I’m sure many readers are well aware how much of a hot topic soils are in recent times – so much so – it seems we are somewhat in the midst of a soil health renaissance. Farmers around the globe are re-engaging with the importance and potential of their most valuable asset.
As valuable as soils may be however, they are also highly vulnerable and some of the statistics on global soil erosion and degradation are alarming to say the least. In whichever way we may define it, soil health is emerging as a one part of a counterbalance. In our attempt to understand, define and therefore protect soils, efforts are being made to advance our grasp of this highly complex world beneath our feet – whether that be using high resolution instrumentation to sense the soil from Space, or simply using a spade and our human senses from the bottom of a soil pit.
Soil Health: A Meeting in the Middle
Throughout my travels I consistently notice this topic of soils or soil health bringing farmers closer together, finding common ground, perhaps more so than anything for many decades. There seems to be an emergence of a ‘middle ground’ between the paradigms of ‘conventional’ vs ‘organic’. These production principles are somewhat overlapping and blurring into one another – but also crystallising into a diversity of hybrid farming systems, no matter the label. Systems that are flexible, adaptable and dynamic but still have applicability across a range of soil types, climatic conditions or environmental parameters. It’s principles that guide these systems, not a one size fits all approach or ‘recipe farming’ where the program of management is the same irrespective of any local or landscape context. Concepts like agroecology, holistic management, biological farming or regenerative agriculture – which each uniquely encapsulate a significant soils focus within their frameworks – are becoming more mainstream and providing a melting pot of ideas, innovation and practical application at the field level.
It’s a pleasure to be joining you for a few articles in Terra Horsch. I am an independent plant and soil health educator with a particular focus on soil biology, plant nutrition and ecologically integrated approaches of food production. The bulk of my work is education and I have been fortunate enough to lecture to farming audiences in many parts of the world. I primarily work in Europe, Australia and Canada [where I am currently based]. I am originally from Australia where I studied Agricultural Science and then worked advising on soil chemistry and plant nutrition programs and the use of biofertilisers. After moving to the UK and working more throughout Europe, I began to engage more deeply with soil biology, composting and farming systems principles such as conservation agriculture and agroecology. These days I try to link all these concepts together into a joined-up approach that focusses on various aspects of soil health, plant health and ecologically integrated production.
Designing with Diversity
Another commonality amongst these middle ground systems is their redesign toward achieving a multifunctional focus beyond solely food production. Indeed, farming systems can achieve many outcomes, food production and ecosystem services do not have to be mutually exclusive. But systems have to be designed for those outcomes and that typically means designing with diversity. The debate does not have to stagnate at which production system is better or worse, but simply acknowledge that all production systems can be improved by designing more diversity into them1. This concept has been nicely conceptualised in Figure 1 – it really doesn’t matter where a production system sits or how it is defined somewhere along that continuum on the bottom of the triangle [from industrial to agrarian] – what is key is how well a production system integrates more diversity [top of triangle]. That could encompass practices like cover crops, intercropping or conversion of field margins/non-productive areas to wildlife habitat. We will return to this theme and expand on some of these concepts in the fourth and final part of this article.
So where to start? A critical first step on the transition toward soil health involves improving input efficiencies. There are many benefits to dialling down the application rates and/or frequency of both fertilisers and pesticides. Improving input efficiencies means less input costs – a win for farm profitability and reduced inputs can also improve many environmental outcomes2. Data from over 800 experiments has shown that on average, only 51% of the fertiliser nitrogen [N] applied to cereal crops was recovered by plants3. Although phosphorus [P] is nowhere near as volatile as N, it is still highly reactive – with applied inputs readily adsorbing onto soil surfaces and locking up in organo-mineral and cation-anion complexes. More than 80% of P applied as fertiliser can become unavailable for plant uptake shortly after application4 resulting in appallingly low P-fertiliser use efficiencies of 10-15%5.
