Additional Soil Fertility and Nutrient Amendments

There are many different types of commercially available soil amendments and plant nutrient sources on the market today.   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.  There are many  categories of such products available for purchase. 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 nature of the material itself, e.g. derived from biological sources. It does not necessarily indicate acceptability for certified organic production. Such materials included in this category are: processed slaughterhouse wastes, leather processing waste, biosolids, papermill sludge, and 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 is a common practice among vegetable farmers. There are many products available on the market, including those used in both conventional and certified organic production.  Foliar feeding is not recommended as a major source of nutrients for a growing crop, but under certain circumstances, it can be used for supplemental feeding. Such circumstances include: 1) when soils are cold and nitrogen and phosphorus mineralization rates are low; 2) at the onset of nutrient deficiency symptoms (verified by properly conducted leaf tissue testing) in rapidly growing plants; 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, or root diseases or even macronutrient (N, P or K) deficiencies that can be limiting micronutrient uptake or availability. Addressing these issues may be a more effective and permanent way to ensure crop micronutrient needs are met.

New England soils are glacial in origin and are considered “young.” For this reason, soils are not typically lacking in micronutrients. In soils with pH greater than 7, metal cations become unavailable 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

According to a recent review article (du Jardin, 2015), a plant biostimulant is “any substance or microorganism applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrients content. By extension, plant biostimulants also designate commercial products containing mixtures of such substances and/or microorganisms.”

There are now hundreds of commercially available products that fall into these categories. This does not include products labeled for pest control purposes, which fall under strict EPA guidelines  each with their own EPA registration number.

One category of these products is familiar to most: various strains of species of Rhizobium inoculants for legumes. 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.

In recent years, there has been a proliferation of Arbuscular Mycorrhizal Fungus (AMF) inoculant products. In cultivated soils, these fungi are symbiotic with many crop plants (excluding Brassicas) and extensive research has shown their beneficial effects on plant nutrition, growth, and stress reduction in field, nursery pot and greenhouse conditions. The fungi live inside plant roots, where they obtain a carbohydrate energy source from plants. In turn, the fungal mycelia explore the soil volume for mineral plant nutrients (more efficiently than plant roots) and transfer these nutrients to plants. In this way, the fungi extend the “rhizosphere”, or immediate micro-environment that surrounds plant roots. Unfortunately, results of testing these various products on the market under real-world conditions are not readily available. It is hard to know whether inoculation has any short or long-term economic impact. It will continue to be a research topic of great interest.

Other microbial inoculant products

There are numerous single-species and multi-species mixtures of soil microbial inoculants available through standard commercial suppliers. Academic research with many of these organisms, both fungi and bacteria, has shown their positive potential. They act to influence the crop plants’ rhizosphere, promoting potential availability of mineral nutrients already present in the soil. This is sometimes accomplished by stimulating plant responses to stresses or diseases. They do not, however, directly supply nutrient elements to plants. Successful field use of many of these products may not be predictable, though they are based on known biological relationships. Industry researchers suggest that these products have to be incorporated as part of a regular program, and care needs to be taken in the use of other soil amendments in order to positively influence successful establishment of these microorganisms in crop soils. There may well be a promising future for microbial inoculants, particularly if it means reduced fertilizer input, but product efficacy is not well documented at this stage.

Compost Tea

From the National Organic Program, the legal definition of compost tea 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.” Strictly as a supply of soluble plant nutrients, compost tea has a very low analysis. The variability of microbial species content is high, depending on the source of compost and the “brewing” conditions. Although it is widely produced and used on farms of various scales, reliable research evidence of its efficacy is inconsistent at best. Benefits of using commercial microbial inoculants are variable, and using compost tea is even less dependable, regardless of its widespread popularity. If you do plan to use tea, care must be taken to avoid cultivating bacteria harmful to human health.

Humates, humic acids, humic substances

As the name implies, these materials are akin to highly decomposed soil organic matter, 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 are not fertilizers, since they contain only small amounts of plant nutrients, but their potential usage has been classified as providing soil physical, chemical, and biological benefits” (Mikkelsen, 2005).

These materials have been studied for over 50 years, mainly in controlled settings, and much has been concluded about their “potential” benefits. Even so, mixed results have come from laboratory and greenhouse studies, with some resoundingly positive reports, many neutral, and a few with detrimental results. Under field conditions, it’s even harder to find positive effects “at typical commercial application rates in representative field soils” (Hartz and Bottoms, 2010). These application rates are extremely low in comparison to the content of naturally occurring compounds in soil organic matter which would effectively perform the same functions, such as chelation of micronutrient metals, and possibly 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 and until recently, their primary benefit was considered to be similar to that of other organic amendments, releasing nutrients through decomposition. It was discovered over 50 years ago that seaweed nutrient content was too low to directly boost soil test nutrient levels and that other growth stimulating mechanisms must be involved (Craigie, 2011). Seaweed has been proposed to have several different effects on the root zone environment (rhizosphere) and on plants themselves.

Though seaweed extracts are used in crop production in large quantities, world-wide, there is surprisingly little published research on their use and effectiveness in real vegetable field settings. One of the more common claims is alleviation of the effects of environmental stresses, such as temperature and moisture extremes. Therefore, when used during typical conditions, their effects are hard to detect. Subtle effects are difficult to measure in the field, alongside of many other possible factors.


Berruti A, Lumini E, Balestrini R and Bianciotto V (2016) Arbuscular Mycorrhizal Fungi as Natural Biofertilizers: Let’s Benefit from Past Successes. Front. Microbiol. 6:1559. doi: 10.3389/fmicb.2015.01559

Craigie, James S. 2011. Seaweed extract stimuli in plant science and agriculture. Journal of Applied Phycology. 23:371-393.

du Jardin, Patrick. 2015. Plant biostimulants: Definition, concept, main categories and regulation, Scientia Horticulturae, Volume 196, 30, Pages 3-14, ISSN 0304-4238,

Guodong Liu, Edward Hanlon, and Yuncong Li. 2015. Understanding and Applying Chelated Fertilizers Effectively Based on Soil pH.

Hartz, Timothy K., and Thomas G. Bottoms. Humic Substances Generally Ineffective in Improving Vegetable Crop Nutrient Uptake or Productivity. 2010. HortScience. 45(6):906-910.

Lyons, Graham and Yusuf Genc. 2016. Commercial humates in agriculture: real substance or smoke and mirrors? Agronomy 6(50).

Khan, W., Rayirath, U.P., Subramanian, S. et al. J Plant Growth Regul (2009) 28: 386. doi:10.1007/s00344-009-9103-x

Mikkelsen, R.L. 2005. Humic Materials for Agriculture. Better Crops/Vol. 89 (2005, No. 3)

National Organic Standards Board Crops Subcommittee Proposal Hydroponic/Aquaponics/Bioponics September 6, 2016. 

Wang, Zheng and Julie Laudick. Microbes in OMRI-listed (February 13, 2017) Products Advertised to Enhance Crop Growth. Vegetable Production Systems Laboratory, Dept. of Horticulture and Crop Science, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, OH.