Soils

Plant Material Maintenance

Soil Soil is the top layer of the Earth’s crust.

Millions of years to make Being lost at a rate of 5.2 tons/acre/year (Iowa state-

wide average)

SOLIDS + PORE SPACE =

SOIL

Pore Space

Two kinds:Macro pore space Micro pore space

air

water

The Ideal Soil

In undisturbed soils organic matter helps bind soil particles into larger aggregates.

Soil Horizons and the Soil Profile:

O – Thin layer of decomposing organic matter.

A – Primarily composed of inorganic materials (sands, silts, and clays) it is also normally rich in organic matter. This horizon has a characteristic dark color.

E – Not present in all soils and not pictured in the illustration to the left.

B – Zone of accumulation where materials that have leached from the surface mix with soil particles from the lower parent material.

C – Composed of partially weathered parent material.

Inorganic (Mineral) Soil Particles

Sand Silt Clay

Properties of sand, silt, and clay:

Size Shape Water Infiltration

Aeration Water-Holding Capacity

Nutrient-Holding Capacity

Sand Largest2.0 – 0.05mmNaked eye

Round or angular

Rapid Good Low Low

Silt Medium0.05 – 0.002mmMicroscope

Round or angular

Slow Poor Moderate Moderate

Clay Smallest<0.002mmElectron microscope

Wafer-like

Moderate to poor

Moderate to poor

High High

Water-holding Capacity

Soils with a large percentage of micropores have a high water-holding capacity - clay

Soils with a larger percentage of macropores over micropores have a lower water-holding capacity - sand

Gravitational Water

Water that drains from the macropores under the force of gravity after a rain/irrigation event.

Field Capacity

A soil is said to be at field capacity immediately after the gravitational water has drained away.

Water that remains held in the micropores is called capillary water. available unavailable

Permanent Wilting Point

When available water is depleted, and no additional water is added, plants may reach the permanent wilting point.

Soil Texture

Determined by the percentages of sand, silt, and clay a soil contains.

Textural triangle.

Loam Soils

Ideal balance of 3 particles Results in high fertility and good water

retention Drains well Desired for gardening

Organic Matter (OM)

Generally only present in very small quantities; 5% or less.

Most OM in the form of humus: stable, decomposed plant and animal life. In sandy soils - increases water and nutrient holding

capacity. In clay soils - improves drainage, air movement, and

helps form aggregates.

Cation-Exchange Capacity

Capacity of a soil to attract and hold nutrients on the surface of soil particles.

Soil and organic matter have negative charges. Nutrients have positive charge, so are attracted

to the negative charges . Clay soils and soils high in organic matter have

higher CEC rates.

pH Scale

Measures the acidity/alkalinity. pH affects the nutrient availability in the soil. Ranges from 0 to 14. pH of 7 is neutral. Less than 7 is acidic. More than 7 is alkaline. pH of 6 is 10 times more acidic than 7 pH. pH of 5 is 100 times more acidic than 7 pH.

Nutrient availability and pH

At certain pH levels, some micronutrients become unavailable to the plant. They become chemically bound to the soil particles.

pH level of 6.5-7.0 is the optimal range for most micronutrients.

Iron is commonly deficient due to high pH soil.

Adjusting pH levels

Low pH soils (acidic)Add lime

High pH soils (alkaline)Very difficult to lower pH on large scaleAdd sulfurAdd organic amendments

Oak leaves, sphagnum moss, pine needles

Mychorrhizal Fungi A symbiotic relationship between plant

roots and fungi. The fungi help roots absorb water and

nutrients. The roots provide the fungi with food

(sugar). Most plants growing in “undisturbed” soils

have mychorrhizal fungi growing in association with their roots.

End