How are chemicals stored and released from soil?

Soil chemistry is largely governed by the soil’s ‘colloidal’ material. Colloids are particles that are larger than individual molecules but small enough to be microscopic. When colloidal particles are mixed with water they remain in suspension in a dispersed state.

The following diagram illustrates that soil is made up of a mineral and an organic fraction. Both fractions contain colloidal material.

It is these colloidal materials that have a great influence on the chemical properties of the soil.

Soil colloids also carry an electrostatic charge: clay minerals, hydrous oxides of iron (Fe) and aluminium (Al), and soil humus. This gives them the ability to hold onto certain chemicals, so that they can store nutrients, acidity and toxins. Cations are attracted to the negatively charged soil colloids, forming a layer of positively charged particles. The ability of a soil to store nutrients is related to both the amount and the type of colloidal material present. The ability of a soil to store nutrients can be expressed in terms of its cation exchange capacity or CEC. Soils with large amounts of clay and organic matter have high CECs whereas soils with sandy textures and low concentrations of organic matter have low CECs.

What determines the availability of acids and nutrients?

Soil colloids have the ability to act as a chemical store because they ‘buffer’ chemical changes by soaking up and then releasing ions from the soil solution.

All acids have high concentrations of H+ ions in solution. The strength of an acid is described using the pH scale, which is a logarithmic scale expressing the concentration of H+ ions in solution. The following shows the effect of soil pH on the availability of some common soil chemicals, which (with the exception of Aluminium) are essential elements for plant growth. The major elements are required in large amounts whereas the trace elements are required in small amounts (parts per million). Absorption of these elements by the plant through the roots occurs in ionic form from either the cation exchange complex of clay and humus or directly from the soil water solution.

Soil chemical name

Abbreviation

pH availibility range

Deficiency problems

Comments

Aluminium

Al

Increasingly available below pH 5.5.

Aluminium trisilicate is a grey clay. Mix sand and horticultural grit with it to improve it's soil texture to one more suitable for most plants.

Liming to pH 5.5 is recommended to avoid toxidicity dangers below pH 5.5.

Boron

B

4.0-7.5

Maxicrop Seaweed Meal provides all trace elements (This is brown knotted wrack seaweed - Ascophyllum nodosum, which has been dried and ground into a powder form). Apply 200 grammes/square metre on beds, trees, hedges and 70 grammes/square metre on lawns as a spring dressing each year.

Trace element.

All trace elements are involved in the enzyme actions.

Overliming to between pH 7.5-8.5 causes deficiency.

Calcium

Ca

Soils with pH between 5.5-6.5 require 200grammes/square metre of Dolodust (Dolomitic Lime) to raise the pH to above 6.5 as a liming agent.

Major element.

This is essential at the growing points of plants and in cell walls.

May be deficient in acidic soils.

Carbon

C

---

No carbon, no growth.

Major element.

This is taken from carbon dioxide in the air.

Chlorine

Cl

---

Maxicrop Seaweed Meal provides all trace elements. Apply 200 grammes/square metre on beds, trees, hedges and 70 grammes/square metre on lawns as a spring dressing each year.

Trace element.

Cobalt

Co

4..0-7.0

Maxicrop Seaweed Meal provides all trace elements. Apply 200 grammes/square metre on beds, trees, hedges and 70 grammes/square metre on lawns as a spring dressing each year.

Trace element.

May be toxic in acidic soils and deficient where pH exceeds 7.0 (below 7.0 soil is acidic, above 7.0 soil is alkaline)

Copper

Cu

4.0-8.0

Maxicrop Seaweed Meal provides all trace elements. Apply 200 grammes/square metre on beds, trees, hedges and 70 grammes/square metre on lawns as a spring dressing each year.

Trace element.

May be toxic in acidic soils and deficient where pH exceeds 7.0

Hydrogen

H

---

Available to plants by their breakdown of water.

Major element.

Iron

Fe

4.0-7.3

Sulphate of Iron applied at 30 grammes/square metre to chalk beds and to all lawns (aids destruction of moss in lawns) every spring will reduce deficiency.

