When are approaches to Plant Nutrient Management actually Sustainable?

When are approaches to Plant Nutrient Management actually Sustainable?
Sustainable Plant Nutrient Management

Torsten Mandal

Agronomist specialised in international sustainable agroforestry, land and soil management

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-Discussion, conclusion, and perspectives of sustainable nutrient management. Principles, terms, schools, and debate for sustainable nutrient management

Sustainable nutrient management is based on the combination of many facts and principles, to ensure nutrients are conserved, applied, recycled, and used efficiently without many losses to the environment. It is not scientifically clearly restricted to using or not using specific groups of inputs, rules, or methods.

Accordingly, it is hard to summarize it all briefly. Another related article in this chapter discusses the principles and terms compared to related terms.

Likewise, one article here by Mandal (2023) summarizes the most important practices and facts. Even strict restrictive principles can make sense, at least for certified marketing of, e.g., “Organic” products sold at premium prices based on clear rules largely based on consumer and grower preferences.  Some high-value products only contain small amounts of nutrients. Likewise, some soils are highly enriched with most nutrients and N-fixing plants can fit into the system.

Likewise, sustainable use of fertilizer, manure, and pesticides requires regulations that are good and followed.

Often, low input can give low output (yields). Terms like agroecology and food sovereignty are sometimes interpreted as commercial inputs like fertilizer that should be reduced or stopped, even in systems where such uses are low. [1] However, not all “inputs” must be bought (e.g., rainwater can be infiltrated, and biological nitrogen fixation promoted). Improved, specific, low-cost solutions can be needed, and not only popular belief in conflicting, imported standard visions of systems and restrictions on the use of commercial inputs.

Not all nutrients even need to be supplied by farmers, even if they are essential (necessary) or beneficial for plants and consumers – because they may be needed in small amounts compared to what is available and remain so. Likewise, not all outputs are used (erosion, burning, leaching deep down with water). Interpretations of the terms and strategies vary, too; read more about them in the article.

Sustainable Plant Nutrient Management among related concepts

While environmental, nutritional, and social factors are essential, avoiding commercial inputs like fertilizer is not easy, as documented in a research review assuming a restrictive definition of agroecology and food sovereignty (from, e.g., international corporations).  [1]

Sustainable plant nutrient management means using plant nutrients efficiently in ways that can be continued without damaging soil and the environment.

Financial and social sustainability may also be considered. In practice, “sustainability” is both used about reducing damage and trying to increase environmental, climate and social benefits. Many conflicting views exist on the definitions and practical use, so farmers must have an overview and specific knowledge based on facts.

Nutrients are often the most important for crop yields, costs, and environmental challenges. Both soil fertility, plant growing methods, and crop factors are important.

The term is, e.g., used by university departments with a focus on understanding and modelling where more nutrients are needed (often in emerging countries) and where too much is wasted (often in rich), and what can be done about each relevant nutrient. This can differ from some interpretations of the revived simple focus on fixed planetary boundaries (revived in the trendy Doughnut Model). Interactions and complexity are considered in Sustainable Plant Nutrient Management (SPNM), but in some cases, scientists may focus on specific nutrients (like Nitrogen (N) and Phosphorus or Phosphate (P)), which are very important worldwide [2]. SPNM covers all aspects of using plant nutrients more effectively and responsibly from organic and/or mineral/synthetic sources. This often requires a better understanding at all levels of crop variations in needs and uptake over time and place (even within fields), of recycling, nutrient release, and fixation. [3] Sustainable soil management focuses more on carbon content, tillage, and physical aspects (see my Wikifarmer pages on soil and water conservation). While advanced university research still contributes, the focus of Wikifarmer Library is explaining methods and principles small farmers can use directly or with help from a laboratory.

Integrated Plant Nutrient Management (IPNM) as part of Integrated Soil Fertility Management (ISFM)

Integrated nutrient management is a related term, but it is mainly used only for a combination of commercial/mineral and organic manure.

However, the way to obtain sustainable nutrient management can vary with the situation. Even certified organic farmers are sometimes or always permitted to use specific types of mineral fertilizers. Likewise, some farmers have an excess of rich organic manure, and some have access to very little or nutrient-poor manure. So far, some fields, for example, in northern Uganda, only visually appear to need nitrogen and more biological N-fixation, recycling, and soil conservation. However, the new soil maps indicates deficiencies can soon occur. Likewise, other fields in some nearby parts of western Kenya need more nutrients that so far cannot be made sufficiently available without “chemical,” soluble, concentrated fertilizers. Transport and price challenges vary too.  The big Vi Agroforestry project concluded fertilizers (or buying fodder) were essential to get a harvest in some areas. The International Fertilizer Association writes that IPN is needed as part of IPSFM (that, e.g., also includes soil conservation) [4].

