Soil Amendments and Their Effects on Sunflower Growth

An interesting topic in agriculture is the search for forms of fertilisation that have a low impact on soil, plants, humans, and the environment. Wastewater treatment plants almost always separate the organic matter, called wastewater sludge or sewage sludge. This sludge is rich in mineral macronutrients such as nitrogen (ammonium or nitrate), phosphorous (phosphate), potassium, and micronutrients. The sewage sludge might provide the majority of necessary nutriments for growing plants, and also provide many beneficial effects when it is applied to soils, resulting in an improvement in the chemical, physical, and biological characteristics. We know that wastewater sludge and soil microorganisms play an important synergetic role on the released and available nitrogen. Although urea is the most accepted type of fertiliser used worldwide, it does have some inconvenient drawbacks such as pH changes, microflora modifications from the soil, and others; these issues must be considered in order to avoid N losses. An interesting alternative might be the use of Plant Growth-Promoting Rhizobacteria (PGPR, soil bacteria that colonize the roots of plants by inoculation onto seeds or roots and that allow enhanced plant growth) as helpers. Examples such as Pseudomonas, Azotobacter, Burkholderia, Klebsiella, and Bacillus, among other genera, have been reported. In this chapter, we focus on specific types of fertilisation or soil amendments and their effects on this important crop. In particular, Bacillus subtilis has showed promising results. It is known that these bacteria might improve plant growth by several direct mechanisms as well as indirect mechanisms, such as controlling phytopathogen organisms, or a combination of both. A regular strain of Bacillus subtilis was co-inoculated with urea in sunflower roots, and it was found that the strain temporarily stimulated sunflower cultivar growth. We found that B. subtilis improves the sunflower establishment, particularly its strength and vigour during the early stages, resulting in an increase of the mass and length of roots compared with the control treatment. Application of either organic or mineral fertiliser improved the crop yield under various conditions, but fertilising the crops with sewage sludge might be more environmentally friendly than using mineral fertiliser. Wastewater sludge has a high NH₄⁺ content that is slowly oxidized to NO₃^- while being absorbed by sunflower cultivars. In both cases (organic amendment or bacterial inoculation with urea), the NO₃^- is not lixiviated or leaked during experiments. Accordingly, the wastewater sludge and B. subtilis might be potential methods of fertilisation for sunflowers or any ornamental cultivars.