46 FOREST REPORT 2005
2.2 Soil
Approximately half of all the forest soil in Swit- zerland is either not acid or only slightly acid.
One third is highly acid, and 5% of the soil is even extremely acid.
About one third of the forest soils in Switzerland contain in at least one soil layer a concentration of aluminium that can endanger the growth of plant roots.
Acid soils
Plants find important nutrients such as nitrogen and phospho- rous in humus.Additional nutri- ents for plantsare made availa- ble in the soil through physical processes: forest soilsaredotted with negative particles (anions) onwhichpositivelychargedpar- ticles (cations) can dock.These docking points are also called
“ion-exchange sites”,and,in the ideal situation,are filledwith nu- trient cations such as calcium, magnesium,orpotassium.Nutri- ent cations are also called “base cations”or“bases”.The more nu- trient cationsdocked at the ion- exchange sites, the higher the base saturation and, according- ly, the better supplied with nu- trients the soil. In acid soil, on the other hand, other cations that areworthless or even harm- ful for plants take over the ion- exchange sites. These cations may be, for example, positively chargedhydrogenparticles(pro- tons) or poisonous aluminium releasedfrom otherwise harmless compoundsbyacidrain orbyac- ids from natural processes. Alu- miniumcationsdisplace nutrient cations from their ion-exchange sites because they adhere more strongly to the ion-exchange sites (strongerconversion energy).As aresult of the displacement the nutrientscan no longer cling to the soilandarewashedout.The base saturation decreases, and with it, the nutrient concentra- tion in the soil.
Soil quality can be meas- uredon thebasis ofbase satura- tion,but determining this isdif- ficult. Therefore, researchers at Swiss Federal Research Institute WSL take thepHvalue of the soil asan indicator of itsquality,be- cause pH values and base satu- rationare related.The lower the pHvalue, the moreacid the soil and,asarule, the lower thebase saturation as well. At WSL the pH values of 258 soils were ex-
47 2 HEALTH AND VITALITY
amined.This is arepresentative selection ofalmost everykindof forest soil there is inSwitzerland.
The researchers measured the pH-valuesat variousdepthsand found that,asarule, the topsoil hadlowerpHvalues,andis more acid than the subsoil.Expertsalso noticedregionaldifferences.The spectrum ranges frombase satu- rated, nutrient-rich soils with a pHvalue of7.5 toveryacidsoils with values that are even lower than3.
In acid soil cations that are worthless or even harmful for plants take over the ion-exchange sites. These cations may be, for example, posi- tively charged hydrogen particles (protons) or poisonous aluminium released from otherwise harmless compounds by acid rain.
2.2.2
pH values of forest soil
Frequency distributionof pHvaluesin thesoilsamples from the topsoil andsubsoil. The topsoil generally hasa higher pHvalue than thesubsoil.
[%]
■Topsoil
■Subsoil 16
12 8 4 0
2,4–2,6 2,6–2,8 2,8–3,0 3,0–3,2 3,2–3,4 3,4-3,6 3,6–3,8 3,8-4,0 4,0–4,2 4,2–4,4 4,4–4,6 4,6–4,8 4,8–5,0 5,0–5,2 5,2–5,4 5,4–5,6 5,6–5,8 5,8–6,0 6,0–6,2 6,2–6,4 6,4–6,6 6,4–6,6 6,6–6,8 6,8–7,0 7,0–7,2 7,2–7,4 7,4–7,6 7,6–7,8 7,8–8,0
2.2.1
Soil particles as ion-exchangers
Acidsdissolve aluminium bondsin thesoil. Thereleased aluminium cations(Al3+)displace the nutrient cations(Ca2+, Na+,Mg2+,K+)in thesoilparticles. Thefree nutrient cationsarewashed out and are then nolongeravailablefor theroots.
