N=Nitrogen , P=Phosphorus , K=Potassium (from medieval latin Kalium = Pot of Ashes)
In agro-industrial farming these 3 macro nutrients are obtained by chemical synthesis and mineral extraction. It is now acknowledged that mineral fertilizers impact negatively soil life and provoke its death in fine, making the soil not suitable anymore for plant growth (compact and not adapted to nutrients uptake). External resource : Read this amazing article to understand why
Organic and sustainable management of the soil requires additional understanding, knowledge and some ecosystem monitoring. This mindmap gives a synthesis of NPK nutrients role, the consequences if not available and the different means to provide plants with them in the process of organic farming. (click on the mindmap to enlarge it)
Nitrogen is available from the air in a stable chemical form. Bacteria are the main organisms responsible for the capture (fixation) and the transformation of nitrogen into a plant edible form.
Mycorrhizal fungi play an important role by transporting biologically fixed nitrogen to plants in organic form, for example as amino acids.
The predominant form of Nitrogen found in the soil is protein (in organic matter and organisms).
The next predominant form is inorganic; (NO3, NO2-Nitrite, NH4)
– Annuals plants need the Nitrate (NO3) form of nitrogen
– Perennials need the Ammonium (NH4) form of nitrogen (feeding trees with nitrate can provoke disease)
Potassium is found in the soil in 4 different forms;
– Potassium as a component of soil minerals
– Fixed potassium
– Exchangeable potassium
– Water soluble potassium
The first 2 forms exist in high quantity in the soil (for example clays of type vermiculite, illite and smectite have a lot of fixed potassium). These 2 forms are not directly edible by plants. Fixed potassium is released in low exchangeable potassium conditions, and mineral potassium is released by weathering, which is a slow process. Recent studies show that silt may represent as well an important reservoir of potassium.
Water soluble potassium which is present in compost is edible by plants but subject to leaching when heavy rains.
Organic matter in the soil favors bacterial, fungal and root activity which accelerate the transformation of non edible potassium into edible one. Macro-organisms (e.g. worms) plays as well an important role by preparing the soil for further bacterial and fungus processing.
To summarize Potassium is an abundant resource which need adequate management (living and active soil ecosystem, good monitoring of compost storage and distribution). Culture rotation may not be needed if the soil ecosystem is very alive and active although guilds will increase biodiversity of the soil ecosystem and concur to its fertility.
Phosphorus is the Achilles’ heel of industrial agriculture and the world food supply (read more at Yale.edu). Reserves of phosphate in the world are now handled by a reduced number of countries (Morocco and China) and Phosphate peak could happen in the next decades. Although fossil fuel replacement and water recycling are predominant concerns and see a larger spectrum of solution every day phosphorus world depletion is ignored by most of the population and represents the real challenge when it comes to aggro-toxic farming.
The challenge is simple; plants cannot grow without phosphorus and this mineral become a limited resource. The only solution is therefore reducing, recycling and increase phosphorous availability in the soil. At the moment industrial agriculture is posing a threat to humanity by wasting enormous quantities of phosphorus which end-up in the rivers and the sea.
– A vegetarian diet represent 0.6kg of phosphorus per year, a meat-based diet requires 1.6kg phosphorus a year
– reducing the usage of phosphate fertilizers to the exact required proportions
– Recycling farm waste
– Recycling human wastes (sewage) Since crops leave the farm to feed the population it is necessary to get the phosphorus back to the location of production. The most advanced countries in recycling organic matter or sewage already perform such a recycling.
Phosphorus concentration varies a lot depending on the soils. As for potassium only a small percentage of it is edible (in solution) by plants. Various techniques allows to increase this availability.
– Increase of microorganisms activity (living and active soil, rich in organic matter). They will mineralize organic matter to make phosphorus available and degrade non edible forms of phosphorus compound to transform them in an edible form.
– Phosphate moves very slowly in soils, deep roots and symbiosis with fungus helps in reaching phosphorus in lower layers of the soil. This can be facilitated by a good soil structure and the mix with perennial plants having in average deeper root system.
To go further and generalize; recycling (mainly avoiding leaching) and soil microbiology are the answers to NPK uptake
Dr Elaine Ingham (microbiology world famous expert) goes further;
– NPK is in enough quantity in the soil for millions of years of agriculture. Just is needed a living soil and its food-web to make it available to plant
– Taking care of NPK fertilizing only is a simplistic vision and more we go more we realize the importance of other chemical elements.
– Classical soil analysis inform only on the ratios of directly available minerals not taking into account the reserve of non directly available minerals that the food web can convert into nutrients. As well soils analysis by giving a PH value does not highlight the fact that PH varies in high proportion at different locations of the plant roots; the plant being able to bio-chemically orchestrate the soil ecosystem (including PH) to feed itself and optimize nutrients exchange at root level.
– Compost, which is an inoculum and not a direct nutrient is key in giving back life to the soil.
Here is Dr Ingham at a conference – a dense and extremely interesting talk