Nitrogen (N) application often exceeds crop needs, particularly in vegetable production, in which the cost of lost yield far outweighs the cost of additional N fertilisation. Yet excessive N use may lead to negative health and environmental effects. While researchers and agronomists agree that many standard methods of making N recommendations do not adequately integrate the N-influencing characteristics of a given field, a comprehensive guide to sustainable N management in vegetable crops has been lacking. The need for such a guide was the impetus for this work, a collaboration between researchers in Quebec and Germany. The guide (Tremblay et al. 2001) demonstrates the use of the N balance approach in tailoring recommendations to crop needs. In so doing, it discusses the behaviour of N in the soil-plant-air system, methods for estimating and measuring the various N inputs and outputs, and the practical use of these in making efficient N recommendations on a per field basis. Also discussed are the environmental and health effects of excessive N fertilisation, and how these effects may be mitigated practically. A unique approach to the development of the guide involved its review by agronomists and researchers across the world to guarantee completeness. Its release as an electronic publication facilitates distribution and navigation within the text, and permits frequent updates that will ensure its continued relevance.

Vegetable crop residues can provide variable amounts of nitrogen to the following crop. However, the effects of the typical Quebec winter on this process are unclear. To understand these effects, a study was conducted using three residue management practices on the residues of three vegetable crops: cauliflower, red cabbage and spinach. The residues were: 1) incorporated into the soil in the fall (FI); 2) incorporated into the soil in the spring (SI) or; 3) mulched and deposited on the soil surface in the fall (M). Shortly after the SI, the entire surface of each plot was sown with wheat. Soil samples were taken throughout the experiment for monitoring nitrate status. Tissue analysis was carried out to obtain the nitrogen concentrations of the residues and the wheat. Residues contributed between 10 and 30 % of their initial N content (equivalent in this case to 27 to 77 kg N/ha) to the wheat. Results also showed that handling residues in the fall (FT and M) led to greater risk of nitrate leaching before the next growing season than spring handling of residues did. Fall handling of cauliflower residues and both incorporation treatments for red cabbage residues were the only treatments that contributed significant amounts of nitrogen to the following wheat crop. Spinach residues provided no significant N contribution to the wheat crop.

Although a lot is known about the nitrogen demand of vegetable crops, many farmers still determine fertiliser requirements by a 'rule of thumb'. Since no soil analysis is carried out, there is no accounting for the mineral N in the rooted layer. Nil-N-plots (where a small part of the field remains without fertiliser) offer a simple method of estimating the N,in content of the soil and a way to adapt top-dressing for vegetable crops. The rest of the field receives a normal basal-dressing (enough for the period of growth between planting/sowing and top-dressing). The amount of top-dressing is determined by the time it takes for the appearance and the intensity of N deficiency symptoms to become visible in the Nil-N-plot, compared to the rest of the field.In a 2-year field experiment with 12 different vegetable crops, the Nil-N-plots method was compared with the N-min method by taking soil samples and adding fertiliser up-to the N-min target value. There was no decrease in yield when using Nil-N-plots method. However, when the N-min content of the soil was high at the beginning, the total fertiliser applied was higher than the N-min target value. Nevertheless Nil-N-plots are better than using 'rules of thumb'.

A major cause of uncertainty in predicting the demand of nitrogen (N) fertilizer by means of mathematical models is the treatment of apparent net N mineralization (ANM) and apparent recovery rate (REC) of mineral N supply (N fertilizer plus soil mineral N at planting). REC and ANM were analysed in 29 multi-level N fertilizer trials conducted with a range of vegetable crops over a period of 11 years. REC differed substantially between experiments (0.29 to 1.24). ANM ranged from -110 to 140 kg N ha(-1); it decreased with an increasing N supply, and increased with the time between planting and harvest. A simple regression model was used to predict ANM. The model consisted of both a N supply-dependent and supply-independent term, and used N supply and growing time as input parameters. Whereas the model significantly reduced the variance in measured ANM (r(2) = 0.45, n = 129), the residuals showed a considerable variation (standard deviation (SD) = 37 kg N ha(-1)). It is not feasible to predict N fertilizer demand without predicting net N mineralization and fertilizer recovery. Therefore, the regression model is recommended despite its inaccuracy, which is taken into account by adding a security margin to the fertilizer recommendation.