1. Acid deposition (e.g. via acid rain) dissolves aluminium bonds and releases Al3+. Soil particles as exchangers.
2. Al3+cations displace nutrient cations in the soil particle.
Al3+
Al3+
Mg2+ Mg2+
Mg2+
Mg2+
Ca2+
Ca2+ Ca2+
Ca2+
Ca2+ Ca2+
Ca2+
Ca2+
K+
Na+ Na+ K+
Soil particles as ion-exchangers
Nutrient-rich soil with
100%base saturation Slightly nutrient-depleted soil with approx. 55%base saturation
3.Nutrients are then washed out (see main text for further explana- tion).
[pH]
Legend:
Al3+ aluminium cation Ca2+ calcium cation
Na+ sodium cation Mg2+ magnesium cation
K+ potassium cation
48 FOREST REPORT 2005 2.2.4
Classification of soil acidity
Definition, frequency andmeanbasesaturationof the individualcategories.
Category
(n=258) pH valueof soil horizon Meanbase
saturation% Frequency %
alkaline pH value of all soil horizons over 7 100 12
slightly acid pH value of soil horizons between 4.6and 7 99 33
moderately acid pH value of fewer than half the soil horizons
less than 4.6 70 18
highly acid pH value of more than half the soil horizons less than 4.6, but fewer than half of them under 3.8
39 32
extremely acid pH value of more than half the soil horizons
less than 3.8 33 5
2.2.3
Soil profile
Soilprofile in apine-mixedforest inCanton Valais. The topsoil is rich in humusandvery darkincolour.Belowit isabrown subsoil(weatheringhorizon)with a limeborder40centimetres deep. Thefloorisacalcareous bedrock.
Subsoil
Bedrock Topsoil
Base saturation
SoilswithpHvalues over5.5are almost completely base saturat- ed. As the pH values decrease, however,base saturationdeclines rapidly. Slightly fewer than half the forest soils studied are ba- sic to slightlyacid.One thirdare highly acid and 5% of the soils are even extremely acid. In the highly acid and extremely acid soils theaverage base saturation of the fine earth is less than half as highas in slightlyacidorba- sic soils.However,even in high- lyacidsoils it is still40% onav- erage.
Research at WSL has also shown that acid soils on calcar- eous rock are significantly rich- er inbasesand thus richer in nu- trients than similar acidsoils on siliceous rock. This shows that it is not thepHvaluealone that determines base saturation.The difference inbase saturationbe- tweencalcareousandother soils can be as much as60%.This is because the lime dissolved in water neutralizesacidsand thus protects the soil.
49 2 HEALTH AND VITALITY
Plant growth
The ratio of the nutrientscalci- um,magnesium,and potassium to the toxic element aluminium influences plant growth. If this so-called BC/Al ratio in the soil solution fallsbelow the value 1, i.e. if there is more aluminium than nutrients in the soil solution, sensitive plants like the beech, for example,canbe harmed.Be- cause thecomposition of the soil solution hasbeendeterminedfor onlyafewsoils,WSLfocuses on theBC/Al ratio on thecation-ex- change sites. If the aluminium cations replace the nutrient cat- ions on the ion-exchange sites, the ratio of nutrients toalumin- ium becomes unfavourable. A BC/Al ratio of less than 0.2at the ion-exchange sites indicates that the aluminium concentration in the soil solution isat alevel that candamage the roots of sensitive plants.
Researchers at WSL found no shortfall in the BC/Al val- ues in most of the258soils tak- en for this purpose. In 83 soils, however, thevalue inat least one of the soil layerswas too low;in 7 soils the value was even too low throughout all layers. If the BC/Alvaluedeclines, then so too does thebase saturationand thus the concentration of nutrients.
If the BC/Al ratio inall the lay- ers of soil is more than 0.2, then thebase saturation is onaverage higher than 90%.In soilswithan unfavourable ratio between nu- trient cationsandaluminium,on the other hand, thebase satura- tion maysink toaround5%.
FURTHERINFORMATION Swiss Federal Research Institute WSL 8903 Birmensdorf
Research Dept. FOREST Section Soil ecology (0) 44/ 739 22 65
