Phosphorous and Potassium

Chapter 14

Phosphorous

Second to N as limiting nutrient

Taken up asH2PO4

- and HPO42-

Importance to Plants

Deficiency stunts growthPurplish color

The only thing special about these phosphatespecies is that they are the dominant onesin the typical range of soil pH, with the singlyionized phosphoric acid dominant at lower pHsand the doubly ionized one at higher pHs.

Fertility and Environmental Problems

Fertility

Little P in soilAvailability lowNo natural inputs

Leaching slight

Conditions to left are for the naturalstate. The saving grace is that the various forms of P in the soil tend tobe strongly retained (at least whenthe total level of P in the soil is low)and not leach out of the rooting zone.Also, nutrient cycling is important inkeeping P in the upper part of the soil.

However, with the advent of P fertilizersand the perhaps over-use of organicfertilizers (manure and like materials),high levels of P have built-up in someplaces. This is thought to pose a riskto water quality, as you shall see.

Concentration in solution low0.001 ppm to 1 ppm

Low solubility of inorganic P compounds

Surface adsorption reactions

While the reference text says [P] inthe soil solution may be up to 1 ppm,this is likely way higher than the naturalstate. The 1 ppb number is more likely.

Low solution concentration is due toprecipitation and adsorption reactionsthat phosphate undergoes. See figure.

Low pH Al and Fe phosphates precipitate

Mid pHBound to Al / Fe oxides and silicate clays

High pH Ca phosphates precipitate

Like in the above figure, at low pH (acidic), P is precipitated by soluble Al and Fe,giving minerals of very low solubility, and at high pH, P is precipitated as Caphosphates that are even less soluble. Throughout the pH range, P is adsorbedonto soil mineral surfaces and this form is strongly held, i.e., little released.

P in acid soils

Reaction with Al3+ or Fe3+

Al3+ + H2PO4- + 2H2O → Al(OH)2H2PO4 + 2H+

Very low solubilityThis is an example reaction. There are various such. What makes it workis the abundance and solubility of the acidic metals, Al and Fe, at low pH.

P in acid to alkaline soils

Adsorption on oxides and silicate clays

Anion exchange

█Cl- + H2PO4- █H2PO4 + Cl-

P slowly availableThis, too, is an example (anion exchange) reaction. The phosphate isstrongly held by the + sites on soil mineral surfaces, more strongly saythan is Cl-, even SO4

2- held but it is released back into solution to someextent.

Displacement of bound OH- or H2O

Al-OH + H2PO4- Al-H2PO4 + OH-

Al-OH2+ + H2PO4

- Al-H2PO4 + H2O

P release very slow OH │-O― P―OH ║ O

These are types of surface adsorptionreactions that entail covalent bonding.In some instances, the phosphate isbonded to two adjacent Als (or Fes)forming an especially strongly bond formthat is not susceptible to release back into solution.

P in alkaline soils

Insoluble Ca phosphates form

2H2PO4- + Ca2+ + 2CaCO3 →

Ca3(PO4)2 + 2CO2 + 2H2O

There are several of these Ca phosphates and the tendency is fora moderately insoluble form to first precipitate but this form slowlybe converted to more and more insoluble forms.

Greatest availability pH 6 - 7

Taking all this into consider-ation, it turns out that thesolubility of P is greatestin this pH range, interesting.

Plant uptake aided by mycorrhizaeThis figure issupposed to showhow mycorrhizalfungi aid P (andcertain micro-nutrients) nutrition.

P from the granulediffuses to the rootbut its concentrationis greatly reducedalong the way bythe afore fixationreactions.

Fungal hyphae effectively shortenthe diffusion distance.

P in the fungal hyphae is then translocated to the root.

Eutrophication

Over-application of P may lead tofreshwater eutrophication

P in runoff and sorbedon erodedparticles

Photo is inaccurately overly dramatic and probablyhas nothing to do with P, however, in principle, it could. It would have to do with an ecological change in the water body brought about by added P.

