Organic fertigation for greenhouse crops

Publikation: ForskningPh.d.-afhandling

  • Bhaniswor Pokhrel
    Bhaniswor Pokhrel
Production and consumption of organic food is on the rise globally mainly due to a greater consumer awareness of issues related to health and the environment. However, the productivity of organic farming systems is considerably lower than for conventional systems. A key factor behind the low productivity is suboptimal nutrient management resulting from poor synchronization between crop nutrient demand and nutrient release from organic fertilizers, affecting the physical, chemical and biological characteristics of the root zone environment, and thus plant growth and productivity. Compared to solid organic fertilizers, the application of liquid organic fertilizers potentially more accurately addresses the nutrient demand, because nutrients are readily available and different fertilizers are easily mixed. This PhD work explores the possibilities and challenges related to the application of liquid organic fertilizers in organic greenhouse crop production.
Four greenhouse experiments were designed where different liquid organic fertilizers were prepared: acidic extraction or anaerobic digestion of red clover and white mustard silage, water extraction of composted chicken manure and flushing of acidic water with ammonia. These fertilizers and commercially available lupin sap as well as pH-controlled chicken manure extract were applied either alone or in combinations to tomato, parsley or coriander grown in a peat-based medium. Their effect on nutrient availability, pH, electrical conductivity (EC) and oxygen concentration in the growing medium was studied. Further, crop growth and biomass yield were measured using leaf area, fresh and dry weight, photosynthesis and carbohydrate content in plant materials. The study showed that acid extraction of red clover and white mustard silage resulted in higher concentrations of most macro- and micronutrients than anaerobic digestion of the same biomasses. Only the concentration of nitrogen was lower when acid extraction was used, leading to a high EC in the fertigation and growing medium solutions. Fertigation solutions prepared from anaerobic digestates had a very high pH of ca. 8 and fertilizer produced by both extraction methods resulted in a lower biomass yield of parsley than chicken manure extract and inorganic fertigation. The lower yield of plants receiving organic fertigation was due to smaller leaves, smaller number of leaves, and reduced fresh and dry weights of aboveground parts. This was not caused by a reduced net photosynthetic rate or altered carbohydrate composition, but all
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organic fertigation strategies resulted in high pH, high Na and Cl concentrations, high NH4/NO3 ratios, and nutrient imbalances in the root zone. Further, lupin sap fertigation resulted in high EC and very low P and micronutrient concentrations. These components may either individually or collectively have been responsible for the reduced plant growth and lower biomass yield compared to inorganic treatments. The pH of the growing medium solution could to some extent be controlled by the addition of sulphuric acid in the fertigation solution, but it was not possible to control the pH in either the fertigation or the growing medium solutions by the addition of organic acids. Another reason for the reduced yield in peat-based growing media in pots could be poor root development in the lower layers due to increased microbiological activity consuming oxygen (low oxygen), increased sedimentation of organic matter from the organic fertigation solutions and/or a high density/biomass of microorganisms leading to changes in growing medium properties. The study demonstrated that yields comparable to a conventional system can be achieved with chicken manure extract or a combination of plant-based fertilizers with N-enriched water.
OriginalsprogEngelsk
Antal sider129
ISBN (trykt)978-87-93398-76-4
StatusUdgivet - 14 mar. 2017

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