P-K-Mg Fertilization
The objective of soil fertility management is to meet the nutritional needs of plants by supplementing the soil's supply of mineral nutrients if necessary, while minimizing the leaching of nutrients deep below the rooting horizon or on the surface by runoff or erosion.[1]
Fertility management is based on 4 criteria identified as the most decisive : soil analysis, the crop requirements, the recent fertilization history and residue management. The reasoning mainly concerns the crops on which it was built, i.e. the field crops, the forage crops, including temporary grasslands in rotation less than five years old.
Scientific basis of fertilization reasoning
Plant nutrition
The total quantity taken up under optimal conditions (also called "total needs"), which can be broken down into exported quantity and returned quantity, is based on a flow of bioavailable elements supplemented by possible fertilization.
With elements P, K, Mg, biomass production increases with soil supply, up to a "non-response threshold" where yield plateaus and where increasing element supply only leads to so-called "luxury uptake" because there is no yield benefit. Achieving potential yield means that, given the element concentration in the soil and the roots' ability to take it up, the supply was sufficient relative to demand during the critical phases determining yield.
For a given soil and mineral, the non-response threshold, as well as the slope of the curve below the threshold, will depend on the plants' ability to more or less easily meet their mineral needs and to remobilize their reserves to growing zones. The plant's sensitivity to "requirement" can thus depend :
- The relative size of the root compartment compared to the aerial parts.
- The chemical and biological environment created by the roots, more or less favorable to mineral mobilization.
- The roots' own absorption capacities.
- The mycorrhization abilities, etc.
P-K-Mg in the soil
Plants take up nutrients in their ionic form present in the soil solution. For K and Mg, these are the ions K+ and Mg2+. For P, these are ions from the phosphate family (H2PO4-, HPO4 2-, in variable proportions depending on soil pH) which are the very dominant stable form of phosphorus in the biosphere. Plants do not absorb organic compounds, which must first be mineralized by soil biological activity (worms, arthropods, bacteria, fungi…). Phosphates in solution are generally in dynamic equilibrium with sorbed* forms on the soil matrix, which replenish the available fraction in the soil solution.
Whatever the element (P, K or Mg), one can schematically represent soil stocks as a pyramid topped by directly assimilable forms (elements in ionic form in the soil solution), themselves in dynamic equilibrium with forms whose solubilization involves only simple ion exchange (K, Mg), or low-energy desorption (P). These exchangeable ion compartments or easily desorbable forms are themselves in equilibrium with other mineral fractions of the soil, but less reactive, and so on.
The fraction of elements that can effectively participate in crop nutrition is called "bio- or phyto-available", i.e. the mineral portion that can go into solution during the crop cycle. For K and Mg, the chemical species considered bioavailable are the solution and exchangeable fractions and possibly a bit beyond. For P, one cannot a priori know the list of chemical species that will contribute to plant nutrition.
Bioavailability of elements supplied by different sources
- Mineral fertilizers : Only the total content, generally water-soluble, of mineral fertilizers matters. For phosphorus, regulations classify phosphate fertilizers according to their solubility in conventional reagents. Water solubility indicates immediate availability, ammonium citrate solubility indicates rapid release. Solubility in other reagents (e.g. 2% formic acid cited for natural phosphates) indicates availability over the crop cycle, depending on soil acidity conditions where the fertilizer is applied (GFR chap. 8.I).
- Crop residues : Their mineral content becomes available within weeks or months after incorporation, depending particularly on chopping and incorporation conditions that facilitate degradation. This availability is almost immediate for K which, remaining ionic in plants, is not bound to organic molecules. For other minerals with variable organic fraction, availability requires a somewhat longer process involving first degradation followed by mineralization of residues.
- Organic fertilizers and amendments : This is an intermediate situation between fertilizers and residues : K is immediately available, Mg and P are mostly present in mineral form. The availability of complementary organic P depends on the overall composition of the amendment (notably C/N/P ratio), relative to that of humus.
As with nitrogen, the fertilizing value of a fertilizing material can be characterized by the Real Utilization Coefficient (CRU), the Apparent Utilization Coefficient (CAU) and the equivalent fertilizer coefficient (Keq).
- CRU represents the fraction of the nutrient input actually taken up by the crop, which requires the use of isotopically labeled elements to be calculated. It is therefore rarely measured.
