Cultivated Soils (Arable and Market Garden)

Recognising soil physical problems

To recognise soil physical problems three main areas can be considered: crop performance, the characteristics of the soil surface, and careful soil profile examination after digging a hole.

Symptoms of soil physical problems limiting crop growth can sometimes be similar to symptoms of other problems (e.g. inadequate soil fertility). Even so, some common symptoms are listed below.

Crop performance
  • A steady decline in crop yields over a number of years, or a greater variation in yields in later years.
  • Establishment and early growth of crops is slow.
  • Crop emergence and maturity are uneven.
  • Crops appear to suffer from water stress more than would normally be expected, resulting from the onset of dry conditions in the root zone.

Characteristics of the soil surface
  • A greater number of passes is required to form a good seedbed than in earlier years or in other paddocks.
  • Formation of a smooth capping layer (crust) of the soil surface following rainfall or irrigation. This can be confirmed by probing and levering up the surface with a pointed blade.
  • Emergence of seedlings is uneven and prevented in some places by the crusted surface layer.
  • Infiltration of rain and irrigation water is poor, commonly causing surface ponding. As a result, irrigation water must be applied in small quantities and at frequent intervals.
  • Presence of deep wheel tracks on the surface.
  • The surface is becoming increasingly prone to soil loss through wind or water erosion.

Examine the soil profile

Several holes need to be dug in a paddock to a depth of about 50 cm. Unless a specific problem within a paddock is being investigated, care should be taken to avoid gateways, tracks, and headlands wheelings and other disturbed ground. A typical piece of land should be selected. Once the hole has been dug carefully, "pick" at the side of each profile pit with a knife or trowel to expose a natural soil surface. The reason for this is that the natural surface may otherwise be obscured by "smearing effects" caused by action of the spade when the hole was initially dug. Study, in detail, the condition of the cultivated (plough) layer, and the undisturbed subsoil beneath it. Some important characteristics to look for are summarised in Figure 1.

1. Cultivated layer

  • cultivated layer is the layer of soil (the top 15-20 cm) that is disturbed fairly regularly by cultivation equipment. In relation to structural degradation, questions that need to be asked include:
  • does this layer seem rather dense with weak root growth, low earthworm density and low organic matter content (i.e. not dark brown like a pasture soil)?
  • following cultivation does the plough layer consist of large clods with smooth, dense crack faces along with a significant amount of fine, structureless material? A well-structured plough layer will consist of many porous crumbs.
  • is there a smeared, denser layer at the base of the cultivation layer? This can often be seen as a smooth, slightly shiny surface, and
  • does the lower part of the cultivated layer become waterlogged during wet weather? A smeared layer can hold water above it.

2. Just below the cultivated layer

  • zone just below the cultivated layer can be critical in relation to successful crop production. The soil below it contains reserves of moisture and nutrients to sustain crop growth during the season. The most common problem in the zone just below cultivation depth is soil compaction - a "plough pan". However, it is not only ploughing that is responsible since the pressure under wheels of tractors and loaded trailers may also penetrate to this depth. The thickness of cultivation pans may vary from 1 cm or less to 10 cm or more. The presence of a cultivation pan can be identified by:
  • a sudden change in structural form with horizontal layering of soil within the compacted layer. Sometimes there is a smooth shiny upper surface on the pan (i.e. a smeared layer; see above),
  • a high density in this region determined by probing and levering the soil with a knife,
  • the absence of pores, fissures, penetrating roots or earthworm holes within the region,
  • an accumulation of roots and/or horizontal root growth immediately above the pan. Little or no root growth below the pan, and
  • perching of water above the pan. Waterlogging in this region may bring about the onset of anaerobic conditions. This may show up as greying (blue-grey pockets of soil) or as rusty-coloured mottles within the soil.
  • is important to assess this zone carefully, particularly to see if crop roots have penetrated in significant numbers. The boundary between tilled and untilled soil can sometimes be mistaken for a soil pan, particularly in late summer when subsoils may be dry and hard.

Fig 1

Figure 1: A summary of important features to look for when examining a soil profile

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