Site Selection and Soils

The plants and plant communities which grow on a site are to a large extent governed by the characteristics and location of the site and soil in which they are rooted. Plant species have evolved strategies to exploit different combinations of climate, soil conditions and disturbance by animals or man: some species have specialised to thrive in particular conditions such as occur on chalk soils, other species are more generalist. For any set of conditions there will be species that thrive and survive, and others which do less well or fail; through this process a natural balance between species is established. Recognising and understanding the characteristics of your site and soil is clearly useful in deciding which species are most likely to do well.

Work with nature: The most rewarding approach to using wild seeds is to work with nature. If you aim to sow a selection of species that are naturally suited to your soil and site conditions, and if you manage the resulting plant community sympathetically allowing natural selection to determine the balance of species appropriate to your site, you have the best chance of attaining the maximum potential from your site and with the least effort. The alternative gardening approach by comparison requires a struggle for control with nature to impose a specific idea of aesthetic and balance that may have no basis in nature.

Plant assessment: Much can be learnt from looking at the plants already growing on site or on similar soils and situations in the locality. A preliminary assessment of local vegetation should be undertaken at an early stage to establish what type of plant communities and approaches are likely to be suitable, as well as identifying any pre-existing habitat or plants that ought to be retained and conserved.

Soils and fertility: Of the factors that influence plant community development, soil characteristics and in particular the fertility and acidity of the soil are probably the most fundamental from the point of view of specifying and using wild seeds. The reason for this is that there is a generally observed principle that the most productive fertile sites and soils tend to have the least potential for plant diversity, whether open grassland, woodland or wetland. Changes in light or water levels will change the type but not necessarily the number of plant species that grow well, but increases in the nutrient status of a site (such as the addition of fertiliser to flower-rich grassland) leads directly to a loss of diversity as the additional resources fuel the growth of the most competitive components at the expense of others. For this reason the emphasis in the following notes will be biased to a discussion of soils and their importance to wild flower seeding.

Understand site potential and limitations: Unfortunately whilst all sites are capable of supporting a naturally balanced plant community, not all sites have the same potential for supporting a diversity of plants, or an attractive range of plants. Where a choice of soil or location is available or can be engineered by earthmoving and land forming (eg in road building) it is useful to be able to identify or specify the most suitable.

Where the choice of soil and location is fixed then understanding the potential or limitations of the site will inform expectations as to the results that can be achieved and the effort required. This may include a decision as to how suitable or cost effective the site is with regard to the desired outcome.

Soils and Soil Fertility

Soil characteristics, especially those which affect productivity, have a controlling influence on the diversity and balance of plant species on a site. The soil characteristics that are most important are:

  • Soil texture and structure
  • Soil fertility
  • pH acidity/alkalinity

Soil Texture & Structure

The physical character of soils is determined by the balance of clay, silt and sand particles and by the organic humus content of the soil. For practical considerations soil texture and related soil structure influence soil workability, drainage and management.

For the purposes of choosing a seed mixture or a planting plan a detailed soil textural analysis is not required. A general understanding of the type of soil on a site, for example whether it is a heavy wet clay, or a light free-draining sand, is all that is required. Many soils in gardens, landscape schemes and meadows are loams:  a mixture of clay, silt and sand with none predominating.

Some guidance as to soil structure and associated mixture choices can be found alongside the descriptions of our meadow mixtures for different soils.

Soil Fertility

The most diverse grasslands in Britain are usually associated with soils of low fertility that have not been agriculturally ‘improved’ by additions of fertiliser. In conditions where nutrients are in short supply niche opportunities arise for a wider range of specialist plant types, each species having its own strategy for scavenging the resources it needs.

For most situations the best assessment of site and soil fertility can be derived from knowledge of the history of a site and observation of the vegetation growing there and in similar conditions in the locality. For example the following are all indicators of above average fertility:

  • the site is a garden or farmland that has been cropped and fertilised in the past
  • the soil usually produces good crops and grass grows well
  • there are weeds which indicate fertility: nettles, docks, cleavers, thistles
  • the soil is deep and well structured

Chemical analysis of soil fertility is a complex subject and is often of limited practical use for wild flower growing.  There is for example no simple way of assessing the ability of a soil to supply plants with the most important soil nutrient Nitrogen, and most established soil test methods have been developed to guide farmers and growers as to how much fertiliser to add to obtain maximum crop yields. Within the 0-9 range over which farm test results are classified the natural levels of most interest for diversity are found in the lowest 0 or 1 class.

Soil phosphorous (P): The phosphate status of soil is considered to be the most useful chemical indicator of fertility and thus potential plant diversity. This is not because phosphorous by itself can greatly enhance productivity (= low diversity), but that its status is frequently the factor that limits plants ability to exploit other resources (principally Nitrogen).

Soils in Britain naturally contain limited amounts of P, and species rich grassland communities are typically associated with soils with a P index of 0 to 1 (15ppm or less). Human activity: human and animal wastes, the addition of basic slag and artificial fertilisers have together augmented the level found in gardens and farmed land to a level 3 or more (many times higher than is ideal for creating diversity). Unfortunately once raised soil P levels decline incredibly slowly, so slowly in fact that elevated P levels are used by archaeologists to detect ancient settlement patterns and abandoned farmsteads – these often reveal themselves as patches of nettles which thrive on high phosphate levels.

Fertility Reduction

A variety of techniques to return fertility to natural levels have been tried. All, with the exception of drastic soil removal, are of limited or variable success.

Weathering / leaching of soil: soluble nutrients like nitrates are lost quite quickly from soils. Insoluble minerals like phosphorous however are only washed out of soils with very high concentrations (index 5+).

