Mineral Fertilizers are Ecological and Sustainable Plant Nutrition Tool

Chemical fertilizers are often portrayed as unsustainable and nonecological components of present agronomical management. The purpose of this paper is to challenge this view, by arguing that mineral fertilizers are actually ecological and sustainable plant nutrition tools that can fully complement organic originating nutrients, found in composts and manures. But, before entering into the arguments, one must take into account that worldwide population is struggling against twin problems of the shortage of fertile soil, and of adequate quality irrigation water.

Table 1: Global reduction in per-capital arable land between mid-20th and mid-21st centuries

Therefore, the question of chemical and organic plant nutrition must be tightly connected with the urgent need to produce an increasing volume of nutritious food for the 9.7 billion people who are forecast to inhabit planet Earth by the year 2050.

Let’s examine the relevant aspects of chemical and organic fertilizers, judging them by their chemical economic, nutritional, ecological and sustainable features.

It’s All Chemistry Urea is one of the most concentrated nitrogenous fertilizers (46% N w/w), which explains immense worldwide demand, and positioning it as the most - used nitrogen fertilizer in the world (annual production of ~ 179 million tones, ref. Yara , 2022 ) . It is industrially produced by reacting ammonia and CO₂ , in many worldwide commercial facilities . Meanwhile, naturally occurring urea is the chief nitrogenous end - product of the metabolic breakdown of proteins in all mammals and some fish species. Urea occurs at appreciable concentrations in the urine of all mammals; hence, it is found at marked concentrations in organic manures utilized in organic agriculture and horticulture. But is there any difference between a urea molecule that has been produced in a cow’s liver , excreted and applied to the soil in the form of cow manure , compared to a urea molecule that has been industrially produced by reacting ammonia and CO₂ in a commercial nitrogen manufacturing facility ?]

From the perspective of a plant root, the answer would be in the negative. The chemistry is the same. When plant roots sense nutrients, such as nitrate, ammonium, urea, phosphate, sulphate, etc., in their rhizosphere , it will not matter whether their sources are chemical or organic . The plant will take them up by the same absorption mechanism, and NUE (nutrient use efficiency ) , regardless of their original source . The plant will also equally integrate these raw materials into its normal metabolic processes.

The environmental aspect of urea production
Putting aside the different caloric values of meat and crop production for human diets, both involve emissions. It is well documented that the gas methane (CH₂) is a byproduct of meat and dairy production. This gas is produced by microbes in the cattle’s stomach during its normal digestion process. hence it is called enteric.

The majority of methane is excreted through its respiratory tracts Methane is a 28-fold stronger greenhouse gas than CO, Figure 1 shows methane emissions from livestock in 2020 in the US, showing that beef cows were responsible for some 72% of the country s enteric methane, while dairy cows share amounted to approximately 25%.

Figure 2 shows that enternc CH4 was the protagonist, representing 70% of all GHGs, well above the 23% manure derived CH, while the share of NO, was lower at 7%. The total from Figure 1 is 175 million metric tonnes CO, equivalent.

Industrial production of urea consumes CO, as a feedstock, at a 1:1 ratio. The nitrogenous feedstock for synthetic urea is ammonia, and an ammonia plant will produce combustion-derived CO2, in the production of ammonia, so the consumption of CO₂ in urea production is one reason that ammonia and urea production units are co - located.

Although there are emissions related to urea and ammonia production because of energy intensive nature of process, it is worth remembering that each tonne of urea involves the consumption of 0.73 tonne of carbon dioxide. This process, however, cannot be considered as environmental carbon sequestration, because once it is applied in the field, it will soon break down , releasing the carbon dioxide that was fixed during production.

The environmental price of the distribution of manures and composts.
Since urea is the most concentrated solid nitrogenous fertilizer (46 % N), its transportation from the production plant to its application fields is very economical. But the situation is very different when manures and composts are used as nitrogen carriers because they are normally around 15-fold poorer in nitrogen contents, making its transportation to remote fields very costly in terms of carbon footprint In many ways, these delivery emissions are unavoidable unless nearly all fertilizers required for the crops’ nutrition are manufactured locally.

