Excerpt From The Course
UNDERSTANDING SOILS
Soil provides plants with the following:
- Nutrition: the plant derives much of its food from nutrients in the soil.
- Support: the soil holds the plant firm and stops it falling over.
- Water and air: the roots absorb both water and air and so the soil must contain both. Soil with too much air leaves the plant starved for water. A soil with too much water leaves the plant starved for air.
Soils vary with respect to the above factors. For example, a sandy soil provides less support than a clay soil. A clay soil generally provides less air than sand but has a greater capacity to hold water. An organic soil usually has a good ability to hold water, but it does not always provide good support.
Soil is made up of organic particles that were once living plants or animals, and also of inorganic particles that were once rocks which have now broken down through chemical and physical weathering. As soils play such an important role in the health and growth of plants, a nursery manager should be aware of the following soil characteristics:
- Texture
- Structure
- Porosity
Soil Texture
Sand, loam and clay describe the texture of a soil. The type of material that makes up a soil affects the movement of water and air through the soil, the root penetration into the soil, and also the looseness and workability of the soil.
Components
The soil profile describes the various horizontal layers that a soil is made up of. Each soil layer may contain soils of different textures. Soils are not evenly made up of individual particles of sand, silt, clay and humus. These individual particles are found in groups called crumbs or aggregates throughout the soil. You can see these crumbs or aggregates when you sift the soil through your fingers. The individual particles in the soil (sand, silt, clay and humus) hold together more firmly than the various aggregates (groups of sand, silt, clay and humus). Aggregates come in different shapes and sizes and are also arranged in different ways to give soils their characteristic structures. Soil aggregates may include the following:
- Gravel: particles larger than 2mm
- Sand: particles between 0.02 to 2mm in diameter
- Silt: between 0.02 and 0.002mm in diameter
- Colloids: less than 0.002mm in diameter (these are either clay or organic).
Soil will also include a certain amount of organic matter.
Colloid particles are small enough to disperse in water. Improving the soil structure has the effect of flocculating the particles, which means clumping the particles together.
Texture can be classified into 7 classes and 16 grades. The following table gives examples of the various classifications. The last word in the classification indicates the dominant component of the soil. Loam has an equal percentage of sand, silt and clay.
Classification of soil texture
| Soil |
% of clay |
| sand |
<< 10% commonly < 5 % |
| loamy sand |
5-10% |
| clayey sand |
5-10% |
| sandy loam |
10-15% |
| light sandy clay loam |
15-20% |
| loam |
25% |
| silt loam |
25% clay 25% silt or more |
| sandy clay loam |
20-30% |
| clay loam |
30-35% |
| silty clay loam |
30-35% clay 25% or more silt |
| sandy clay |
35-40% |
| silty clay |
35-40% clay 25% or more silt |
| light clay |
35-40% |
| light medium clay |
40-45% |
| medium clay |
45-50% |
| heavy clay |
50% or more |
Percolation Rates
The rate of water percolation is another way to describe the texture of soil. Percolation is the natural movement of water through the soil, and soils percolate water at different rates. Soil should be watered only as much, and as fast, as the soil can absorb without runoff.
Sandy soil absorbs more than two inches (5cm) of water per hour. It is very porous, with large spaces between soil particles. Little water is retained and the sandy soil dries out quickly.
Loam soil absorbs from 0.25 inches to 2 inches (6mm to 5cm) per hour. The soil is loose and porous and holds water quite well.
Clay soil absorbs less than 0.25 inches (6mm) of water per hour. Clay soil is dense with few air spaces between particles and holds water so tightly that little water is available for plants.
Soil Structure
Soil structure is described according to shape, size and grade, where grade refers to soil strength and degree of development.
Good structure in a soil means that the mineral particles in the soil are bound together in crumbs, known as peds, of various sizes which are loosely arranged into larger groupings. This gives a well-structured soil its crumbly feel or appearance. This is known as a friable soil and provides plenty of pore spaces between the crumbs allowing good water penetration, aeration, and ease of penetration for plant roots and other soil life.
Crumb formation can be enhanced in a number of ways:
- The addition of organic matter
- The addition of clay in low clay soils
- The addition of iron and aluminium: for soils low in these elements
- The addition of exchangeable calcium: usually applied as lime or gypsum.