- CAU represents the ratio of the additional nutrient taken up by a fertilized crop compared to the same unfertilized crop (control), over the total amount of nutrient applied. It depends on application timing, crop, and mineralization duration considered. In the long term, all P from an organic product will eventually be available, except for insoluble mineral forms it may contain (e.g. some manures rich in CaCO3 with possible P precipitation).
- Keq, which corresponds to the ratio of the CAU of a mineral or organic fertilizer to that of the water-soluble mineral fertilizer used as reference (superphosphate triple for P, ammonium nitrate for N).
Evaluation of the fertilizing qualities of organic fertilizers and amendments should take into account the P mineralization kinetics (estimation under standardized laboratory conditions).
Implementation and COMIFER method
Crop requirements
Field trial results have shown that not all crops respond the same way under nutrient deficiency. Crops for which yield is relatively more impacted are considered "demanding".
The concept of requirement depends on the nature of the harvested organs. For example, total biomass of maize (maize forage) is more affected by P deficiency than grain yield : forage maize is thus more demanding in P than grain maize. The concept of requirement is not related to the quantity or speed of uptake by plants. It expresses the result of a set of mechanisms involved in mineral uptake from the soil and their use in the plant, whose overall coordination is not yet fully understood.
The criterion to quantify this requirement is the relative production loss :
(maximum yield – actual yield under nutrient limitation) / maximum yield
| Requirement | Regarding phosphorus | Regarding potassium |
|---|---|---|
| High | Sugar beet, rapeseed, Alfalfa, potato | Sugar beet, potato |
| Medium | Durum wheat, forage maize, barley, | Rapeseed, alfalfa, grain maize, forage maize, pea, ryegrass, soybean,
sunflower |
| Low | Oat, soft wheat, grain maize, | Oat, soft wheat, durum wheat,
barley, sorghum |
Soil content at analysis
A recent soil analysis is essential for fertilization reasoning in P, K and Mg. For phosphorus, several analytical methods exist in France giving very different results for the same soil sample. Care must be taken with interpretation of values which must always be relative to the same analytical method. The thresholds used in the COMIFER grid are linked to the type of trials exploited for the development of this reasoning scheme, with their possible experimental biases.
These thresholds may vary depending on crop requirement level.
Age of last fertilizer application
Older fertilizer applications tend to dilute in the soil solid phase, or even undergo chemical changes making them less bioavailable over time. This is called fertilizer aging in the soil. Older nutrient applications are less bioavailable than recent ones. In other words, soil fertility decreases as part of the inputs may become poorly soluble or at least forms for which diffusion to the soil solution is slower.
While a very recent soil analysis should allow estimating the current availability status of elements in the soil, sometimes reasoning must be based on an analysis several years old. In this case, even if barely detectable by analysis, the decrease in bioavailability can impact yield. Although this phenomenon seems to depend greatly on soil type, it is guarded against by increasing the multiplier coefficient according to the age of the last fertilizer application.
Crop residue management
In the case of potassium and even magnesium, present in greater quantity in vegetative parts (stems, leaves, etc.) than in reproductive parts (seeds) of plants, export of these residues can represent a K or Mg flux much greater than that exported by harvests, especially grain. It is therefore necessary to take this into account. The 2009 COMIFER Grid thus recommends applying to the following crop a K dose corresponding to straw export, if the initial soil supply is below Timp (depletion threshold), i.e. aiming at least to stabilize soil content. In soils with higher content, no compensation is made, relying on soil stock consumption.
Crop residues export a relatively small amount of P compared to harvested parts, due to their lower P content. Nevertheless, as with K, compensation of exports is recommended if soil content is below Timp, in which case a phosphorus supplement equal to straw export is applied.
Dose capping
More generally, total input in field crops is capped because very high inputs, which could be calculated in situations of low soil content and high yield target, are not as effective as expected. It is preferable in this case to make regular applications. It is also capped at a lower level for forage crops. The high K content of forage crops, combined with high harvested tonnage, can lead to very high dose calculations, which can be limited because very high doses calculated by the usual procedure have been experimentally shown unnecessary to reach the target yield.
Valorization of residuals from previous inputs
In the case of livestock effluent applications, the farmer may apply more K or P than necessary for the receiving crop in a given year, leading to the build-up of a residual pool of unused fertilizing elements. The same applies to the practice of blocking fertilization at the start of rotation.
