Landscaper Pro Advice: What makes a perfect soil?

Soil plays a vital role in the development and longevity of a healthy lawn.  Not only does it provide a physical support for the plants but it also contains essential water, air and nutrients for growth.  The perfect soil will vary depending on the situation, and in particular what it is to be used for.

Soil is made up of solid particles which are either organic or mineral in nature.  Mineral matter is derived from the mechanical and chemical weathering of rock particles into smaller fragments.  Fragments vary in size from stones and gravel through to smaller sand, silt and clay particles.

Organic matter is formed from dead vegetation such as decaying leaves and roots together with the remains of insect larvae, fungal hyphae and micro-organisms.  The particles or organic matter vary from recognisable plants remains (raw organic matter) through to more finely divided material (humus) depending on the extent of decomposition.  Organic matter breakdown is brought about by the micro-organisms within the soil and results in the release of nutrients.

Organic matter tends to accumulate within the top few inches of the surface.  If too much organic material is present the soil will be prone to holding water which will influence the health of the turf, particularly in terms of root development and susceptibility to disease infections.

Soil texture is important

Soils are categorised according to the size of particles they contain (i.e. sand, silt and clay).  The texture of a soil significantly effects the properties of that soil and how it performs as a growing medium.  A material with a uniform particle size (i.e. 90 % medium sand) will facilitate the rapid movement of water and air in the soil.  In other words such a soil will have good drainage properties but will be prone to drought and vulnerable to leaching vital nutrients for healthy plant growth.  A soil containing mixed particle sizes will have a greater number of small pores as the tiny clay particles sit in between the larger sand pieces.  Water will be held for longer within such a soil as it does not drain easily.  Such soils will be vulnerable to waterlogging at times of high rainfall.  The quantity and size of pore spaces within a soil is called its ‘porosity’ and is influenced by the texture of a soil as well as by its structure (see below).

Texture can be assessed in a laboratory using a set of sieves or in the field by rubbing moist soil between your fingers.  Sandy soils feel gritty whereas a silty soil should feel soapy or silky.  Clay soils are easily moulded into sticky balls.  A loamy soil, which will contain around 40 % sand, 40 % silt and 20 % clay, will provide a very good growing medium but will tend to suffer where surface traffic is very heavy.  In such instances a material with a higher level of sand in relation to silt and clay would be a better choice but could suffer if rainfall or irrigation is not supplied.

Soil structure is important

Soil structure can be defined as ‘the way in which the soil particles (sand, silt and clay) are arranged’.  The mineral and organic matter combine with each other to form small lumps or ‘aggregates’.  In a well-structured soil the pore spaces between the aggregates will be large enough to allow the movement of water and air.  If the structure is damaged as a consequence of heavy wear (large machinery, heavy foot traffic) the soil becomes compact and the porosity of the material is reduced.  This is as a consequence of the smaller aggregates of soil merging to form larger ‘blocky’ structures which impede the vertical movement of water, air and plant roots.

Can I influence soil texture and structure?

It is possible to improve both the structure and texture of a soil through the introduction of sand in conjunction with mechanical aeration procedures.  It is essential, however, that the correct grade (particle size) and shape of sand is used and that enough sand is applied to allow the sufficient dilution of smaller particles.  Using the wrong sand and applying too little may result in a higher level of ‘inter-packing’ particles leading to a reduction in porosity and a more water retentive material.

Sand can be applied as a dressing or integrated into the upper profile of the soil during seedbed preparation.  A greater quantity of material can be applied to the seedbed than can be used for a top dressing which must be completed on a little and often basis to prevent smothering the grass plants.

Mechanical aeration techniques (i.e. slitting, solid tining, hollow coring, deep decompaction) are integral to integrating the applied material into the upper profile and ensuring that a surface layer is not created that could lead to a capillary root break being created.  The mechanical operation also works the soil to help improve its structure and improve the porosity to provide a greater movement of air and water

Does soil pH matter?

pH is the measure of acidity or alkalinity brought about by the concentration of hydrogen ions in solution and on negative exchange sites (clay minerals and organic matter particles).  The lower the pH value the more acid the soil.  A soil with a pH of below 6.0 is considered acid and above 7.0 alkaline.  A pH value of between 6.0 and 7.0 is classed as neutral.  You can measure the pH of your soil by using a simple soil pH kit available from many garden centres, or by sending a sample off to a laboratory.

The pH of a soil will impact on the availability of some nutrients (i.e. phosphate) and will influence the level of microbial activity responsible for organic matter breakdown and nutrient recycling.  The ideal pH will vary depending on the grass species composition of the turf but should ideally be kept at between 5.0 and 7.0.  More specifically, an ornamental lawn containing fine browntop bent and fescue grass species should ideally be maintained at a pH of 5.0 -5.5 to give it an advantage over annual meadow grass that has a preference for a higher pH of between 5.5-7.5. Amenity grassland areas dominated by perennial ryegrass prefer a pH of 5.5-7.0.

An acid pH can be raised by the application of lime, either in the form or Dolomitic limestone (magnesium carbonate) or calcium carbonate.  The application of nitrate fertilisers and alkaline irrigation water will also have an influence but the effect will be more gradual.  Alkalinity can be reduced using sulphates, primarily ammonium sulphate fertilisers and/or iron sulphate.

Tips for top quality lawns:

For healthy turf it is integral that the plants develop of good root system to optimise the uptake of water and nutrients, especially during periods of stress (hot, dry weather).  The ability to create such a root system is heavily influenced by the soil in the following ways:

• Improve structure and maintain porosity through regular aeration. Poorly structured compact soils make the development of a healthy root system difficult as it forms a physical barrier to growth as well as restricting air and water movement.

• A poor textured soil may result in restricted drainage and causing waterlogging during wet conditions and restricting the availability of air (oxygen) within the soil profile. Top dress with sand and aerate to improve infiltration rates and the porosity of the material.  If necessary install subsurface pipe drainage to a positive outfall.

• High organic matter levels will restrict the movement of water creating an imbalance of air and water in the soil profile and reduce microbial activity therefore perpetuating the problem. Control organic matter through physical removal (hollow coring, scarification), top dressing and controlling fertiliser and irrigation inputs.

• Acid pH will restrict the availability of nutrients within the soil solution and reduce the longevity of fertiliser inputs. If necessary influence the pH of an area through the use of appropriate amendments (see above).

There are many aspects of the soil that need to be considered therefore to ensure the successful development and long-term maintenance of a turfed area.  If possible, amendments to the soil should be made prior to sowing whilst it is easier to make changes without worrying about the possible effects on turf health.  Of particular consideration should also be the correct conditions in which to carry out work.  If soils are worked during wet conditions there is a strong likelihood that the soil structure will be damaged which will influence the performance of that soil as a growing medium for a number of years.