Continuing, we want the upperscenario, not the lower. While there is evident growth of someaquatic vegetation in the upper,the lower has much more, including a lot of algae. Moreaquatic and algal productionmeans more organic matterin the water body, and this meansmore decomposition of organicmatter and the potential for reduced dissolved O2 associatedwith such decomposition. Notgood, especially when hot, which is a doubly whammy withrespect to dissolved O2 (youknow, lower gas solubility withincreased temperature). Thealgae themselves may also impairwater quality directly.

If P limits algal and aquatic plantproductivity, want to limit P innon-point source pollution.

More P added than removed in harvest tocompensate for P fixation

Organic materials used for fertilizers

Rate based on N needs of crop soexcessive P added

Where does all this P come from?

This is long-term build-up of soil P by continual fertilization. Those fixation reactionsreduce P availability so to compensate a bit extra P has been recommended. Insome places, it has added up.

The downside to manure is relatively high content of P / N compared to plant needs.

So, basing application rate on N content leads to soil P build-up. Do it for a fewdecades and you may have a problem. There is also concern about P lostdirectly from the manure in runoff.

More P is lost from agricultural soilthan from grassland or forest soil(True / False).

More P is lost from conventional-tillsoil than from no-till soil (True / False).True and true. First, there is little soil P without fertilization. Second, a tilled soilis mostly bare so there is more runoff and erosion, carrying more P away. However,most of it is bound to eroded particles. Some studies indicate that while the secondis true, there is more dissolved P lost from no-tilled soils. This general form is morebioavailable, i.e., more effective in inducing eutropic conditions in downstreamwater bodies. Note: most believe freshwater is more susceptible to eutrophication by P than by N, and visa versa for salt water.

P Fixation Capacity of Soils

Soils differ in their capacity toprecipitate and adsorb P, thecombined processes calledfixation or sorption. Here, Bmore than A, clearly.

Factors affecting P fixation include

Amount of clay

Mineralogy

Al and Fe oxides >Carbonates >1:1 silicates >2:1 silicates

pH

More clay, more surface area, more adsorption.General rule, no?

However, the types of surfaces, i.e., types ofclay minerals, affect the extent of adsorption.

Further, if there are a lot of Al and Feminerals, particularly at low pHs, notonly is there adsorption but also precipitation of P as Al and Fe phos-phates. Carbonate minerals can adsorb P and where there are carbon-ates, there is abundant Ca in solutionso that Ca phosphates precipitate.

The aluminosilicate minerals have alot of surface area for adsorption of P.

Add less but place it near plant rootsBanded fertilizer application

Adjust pH to 6 to 7

P Management

Add whopping amount of P to saturatefixation capacity

The first option is not economically norenvironmentally sound. The second isillustrated to the right. A strip of P fertilizeris applied along a planting row. High concen-tration of P in the vicinity of the strip saturatesthe fixation capacity there and a portion ofthe root system near the strip can supply thewhole plant. You can also adjust pH to maximize P solubility.

Potassium

After N and P, K is the most likelymacronutrient to limit productivity

Importance to Plants

Deficiency causes chlorosis, then necrosisof leaf tips and margins

K Fertility

K abundant in soil but different sourcesvary in availability

Micas Not 90-98 %Feldspars available

Illite Slowly < 10 available

Exchangeable Available 1 - 2

This explains the K paradox –a lot of it in soil butstill a need to use K fertilizer. The overwhelmingamount of K in soil is in the structure of soil mineralsand so is unavailable.

The plant takes up K from the soil solution and exchangeable K replenishes thesoil solution. You add K as KCl, giving K+ which is mostly adsorbed onto – sites.

Luxury consumption may be a problem

Uptake in excess of crop needs

This is another factor contributing to theneed for K fertilization.

High rate of harvest removal may exceedrelease of K+ by weathering

Old study fromNY. Land givento ExperimentStation. Wornout soil.

Soil test saysway too little K.Agronomistsinstall plotstudy with trees,some fertilizedwith K, somenot.

Pre-1940 databackwards extrapolated.

Soil sampled foravailable K, i.e.,exchangeable.

Found to go upin surface soilwhere no Kadded.

Maybe weather-ing of minerals.Uptake by deeproots and litterfall certainly atwork here.