Define soil structure and describe the root environment.

Define the term ‘soil structure’.

SOIL STRUCTURE Soil structure is how the particles (sand, silt and clay) are built, glued, cemented, or aggregated together to ideally form a “crumb structure” (peds).

The glues of the soil are: “Humus” and to a lesser extent bulky organic matter which in time becomes Humus; Clay colloids; Minerals such as calcium, magnesium, potassium; Gums secreted by some plant roots; Adhesion between particles is via electrostatic force (flocculation) or cementing substances.

SOIL POTENTIAL Determined by soil-structure

Crumb structure and its influence on plant growth: crumb formation and destruction.

SOIL STRUCTURE Soil structure is how the particles (sand, silt and clay) are built, glued, cemented, or aggregated together to ideally form a “crumb structure” (peds).

The glues of the soil are: “Humus” and to a lesser extent bulky organic matter which in time becomes Humus; Clay colloids; Minerals such as calcium, magnesium, potassium; Gums secreted by some plant roots; Adhesion between particles is via electrostatic force (flocculation) or cementing substances.

SOIL POTENTIAL Determined by soil-structure

CRUMB STRUCTURE how the soil particles hold together/glue (how the soil particles hold together/glue) is described as “good” or “bad”. A good surface structure is call a “tilth”. Good crumb structure should be quite stable.

CLAY COLLOIDS

FLOCCULATION Adhesion between soil particles by via electrostatic force

TILTH A good soil crumb surface structure is call a “tilth”. See soil-structure

FINE TILTH

IMPROVING SOIL STRUCTURE

Digging heavier soils in the autumn, leaving as rough as possible to aid “weathering”; The addition of “bulky organic matter”; Short manure (well-rotted) to light sand soils to bind it together in the first case (physical effect) then becoming Humus (glue); Long manure (half rotted) to open up heavy soils in the first case (physical effect) then becoming Humus; Subsoiling to improve drainage and to break up soil pans (chemical (iron) or cultural plough pan). Garden methods: deep digging and installing drainage; Addition of lime to heavy soils to flocculate the colloids (hydrated lime will alter the soil Ph, gypsum will not); Spiking or tining of lawns for aeration and root penetration; Addition of grit or coarse sand to heavier soils and digging in. Tends to improve surface penetration and surface drainage (but limited effect if the movement of water is impeded).

NO DIG GARDENING

Often as raised beds; Apart from the normal soil disturbance of sowing planting or harvesting, the soil is left and not dug in the normal way. Once created, bulky organic matter is shredded and applied to the surface as required in the normal way of the rotation. It is naturally drawn down into the soil by the action of worms. This allows for the soils’ fertility to build up as the soils’ bacteria are undisturbed and operate in the soil at their preferred level. Weed seeds are not brought to the surface to germinate.

SHORT MANURE

LONG MANURE

WINTER DIGGING

SUBSOILING

SOIL DAMAGE

Over cultivation of the soil, partially mechanical (constant use of a rotavator) physically breaking down the crumb structure, possibly creating a cultivation pan (non-dig concept); Loss of Humus by not adding it to the soil. Example - American dust bowl Also burnt up by over-aeration of the soil by cultivating (see above). Example -Fen peat loss. Burnt up if applied with lime; Compaction caused by walking on wet soil (puddling) physically breaking the structure, driving out the air; Use of heavy irrigation, breaking down the crumb.

AIR FILLED POROSITY Air filled porosity is the relation between pore space, air and water. For good growth of roots and the plant a good balance of air and water is ideally maintained in the soil.

MACROPORE

The pores that are too large to have any significant capillary force. These pores are full of air at field capacity. Macropores can be caused by cracking, division of peds and aggregates as well as plant roots and zoological exploration. Size >75 μm.

MESOPORE

The pores filled with water at field capacity. Also known as storage pores because of the ability to store water useful to plants. They are subject to capillary forces but make water available to the plant,. These mesopores are ideally always full or contain liquid to have successful plant growth. The properties of mesopores are highly studied by soil scientists to help with agriculture and irrigation. Size 75 μm–30 μm.

MICROPORE The pores that are filled with water at permanent wilting point. These pores are too small for a plant to use without great difficulty.
The water associated is usually absorbed onto the surfaces of clay molecules. The water held in micropores is important to the activity of microbes creating moist anaerobic conditions. The water can also cause either the oxidation or reduction of molecules in the crystalline structure of the soil minerals. Size <30 μm. μm = micron or micrometre.

FIELD CAPACITY the amount of water a soil is holding (capillary water) usually 2-3 days after prolonged rain or irrigation has ceased and the gravitational water has drained away;

SOIL PED see ped

PED Soil peds are natural, relatively permanent aggregates, separated from each other by voids or natural surfaces of weakness. Peds persist through cycles of wetting and drying. Soil Fragments and Clods are artificial structural units, formed at or near the surface by cultivation or frost action, and are not peds.

STORAGE PORE see mesopore

SATURATION POINT After prolonged rain or irrigation, all pore spaces in the soil are filled with water and gravitational water is flowing away.

PERMANENT WILTING POINT Permanent wilting point: the point when the available water content of a soil (capillary water) has been used up and the plant wilts;

AVAILABLE WATER CONTENT The amount of water that can be used by a plant between field capacity and permanent wilting point;

SOIL MOISTURE DEFICIT Available water in soils is lost via a combination of: evaporation and plant transpiration (evapotranspiration).
For irrigation it is possible to measure this deficit (potential transpiration) and then replace it by irrigating, the idea being to bring the soil back up to field capacity only and not to over-irrigate as water applied after this point is reached will drain away (gravitational water).

CAPILLARY WATER The available water content of the soil.

EVAPOTRANSPIRATION Available water (capillary-water) in soils is lost via a combination of evaporation and plant transpiration

GRAVITATIONAL WATER water that drains away under gravity

POTENTIAL TRANSPIRATION estimate of amount of water lost by transpiration that can be replaced by irrigation

IRRIGATION Watering plants to ensure water supply at the roots Water-in plants after planting and when necessary to maintain growth and flowering. Too much = weeds & little flowers. Some plants like it drier – Marigolds, Gazania, Geranium. Some plants like it wetter – Begonia, Fuchsias.

Drought stress is common with newly planted trees and shrubs. Even in a cool, wet summer, the rain rarely replenishes soil moisture stores fully. The soil may be dry around the roots even when the surface appears moist;

Dry, windy conditions are especially likely to lead to water shortages. With experience, it is possible to detect the dull, lifeless foliage indicative of drought stress but by then the tree has already been damaged. Ideally anticipate water loss, and irrigate to prevent damage.

WATER REQUIREMENT (PLANTS) For good growth of roots and the plant a good equal of air and water is ideally maintained in the soil.

All plants have a preference for the amount of water they like in the soil. Begonia and Fuchsia like it on the wetter side.

Where African Marigolds Tagetes erecta and Rudbeckia like it on the dryer side.

For sturdy/strong growth, Chrysanthemum are grown on a wet/dry growing regime.

If constantly wet growth will be lusher and possibly more susceptible to diseases, to the point where if water-logged roots may be killed.
If constantly kept too dry, plants will be stressed and will not grow well. If allowed to wilt leaves may be scorched and flower buds may drop (Fuchsias).

Relation between pore space, air and water. Saturation point, field capacity, available water content, permanent wilting point, soil moisture deficit.

Importance of an appropriate balance between air and water for the healthy growth of plants.