In this article, Daniel Ratling, BSc Hons. PCert TSC, Head Groundsman at Whitgift School in London, considers plant-soil interactions and how we turf managers might improve this vital region of the turfgrass ecosystem.
Broadly speaking, plants are immobile and are limited in their capacity to obtain water, nutrition and other beneficial substrates. Above ground is the aerial portion of the plant. This is the lungs, the powerhouse, converting the suns energy to food in the leaf to provide energy for the plant to metabolise. This could not happen without the below ground activity of roots.
As well as offering structural support and facilitating transpiration, roots also exude compounds, beneficial to organisms within the soil, with the aim of enhancing the environment in which the plant is growing.
Plant roots can move soil particles as they grow, create pore space, and physically reorganise their environment, although this is restricted by compaction of the soil and the soils mechanical resistance to root penetration. A well aerated, friable soil offers less resistance to root penetration and allows air (oxygen) into the soil. Oxygen is critical to the success of both roots and soil microbes. Total soil pore space can range from 35% to 55% by volume in an ideal mineral soil, but may be less due to compaction. Total pore space is less important than size and interconnectivity of pores. Pore size can be broadly divided into micropores and macropores. Macropores drain water via gravity, allow for aeration of the soil and gaseous exchange with the atmosphere. Micropores retain water under tension, therefore water losses and gaseous exchange are slower. In practice, most sports pitches will have a complex matrix of different pore sizes, the makeup of which may be determined through the careful selection of appropriate sand-soil materials during design and construction of high end sports pitches, or can be influenced in the fullness of time via long-term management of native soil pitches.
Soil is not only complex in structure, but also in the complexity of life it supports. It is a diverse ecosystem containing a wide range of organisms including Bacteria, Archaea, Fungi, Nematodes, Protozoa, as well as more complex organisms such as Ants and Earthworms. Distinct from the 'bulk soil', an area of great interest is the rhizosphere. The rhizosphere is the area at the root-soil interface. It is on a scale of millimetres but can sustain an immense variety of organisms. It is directly affected by the plant itself and is the habitat of many soil-borne microorganisms that facilitate plant life and therefore life itself. Typically, the microbial community in the rhizosphere is more populous compared to the bulk soil but is less diverse due to the selective pressure of plant rhizodeposition.
Rhizodeposition is the release of root exudates into the rhizosphere. Root exudates may take the form of simple sugars formed during photosynthesis, amino acids, protein complexes or organic carbon residues. Some plants direct a significant proportion of photosynthetic product to rhizodeposition. Increases in organic soil carbon rhizodeposits have shown bacterial increases of up to 5 to 10 times when compared with the bulk soil. Soil organisms confer significant benefit to the plant, so much so that plants are reliant upon them to survive. These relationships are 'active' and over millennia have evolved complex chemical signalling pathways and exchange of substrate for the mutual benefit of both organisms.
There is evidence to suggest that the maturity of the plant influences selection of rhizodeposits. For example, sugars may be exuded in greater quantity at seedling stage, potentially attracting a broad range of microbes during plant infancy. This will typically boost the overall population numbers of organisms in the soil and critically within the rhizosphere. In contrast, rhizodeposition of amino acids and organic compounds have been shown to increase with maturation of the plant, possibly as it selects for specific beneficial organisms at various stages of its life cycle. Different plants have been shown to select for a core range of organisms within the rhizosphere through exudate deposition. These organisms can form a protective layer around the root system known as a biofilm. In doing so, groups of organisms act on mass to protect the plant from pathogen attack and confer various other unique benefits.
Plants can both enhance microbial function through secretion of specific exudates or increase the number of beneficial microbes to perform a specific function. Some microbes promote plant systemic response to abiotic stresses and allow the plant to better tolerate drought or heat stress. Others directly boost the plant's own immunity against attack from pathogens or act directly on the pathogen to the benefit of the plant. The array of molecules, including naturally occurring antibiotics, produced by some Plant Growth Promoting Rhizobacteria (PGPR) to combat soil borne pathogens is staggering! This is before you even consider the interplay of plant roots, Mycorrhizal Fungi and the myriad of other plant-soil interactions known and unknown. This is an important and exciting area of research and one that is increasingly relevant in our industry. Already we have lost the use of several pesticides, it seems we will face increasing legislative and public pressure on our use of resources, as well as the quantity and type of inputs. Additionally, we will increasingly have to consider how we interact with, and impact upon the environment. We need to demonstrate that we are a force for good. Key to this is understanding how the soil works. How its existing function could be utilised, even encouraged, to improve pitch performance and playability, this is critical if we are to meet these challenges head on.
