield. Thus, neither pH nor nitrogen rate significantly affected carrot cull rate (percentage of over-or undersized, broken, split, forked etc.).
Carrot leaf tissue levels of phosphorus, magnesium, and potassium were unaffected by soil pH. The higher nitrogen rate tended to depress potassium levels. Calcium levels increased with both pH and nitrogen rate. Zinc and manganese levels decreased with increasing pH, with zinc levels being possibly deficient at pH 6.4 and above. Manganese levels at pH 5.0, while double those at 6.4 - 6.6, were not in the toxic range.
III. Lettuce
Lettuce head size increased dramatically with increasing pH to a maximum of 1.4 pounds at pH 6.4 and 150 pounds/acre of nitrogen. There was a trend for the high nitrogen rate to suppress head size at low pH and increase it at high pH.
Leaf tissue levels of magnesium, potassium, and calcium were unaffected by pH and nitrogen. Phosphorus levels increased with increasing pH while manganese and zinc levels decreased with increasing pH. Leaf tissue manganese decreased from nearly 200 ppm at pH 5.0 to less than 40 ppm at pH 6.4. The 200 ppm level may well be in the toxic range. Lettuce stands were suppressed by both low pH and high nitrogen rate.
In summary, bean yields were not greatly affected by pH or nitrogen rate because of great variability between and within plots. Carrot yields increased with increasing pH but were depressed by the high nitrogen rate. Lettuce yields responded dramatically to increasing pH and manganese toxicity may be a factor in poor lettuce growth at low pH. Stands of lettuce and carrots were improved at higher pH but suppressed by high nitrogen. Data from these experiments will be published in the 1979 Proceedings of the Western Washington Horticultural Association.