Crumb formation is reduced with increasing levels of exchangeable sodium which is common in areas with salinity problems. For example, leached clay soils which are high in sodium tend to have poor structure. Soil structure can also be readily damaged by:
- Over-cultivation or poor cultivation techniques: particularly when the soil is very wet
- Compaction: for instance, by repeated trafficking of machinery
- The killing of soil life with repeated applications of chemicals.
A well-structured soil has aggregates arranged in such a way that the soil is resistant to crushing and compaction. There should be many spaces and channels, known as pores, between the aggregates to allow oxygen to reach the roots as well as movement of excess water through the soil.
Diagnosis of Soil Type
The following table gives a diagnosis of soil type depending on how it feels between the fingers.
|
Does the soil stain the fingers? |
Does the soil bind together? |
Does the soil feel gritty? |
Does the soil feel silky or sticky? |
Does the soil make water cloudy? |
| Sand |
No |
No |
Yes |
No |
No |
| Sandy Loam |
No |
Yes |
Yes |
No |
No |
| Loam (or silt) |
No |
Yes |
No |
No |
No |
| Clay Loam |
Yes |
Yes |
No |
No |
Yes |
| Clay |
Yes |
Yes |
No |
Yes |
Yes |
Organic soils are soils containing a large proportion of organic matter (more than 25%). These are usually black or brown in colour and feel silky. It is possible to get organic types of all of the above soils. A simple test of the organic matter content of a soil is to place a small amount in a container of water. Organic matter tends to float to the soil surface, so the more material that floats, the higher the organic matter content.
Pore Space in Soils and Growing Media
Aeration and drainage of a growing media will mostly depend on the amount of pore space available in the growing media. The pore space can be defined as the percentage of the media’s volume that is not filled with solids. For example, a total pore space of 50% means that in every litre of media there is 50% pore space and 50% solids. Total pore space can vary from as low as 30% in a heavily compacted soil up to 95% in some peat. Good garden soils contain about 50% total pore space while good potting mixes and propagating media may have up 60-80% total pore space.
Pore shape and size are also important. Large round or irregularly shaped particles result in bigger air spaces than flat or small particles. Large pore spaces allow greater movement of air and water. However, if the growing media consists of just large pore spaces, the water holding capacity of the media will be poor. The ideal situation is a combination of small and large pores spaces that provide good aeration and drainage but sufficient water holding capacity.
Soil Temperature
The rate of absorption of water and nutrients is affected by the temperature of the soil. Too much heat or cold will slow the whole metabolism down. Soil temperature is not always the same as atmospheric temperature. Mulching a plant or adding organic matter to the soil will even out, or lessen, the fluctuations in soil temperature. As with most organisms, plant roots will grow within a particular range of tolerance which will vary from one species to another.
Soil pH
In rough terms, soil pH can be described as a measure of the relative proportions of positive and negative ions in the soil. In pure water they are equal and so the pH of water is normally 7. A pH of 7 is neutral. A scale of 0 to 14 (called the pH scale) is used to record this measurement of pH.
0————Acid————–7——–Alkaline——–14
Most plants prefer a pH of 6 to 6.5 (i.e. slightly acid), although, there are many exceptions. Plants may grow outside of their ideal pH range, but they will not grow as well. If the pH is below 4.5 or above 8 it is no good for the vast majority of plants. You should check the pH preferences of individual plants and the pH of the soil into which they are to be planted before adjusting the pH or planting.
Soil pH can be adjusted by the use of chemicals known as soil ameliorants. These are soil additives used to improve soil characteristics. Ameliorants include Lime (to raise pH) and Sulphate (to lower pH). However, the soil will tend to buffer, or modify, the effect of these chemicals and so calculation of the amounts required is often difficult. The general rule is to apply small amounts until the required result is obtained. It is better to add too little than too much!
When lime is added to break up hard clay soils, it will also raise the pH of the soil making it more alkaline. The addition of organic matter such as manure or compost, which contains weak acids, will cause the pH to drop. If fresh manure is used it can cause a drastic drop in pH. Sulphate of ammonia will also cause pH to drop.
Susan Stephenson