&Quot;on Site&Quot; Diagnosis of Nitrogen Fertility to Find Nitrogen Fertilizer Requirements of Leafy Vegetables

News source: Taiwan Agricultural Research Institute, Taiwan ROC
See PDF file for e-mail address, 2002-04-01

Avoiding Nitrate Pollution

Leafy vegetable crops often receive heavy nitrogen applications. To avoid contamination of the environment with nitrate or nitrite, we need quick test methods for available soil N, to use for on-site diagnosis and fertilizer recommendations in leafy vegetable fields. In laboratory tests of incubation leaching and chemical extraction methods, the results indicated that the level of available N detected by chemical extraction of 10% KCl, hot KCl and hot water, showed a positive and significant correlation with the levels obtained by incubation leaching.

Nitrogen Content and Yield

The relative yield of crops not given any nitrogen treatment reached 100%, when the N content detected by chemical extraction fell in the ranges 100 ppm, 150 ppm and 200 ppm. Moreover, in field and pot experiments, there was a highly significant linear relationship between the available N content and electric conductivity EC, 1:5 of the equations:

Where Y = 252.08X + 2.0063 n = 23,R² = 0.7643**, from pot experiments, and Y = 250.39X + 0.2584 n = 22, R2 = 0.6363**, from field experiments. Here, Y was equal to the EC value; X was equal to the nitrate (NO3-N) extracted by 10% KC1.

Nitrogen Requirements Based on Ec

Based on these results, on-site N diagnosis could be implemented in leafy vegetable fields, provided the necessary facilities were available. Growers could be recommended to withhold N fertilizer when the EC is higher than 0.4 ms/cm, suggesting a level of nitrate in the soil of more than 100 mg/kg. This may be found in plastic houses susceptible to salt problems, because of the high evaporation rate and lack of rainfall leaching, or in poorly drained fields where salts have accumulated.

When EC values fall to the ranges of 0.4 _ 0.3 ms/cm, the N fertilizer rate should be reduced by three-quarters. At 0.3 _ 0.2 ms/cm, the rate should be reduced by half, and at 0.2 _ 0.1 ms/cm, the N fertilizer rate should be a quarters of the conventional rate cited in the Fertilization Handbook. If the EC is less than 0.1 ms/cm, the recommended N use should follow that in the Fertilization Handbook.

Effect of Drainage

However, the recommended rate of applied N based on rapid diagnosis should be adjusted and increased in fields which are well-drained, where the soil is subject to N losses from infiltration, or where there is a low soil organic matter of less than 3.0%. The dynamics of soil N are profoundly influenced by levels of soil moisture and soil organic matter. In summary, the rapid soil testing kit not only promotes the rational use of fertilizer, thus reducing production costs, but also helps to conserve the environment.

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