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  • Author or Editor: David C. Ditsch x
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High-value crops (tobacco and sweet corn) often receive high levels of N fertilizer during the growing season rather than risk yield and/or quality reductions. Following harvest, small-grain winter cover crops are sown to reduce soil erosion and recover residual fertilizer N. Fall cole crops, such as cabbage, grow rapidly in early fall, respond well to N fertilization, and have the potential to be sold for supplemental income. The objectives of this study were to 1) compare fall cabbage and winter rye as scavengers of residual fertilizer N and 2) determine if a relationship between fall soil mineral-N (NO 3 +) levels and fall cabbage yield response to N fertilization exists. Soil mineral N levels following sweet corn and tobacco ranged from 22 to 53 mg·kg–1 in the surface 30-cm and declined with depth. Fall cabbage appeared to be as effective as rye at reducing soil mineral N levels. No fall cabbage dry matter yield response to applied N was measured in 1993 and 1995. However, following sweet corn in 1994, a small cabbage yield response to N at 56 kg·ha–1 was measured when the soil mineral level, prior to fall fertilization, was 22 mg·kg–1.

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Tomato fertility trials (1992–94) showed no yield response to fertigation N rates between 101–393 kg·ha–1. In 1995, soil Cardy NO3-N readings taken just prior to fertigation showed 53 kg NO3-N/ha in the top 30 cm. Laboratory test on the same sample showed 72.4 kg/ha (NO3 + NH4-N). Forty percent of the available nitrogen was NH4-N, which is not detected by Cardy meters. Soil mineral N levels were measured at fourth injection, second harvest, and 9 days after last harvest. On these dates the 0 kg N/ha treatment had 28, 24, and 8 mg N/kg available in the top 15 cm of soil, similar to the N fertigation treatments. As the growing season progressed, soil mineral N levels decreased, and 9 days after the last harvest residual soil N levels were close to those seen initially. Tomato petiole sap Cardy NO3-N readingsshowed a significant difference between the 0 kg·ha–1 treatment and those (84, 168, and 252 kg·ha–1) receiving N (512 ppm vs. 915, 1028, and 955 ppm NO3-N, respectively). Treatments receiving fertigation N gave petiole sap NO3-N readings higher than those listed by Hochmuth as sufficient for tomatoes. While the data showed a clear separation between the three N treatments and 0 N rate, no significant difference in yield of US #1 or US #2 large fruit occurred. This suggests that adequate N fertility was provided from O.M. mineralization. The highest N rate also had significantly more US #1 small and cull tomatoes than the other treatments. Some Kentucky soils have adequate residual N capable of producing commercial fresh-market tomato crops with little or no additional N. In addition to potential ground water pollution, overfertilization of tomatoes may decrease fruit size and reduce fruit quality by causing NH4-K + ion competition, as well as increase the risk of certain fungal and bacterial diseases.

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In Kentucky, fresh-market tomato production is a 3-million-dollar crop involving 405 to 486 ha. During the 1980s, on-farm demonstrations showed yields and grower returns increased when intensive production practices were followed. Fertigation recommendations were based on a 1 N: 2 K ratio with a total of 225 kg N/ha. Symptoms of Mg deficiency and blossom-end rot sometimes were seen, and we were concerned about potentially high fertilizer concentrations in the plant root zone. Farm fertility trials (1992 to 1994) showed no yield response to applied N rates between 101 and 393 kg·ha–1. In 1993, a presidedress N test (PSNT) (NO3 + NH4-N) indicated 131 kg N/ha was available in the top 31 cm of soil. At the final tomato harvest, 343, 529, and 647 kg NO3 + NH4-N was measured in the top 46 cm of soil for the three N rates tested (191, 298, and 391 kg N/ha). In 1994, the PSNT showed 86 kg NO3 + NH4-N/ha was present in the top 31 cm of soil. At final harvest 58, 124, and 157 kg NO3 + NH4-N/ha was measured in the top 91 cm of soil for the 140, 225, and 309 kg N/ha applied. Tomato phenology vs. petiole NO3-N concentration showed that all three N levels gave similar values, with no clear distinction between petiole NO3-N and the N rates tested. Future tests need to include a zero applied N rate to determine if a predictable relationship exists between a PSNT or petiole NO3-N levels and a yield response to fertilizer N.

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