Clearly there is much that nature and evolution has put in place a long time before sports turf managers came along. In an undisturbed grassland, the natural ecosystem may maintain equilibrium that requires no outside interference or supplementation. Of course, an undisturbed natural grassland is unlikely to be robust enough for sport or offer adequate playing surface performance or quality. In many sports turf situations, soils may be compacted through play or maintenance operations. Typically, monostands of grass conferring desirable traits are sown, and in some instances soil profiles may be artificially constructed using materials offering benefits to playing characteristics and pitch carrying capacity. Of course, this is all entirely necessary for the good of sports provision and is where we turf managers come in, attempting to not just manage nature, but work with it to produce high quality natural turf surfaces.
Given the very clear benefit of a healthy soil ecosystem, as well as the known and unknown impact we may have as sports turf managers. It seems sensible to give some thoughts as to how we can best improve or enhance life in the soil and thus the performance of our sports surfaces.
Of all cultural practices, apart from mowing, aeration would seem the most important. Allowing gaseous exchange to take place is critical. For plant or microbe, a soil devoid of oxygen is not a good place to be. Often, aeration will be coupled with a means of decompaction with the resultant reduction in soil bulk density and increase of air-filled pore space, a proportionally higher number of macropores and associated gaseous exchange and drainage. Aeration not only directly benefits the grass plant by allowing air and water to the roots, but also benefits the diverse ecosystem that lives within the soil and confers all the essential benefits the grass plant and wider soil ecosystem could not do without.
Soil fertility is important for successful grass plant growth and, if lacking in any of the essential nutrients, grass plants will be limited by that deficiency, may suffer stress and potential failure. Therefore, fertiliser application may be necessary to maintain nutrition at an acceptable level. Evidence suggests that, as nutrition increases, the composition of microbial life within the rhizosphere and the bulk soil changes. This may be for a number of reasons; perhaps the benefit of 'good' soil fertility and the selective pressure of plant rhizodeposits alters the microbial composition, or in some instances it may be a negative response to accumulations of inorganic molecules and salt damage within the soil ecosystem. I have written previously on Salt Indices and Soil Microbiology, so I will not go over old ground here. However, it is worth reiterating that nutritional inputs should be well thought through and considerate.
If possible, limit the use of plant protection products (PPP) as much as possible. Clearly not requiring PPPs is an ideal scenario, but in some situations that may be entirely unrealistic without significant grass failure or unacceptable levels of pest/disease outbreak. An Integrated Pest Management strategy should be in place and, following that process, applying a Fungicide or similar should be the last step anyway. For many, using PPPs is not affordable so isn't an option. In some cases, tolerable levels of pest or disease incidence can be achieved using sound cultural, mechanical and biological practice.
Biostimulants and Plant Health Elicitors are viewed as sustainable alternatives, or supplements to chemical fertilisers, and in some instances fungicides. They are typically applied at low rates compared to fertilisers but are often expensive in comparison. Biostimulants differ from fertilisers in so much as they attempt to improve plant development or health without reliance on essential nutrients. They do this by interacting with plant signalling pathways, reducing the onset of stress, or promotion of existing/introduction of new beneficial microbial communities. A huge range of biostimulants are available for use, the following list is not exhaustive but includes - sugars, seaweeds, humus derived products, protein hydrolysates from both plant and animal sources as well as beneficial fungi and bacteria.
Some, like seaweed, have been used in agriculture and horticulture for millennia. Others, like PGPR are new commercially, although of course exist naturally within the soil and have done for aeons. Plant Health Elicitors include the likes of Phosphite and are used to activate plant systemic response mechanisms. Phosphite has been proven to prime the plants defences against Microdochium nivale pathogen activity through the formation and accumulation of defence compounds at fungal entry points, leading to M. nivale suppression.
Many biostimulants will offer some benefit to plant and soil health given the right conditions during and after application, but that poor product selection, timing, misapplication, or inadequate cultural practices could render them ineffective. If using biostimulants, they should be regarded as a component part of an overall management strategy and not as a replacement for sound cultural practice and fertility management.
Resources are often tight, and we have a duty to get best value for our employer, not to mention our responsibility to manage sports surfaces in a sustainable and environmentally sensitive way. As sports turf managers, what we are trying to achieve is to bridge the gaps in nature on often artificially created, certainly artificially managed and sometimes stressful growing environments. Keeping our pitches aerated, considerate fertilisation and the promotion of good soil biology makes sense and is often economical in the long term. The information garnered from ongoing research into soil microbiology and the plant-soil interactions of the rhizosphere should only serve to empower our decision making and further our knowledge of how our sports surfaces might perform at their best. Get the soil functioning right and everything up top gets a whole lot easier.