There are a range of strategies that farmers can implement to improve their input efficiencies and I’d like to focus on four of these in this article:
- integrated nutrient management,
- carbon based inputs,
- seed treatments and
- foliar applied solutes.
Of course, every farm whether they have a focus on soil health or not should be working to improve nutrient use efficiencies, so these strategies have universal appeal to almost all production systems.
Firstly, integrated nutrient management [INM] refers to combining multiple strategies and all possible sources of inputs to manage production. This might include integrating together the use of inorganic nutrients, organic amendments, biofertilisers, biostimulants, waste/by-products, green manures, cover crops, and intercrops [particularly with legumes]6. As well as the use of multiple types of inputs, INM aims to match the nutrient amount and timing with crop requirements – often monitored and evaluated with a combination of soil and leaf analysis – with the aim of fine tuning fertilisation rates and timings while reducing losses, improving input efficiencies and yield.
Secondly, the use of carbon based inputs refers to including a carbon [C] source with all inputs – commonly fertilisers and pesticides. Eminent soil scientist Rattan Lal, from the Carbon Management and Sequestration Center at Ohio State University has often stated that the NPK revolution should have been a CNPK revolution which would have also included a more balanced focus on the role of C in soil fertility management. A traditional example of this would be the use of synthetic fertilisers used with organic manures. Other carbon sources that are often mixed with inputs include molasses, humic & fulvic acids, seaweed/kelp extracts, amino acids, fish hydrolysates or other plant extracts7. These C sources can be mixed with liquids for in-furrow injection or foliar application or also used to coat granular fertilisers. C sources can also be blended with traditional synthetic nutrients and formulated into granular C-based compound fertilisers that are ready to use8,9.
Thirdly, seed treatments offer a particularly efficient mode of delivery – targeting hyper low doses of inputs directly around the seed can improve efficiency of uptake vastly when compared to soil applied inputs. There has been a significant body of work exploring mineral nutrient treatments10,11 though I notice a trend from farmers towards using more biofertilisers and biostimulants as these biological based inputs appear to support the soil microbiota more effectively, as anecdotally evidenced by some fantastic rhizosheaths I have personally seen from these bio-inputs. Examples of these include biofertilisers such as compost and vermicompost extracts, specialist microbial inoculants such as mycorrhiza or N-fixing bacteria or a consortium blend of various known microbial species. The biostimulant options often include materials such as molasses, humic acid and kelp extracts for example.
Finally, foliar applications are commonly suggested to be considerably more efficient and economic than soil applied inputs12. Soil applied nutrients are more prone to leaching, volatilisation or locking up with other antagonistic minerals and so direct delivery of nutrients to the plant foliage can bypass these soil interactions and imbalances. Although foliar inputs can be more effective, they can also be rather variable and there are a few important limitations and considerations to ensuring adequate uptake and success – let’s explore a few of these factors in more detail.
Targeted Efficiencies with Foliar Applications
There are many variables that influence the success of a foliar application and consequently many factors to consider when formulating and applying foliar inputs. This topic could easily be a full length article on its own, but for introductory purposes, I will summarise a few of the key considerations. Nutrients are absorbed through the leaf via two main pathways – the stomata and micropores in the cuticle. Understanding the details of these pathways helps develop more targeted applications. As stomata are on the underside of the leaf and micropores found on both sides [and particularly at the base of trichomes] it is important to ensure you target the foliar spray on both sides of the leaves – this ultimately equates to more surface area for absorption via both pathways. The opening of both stomata and micropores is maximised with greater humidity so foliar applications should ideally be timed for the early mornings and late evenings13 [Figure 2]. Spraying during high temperatures and in the middle of the day should be avoided – I realise this may be difficult for larger operations so prioritise morning and evening applications to your poorer performing fields to give them the best possible chance of a boost. Regarding the spray formulation, consider factors such as nutrient solubility, chelation, nutrient concentration, spray pH and wetter stickers. A brief comment on each of these is below:
- Solubility: Inputs must be water soluble for optimum diffusion through the leaf.
