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:
• texture and structure
• pH acidity/alkalinity
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.
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.
Assessment of fertility
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 (10ppm 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 2 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.
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).
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.