Nutrient management in organic greenhouse production: Navigation between constraints

Organic greenhouse production within the European context is limited both by its own principles, among them restrictions concerning soil and fertilizers, and by legislative restrictions on manure, compost and fertilizer applications. Results of a monitoring project on organic greenhouses were evaluated concerning the various legal constraints. Important bottle necks are related to the limitations in the N and P input, the unbalanced input of nutrients and the restrictions on irrigation with consequently increased salinity problems. The development of an organic fertilizer database and a decision support model for fertilizer and organic matter application is evaluated. Balanced mineral supply is possible within the prevailing legal mineral targets, but risk of salinity is a concrete concern. Challenges in the very near future will be the upcoming targets on N and P emission and new regulations on organic manure. INTRODUCTION Organic vegetable production as expressed by the IFOAM Basic Standards (IFOAM, 2007) discerns itself in many aspects from conventional production. One of the key issues is the approach to soil and soil fertility, as is manifest in the regulations on permitted fertilizers and soil amendments in the EU regulation for organic food production (Anonymous, 2002). This entails restrictions for crop production, for the goal in organic production is equilibrium between the production capacity of the soil and other production factors, rather than maximizing yields. Organic greenhouse production operates under the same principles; however, greenhouse crops are characterized by high production levels and consequently require high inputs of nutrients. High yields are considered necessary to recoup investment costs. This aspect, however, conflicts with basic organic principles and guidelines. In addition to the guidelines and standards of the organic label itself, European and national regulations on fertilizer inputs, as derived from the Nitrate Directive (ND) (Anonymous, 1991) or the WaterFramework Directive (WFD) (Anonymous, 2007), will restrict organic greenhouse production even more. This may sound strange, since organic production is often thought to be synonymous with sustainability and an environmentally friendly way of agricultural production. However, the generic way the regulations are described, by which there is no differentiation between greenhouse and field crops, and the strong dependence on organic fertilizers and composts, makes fertilization in organic greenhouse production quite complicated. In this paper, it will be discussed how greenhouse growers have to deal with the various constraints for fertilization management and which constraints have to be faced in the near future. The focus will be within the Dutch context. FERTILIZATION PRACTICE The current fertilization strategy in practice is characterized by a base dressing of compost, and in many cases also animal manure, before planting, partly to amend the soil Proc. First IC on Organic Greenhouse Hort. Eds.: M. Dorais and S.D. Bishop Acta Hort. 915, ISHS 2011 76 micro-organisms and maintain soil structure and partly for nutrient supply. Top dressings are applied throughout the growing season, using a variety of additional fertilizers like dried dairy and chicken manure, dried slaughterhouse waste (bloodmeal, feathermeal) or dried fertilizers derived from plant sources (e.g., remains from malt, sugar beet, ricinus industry). Occasionally, fertigation with liquid organic fertilizer which is dissolved in water with epsom salt or potassium sulphate is used to establish a certain electrical conductivity level (EC) to allow dosage rates to be automatically controlled. The rates applied are mainly based on soil analysis and the grower’s own experience (Janmaat and Cuijpers, 2005). From 2002 on, the fertilizer application and soil management was monitored at 8– 10 organic greenhouse companies. In some years, the total net crop nutrient removal was determined by fresh and dry weight of crop plants and plant tissue analysis aimed at estimating the mineral balance. The total available N was estimated using the mineralization model established by Janssen (1984). The greenhouse companies observed had intensive fruit vegetable crop rotations, consisting of tomato, cucumber and sweet pepper and a limited number of smaller crops like lettuce, cauliflower, beans and egg plant. Parts of these results have been described earlier (Cuijpers et al., 2008). The average total supplied N was much higher than the removal by the crop. At the same time, the N supplied was in the same order of magnitude as the estimated total available N, showing that the unavailable fraction of organic N in the yearly applied soil amendments was compensated by the decomposition of the soil organic matter versus the historic soil amendments (Fig. 1). The average application was about double the crop demand. Part of this excess will be lost by denitrification, the other part by leaching and a small fraction will be fixated in the stable organic matter pool. de Visser et al. (2006) showed that denitrification losses could be considerable; it was found to be responsible for as much as 25% of the total N balance output. For P, the results were also troubling; the total supply was almost three times the estimated crop removal, resulting in a significant P excess. Since leaching of P is not to be expected, the majority of P will have accumulated in the soil. The excessive P accumulation in organic greenhouse horticulture was observed previously (Voogt, 1999). Next to the N and P surpluses, it was established that the mutual ratio between N, P and K in the total organic fertilizer supply was not balanced with the ratio in the crop demand (Fig. 2). This was caused mainly by the relatively lower levels of K in compost and manure (45 and 50%, respectively), compared with the crop uptake (57%). Moreover, the portion of P in compost and manure (11%) is relatively much higher than the crop demand (6%). In addition, growers added more specific N fertilizers like blood and feathermeal to compensate for N losses not due to crop uptake but other processes in the N cycle, like denitrification (Cuijpers et al., 2005). REGULATIONS Dutch organic greenhouse production has to meet the following standards and regulations: A. The principal aims of organic production and processing, as formulated under IFOAM basic standards, are of course leading for organic greenhouse production. Within EU, the EU regulation for Organic Food (837/2007) is leading. This does not need further specification within this context; the fertilizers allowed are certified by SKAL, the Dutch certification body (Anonymous, 2002). B. Manure: resulting from the European Nitrate Directive, applicable since 1998. The application of manure is limited to the equivalent of 170 kg N ha yr. This also holds for any additional (dried) fertilizer derived from animal manure, even for the manure component in composts amended with manure like mushroom compost (Anonymous, 1986; Anonymous, 1997; Anonymous, 2010). C. Compost: Since 2008, the application of compost is regulated under the new fertilization law. Compost should contain at least 10% organic matter (dry matter) and must comply with the EU standards for heavy metals. There are no more limits in the applied quantity but these are restricted by the application standards for nutrients (D)