The industry-controlled International Fertilizer Association welcomes agroecology in principle, but it recommends that many typical agroecology methods are integrated with the right source, timing, placement, and dose (4 R) of mineral, manufactured fertilizers (Integrated Plant Nutrient Management). They have free publications via the same link on improved, safer fertilizer use [4]. Still, IFA represents fertilizer companies and focuses on fertilizer aspects. Likewise, precision agriculture typically uses high tech to apply fertilizer and other inputs where it is most needed in time within the fields – as small-scale African farmers have traditionally done in their own ways.

Regenerative farming or agriculture

Unfortunately, the term “sustainable” can indicate continuing with the current level of soil fertility. “Regenerative” farming has been proposed as a term instead, to indicate the aim at improvements, not just maintenance. It is like “agroecology” used both broadly and in a restrictive sense. The focus is on regenerating higher organic matter content, microbial activity, soil cover, biodiversity, and reduced or no tillage. Regenerative agriculture is interpreted in many ways and is not a restrictive certification system suited for premium prices. It may transition to organic farming but avoiding both mechanical and chemical weed management is a big challenge incl. local learning process. Likewise, soil in the humid tropics depleted for nutrients and lime for many millions of years differs much from younger, temperate soils that may have been overfertilized for decades with some or many nutrients. [5]

Agroecology and food sovereignty

Sustainable nutrient management is, e.g., a key part of the broader Agroecology Department’s activities at public Aarhus University. Again, regenerative and agroecology have different meanings to different people. Some definitions focus on combined agriculture and ecology and make room for diverse realities and strategies. Some are more restrictive and demanding [3]. Diverse, equitable, resilient, traditional, local, healthy, and fair, “food sovereignty” can also be included and sometimes in contrast to “international corporate agribusiness” and free trade agreements. FAO’s overview of agroecology from 2018, with comments and complementary information, includes 10 principles that can reduce the need for mineral fertilizer and its losses of it [6]. See Figure 2.

In principle, the fertilizer industry and most soil fertility researchers embrace agroecology interpretations that are combined with “chemical” fertilizer use -at least when fertilizers are needed; see Integrated Plant Nutrition above [7].

Figure 2. Agroecology as defined by FAO includes agronomic, ecological, and socio-economic strategies rather than rigid restrictions. Credit FAO, 2018 The 10 Principles of Agroecology. FAO: The Agroecology Hub.

Nutrient replacement and commercial, “chemical” or “synthetic” fertilizers

An old, simple, conventional idea of sustainable nutrient management is to fully replace all the nutrients removed with fertilizer and other methods. Some still promote nutrient balance as the key to sustainability. Actually, it can make sense to maintain adequate or optimal levels of several nutrients, but e.g. for several micronutrients, availability rather than the total amount is often important.  Removals of nutrients can mean harvesting, leaching, evaporation, fire, and erosion. Phosphate is often fixed too hard to the soil for supplying young plants in e.g. cold or dry soil. This is a challenge too. It is a fundamental challenge to some sustainability ideas focused on stopping the use of what many call “synthetic,” “commercial,” and/or “chemical” fertilizers. However, in organic farming rules, it is rather directly water-soluble mineral fertilizers that are usually not allowed (natural or not). It can be hard to replace all nutrient losses by conserving, recycling, and biological N-nitrogen fixation. Particularly in the humid tropics, total nutrient contents are low after millions of years of leaching them out with hot water. Likewise, small farmers in the tropics may not be able to buy nutrients in the form of fodder or manure from other farms. Access to nutrients from grazing wetlands, pastures, forests, and out-fields is also reducing as well as access to land available for unproductive fallows to release nutrients tightly bound to minerals.