- Chelation: Always combine a carbon source to chelate or complex mineral nutrients, this improves the diffusion and prevents antagonistic interactions between nutrients – molasses, fulvic acid, amino acids, kelp are perfectly suitable.
- Nutrient concentration: if spray concentration is too dilute, absorption across leaf surfaces will be slow. A spray EC [electrical conductivity] of 1.5-3 mS/cm is a useful guideline and can easily be measured with an EC meter.
- Spray pH: Generally, around 6 is ideal however targeted high or low pH’s for specific inputs or functions also exist. Very hard water with a high pH should be treated prior to mixing in nutrients.
- Wetter Stickers: Increases the adhesion time and rainfastness.
Careful consideration of the spray formulation and the timing of application can go a long way toward improving plant response to foliar inputs. Understanding the factors that influence the uptake and utilisation of leaf applied solutes can help overcome a ‘hope for the best’ or ‘spray and pray’ approach.
Where to Next?
Improving input efficiencies and thereby utilising less inputs in the production system is an ideal starting point on a transition toward soil health. There are both economic and environmental win-wins that emerge with this process. As farmers work through this first transition, they can also integrate a process of input substitution – substituting fertilisers and pesticides for more biologically based inputs. For example, substituting N fertilisers with N fixing bacteria or fungicides with fungicidal plant extracts. These bioalternatives come in a huge array of compounds and substances and we will explore the use of these materials in the next article [published online]. The third part in this series, also published online, will cover soil organic matter [SOM], with a particular focus on the emerging paradigm of SOM formation. There has been a great deal of interest in soil organic matter in recent years and many new studies and new ideas are emerging into the nature of SOM which challenge some of our previous thinking. These first three articles all have a focus on improving soil health in one way or another. They will aim to lay a foundation for the fourth and final piece [to be published again in Terra Horsch], which will focus on system redesign – redesigning the production system by integrating more biodiversity and ecological thinking. This will primarily include discussions on increasing plant species diversity within production areas but will also touch on the role of grazing livestock, trees and management of non-production areas such as field margins.
Read more about soil analysis and soil activity.
- Organic and Conventional Agriculture: A Useful Framing? (2017). doi: 10.1146/annurev-environ-110615-085750
- Reducing pesticide use while preserving crop productivity and profitability on arable farms. (2017). doi: 10.1038/nplants.2017.8
- Recent Developments of Fertilizer Production and Use to Improve Nutrient Efficiency and Minimize Environmental Impacts. (2009). doi: 10.1016/S0065-2113(09)01008-6
- Phosphorus activators contribute to legacy phosphorus availability in agricultural soils: A review. (2018). doi: 10.1016/j.scitotenv.2017.08.095
- Phosphorus cycling in UK agriculture and implications for phosphorus loss from soil. (2006). doi: 10.1111/j.1475-2743.2001.tb00020.x
- Integrated nutrient management (INM) for sustaining crop productivity and reducing environmental impact: A review. (2015). doi: 10.1016/j.scitotenv.2014.12.101
- The Use of Biostimulants for Enhancing Nutrient Uptake. (2015). doi: 10.1016/bs.agron.2014.10.001
- A slow release brown coal-urea fertiliser reduced gaseous N loss from soil and increased silver beet yield and N uptake. (2019). doi: 10.1016/j.scitotenv.2018.08.145
- Nitrogen Dynamics in Soil Fertilized with Slow Release Brown Coal-Urea Fertilizers. (2018). doi: 10.1038/s41598-018-32787-3
- Micronutrient application through seed treatments - a review. (2012). doi: 10.4067/S0718-95162012000100011
- Seed treatments for sustainable agriculture-A review. (2015). doi: 10.31018/jans.v7i1.641
- Foliar fertilization of crop plants. (2009). doi: 10.1080/01904160902872826
- Uptake and Release of Elements by Leaves and Other Aerial Plant Parts. (2011). doi:10.1016/B978-0-12-384905-2.00004-2
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