In addition to fertilizers, manures, and composts, there are several other types of commercially available soil amendments and plant nutrient sources. While some products contain detectable quantities of nutrients that become available to plants in the near term, other products may instead increase availability of existing plant nutrients in the soil. Many have not been well tested in controlled studies. Some, but not all, have been approved for organic production.
Organic Residuals
Organic by-products of industrial processes fall into this category. Note that the use of the word “organic” in this case refers to the nature of the material itself, i.e., derived from biological sources. It does not necessarily indicate approval for use in certified organic production. Materials included in this category include processed slaughterhouse wastes, leather processing waste, biosolids, papermill sludge, as well as composts. In general, as these products decompose, plant nutrients are released. Many are sold with the nutrient analysis content listed, which is generally very low on a percent-by-weight basis. The greater benefits are usually for soil conditioning, and in some cases, liming activity. Not all of these products are acceptable for certified organic production, and acceptability for use in food production should be verified.
Foliar Amendments
Foliar feeding has become a more common practice among some vegetable farmers. Many products are now available on the market, for use in both conventional and certified organic production. Foliar feeding is not recommended as a major source of nutrients for a growing crop, but it can be used for supplemental feeding under certain circumstances. Such circumstances include: 1) when soils are cold and N and P mineralization rates are low; 2) at the onset of nutrient deficiency symptoms in rapidly growing plants (verified by properly conducted leaf tissue testing); and 3) during periods of high nutrient demand, especially fruiting. Even so, nutrient deficiencies often result from indirect causes, such as water issues, soil compaction, pH, root diseases, or even macronutrient (N, P, or K) deficiencies that can be limiting micronutrient uptake or availability. Addressing these issues is likely a more long-term, as well as time- and cost-effective way to ensure crop nutrient needs are met.
New England soils are glacial in origin and are considered “young”. For this reason, our soils are not typically lacking in micronutrients. In soils with pH greater than 7, metal cations become less available to plant roots, and plants may show signs of deficiency. Most soils in New England are acidic, requiring periodic lime applications. Where soils are alkaline, the best way to correct deficiencies of Zn, Mn, Fe, and Cu may be to apply foliar sprays of these nutrients in chelated form. Certified organic growers should ensure that they are using forms allowed under organic certification. In some cases, it may be necessary to lower soil pH using products such as elemental sulfur, aluminum sulfate, or ammonium sulfate.
Plant Biostimulants, Biofertilizers, Microbial Biostimulants, Microbe-containing Bio-products
A biostimulant is a substance or microorganism (or mixture of one or more of these) applied with the intent of enhancing a crop’s nutrient efficiency, abiotic stress tolerance, and/or quality traits, regardless of the material’s nutrient content. There are now hundreds of commercially available products that fall into these categories. Products labeled for pest control purposes do not fall into this category.
Rhizobium inoculants for legumes are one well-known type of these products. Research has consistently shown the benefit of legume inoculation to realize full nitrogen fixation potential of legumes, provided that the plant and bacterial species are properly matched.
There has been a proliferation of mycorrhizal fungus inoculant products. These fungi are symbiotic with many crop plants (excluding brassicas and a few others). While extensive research has shown their beneficial effects on plant nutrition, growth, and stress reduction in field, nursery pot, and greenhouse conditions, it is still unknown whether inoculation has short- or long-term economic impact in annual vegetable production. The fungi live inside plant roots, where they obtain a carbohydrate energy source from plants. The fungal mycelia grow out from the plant roots into the soil and extract water and mineral nutrients, which are then transferred, in part, to the plant.
There are numerous other soil microbial inoculant mixtures available from commercial suppliers. Peer-reviewed research with many of these organisms has shown some positive potential. They are intended to influence crop plants’ rhizosphere, promoting potential availability of mineral nutrients already present in the soil, sometimes by stimulating plant responses to stresses or diseases. They do not, however, directly supply nutrient elements to plants. There may well be a promising future for microbial inoculants, particularly if it means reduced fertilizer input, but product effectiveness has not been well documented at this stage.
Compost Tea
The legal definition of compost tea from the National Organic Program is “a water extract of compost produced to transfer microbial biomass, fine particulate organic matter, and soluble chemical components into an aqueous phase, intending to maintain or increase the living, beneficial microorganisms extracted from the compost.” Microbial species content is highly variable, depending on the source of compost and the “brewing” conditions. Compost teas have a very low analysis of plant nutrients. Although they are widely produced and used on farms of various scales, research evidence of their efficacy is inconsistent at best. If you do plan to use compost tea, care must be taken to avoid cultivating bacteria harmful to human health (e.g., start with finished compost, use potable water, and avoid using additives like molasses).
Humates, Humic Acids, Humic Substances
These materials are made of very large and complex molecules. Most commercial products are extracted from peat or soft brown coal deposits of lignite. Extraction processes and treatments vary widely, so it is difficult to make comparisons between various products on the market. Humic materials contain only small amounts of plant nutrients, thus are not considered fertilizers. Their usage has been promoted by some to provide physical, chemical, and biological benefits to soils. These materials have been studied for over 50 years, mainly in controlled settings, with mixed results from laboratory and greenhouse studies. Under field conditions there are few documented positive effects from their usage. Naturally occurring compounds in soil organic matter effectively perform the same functions, such as chelation of micronutrient metals, and possibly producing plant hormonal effects. Nevertheless, there are many commercial products available, and little consistency among them.
Seaweed Extract Products
Seaweeds have been applied to agricultural land for at least a few thousand years. There is surprisingly little published research on their use and effectiveness in field settings. The nutrient content of seaweed is too low to directly increase soil test nutrient levels but it has been proposed that seaweed has several different effects on the rhizosphere; one common claim is that seaweed extracts alleviate the effects of environmental stresses, such as temperature and moisture extremes. The effect of seaweed applications are hard to detect when used during typical conditions and published research has not yet proven these effects.