Claims that diverse, lasting plant cover, microbes or other “bio-stimulants,” and “compost teas” can solve the problems (and not just contribute at best) are often strongly promoted by people with commercial interests. They are often poorly documented – particularly for nutrient-poor soils in the tropics. In temperate countries, soils have been enriched by ice ages and sometimes sediments. Also, more phosphorus has been applied than taken up because young plants with short roots need higher doses when cold. Many microorganisms and substances can be documented as important to add for nutrient release and uptake in sterile laboratory studies, but they may be sufficiently available in usual soils. Acidifying bacteria or root excretions can, e.g., make phosphorus more available in some cases. However, it may not be enough. particularly in depleted old, acid, or sandy soil, particularly for young, demanding crops when soils are, e.g., cold, or dry and food crops are removed with good yields. Even in fertile mineral soils, it can be a problem.

Nitrogen and phosphate sustainability at the global scale

The sustainability of natural gas used today for making nitrogen fertilizer is also a problem due to high economic and environmental costs. However, a large fertilizer company is now starting to turn to renewable energy.

Depletion of limited natural resources is a concern too, but that is mainly relevant for good, relatively soluble sources of phosphate low in cadmium (Cd) [8]. Untreated rock phosphate permitted in organic farming is often hard to transport and use efficiently. Cadmium can be removed by processing (reduction and heating), and its content is usually well below official safety limits.

Recycling nutrients is used increasingly from biomass ash, sewage pipe, and sorted kitchen waste, e.g. in Denmark. In developing countries, so far, less of the consumed nutrients are returned to the fields on small farms with “modern”, deep pit latrines than traditionally. Health concerns are manageable but important, particularly for long-lasting resting stages (cysts, spores) of parasites and some bacteria.  The partly spiritually-/astrology-based old biodynamic movement is against any recycling of human faeces partly for spiritual reasons.

Biodiversity can be reduced by water pollution by too high amounts of growth-limiting nutrients for plants and bacteria in water. It is usually Nitrogen and/or Phosphorous causing such nutrient pollution (eutrophication). The problem gets visible when many plants rot on hot days with little wind and oxygen, so toxic gasses are dissolved and released killing or moving life in the water. In landscapes with much nitrogen from farms in the air and rain, like Denmark, small flowering plants needing a shade-free (infertile) site can be threatened even by ammonia N from the air. Thus, some endangered wild species need low soil fertility to compete.

In contrast, higher crop yields can reduce the need for farmland and may make more space for nature, forestry, and pastures possible (but may also make it more profitable to cultivate it). More plant growth can mean more carbon capture too.

Nitrogen fertilizer is despite several big challenges made from the air and a source of energy.

Some nutrients are sufficiently available in some biologically active, healthy soil, so it may not be a practical resource sustainability problem for several years, that the total or available amount of such nutrients may reduce.

However, if soil mining in the field is taken for granted as sustainable, the great dust bowl in the 1930s Midwest in the USA should be kept in mine.

High use of phosphate can be a sustainability problem on a global scale because good resources are depleted. However, phosphate in acid-treated directly-water-soluble forms is normally more effectively used than most rock-phosphates permitted in organic farming.

For seasonal crops, early P-availability is most critical, and soils may not be acidic. Transport is also less demanding for the more concentrated forms, but transport of strong, nutrient-containing acids (H2SO4, H3PO4, and HNO3) and the production includes some risks. Sometimes organic manures need little or no transport. In principle, some relatively soft rock phosphates can be used for several crops, while most are only suited for tree crops in acid soils – or for processing to more soluble forms.

Combined application with sulphur and special bacteria may acidify rock-phosphate in the root zone, –if both are locally available, -if the method is known, -if it acts fast enough, -if it is not harming roots, and, -if soil acidification is acceptable. Most other fertilizer nutrients are not from resources depleted in the nearest centuries at least.

However, nitrogen fertilizer is usually made with the use of much fossil energy and natural gas, but it can be made again with hydropower, or perhaps where natural gas would otherwise be flared. Renewable energy can be used with air (78% N2) for the Haber-Bosh process, but at present, using natural gas is a bit cheaper. It can even add value to renewable energy when supply exceeds demand and is again starting to be used. It is an artificial synthesis similar to biological nitrogen fixation – also from air to ammonia [9].

What is sustainable is debated among some, but often taken for granted. Oversimplified generalisations are easiest to communicate and regulate. Many aspects can be important, but it is important to know which are most important in specific situations and practical ways to handle them appropriately for each farmer, crop, or part of a field.

Sustainable nutrient management is not in conflict with current mainstream agricultural research, but it often differs from simply applying enough commercial fertilizer from some beliefs developed in opposition to commercial farm inputs.