It is accepted that there may be a loss of bioavailability over time, so in the case of excess fertilization in organic or mineral form, only a portion of the potential residual will be retained and deducted from the needs of subsequent crops. By default, a value of 80% is accepted (COMIFER 1997). This should be evaluated by soil type as not all lead to a visible loss of availability.
Adapting reasoning to multiple practices
New fertilizing materials
The range of fertilizers available on the market has expanded, due to the use of multiple sources, themselves subject to different treatment processes. Examples include struvite, ashes, digestates, composts, sludge, etc. Alongside these mostly recycled materials, there is also use of some raw ores, mainly phosphates (natural phosphates).
When applied to the field, the chemical elements contained in fertilizing materials are integrated more or less rapidly into the soil mineral compartments. They are first solubilized – more or less quickly – transiently in the soil solution then may bind more or less strongly to the soil solid fraction (adsorption, precipitation, …). The kinetics of these phenomena is very variable. It depends on the elements involved, the nature and fineness of the product, environmental characteristics, climatic factors and cultural techniques (incorporation, soil mixing, …). This must be taken into account in fertilization reasoning. When fertilization aims to supplement a short-term insufficient soil supply to meet plant needs, it will be necessary to use products with the highest Keq.
It was once thought that processes aimed at stabilizing the evolution of certain organic products such as liming of sludge could have detrimental effects on phosphorus availability. Laboratory and vegetation studies have shown that no hasty conclusions should be drawn : some products like limed sewage sludge have significant effectiveness from the first year (85% availability). Moreover, conversion to organic form (composting) does not lead to better bioavailability.
Localization of applications
Applications made on the seed row (starter fertilizers) have shown their effectiveness in soils with low content for spring crops (mainly maize), particularly on the vigor of young plants in situations where there is a risk of unfavorable conditions early in the cycle. This technique is also studied to promote establishment of autumn crops such as rapeseed. Localized increase of mineral concentration in solution, especially in soils with high buffering capacity, will help the plant acquire the nutrients it needs despite a still poorly developed root system. Rapid establishment of leaf area index will allow sustained juvenile growth.
Localized limited application then secures crop establishment. However, yield gains are not always observed in the end. Where fertilization is necessary, localized application improves fertilizer efficiency early in the crop cycle but does not reduce fertilization needs in the medium term. Localized in-row application at sowing also provides a solution in no-till situations where surface fertilizer application is not followed by incorporation or soil mixing. Evaluations of these practices are ongoing to avoid excessive extrapolation of isolated cases to all crops.
Simplified cropping techniques
The development of simplified cropping techniques raises questions about extrapolating the diagnosis established based on a reference framework of regularly plowed trials. Indeed, after 5 to 10 years without plowing, a differentiation in nutrient contents according to depth is observed on the former plowed horizon. The gradient of P, K, Mg content is such that the first centimeters are enriched thanks to the inputs of fertilizing materials and the return to the soil of non-buried crop residues. Therefore, contents measured at different depths would lead to erroneous reasoning :
- In the only enriched superficial horizon, one might believe in an increase in soil fertility, whereas there is likely a depletion phenomenon in the underlying horizon. This compartment, also explored by roots, contributes all the more to plant nutrition as it remains moist longer than the superficial horizons during dry periods.
- In the only underlying horizon, one might consider a decrease in fertility, whereas uptakes at the young plant stage which are generally decisive, could take place under good conditions – provided that surface soil moisture is adequate – since they are essentially carried out in an enriched horizon.
Today, following a study conducted in 2014 by COMIFER and GEMAS, the laboratories members of GEMAS recommend – pending precise references acquired in no-till over the long term – to continue sampling at the former plowing depth to avoid these biases. In practice, if this depth is not known precisely, sampling at 0-20 cm is advised because it remains interpretable by the current reference framework.
It is obvious that in the context of these new reduced tillage methods, many other factors must be examined with much more attention than for plowed soils :
- Homogeneity of the distribution of fertilizing materials and notably organic products.
- Impact of a drought which can cause roots to quickly descend in depth below the enriched horizon.
- Absence of rainfall after an application which can hinder the dissolution of fertilizer particles.
Contents, thresholds, and dose calculation grid can be found in this document "Phosphorus – Potassium – Magnesium fertilization. COMIFER".