Losses occur most rapidly from sandy soils with low organic content; soils with high clay or organic content tend to hold on to minerals. Leaching as a strategy is not recommended as losses of minerals (e.g. nitrates) from soils to watercourses is a major source of pollution.

Repeated removal of bulky crops: cropping may mop up a short term surplus of nitrates but have little impact on long term reserves of phosphates. In practice any significant reductions in P levels require years/decades of continuous removal.

Deep ploughing to bury nutrients: where the topsoil layer is shallow (<20cm) and overlies poor subsoil deep ploughing can bury and dilute the nutrient store of the topsoil. However as the store of nutrients is redistributed and not removed observed benefits of this technique tend to be short lived as after 4 years competitive plants will root deeper to access these reserves.

Adding material to dilute nutrients: adding materials such as chalk rubble or crushed concrete to the surface to bury or dilute nutrients can produce interesting results. However as with deep ploughing diluting the nutrient pool often has only a temporary effect. Even where depths of 50cm+ (1 tonne/m2) of chalk are laid over good soil, deep rooted plants will eventually find their way through. This approach however can yield some interesting results where the site is not too fertile to begin with and the added material, as with chalk, also changes the soil chemistry and structure.

Land-forming to remove topsoil: this is the only truly effective way of removing nutrient stores from a site. As this approach is irreversible and expensive it is not for the fainthearted. Before embarking on this approach it is important to consider the long term implications for the site and assess whether there is any potential to damage buried archaeological features. This approach is most appropriate to landscape projects where overall there may be a shortage of good topsoil so that re-profiling can be arranged to build up topsoil on amenity areas where it is needed and remove it from areas designated for permanent wildflower mixtures (e.g. road embankments).

pH: Acidity/Alkalinity

One of the most obvious changes of vegetation character is that between acid soils with heather and gorse, to alkaline chalk and limestone soils with its profusion of flowering plants. A pH test of your soil can be useful in confirming with other observations the character of your soil.

Most natural soils fall in the range pH 5 – 7.5.

  • pH >7.0 Calcareous
  • pH 6.5-7.0 Neutral-calcareous
  • pH 6.0-6.5 Neutral
  • pH 5.0-6.0 Acid-neutral
  • pH <5.0 Acid

Calcareous soils with a pH 7.5+ potentially support the greatest diversity of plant species. The most diverse calcareous grassland containing chalk or limestone specialists will be found on thin soils in which the chalk/limestone is significant and obvious.

It should not be assumed that soils overlying chalk or limestone are calcareous as the topsoil could be derived from glacial drift which is naturally more acidic, or the surface layers may have acidified. Generally over time soils in Britain tend to become more acidic as a result of the acidifying effects of rainfall, leaf fall and natural soil processes. Some upland limestone meadows of the Pennine dales perhaps owe their floral richness to historic liming reversing the acidifying effects of high rainfall.

Clays and other soils not derived from calcareous bedrock tend towards acid or acid-neutral but may have had their pH raised by past liming.

Acid soils generally support a lower diversity of species as fewer species have evolved to cope with acidity and its effects.

Organic humus content: Organic matter in the soil is essential (even for most wild plants) for soil structure and the retention of moisture and minerals. Raw soils such as subsoils, damaged soils, manufactured substrates for roof schemes and quarry reclamation materials which lack organic material and structure may have physical problems limiting plant growth.  Remediation may be required before sowing which may include the addition of organic material as well as deep ripping to break any compaction.

Other Site Factors 

Drainage and Hydrology

Plants vary considerably in their ability to tolerate drought or water-logging. The seasonality of flooding or drought on a site also has a big effect on plant communities, and is quite a complex subject.

Wet soils: From a practical point of view wetland areas if not overloaded with nutrients from adjoining land can produce valuable and diverse habitat and developed and can be sown with a species selection such as our Pond Edge mix EP1. The kinds of plants that establish readily from seed tend to be marginal plants rather than true aquatics: they possess a degree of adaptation to flooding and waterlogging but do not need to be in water to grow. Seasonal flooding may have practical implications for timing of sowing and establishment: this may typically favour sowing in spring to get good establishment before the winter rains.

Drought prone soils usually develop characteristic plant communities. Drought can be caused by soils that drain feely (like sands) and cannot hold moisture (eg low in organic matter). Drought can also occur where the depth of soil is very shallow (as in green roof schemes)  and cannot hold enough water in reserve between showers (particularly in drier parts of the UK). Plant species that cope with surface drought by deep rooting can survive in the first situation, but may not cope where the roots are restricted and cannot grow to find water. In the latter case plants which seed freely and can regenerate from seed after drought (like annuals) can avoid drought in this way.

Sowings on dry sites are usually more successful in autumn giving a longer period for root establishment ahead of droughts in summer.

Shade and Tree cover

Apart from the obvious effect from shading, trees can also effect ground vegetation through their roots taking moisture, and from autumn leaf-fall which smothers plants and prevent seedling growth. Mixtures of species suitable to these areas like EW1 woodland mix can be sown  Because of the effects of trees establishment of sown species may take longer and be more patchy than sowings in the open and this should be taken into account when devising schemes. Long established tree and shrub cover may also cause localised soil enrichment either directly from leaf fall, or as in old parkland as a result of cattle congregating beneath – such areas are best avoided if possible.

Steep slopes and banks

Wild flower mixtures can produce good results on embankments, the additional drainage and shallow soils from years of soil slippage sometimes gives better results than comparable sowings on the flat. The results obtained from a sowing may vary depending on whether the slope faces North or South. Initial establishment on steep slopes can be tricky as seed is prone to washing off in heavy rain or being exposed to drought in summer.