Environmental factors and cultural measures affecting the nitrate content in spinach

English. Ingestion of high amounts of nitrate by man can be considered hazardous to human health. In the human body, nitrate can be reduced to nitrite which may cause methemoglobinemia. Furthermore, the possible formation of N-nitroso-compounds from nitrite and secondary nitrogen compounds in the human stomach constitutes a risk, as for laboratory animals many N-nitroso-compounds have been shown to act as potent carcinogens.In the Netherlands, acceptable daily intakes of nitrate and nitrite, as set by the FAO/WHO for additives, are sometimes exceeded. The intakes of nitrate and nitrite therefore should be reduced. As nitrite intake is linked to nitrate intake which for about 75% is of vegetable origin, the nitrate contents of vegetables should be suppressed as much as possible.Spinach is one of the vegetables frequently having a high nitrate content. In the Netherlands, most spinach is grown for the processing industry and a portion of the processed spinach is used as baby food.Many environmental factors and cultural measures can affect the nitrate content in spinach. A review of relevant literature is presented in chapter 2. In the present study, some of these environmental factors have been investigated with respect to their influences on nitrate content and yield of spinach. Most attention, however, has been given to cultural measures, especially those concerning application of nitrogen fertilizers.The materials and methods used in this study are discussed in chapter 3 .In chapter 4, the results of experiments carried out indoors, with water-, sand- and soil-cultures, are described. In water-culture experiments, the cation:anion uptake ratio for spinach supplied with nitrate as sole nitrogen source, proved to be close to unity. When nitrogen (NO 3 ) was withheld from the spinach plants over a 3- or 9-day period, the nitrate contents in the aerial parts significantly decreased (section 4.2).In sand-culture experiments spinach showed very poor growth when supplied with ammonium-nitrogen only. With both ammonium- and nitratenitrogen added, the nitrate contents in spinach proved to be not always lower than with nitrate-nitrogen only. On a whole-plant basis, the carboxylate (C-A) : organic nitrogen ratio was found to reflect the extent of nitrate- or ammonium nutrition fairly closely (section 4.3).In soil-culture experiments both the nitrate contents and dry-matter yields of spinach were found to be strongly affected by the amount of nitrate applied. Yields usually showed a maximum, whereas nitrate contents did not. The nitrate contents in petioles of spinach were considerably higher than in laminae, and nitrate accumulated mainly in older leaves. In most soil-culture experiments, the nitrate contents in the aerial parts of spinach decreased with increasing age of the plants. Nitrate contents in autumn-grown spinach were higher than those in spinach grown in spring. Variations in timing of application of nitrate-nitrogen did not affect the nitrate contents. Compared with a normal soil- moisture content, a low soil-moisture content was found to increase, and a high sod- moisture content was found not to affect the nitrate content in spinach. Liming a sandy soil, resulting in increased soil pH-values, caused the nitrate contents in spinach to decline. Molybdenum applied as a spray onto the leaves of spinach, grown on a sandy as well as on a clayloam soil, did not have any effect on the nitrate contents in the aerial parts, but differences in soil type appeared to affect these contents (section 4.4).In a soil-culture experiment with a sandy soil, NH 4 -N dressings produced higher nitrate contents and lower dry-matter yields of spinach than did NO 3 -N-dressings. With the use of a nitrification inhibitor (DCD), however, lower nitrate contents were found with NH 4 -N-dressings. With a clay-loam soil used in a growth-chamber experiment, replacement of about 30% of the NO 3 -N by NH 4 -N with a nitrification inhibitor (nitrapyrin) added, did not result in a decrease in nitrate content, but replacement of about 60% of the NO 3 -N by NH 4 -N did. In the same experiment, a decrease in light intensity from 70 to 33 W per m 2and a rise in temperature from 12 to 22°C caused the nitrate content in spinach to increase (section 4.5).In a comparison of different N-carriers in a soil culture, positive results were obtained with sulphur-coated urea (section 4.6), with farmyard manure and pig- manure slurry (section 4.7), when plant-available N was taken into account. Variations in P-dressings as well as in soil P-status did not affect the nitrate content and yield of spinach (section 4.8). K-dressings in general increased the nitrate contents and yields of spinach, with K 2 SO 4 more than KCl being responsible for increases in nitrate contents (section 4.9).Large differences among spinach varieties were found with respect to nitrate contents in leaves (section 4. 10).In chapter 5, the results of experiments conducted outdoors are described. In the field experiments, yield, NO 3 - and N(total)-contents and total N in the aerial parts of spinach increased with increasing amounts of NO 3 -N applied. With the highest NO 3 -dressings, NO 3 -N accounted for 9-27% of the total N in spinach. The corresponding NO 3 -contents ranged from 300 to 1100 mmol per kg DM. In one experiment, varietal differences in N03 -content ' could be attributed to differences in age of the crop. Differences in NO 3 -contents between 'morning' and 'evening' harvests were found only on a bright day with high irradiation and with high NO 3 -levels in the plants.Available N in the sod profile before sowing ranged from 32 to 72 kg N per ha in the five spring experiments and from 40 to 132 kg N per ha in the two autumn experiments. In general, the net mineralization of organic N during the experimental periods was about 35 kg N per ha. Nitrogen appeared to be taken up by spinach mainly from the top 60 cm of the soil profile. Of the plant nutrients other than N only the contents of SO 4 and H 2 PO 4 were not affected by variations in NO 3 -dressing (section 5.1).Variations in timing of nitrogen applications affected yields in one, but not in another field experiment. NO3- and N(total)-contents and total N in the aerial parts of spinach in general decreased due to a partitioning of total N applied in one field experiment, whereas in another one, top-dressed N applied as NH 4 or NH 4 +NO 3 resulted in lower values than did comparable dressings of NO 3 (section 5.2).Due to partial or complete replacement of NO 3 -N by NH 4 -N, yields, NO 3 - and N(total)-contents, total N and the ratio (C-A):organic N in the aerial parts of spinach decreased in three field experiments, the effects in general being more pronounced with than without a nitrification inhibitor added. In two other field experiments, the effects of variations in NO 3 :NH 4 -ratio were much less pronounced or absent (section 5.3).On farmers' fields with 'low' N-dressings, plant-available N (= N-dressings plus the amount of available N in the soil before sowing) ranged from 135 to 175 kg N per ha, while with 'high' N-dressings plant-available N ranged from 200-415 kg N per ha. Compared with 'high' N-dressings, yields with 'low' N-dressings on the average were 17% lower, while NO 3 -contents were 64% lower (section 5.4).A discussion of the results is presented in chapter 6. Measures aimed at utilizing or manipulating environmental and soil conditions, at exploiting varietal differences and those aimed at regulating nutritional conditions, other than the ones governed by nitrogen, with the purpose of growing spinach with an economically acceptable yield level and with a low NO 3 -content, are dealt with in section 6.1.Experimentation aimed at finding the proper amount, the proper form and the proper timing of application of N for attaining the above-mentioned goal, is discussed in section 6.2. In the latter, special attention is paid to the relationship between the quantity of plant-available N (= N-dressing plus the amount of available N in the soil before sowing) and the NO 3 -content in spinach leaves. Based on data from spring field experiments and from experiments on farmers' fields, the amounts of plant- available N with which critical levels of NO 3 -contents are not surpassed, are presented. Recommendations for practical use in commercial spinach growing are formulated in the final portion of section 6.2.

Dutch; Flemish. Proefschrift over de invloed van milieufactoren (licht, temperatuur, vochtigheid, beschikbaarheid van water) en de verhouding van toegediende voedingsstoffen op het nitraatgehalte van spinazie