So what are biostimulants?
Basically they are biologically based products that can be applied to the soil or directly to plants, which result in a desirable effect on plant growth. Biostimulants are neither pesticides, soil improvers or nutrients but manufacturers of biostimulants often claim that they will improve the efficacy of these products or can in some instances replace them.
Any products that claim to control a particular condition or have beneficial effects on plant growth must be registered by the Australian Pesticides & Veterinary Medicines Authority (APVMA). The definition under the Agvet Code is based on the way a product is intended to be used, its ingredients and the claims made as to what it does. Biostimulants may contain nutrients but are not classified or registered as fertilisers, as any fertiliser must provide nutrients as its main function and biostimulants by their very definition promote plant growth by other means than providing nutrients.
Biostimulants can be classified into a range of categories which include:
• Organic acids
• Inorganic compounds
• Seaweed extracts and botanicals
• Humic substances
• Chytosan and other bipolymers
• Beneficial fungi
• Beneficial bacteria
The diversity of products in each category is quite broad and each plays a different role in relation to their impact on growth. Biostimulant functions can enhance nutrient uptake and efficiency, improve abiotic stress tolerance such as drought or frost tolerance and plant quality traits such as increased fruit yield and environmental benefits through increasing beneficial bacteria in soils and growing media. With an unprecedented increase in the incidence of extreme weather patterns due to climate change, biostimulants are becoming increasingly popular as part of garden maintenance schedules. Of all the categories listed, seaweed extracts and beneficial fungi provide the landscape manager with the greatest range of products that are readily available in the retail market.
From the ground up
Every landscape manager knows that soil biology is the key to plant success. Soil biology contributes to the fertility, productivity and sustainability of soils through organic matter turnover, improved soil structure, increased water holding capacity and a reduction in nutrient leaching and suppression of diseases. Biostimulants containing beneficial microbes can assist in meeting these requirements of good soil health if applied correctly and at the right time in the growth cycle of plants.
One of the most popular biostimulants used in ornamental horticulture in Australia are seaweed extracts. Due to the plant growth promoting effects and effect on plant tolerance to stress factors such as salinity, temperature extremes, nutrient deficiency, and drought, seaweed extracts have established themselves as the go to product when it comes to biostimlants. Apart from the growth promoting effects seaweed extracts have on plants, they can also improve the physical, chemical, and biological properties of the soil. Even at low concentrations seaweed extracts have been found to have a positive effect on plant growth. Scientific research continues into seaweed extracts and like most products there are those that have better qualities than others. Liquid seaweed extracts were regarded as a plant tonic because of their medicinal-like properties for enhancing plant growth. However, current research reflects a more sophisticated understanding of the metabolic compounds in seaweed extracts that have a direct effect on plant metabolism or indirect effects through the soil microbiology or by the interaction with plant pathogens. For centuries farmers and gardeners alike have exploited the beneficial properties of using seaweed in cultivating plants to improve soil nutrition and structure through composting. The current formulation of liquid seaweed extracts became available from the 1950s and have increased in their application and popularity ever since across agriculture, production horticulture and ornamental horticulture since. In a scientific paper published in 2015, Tony Arioli indicates that the first Australian company (Tasbond Pty Ltd.) began the manufacture of a liquid organic seaweed extract in 1970. In 1974, the company’s first commercial production of the liquid organic seaweed known as Seasol™ was in Tasmania (Australia). Manufacture was solely based on a local Tasmanian giant bull kelp (Durvillaea potatorum). The bull kelp was sourced from harvestors that collected the storm-cast seaweed from the pristinebeaches of King Island Australia. Tasbond Pty Ltd. was later purchased and since 1984 traded as Seasol International Pty Ltd. It has successfully pioneered the use of seaweed products in the Australian commercial and home garden segments and exported the liquid organic seaweed product for over 30 years.
There are of course other formulations of seaweed extract available and powdered formulations are now commonplace in the market. There are differences between products with a range of seaweed species being used in production however, it is products that have been derived from Durvillaea species that has proven to be the most beneficial to date. Water soluble powdered formulations derived from Ascophyllum nodosuma species of seaweed from the cold waters of the North Atlantic also provide the same benefits as other kelp products.
Beneficial Fungi and bacteria
Another range of biostimulants that are receiving interest are soil inoculants such as Mycorrhizal fungi. The term mycorrhizal is derived from the Greek origin of the two words mykes, meaning fungus, and rhiza, meaning root. These types of fungi have established symbiotic (or mutually beneficial) relationships with a wide variety of plants over time. According to research more than 90 per cent of plant species in their natural environment form a symbiotic relationship with beneficial mycorrhizal fungi. The networks created in the root system of plants
by the mycorrhizae essentially create more surface area for the roots to be able to access and absorb more water and nutrients from the soil. It stands to reason that when applied to engineered soils or container growing media, Mycorrhizal fungi will have a significant impact on plant establishment and growth.
Beneficial bacteria also make up a large component of the soil or growing media matrix. It is well documented that beneficial bacteria play a critical role in plant health and growth. The colonisation of the root zone of plants (the rhizosphere) by beneficial bacteria and fungi will have a positive impact on plant growth. Inoculating the soil or growing media with beneficials such as Trichoderma, Bacillus spp. and Rhyzobia spp. can aid in nitrogen fixation, phosphorus conversion to soluble form and increasing auxins that stimulate plant growth.
Humic substances include Humic acid, which is often grouped together with fulvic acid and humin. All three substances make up what we call humic content, or humus in relation to landscape soils. Humin is organic material that is completely stable, it cannot be broken down further or dissolved or altered by other elements in the soil. As such, humin does an amazing job of increasing the water holding capacity and improving the structure of soil. However, it contains no nutrients and doesn’t play a direct role in plant nutrition.
On the other hand, Fulvic acid is a nutrient facilitator. When applied to plants as a foliar spray combined with liquid organic fertiliser, its small particle size helps plants draw in and utilise those nutrients better. In soil, fulvic acid has what’s called a “chelating” effect on the nutrients plants need to grow. That means it keeps things like phosphorous or iron in a soluble form available to plants preventing them from binding together and becoming insoluble. Humic acid is like a cross between humin and fulvic acid, it provides some benefits to improve soil structure and hold water, though not as well as humin. It also helps plants uptake and absorbs nutrients, though not quite as well as fulvic acid.
Basically, humic acid is an all rounder when it comes to soil health.
Spoil your soil
It pays to spoil your soil in order to improve plant growth and appearance and biostimulants may be just the ticket when it comes to pampering plants. The problem is the number of products available of differing quality and efficacy. It’s therefore important to weigh up the cost of biostimulants against the expected gains and understand the benefits if they are used in the right situations in the right way. In most instances biostimulants are a good value proposition when it comes to increasing plant health as part of plant maintenance scheduling.