Sustainability terms sometimes include social and economic sustainability. Social sustainability can require that most small farmers are not rapidly excluded from competing despite they are using only the inputs, knowledge, and land that they can get access to and afford  — or despite land remaining available for purposes other than agriculture. Economic sustainability means yields are high and resilient compared to costs.

Sustainability for climate complicates nutrient management as well.

High nutrient use efficiency helps, and high nutrient uptake by plants can result in more carbon capture by plants.

The use of fertilizer can increase energy consumption to produce and transport it and emission of the greenhouse gasses like the nitrous oxide N2O from seasonally wet N-rich soil.

Fertilizers can both be used to save, boost, or cultivate marginal areas like pastures and forests. Likewise, more heads of livestock can be fed and sold affordably too, and they emit greenhouse gases. However, better nutrient supplies of plants and animals can mean faster CO2 uptake, less need for land for food and fodder, and less greenhouse gas emission per kilo of livestock products, than if the livestock grows slowly. Some find livestock essential for organic and regenerative farming.

Recent, peer-reviewed, scientific literature reviews by soil fertility and plant nutrition researchers usually underline the combined need for: -better use of local needs and variations, and -often use of both commercial fertilizers in improved ways, and, -many other soil fertility strategies. Such strategies can be recycling, conservation, intercropping, agroforestry, integration of livestock and crop production when relevant, plus suited plant species and varieties/cultivars.

Combining manure and fertilizer often helps yields, soils, and profits, but synergy (positive interaction) is hard to document, and combinations are not always needed or possible. Sometimes, organic manure is most appropriate, sometimes fertilizer, and sometimes a combination. Likewise, many other methods can help too. [10] [11]

Some consumers (mainly in advanced markets) are willing to pay extra if crops are certified as grown “organically” e.g. without water soluble, mineral, fertilisers and modern types of chemical pesticides and some growers and authorities prefer it too.

To conclude, rules and beliefs can be relevant, but sustainable plant nutrient management is a broad goal with many tools to consider, not a set of rules or general beliefs that can briefly be explained and easily used for marketing certification. Therefore, farmers and advisors must understand the related practical principles and facts.

References:

[1] Mudombi-Rusinamhodzi G and Rusinamhodzi L (2022): Food sovereignty in sub-Saharan Africa: Reality, relevance, and practicality. Front. Agron. 4:957011. doi: 10.3389/fagro.2022.957011

[2] Sustainable nutrient management – WUR

[3] Sustainable Nutrient Management (au.dk)

[4]  IFA (2018): Integrated Plant Nutrient Management. 9 pp. IFA. Link: Publication Detail (fertilizer.org)

[5] Grant T (2022): Exploring regenerative agriculture in coffee production – Perfect Daily Grind

[6] FAO 2018: 10 elements | Agroecology Knowledge Hub | Food and Agriculture Organization of the United Nations (fao.org)

[7] Agroecology (fertilizer.org) Fertilizer use, Sustainability, IFA accessed 2023.

[8] Cadmium and Phosphorous Fertilizers: The Issues and the Science – ScienceDirect

[9] Fertilizer Industry Handbook 2022, Yara.com.

[10] Mudombi-Rusinamhodzi G and Rusinamhodzi L (2022): Food sovereignty in sub-Saharan Africa: Reality, relevance, and practicality. Front. Agron. 4:957011. doi: 10.3389/fagro.2022.957011

[11] Mucheru-Muna M, Mugende D, Pypers P, Mugwe J, Kung’u J, Vanlauwe B, Merckx R  (2013): Enhancing maize productivity and profitability using organic inputs and mineral fertilizer in central Kenya small-holder farms. Experimental Agriculture, 50, 250-269. doi:10.1017/S0014479713000525

See also; IFA (2018): Integrated Plant Nutrient Management. 9 pp. IFA. Link: Publication Detail (fertilizer.org)

Sustainable Plant Nutrient Management (SPNM): An overview

Sustainable Nutrient management: Introduction to concept, strategies, and principles

Nutrient conservation and cycling

Mineral fertilizers (including ash) and sustainability

Nitrogen (N): Essential for plant growth and yield but can have high costs to farmers, the environment, and health

Biological Nitrogen Fixation and seeding Legumes for Soil Fertility

The importance and management of Phosphorus (P) and Potassium (K) in plant production

Ion charges and secondary (=meso) nutrients: Calcium, Magnesium and Sulphur

How important are the Micronutrients for plants

Soil and plant analysis and field observations

When are approaches to Plant Nutrient Management actually Sustainable?

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