Relative numbers of atoms of the essential elements in alfalfa at bloomstage, expressed exponentially. Note that there are more than 10 millionhydrogen atoms for each molybdenum atom. Even so, normal plant growthwould not occur without molybdenum. (Modified from Viets, 1965).

Il K non subisce variazioni nello stato redox ed è presentesempre come ione potassio (K+) e quindi non entra a farparte dell’organizzazione strutturale delle molecoleorganiche. Il suo dinamismo nel suolo è costituito dalloscambio cationico e dai processi di alterazione, non dalleattività microbiche.

• il tipo di matrice litologica che ha originato il suolo

• il grado di alterazione

• le condizioni climatiche

• l’erosione

• il ruscellamento

• il pH del suolo

• la concimazione potassica

Litosfera Cellula vegetale-------------------- % (p/p) ---------------------

Ossigeno (O) 46.7 Carbonio (C) 45Silicio (Si) 27.7 Ossigeno (O) 45Alluminio (Al) 8.1 Idrogeno (H) 6Ferro (Fe) 5.1 Azoto (N) 1.5Calcio (Ca) 3.7 Potassio (K) 1.0Sodio (Na) 2.8 Calcio (Ca) 0.5Potassio (K) 2.6 Magnesio (Mg) 0.2Magnesio (Mg) 2.1 Fosforo (P) 0.2Altri 1.2 Zolfo (S) 0.1

Processi di natura chimica, chimico-fisica e biologica ne modulanola disponibilità, cioè la loro concentrazione nella fase liquida.

Listed in increasing order of plant availability, soil K exists in four forms:

mineral 5,000-25,000 ppm (0.5-2.5%)

nonexchangeable 40-600 ppm

exchangeable 50-75 ppm

solution 1-10 ppm

Mineral K accounts for 90-98% of total soil K,whereas slowly available (nonexchangeable) andreadily available (exchangeable and solution)represent 1-10% and 0.1-2%, respectively.

The total amount of K in a soil and its distributionamong the four pools is largely a function of the kindof clay minerals present in a soil.

Il potassio ha un ciclo biogeochimico sedimentario(da Weil & Brady, 2017)

Mineral K and NonexchangeableRemaining soil K is comprised of nonexchangeable and mineral K.

– K in primary minerals. The rate ofnonexchangeable K release to solution andexchangeable K is largely governed by weathering ofK-bearing micas and feldspars.

feldspars orthoclase, microcline: KAlSi3O8

micas muscovite: KAl2(AlSi3)O10(OH)2

biotite: K(Mg,Fe)3(AlSiO3)O10(OH)2

phlogopite: KMg3(AlSi3)O10(F,OH)2

– Nonexchangeable K in secondary minerals. Kfixation represents the entrapment of K+ betweenthe layers of 2:1 clays, predominantly hydrous mica.

Exchangeable K

– K+ is adsorbed to negatively charged soil colloids by electrostatic attraction to three types of exchange sites or binding positions of either clay minerals or humic substances.

Soil solution K

– Plant roots absorb K+ from the soil solution. Soilsolution K+ concentration optimum plant growthranges from 1 to 10 ppm, depending on crop yieldlevel.

– The quantity of K transported to the root surfaceby diffusion and mass flow is related to K intensity.

– K diffusion is a slow process compared with massflow and is limited to distances of only 1-4 mm.Diffusion accounts for approximately 90% of Kabsorption by roots.

– Soil solution K and exchangeable K are in dynamicequilibrium. This equilibrium is of great practicalimportance.

La quantità dei nutrienti presenti in forma solubile nella faseliquida del suolo è generalmente poco elevata, non bilanciata evaria secondo le caratteristiche pedoclimatiche del sito.

Solution K+ in fine-textured soils may be much lower than incoarse-textured soils at any given level of exchangeable K.

Type of clay is also decisive in controlling exchangeable K.

Clay content and type

Factors affecting K availability

Factors affecting K availability

Soils containing vermiculite, montmorillonite, or micahave more K than soils containing mainly kaolinitic clays,common in highly weathered soils.

Clay content and type

Factors affecting K availability

Potassium is much readily lost by leaching than phosphorus.In humid-region soils growing annual crops, the annual lossof K by leaching is usually about 25-50 kg/ha, the greatervalues being typical of acid, sandy soils.

Leaching losses: climate

Factors affecting K availability

Leaching losses: soil acidity

Factors affecting K availability

A 60-Mg/ha yield of corn silage may remove 160 kg/ha ofpotassium.Harvest of high-yielding legume hay may remove 400 kg/haof potassium each year.

Plant uptake and removal

Uptake and removal of potassium can be exaggerated bythe tendency of plants to take much more potassium thanthey need if sufficiently large quantities of easily availablepotassium are present. This tendency is termed luxuryconsumption. It is likely to depress Ca and Mg uptake.

Factors affecting K availability

The general relationship between available potassium level in soil, plant growth, and plant uptakeof potassium. If available soil potassium is raised above the level needed for maximum plantgrowth, many plants will continue to increase their uptake of potassium without anycorresponding increase in growth. The potassium taken up in excess of that needed for optimumgrowth is termed luxury consumption. Such luxury consumption may be wasteful, especially if theplants are completely removed from the soil. It may also cause dietary imbalance in grazinganimals. (Diagram courtesy of Ray R. Weil).

La concentrazione di K nei tessuti vegetali

La quantità di potassio assorbita dalla coltura (g/kg s.s.) ed accumulata neidiversi organi vegetali cambia nel corso degli stati fenologici della pianta.

La concentrazione di K nei tessuti vegetali

Variazione della quantità di potassio assorbito nel tempo dalleparti epigee di piante di riso coltivato in asciutta.

La carenza di potassio

Il contenuto totale di potassio nell’intero profilo di suolo è elevato

Nel breve termine una quantità molto ridotta di potassio è disponibile per la nutrizione minerale della pianta

Il potassio può subire significative perdite per lisciviazione

L’assorbimento di potassio cambia nel corso delle fasi fenologiche della coltura

Il potassio può subire asportazioni significative specialmente quando la pianta opera un consumo di lusso in risposta ad abbondanti concimazioni e non viene prevista la restituzione dei residui colturali

Occorre considerare le interazioni sia positive (Cu, Mn, Fe) sia negative (Ca, Mg, B) con altri nutrienti