Know your soil, types, structure and condition

Soil, mud, dirt is truly a fascinating subject which can be very complex, yet is often overlooked as a dry, boring and simple topic, but without the complex relationships, chemistry, types and structures of soil we would not have the myriad of flora which graces the variety of habitats on earth.

Sand Silt Clay Loam *
Water retention Sandy soils do not retain water well Good water retention Clay soils retain water well but are prone to getting water logged Has good water retention due to addition of clay
Nutrients Sandy soils are inert and have low cation exchange capacity Low cation exchange capacity meaning it does not hold on to nutrients Clay soils have a high cation exchange capacity meaning they hold onto nutrients well Has relatively good cation exchange capacity due to the addition of clay
Warming Sandy soils have low water retention making them faster to warm up than clay soils Relatively slow to warm due to water retention Clay soils are heavy and retain water as such they are slow to warm up Medium warming (not as fast as sand but faster than silt can clay)
Cultivation Sandy soil is advantageous for growing root crops such as carrots; where problems such as Phytophthora won’t affect your crop unlike planting root vegetables in clay based soils.

Sandy soils will warm faster than clay based soils due to less water logging, as such seeds can be planted out and germinate earlier.

Can be easily compacted Difficult to work due to clay soils being “sticky” and hard to work, also warms slower than sand/silt so the growing season for clay soils can start later (this is due to the high water retention of clay soils) Easy to work with but can become harder when the mixture is higher in clay
Aeration and drainage Sandy soils are very free draining Drains poorly and retains water longer Clay soil is slow to drain and have low AFP (air filled porosity) Drains well due to composition (sandy soil) and has good AFP
Additional information Silty soil is very similar to clay soils

* Loam soils are a combination of sand, silt and clay in almost equal proportions, due to this mixture it helps to remove the negative aspect of each soil type.

Sandy Soil

  • Largest particles among soil types (over 0.2mm)
  • Dry and gritty to the touch
  • Due to large particle size, it is not good at retaining water
  • Water drains rapidly
  • Nutrients are less available to the plants as they are washed out with the draining water
    • Lacking in nutrients
  • Sandy soils are inert (Little chemical activity/little nutrient bonding)
  • Free draining; high AFP (Air filled porosity)
  • Easy to cultivate (light)
  • Warms early in the year
  • Suitable for growing roots crops
  • Suits plants which are adapted to hot dry climates

Silt Soil

  • Like clay soils
  • Smaller particles than sandy soil
  • Smooth to the touch
  • When moistened, it becomes soapy to the touch
  • Retains water longer
  • Can’t hold onto nutrients as well
  • Due to moisture retention, silty soil is cold and drains poorly
  • Can be easily compacted
  • Little chemical activity (little nutrient bonding)
  • Found in plains/valleys due to transport by water

Clay Soil

  • Smallest particles out of clay, silt or sandy soil (less than 0.002 mm)
  • Good water storage properties
  • Due to small particle size it has a tendency to settle together and limit air movement
  • Slower to drain and locks up nutrients (thus being rich in plant nutrients)
  • Very chemically active (Good nutrient bonding)
  • The cation exchange properties of clay allow for good water and nutrient retention
  • Heavy; cold and tends to be water logged
  • Low AFP
  • Difficult to work, wet and sticky in winter and bakes hard in summer
  • Slower to warm up (due to texture and water retention)

Loam Soil

Loam soils are a combination of sand, silt and clay in roughly equal parts, the presence of different soil types helps to offset the negative aspects of other soil types. For instance, clay soils, can become water logged as the clay particles hold onto soil readily whereas sandy soils are very free draining and don’t hold onto water so by combining the properties of clay and sand you can improve the drainage.

Soil Structure, Condition and Drainage

Nutrient availability is a vital aspect of plant growth as inadequate nutrient availability will lead to deficiencies, stunted growth and ultimately plant death. Deficiencies can show up as spots or yellowing / discolouration of leaves and or fruits. As such it is important for a soil to have a good cation exchange capacity (which is the ability for the soil to hold onto and exchange nutrients). Clay and silt soils have a high cation exchange capacity which means they will hold onto nutrients and make them available to growing plants while sandy soils are inert and will not hold on to nutrients, as such sandy soils are often lacking in nutrients and will not hold onto nutrients from feeds.

Pans/compaction is an impediment to plant growth as the roots encounter a hard almost impenetrable barrier which they cannot pass through (some plants such as Cannabis sativa and some clovers can, with great energy expenditure break through and help to break up cultivation pans). Sub surface pans are especially damaging to root crops such as carrots, which will hit pans at which point the vertical growth will become stunted as the root crop attempts to push through the hard-compacted barrier.

Water is a vital part of plant growth, but too much of a good thing can become bad! Waterlogging reduces the availability of oxygen within the soil by filling all the micro and mesopores with water, if these pores remain filled with water for too long then the plants will be unable to “breath” eventually dying. Clay soils are prone to water logging due to their high cat-ion exchange capacity, while sandy soils are very free draining often leading to the opposite of water logging which is also detrimental to plants development. Lack of water during development will lead to stunted, poor development, low yield of fruits and damaged leaves such as yellowing in cabbages or chard.

Soil structure is important to the health of plants and how well they develop, poor structure will lead to a low AFP or an imbalance between micro, meso and macropores which will affect the availability of water and oxygen to plants. Clay soils have a tighter structure compared to sandy soils and as such have a lower AFP value.

Temperature has a role to play in germination and plant health, as too low temperatures will lead to slow germination and slow growth. If the temperature is too low then the seeds may never germinate. Clay soils are slow to warm which means the growing season will start later in the year compared to sandy soils which are fast to warm up (due to the low water content). This means the growing season with sandy soils will start sooner than clay based soils.

