Soils: properties which impact on nutrient availability
Where do plants derive the inorganic minerals described earlier?
- majority come from weathered rock,
- approximately a 1/2 come from water and air,
- and a small portion ( less than 10%) from humus, decomposing but not decomposed organic material.
Minerals derived from the soil must be dissolved in water. Otherwise the plant will not be able to obtain them.
How does soil structure impact on nutrient availability?
Soils are characterized by the particles which make up their texture; they may be described as clayey ( common enough in CC, or sandy ( on the eastern shore) or more ideally as loamy.
clay (micelles) : less than .002 mm in size
silt: .02-.002 + sand + clay
sand: 2 mm- .02 mm
Very fine particles. Very large surface area; charged to hold ions. Good water retention and ability to hold onto nutrients.
Can compact greatly thus effecting root growth by not allowing enough pores for air and water. Think of swampy soils
This soil is excellent for it combines the high surface area and water retention of clay with the air spaces of sand.
Very large particles which have large pockets between them. This is great for aeration
Can be detrimental for the plant because there is very little surface area. Thus nutrients will not bind to it and water just runs through.
Additionally, the age of the soil influences its organization. As soils mature they develop distinct layers or horizons ( this given they are not disturbed). From the base parent rock breaks down with weathering ( water which freezes and defrosts causes cracks and eventually physical breakage as well as organic acids which break down chemical structure.
From the top down, biological organisms add organic materials ( feces, fibrous content -> dead bodies), aerate with tunnels, excrete organic acids all which add not only to minerals but also organic content with high water holding capacity. With rain/snow materials from above can leach down and with drought/drying moisture can move up the soil column carrying compounds from below to the surface. Plants with deep root systems ( grasses and some desert tap root plants) are also responsible for bringing up large amounts of nutrients from below.
Soil is never static... its properties can change either slowly or even more rapidly with human or natural disturbances.
Organic; surface litter
Topsoil; humus, great many organisms live here
Zone of leaching; area through which dissolved and suspended materials move downwards
Subsoil; Accumulation of Iron and Aluminum compounds which were leached from A and E levels
Parent Material; Partially broken down rocks and inorganic materials
How do plants obtain the nutrients in the soil?
As rock weathers the - charged components are more resistant, so the clay micelles develop a - charge. Due to this, cations released by weathering do not completely leave the rock but are held by weak charges.
The cation exchange capacity (CEC) of a soil is simply a measure of the quantity of sites on soil surfaces that can retain positively charged ions (cations) by electrostatic forces. Cations retained electrostatically are easily exchangeable with other cations in the soil solution and are thus readily available for plant uptake.
CEC is important for maintaining adequate quantities of plant available calcium (Ca2+), magnesium (Mg2+) and potassium (K+) in soils.
Under acid conditions (pH <5.5) as in tropical soils with high organic content, aluminum (Al3+) may also be present as an exchangeable cation.
Some cations will have a preference over others for the negative surface charge of a soil colloid. In general this will be related to the charge of the cation and its hydrated radii. For example the following order of preference is observed for cations with a different valence:
Al+3 > Ca+2 = Mg+2 > K+ = NH4+ > Na+
In the case of cations with the same charge or valence the order of preference follows the order of decreasing hydrated radii:
K+ > Na+ > Li+
The greater the charge the greater the force of attraction and the greater the distance between the charges the lesser the force of attraction. In highly weathered soils such as the Southeastern USA and the tropics the high rainfall leaches monovalent and divalent cations and the exchange complex becomes dominated by Al+3 (low base saturation).
Cation exchange sites are found primarily on clay and organic matter (OM) surfaces. Soil OM will develop a greater CEC at near-neutral pH than under acidic conditions. Additions of an organic material will likely increase a soil's CEC.
Soil CEC may also decrease with time through acidification and OM decomposition.
When a plant respires ( as well as other critters including fungi/bacteria in the soil) it releases CO2 which can then combine with soil water to produce H2CO3 --> dissociates into H+ + HCO3 --> H+ and CO3.
The H+ displace the + charged minerals releasing them into soil solutions which can then be picked up by the root. .
Acid rain unfortunately has the same impact.....If the influx of H+ is too rapid nutrients can leach out too quickly leading to loss to waterways. Once the nutrients are lost to the soil, unless they are returned via decomposition, the soil can become infertile.
Optiona reading: Here is a nice page on effects of acid rain...
How does acidity impact on soil nutrient availability?
As we discussed with cation-exchange, nutrients may be more or less tied to the clay dependent on pH....Note how iron availability is strongly dependent on pH. With liming of soil ( to neutralize the soil), iron which was adequate at pH of 6-7 may be deficient at a pH of 8. In some cases nutrients may become to available and even toxic ( Al).
Plants themselves can control pH of the soil dependent on the organic acids in their leaves. Pine produces a very acidic needle duff layer... this is thought by some ecologists to be a competitive strategy to make enough ions unavailable for hardwoods which require higher mineral availability. This also impacts the microbes... fungi do better under acidic conditions, bacterial under more neutral..
How would this impact the plants in terms of their biological associations? ... see the next page