Trace element.

Magnesium

Mg

5.0-9.2

This may occur on chalk soils, which have not been fertilised with magnesium rich manure since the 1940's. It may be advisable to use the Kieserite from Monro South for most new gardens on a yearly basis for at least 10 years, to overcome possible magnesium deficiencies.

Major element.

Component of chlorophyll which is essential to photosynthesis.

Manganese

Mn

4.0-7.0

Maxicrop Seaweed Meal provides all trace elements. Apply 200 grammes/square metre on beds, trees, hedges and 70 grammes/square metre on lawns as a spring dressing each year.

Trace element.

Molybdemum

Mo

4.5-10.0

Maxicrop Seaweed Meal provides all trace elements. Apply 200 grammes/square metre on beds, trees, hedges and 70 grammes/square metre on lawns as a spring dressing each year.

Trace element.

Nitrogen

N

5.0-8.5

Leaves will be pale green, yellow green or yellow. The cause needs to be ascertained before action taken (see pages 202-205 of Soil by Ken Simpson in Garden Pests section of Library). Generally, if a 3" deep mulch of organic material - Spent Mushroom Compost, Zoo Compost, Horse Manure, Cow Manure - is spread on the garden beds in late autumn, there should be no shortage of Nitrogen.

Major element.

This is a constituent of all proteins in a plant.

Oxygen

O

---

No oxygen, no growth. Usually available from breakdown of carbon dioxide by the plant.

Major element.

Phosphorus

P

5.0-7.0

Most of the soils of the British Isles are naturally deficient in phosphorus. A slow release form called Phosmag is available from Monro South. This has 5 parts Nitrogen, 19 parts Phosphorus, 10 parts Potassium, and Magnesium in a slowly dissolving form. 1 dressing per year is sufficient for both phosphorus and potassium.

Major element.

Required for enzyme actions within a plant.

Liable to be fixed by Iron, Aluminium, Manganese at low pH by combining with those elements; insoluble forms created at high pH, and can inhibit the acceptance of Calcium by plants.

Potassium

K

5.0-10.0

The first symptom is yellowing of the tip and leaf margin. These parts then die as yellowing followed by browning spreads inwards between the veins. The whole leaf then dies. Use Phosmag as recommended above in early spring.

Major element.

Used for regulation of osmotic pressure and turgidity of the tissues.

Deficiencies liable at low pH.

Silicon

Si

---

Trace element.

Sulphur

S

5.0-10.0

Due to coal not being burnt as much nowadays, there is a deficiency in the soil of sulphur. Superphosphate at 105 grammes/square metre per year will restore sulphur levels to a private garden.

Major element.

This is part of some proteins and oils in a plant.

Some reduction at low pH, but sulphur bacteria still active.

Zinc

Zn

4.0-8.0

Maxicrop Seaweed Meal provides all trace elements. Apply 200 grammes/square metre on beds, trees, hedges and 70 grammes/square metre on lawns as a spring dressing each year.

Trace element.

May be toxic in acidic soils and deficient where pH exceeds 7.0

Phostrogen Plant Food provides Nitrogen, Phosphorus, Potassium, Magnesium, Calcium, Sulphur, Boron, Copper, Iron, Manganese, Molybdenum and Zinc in soluble form for applying using a watering can, or it can be used as powder at 70 grammes per square metre as a soil dressing in Spring and again in Summer. Maxicrop Seaweed Meal or Maxicrop Calcified Seaweed can also be obtained from garden centres to provide more of the trace elements each Spring. These two items would provide a fair proportion of nutrients required for most small gardens and it would still be usefull to have a 3-4" deep mulch of organic material on the beds/hedges as well. Small quantities of the J Arthur Bower's Garden Lime (instead of Dolodust for Calcium), Sulphate of Iron, Superphosphate (for Sulphur) and Phostrogen Plant Food (for Potassium and Phosphorus instead of Phosmag) can also be obtained on-line.