This is not very realistic and would also be subject too different levels of efficiency and environmental sustainability.

Nevertheless, it could be argued that largescale nitrogen plants in certain parts of the world are actually a more efficient way of producing nitrogen for the world, and therefore it is necessary to accept some emissions in its transport.

The challenge then moves to maximizing the efficiency of the urea that is applied.

Protecting against nitrogen losses after field application is one way to increase plants nitrogen use Efficiency. There currently exists an extensive array of methods that effectively protect industrially produced nitrogenous fertilizers, from disintegration, and from losing their nutritional value, shortly after field application. Some such methods are formulating them as slowrelease and controlled-release fertilizers. Other effective methods are applying the fertilizers together with urease inhibitors, like NBPT 4-bromophenylboronic acid and acetohydroxamic acid, or with nitrification inhibitors, such as DMPP Nitrapyrin, DCD and Ethoxyquin. These agents are now widely used in agriculture and horticulture, by blending them with chemical fertilizers or coating their granules during their production processes. These methods remarkably increase the NUE of these chemical fertilizers.

Trials intending to apply the latter methods to organic fertilizers have resulted, thus far, in very low success rates , mainly because some components of the manures neutralize the activity of nitrification inhibitors . A recent description of this neutralization pathway reveals that it is based on the competition between DMP ( and its derivatives ) and soil chelates , on acquisition of copper ( Cu ) and Zn ( Zn²t ) cations , which are indispensable for the activity of DMP ( Corrochano Monsalve , et al . , 2021 ) .

Other plant nutrients.
So far , the arguments were focused on nitrogen , because it is , by far , the major nutrient applied in agriculture . But very similar considerations are valid regarding other nutrients applied to plant crops in the form of chemical fertilizers, or as organic manures, namely potassium, phosphate, calcium, magnesium, sulphur and trace elements. Actually the feed stocks of many industrially produced mineral fertilizers are naturally found ores, or precipitates of ancient remnants of plants and living beings, such as calcium, phosphates, mined in in various parts of the globe such as Morocco Florida or in Finland. Therefore, in many cases, even industrial fertilizers originate from geological organic resources. It is necessary to relate here also to the common prejudice that chemical fertilizers contain noxious materials while organic fertilizers are free from such compounds. This is an unfair representation because modern fertilizer producers are now very sensitive towards the quality and pureness of their products, due to their obligation to comply with very strict human health and environmental regulations Organic fertilizers, on the other hand, suffer from low N contents, coupled with their frequent sodic or saline contents, and of herbicides, pesticides and heavy metals contamination. As the preparation of composts and manures is often done on farms that do not necessarily follow health standards and processing regulations, these products may harbour harmful pathogens. Additionally, their low nutrient contents negatively affect their large-scale usage due to very high energy costs and carbon footprint associated with their transportation and application. On the other hand, organic fertilizers contain a wide array of natural chelating agents that enhance the nutrients uptake by plant roots.

Integrative approach
The previous paragraph briefly outlined the various pros and cons regarding the usage of chemical andorganic fertilizers. But this does not imply that growers should stick to one type only. It is strongly recommended that a judicious combination of mineral fertilizers with organic and biological sources of nutrients be promoted for normal agricultural management, at flexible ratios that depend on the local agronomical conditions, actual prices and on the grower’s approach.

Furthermore, the well documented positive impact of biostimulants and biofertilizers, or soil conditioners, Suggests an approach that also integrates them in normal agricultural management, combined with both chemical and organic fertilizers. According to IFA’s publication at July, the world’s forecast consumption for 2021 amounted to some 111 million tonnes of N, 50million tonnes of P and 39 million tonnes of K. The continuous, almost metronomic consumption growth trend of mineral fertilizers leaves no doubt about their central role in agricultural management for decades to come. And the organic fertilizers are a welcome addition to the increasing demand for specialty fertilizers worldwide.