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  Section: Principles of Horticulture » Physical properties of soil
 
 
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Management of main soil types

 
     
 
Content
Physical properties of soil
  Plant requirements
  Composition of soil
  Formation of soils
  Natural soil profiles
  Soils of the British Isles
  Topsoil and subsoil
  Soil components
  Soil texture
  Soil structure
  Cultivations
  Management of main soil types

Sandy soils
Sandy soils are usually considered to be easily cultivated, but serious problems can occur because the particles readily pack together, especially when organic matter levels are low. Consequently many sandy soils are difficult to firm adequately without causing over-compaction. Pans near the surface caused by traffic and deeper cultivation pans frequently occur on sandy soils, resulting in reduced rooting and water movement. Subsoiling is frequently undertaken on a routine basis every 4–6 years, although the need can be reduced by keeping machinery off land while it has low load-bearing strength and by encouraging natural structure-forming agents.

Coarse sands have low water-holding capacity, which makes them vulnerable to drought, particularly in drier areas. This is not such a disadvantage if irrigation equipment is installed and water is readily obtainable. In many categories of horticulture there is a demand for soils with good workability. Coarse sands, loamy sands and sandy loams have the advantage of good porosity and can be cultivated at field capacity. Sands tend to go acid rapidly and are vulnerable to overliming because of their low buffering capacity.

Silts and fine sands
These can be very productive soils because of their good water-holding capacity and, while organic matter levels are kept above 4 per cent, their ease of working. However silts and fine sands present soil management problems, especially when used for intensive plantings, because they have weak structure, are vulnerable to surface capping, and are easily compacted to form massive structures. To achieve their high potential, efficient drainage is vital to maximize the rooting depth. Fine tilths in the open should be avoided, especially in autumn and early spring, because frosts and heavy rainfall reduce the size of surface crumbs. For the same reason, care should be taken with irrigation droplet size that, if too large, can damage the surface structure. Improving soil structure is not easy after winter root crop harvesting or orchard spraying on wet soils, because low clay content results in very little cracking during subsequent wetting and drying cycles. Improvement therefore depends on other natural structure forming agents or on subsoiling.

Clay soils
Clay soils tend to be slow draining, slow to warm up in spring, and have poor working properties (see soil consistency). A serious limitation is that the soil is still plastic at field capacity, which delays soil preparation until it has dried by evaporation. Permanent plantings are established to avoid the need to rework the soil. Playing surfaces created over clays have severe limitations, particularly when required for use in all-weather conditions. Where high standards have to be maintained, as in golf greens, fine turf is established in a suitable growing medium overlying the original soil. However, a high clay content is an advantage for the preparation of cricket squares where a hard, even surface is required but is played on only in drier weather. Increasingly, heavily used areas are replaced by artificial surfaces.

Horticultural cropping of clays is limited to summer cabbage, Brussels sprouts and to some top fruit in areas where the water table does not restrict rooting depth. Under-drainage is normally necessary. In wetter areas most clay soils are put down to grass. Timeliness, encouraging the annual drying cycle of the soil profile and maximizing the effect of weathering to help cultivations are essential for successful management of clay soils.

Peat soils
Peat soils have very many advantages over mineral soils for intensive vegetable and outdoor flower production. Fenland soils and Lancashire Moss of England; peatlands of the midland counties of Ireland; the ‘muck’ soils of North America; and similar soils in the Netherlands, Germany, Poland and Russia have proved valuable when their limitations to commercial cropping have been overcome.

Well-drained peat at the correct pH is an excellent root environment. It has a very much higher water-holding capacity than the same volume of soil and yet gaseous exchange is good. Root development is uninhibited because friable peat offers hardly any mechanical resistance to root penetration. This leads to high quality root crops that are easily cleaned. These cultivated peat lands warm up quickly at the surface because the sun’s energy is efficiently absorbed by their dark colour, with consequent rapid crop growth. These soils have a very low power requirement for cultivation, are free of stones, and can be worked over a wide moisture range.

Plant nutrition is complicated by natural trace element deficiencies and the effect of pH on plant nutrient availability. Peat has poor load-bearing characteristics and specialized equipment is often needed to harvest in wet conditions. Whilst peat warms up quickly on sunny days, its dark surface makes it vulnerable to air frost because it acts as an efficient radiator. Firming the surface and keeping it moist combat this. Weeds grow well and their control is made more difficult by the ability of peat to absorb and neutralize soil-acting herbicides. The high organic matter levels also make the peats and sandy peats vulnerable to wind erosion in spring when the surface dries out and there is no crop canopy to protect it.


 
     
 
 
     



     
 
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