What Makes a Perfect Soil for Lawns?

The perfect soil varies depending on the situation, and in particular what it will 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 forms from dead vegetation, such as decaying leaves and roots together with the remains of insect larvae, fungal hyphae, and microorganisms. The particles or organic matter vary from recognizable 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 microorganisms 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.

Soil Texture is Important

Soils are categorized according to the size of particles they contain (i.e., sand, silt, and clay). The texture of a soil significantly affects 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 water-logging 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 fingers. Sandy soils feel gritty whereas a silty soil should feel soapy or silky. Clay soils are easily molded 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 due to heavy wear (large machinery, heavy foot traffic) the soil becomes compacted, and the porosity of the material is reduced. This is because 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 tinning, hollow coring, or deep de-compaction) are essential for 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?

The pH value 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 centers, 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 and 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 of dolomitic limestone (magnesium carbonate) or calcium carbonate. The application of nitrate fertilizers and alkaline irrigation water will also have an influence, but the effect will be more gradual. Alkalinity can be reduced using sulfates, primarily ammonium sulfate fertilizers, or iron sulfate.

Tips for Top-Quality Lawns

For healthy turf it is essential that the plants develop a good root system to optimize 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 cause waterlogging during wet conditions, 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 fertilizer and irrigation inputs.
• Acid pH will restrict the availability of nutrients within the soil solution and reduce the longevity of fertilizer inputs. If necessary, influence the pH of an area using appropriate amendments (see above).

There are many aspects of the soil that need to be considered 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 while it is easier to make changes without worrying about the possible effects on turf health. The correct conditions should also be considered when carrying 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.