1 Introduction.- 2 Literature.- 2.1 Nitrogen transformations in soil.- 2.1 Nitrogen transformations in soil.- 2.2 Nitrate uptake.- 2.3 Nitrate reduction in plants.- 2.4 Nitrate contents in plants.- 2.5 Cultural measures.- 2.5.1 Nitrogen dressing.- 2.5.1.1 Nitrogen amount.- 2.5.1.2 Nitrogen form.- 2.5.1.3 Time of application of nitrogen.- 2.5.1 Nitrogen dressing.- 2.5.2 Other nutrients.- 2.5.3 Herbicides.- 2.5.4 Spinach variety.- 2.5.5 Harvesting procedure.- 2.6 Environmental conditions.- 2.6.1 Light.- 2.6.2 Carbon dioxide.- 2.6.3 Temperature.- 2.6.4 Water.- 2.6.5 Season.- 2.6.6 Location.- 3 Materials and methods.- 3.1 Indoor experiments.- 3.1.1 General.- 3.1.2 Water-culture experiments.- 3.1.3 Sand-culture experiments.- 3.1.4 Soil-culture experiments.- 3.2 Field experiments.- 3.2.1 General.- 3.2.2 Duiven experiments.- 3.2.3 Lelystad experiments.- 3.2.4 Helden experiments.- 3.3 Weather conditions.- 3.4 Harvest.- 3.5 Chemical analysis.- 3.6 Soil analysis.- 4 Nitrogen nutrition and the ionic balance in spinach; results of indoor experiments.- 4.1 General introduction.- 4.2 Spinach grown in water-cultures.- 4.3 Spinach grown in sand-cultures with nitrate- and ammoniumnitrogen.- 4.4 The effects of nitrate-nitrogen on yield and chemical characteristics of spinach grown on various soils.- 4.4.1 The amount of nitrate-nitrogen applied.- 4.4.2 Distribution of plant nutrients in the aerial parts of spinach.- 4.4.3 Age of plants and season of growth.- 4.4.4 Time of application of nitrate-nitrogen.- 4.4.5 Soil-moisture content.- 4.4.6 Soil pH.- 4.4.7 Mo-spraying on two soil types.- 4.5 The effects of variations in N-form in soil-cultures on yield and chemical characteristics of spinach.- 4.5.1 Ammonium- and nitrate-nitrogen dressings.- 4.5.2 Light intensity- and temperature effects with different amounts of nitrogen and varying N03 : NH4 -ratios.- 4.6 Comparison of the performances of different N-carriers as reflected in yields and N03-contents of spinach.- 4.7 The effects of N in manures on yield and chemical composition spinach.- 4.8 Effects of variations in P-dressing and in soil P-status on yield and chemical composition of spinach.- 4.9 Effects of variations in the size of K-dressings and in type of K-carrier on yield and chemical composition of spinach t1.- 5 Results of outdoor experiments.- 5.1 The effects of variations in nitrate dressings.- 5.1.1 Yields and dry-matter contents of spinach.- 5.1.2 N03-contents in spinach.- 5.1.3 N-contents and total N in the aerial parts of spinach.- 5.1.4 Available N in the soil profile.- 5.1.5 Plant nutrients other than nitrogen.- 5.2 The effects of variations in timing of nitrogen applications.- 5.2.1 Yields and dry-matter contents of spinach.- 5.2.2 N03-contents and N-contents and total N in the aerial parts of spinach.- 5.3 The effects of variations in N03 : NH4-ratio.- 5.3.1 Yields and dry-matter contents of spinach.- 5.3.2 N03-contents in spinach.- 5.3.3 N-contents and total N in the aerial parts of spinach.- 5.3.4 The ratio (C-A) : org N in the aerial parts of spinach.- 5.4 Experiments on farmers' fields.- 5.4.1 Yields and dry-matter contents of spinach.- 5.4.2 N03-contents in spinach.- 6 Discussion.- 6.1 Cultural measures, other than nitrogen dressings, aimed at minimizing N03-contents in spinach.- 6.2 Cultural measures related to nitrogen, aimed at minimizing N03-contents in spinach.
