with Biostimulants
By Michelle Jones, The Ohio State University
Fertilizer prices remain very high and represent a much larger expense for growers than they have in the past. There are increasing concerns not only about fertilizer affordability, but about availability as well. Reducing the use of fertilizers seems prudent, but can this be done while maintaining crop quality and meeting tight scheduling windows?
We are hearing more about biostimulants and how they can promote plant growth, but can fertilizer levels really be reduced? There is a lot more we need to learn about how different biostimulants work and how they are affected by other production inputs and greenhouse environmental conditions, but I think the short answer is yes.
As an industry, we rely heavily on fertilizer applications to produce high-quality crops in a short amount of time. Plants need a complete complement of macro and micronutrients for healthy growth, but how much of the nutrients that are applied through water soluble fertigation methods are really taken up and utilized by the plant? Fertility management for plants grown in containers is highly influenced by the limited volume of the media, which results in a lower buffering capacity for the solution composition and limits the supply of available nutrients. Applied nutrient ions may bind to charged media components or form insoluble complexes with metals like iron (Fe) or calcium (Ca).
Soilless growing media has limited nutrient-holding capacity, which means a fertility management strategy that improves the plant availability and uptake of individual nutrients is needed before fertilizer application rates can be reduced.
This is where biostimulants come in. Many biostimulants include microbial (bacteria or fungi) or non-microbial components that promote plant growth by improving the efficiency of nutrient uptake. Fertilizer application rates can then be reduced if plants can take up and use a larger percentage of the nutrients that are applied.
How Do Biostimulants Improve Fertilizer Nutrient Use Efficiency?
Some biostimulant microbes produce hormones like auxin that stimulate root growth. The larger root systems allow plants to take up more of the available nutrients and water, which results in increased growth. This can be advantageous if early root growth in young plants is stimulated to give them a more vigorous and earlier start. This benefit may be limited in container-grown crops, where the media volume and availability of nutrients is constrained by the container, while plants grown out in the field can continue to expand root growth to areas of increased water and nutrient availability.
Many biostimulants also include microorganisms that can convert the insoluble and unavailable forms of some macro and micronutrients into forms that are available to plants.
One of the best examples of this is with phosphorus. When bound to calcium [Ca3(PO4)2] or iron [FePO4], phosphorus is insoluble and unavailable to plants. Bacteria that can solubilize phosphorus release phosphate ions that are immediately available for uptake and use by plants. Without these bacteria, a significant amount of the phosphorus that is applied in fertilizers will not be used by plants.
Many genera of bacteria can solubilize phosphorous, and phosphate-solubilizing bacteria exist in most soils. But the diversity of native microbial populations and the concentrations of both bacteria and fungi are much lower in soilless media than in soil. The containers of soilless media used to grow greenhouse crops may not contain bacteria that are able to solubilize phosphorus or other nutrients. Microbial biostimulants can be used to build healthy microbial populations in these soilless mixes.
The beneficial effects of humic acids and fulvic acids on plant growth have been well known for decades, but there has recently been renewed interest in biostimulants that contain these humic substances. Humic substances originate from organic matter in soil, water, peat, and sediments.
Humic acids and fulvic acids may be used with fertilizers to improve nutrient use efficiency.
They have a beneficial impact on plant growth by improving the porosity, cation exchange capacity (CEC), and biological activity of growing media. Humic and fulvic acids also form complexes with mineral nutrients, making them more available to plants and facilitating increased uptake, which directly promotes plant growth. The chemical composition and source of the humic substances will determine their bioactivity, so not all humic substance-containing biostimulants will have the same effects on plant growth and nutrient use efficiency.
Plant responses to humic substances and microbial biostimulants are dependent on the plant species and highly influenced by crop management and greenhouse environments, but they can be used to grow plants at reduced fertilizer rates with some experimentation.
Remember that if plants are receiving an excess of the nutrients that they need from higher levels of fertilizers, you may not see any positive effects of biostimulants on plant growth promotion.
At The Ohio State University, we have enhanced our research capabilities for trialing biostimulant products under multiple conditions with an automated imaging system. The Traitfinder Greenhouse system provides nondestructive evaluations of 20 different plant growth and health parameters that can be used to assess the effects of biostimulant applications on plant growth and stress tolerance. With this system we hope to be able to provide crop-specific recommendations for reducing fertilizer rates with various biostimulant applications.
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About the author:
Michelle Jones is a Professor and the D.C. Kiplinger Floriculture Chair in the Department of Horticulture and Crop Science at The Ohio State University.