When calculating fertiliser recommendation sand nutrient balances, growers and advisers usually do not do site specific measurements but use data from look up tables. Data given by older literature sources, however, do not take into account today's cropping practices. Therefore a databank on nutrient contents of total yields, marketable yields and harvest residues of 22 field vegetable crops is presented. This databank was compiled using results from topical field experiments and from recent literature.

In this study, some ornamental crops were tested for their tolerance to increasing Na, Cl and salt concentrations following addition of compost to peat substrates. The results show considerable differences in Cl tolerance among tested plant species. Strong growth inhibition was observed with Primula as well as Saintpaulia. at concentrations above 450 to 500 mg Cl/l substrate. Pelargonium showed growth reduction when Cl concentrations exceeded 880 mg/l while Petunia growth remained unaffected up to 1000 mg Cl/l. The critical values for Na were as follows: Pelargonium: 780 mg Na/l, Begonia: 550 mg Na/l, Petunia: 540 mg Na/l, Primula : 140 mg Na/l substrate. Assuming that 40 % compost will be used and that the critical Na and Cl values should be determined by the most sensitive species, composts used in production of potting substrates may contain up to 1000 mg Cl/l and 225 mg Na/l. Under these conditions 2,5 g salt/l in the mature compost could be tolerated by all tested plant species. These limits are valid for compost: peat mixtures only. Other salt, Na or Cl sources such as high concentrations in irrigation water or other substrate components e.g. coconut fiber have to be taken into account for the determination of quality criteria of composts used in substrates.

Results of many investigations of the last 10 - 15 years on the nitrogen dynamic have made N fertilizer recommendations in vegetable production more reliable. Four important aspects have to be taken into account:1. Measuring the N content of the soil on very plane sites at planting time or before top dressing means a good precision for determining the amount of fertilizer.2. If the mineralization rises strongly because of adding new organic substance, this has to be included to correct the target value.3. The same holds true in the case of extreme summer rainfall with leaching losses of nitrate.4. Approaching mathematical estimates to establish the target values for nitrogen fertilization, it proves useful to consider N-min content of the soil, N release from humus and from fresh organic substance on the supply side and N uptake of the plants, necessary mineral N content of the soil, and N immobilization on the demand side.When these aspects are considered it is likewise possible to plan N fertilization for those crops where no long-term experiments exist and to improve N fertilization on the basis of N-min soil analyses.

When establishing target values for N fertilization (N(min)-Sollwert = optimal sum of soil mineral N at the beginning of cultivation and fertilizer N) experimentally the components of the N balance are restricted to N(min) target value, N(min) supply of the soil and N fertilizer amount. In fertilization experiments any other processes in N dynamics are included in the target value without being noticed.If N(min) target values are to be calculated such simplified reflection is not longer possible. In this case the most important components of the N balance have to be determined and quantified.Up to now the formulas to calculate the N(min) target value have included the N uptake of the crop (also called ''N demand''), frequently the necessary minimum mineral N content at harvest time in the rooted soil layer, sometimes the N release during the cultivation period. Immission, denitrification, leaching and others during the summer half-year have been considered in only a few cases.Calculated N(min) target values from N uptake of the crop plus necessary minimum mineral N content in the soil at harvest time minus N release yields in all cases studied distinctly lower values than determining them by fertilizer experiments. Only if a calculated quantity ''deficit'' is included a satisfactory agreement between the two values is attained. There are indications that this quantity consists mainly of temporarily bound or immobilized nitrogen.