  • Oxygen starvation in root crops, tubers and storage organs (such as potatoes) leads to cell death, this often shows as dark discoloured areas.
  • Lack of root function and water movement in plants (which also means calcium will not be pumped around the plant) will lead to calcium related disorders in plants such as higher instances of blossom end rot in tomatoes and other such crops.
  • Leaching and denitrification (loss of nitrogen through run off or nutrient poor soils such as sandy soils) will lead to nitrogen deficiencies across most crops (very noticeable in green leafy vegetables such as chard and lettuce)
  • Water logging is shown to reduce flower production and increase young fruit abscission and abortion in legumes.
  • Saturated soils can lead to a build up of ethylene which can cause leaf drop, flower drop, fruit drop or early plant decline in many vegetable crops.

Soil Structure

Soil structure refers to how the arrangements of soil separates into units called aggregates. An aggregate is made up of pore spaces and solids which are separated by planes of weakness.

Effect of organic and inorganic fertilisers on soil

Organic fertilisers

Organic fertilisers are obtained from plants (composted green waste) and animal excretions (well-rotted manure) these are slower acting when compared to inorganic fertilisers but are very effective and can have other benefits such as helping to smother weeds and retain water when used as a mulch (which also adds nutrients into the soil).

Organic fertilisers release there nutrients into the soil slowly as they must be released by microorganisms working on the organic matter of the fertiliser; this makes the nutrients less prone to leaching like inorganic fertilisers.

The addition of organic fertiliser such as composted green waste can improve the structure of the growing media by increasing its AFP, water retention, drainage and nutrient retention capacity (cation exchange capacity).

Using organic fertilisers such as straw or composted green waste as a mulch will help to add nutrients into the soil while providing soil coverage thus preventing excess water loss through evaporation.

The use of organic fertilisers derived from manure/farm waste are also causing increasing problems by polluting the water tables and introducing antibiotic resistant bacteria into the soil due to the overuse of antibiotics in animal agriculture. Manure derived fertilisers can also introduce bacteria such as e-coli and salmonella into the soil.

Examples

  • Composted green waste
  • Well-rotted manure
  • Seaweed extracts
  • Nettle/comfrey feeds

Inorganic fertilisers

Inorganic fertilisers are generally more concentrated and faster acting in comparison to organic fertilisers and are produced via artificial methods and chemical synthesis. These inorganic fertilisers do not need to be broken down by soil microorganisms to become available to plants like organic fertilisers, but they are susceptible to leaching (being washed out of the soil into surrounding fields and waterways).

The addition of inorganic fertilisers boosts the nutrient contents of the soil for a short duration but in no way changes the characteristics of the soil such as water retention, nutrient retention (cation exchange capacity), humus content or structure.

Inorganic fertilisers come in different types which vary in mineral and nutrient contents, they can come in balanced/complete, nitrogen fertilisers, potassium fertilisers or phosphorus fertilisers. This makes it more flexible as you can add nutrients into the soil which it is currently lacking without increasing the other nutrients/mineral content too high.

The addition of inorganic fertilisers is less environmentally friendly compared to the use of organic fertilisers as inorganic fertilisers leach out into water ways and contaminate water tables, they can also cause stripping of soil nutrients and cause plant/root burn if used incorrectly. Inorganic fertilisers also do little to help the health, microorganisms and vitality of soil and the overuse or inorganic fertilisers can damage the soils resistance to pests and diseases; over time they can also kill off microbial activity.

Effect of soil pH on nutrient availability
Effect of soil pH on nutrient availability (Figure 1)

Improving Soil to enhance plant growth

Altering the structure and properties of soils such as water retention and drainage can help to improve crop yields by making the conditions more favourable to the selected crop; such as growing C. sativa. When grown in clay based soils the fibre and grain production will be lowered compared to loam based soils as the soil is often too cold with too much water for C. sativa. The addition of composted green waste to clay based soils can improve the drainage and loosen the soil up making the conditions more favourable. The same is true for sandy soils which are inert meaning they cannot hold onto nutrients and have little to none water retention capabilities. C. sativa requires a high nitrogen environment to grow optimally. In sandy soils C. sativa will grow poorly so you should add something like composted green waste to boost the humus levels which will help to increase the nutrient retention and availability along with increasing the water retention capability of sandy soils, increase the water retention and increase the cation exchange capacity of the sandy soil.

References

Figure 1: Unknown., nd. Chart of the Effect of Soil pH on Nutrient Availability. [Image]. Available at: http://www.avocadosource.com/tools/fertcalc_files/ph.htm [Accessed 14th March 2016]

Bibliography

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Amaducci, S., 2008. Growing Swiss Chard. [PDF]. Available at: https://www.votehemp.com/PDF/Characterisation_of_hemp_Cannabis_sativa_roots_under_different_growing_conditions.pdf [Accessed 1st March 2016]
Thomas, L., 2010. SOIL PH AND THE AVAILABILITY OF PLANT NUTRIENTS. [online]. Available at: http://www.nutrientstewardship.com/implement-4rs/article/soil-ph-and-availability-plant-nutrients  [Accessed 3rd March 2016]
GUO,, 2008. Charcoal May Help Improve Soil Quality. [online]. Available at: http://www.npr.org/templates/story/story.php?storyId=89562594 [Accessed 3rd March 2016]
Mallory, JJ., 2010. Evaluating the effect of tillage on soil structural properties using the

pedostructure concept. [onine]. Available at: http://naldc.nal.usda.gov/download/50063/PDF [Accessed 1st March 2016]

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