Ivydene Gardens Soil:
How are Chemicals stored and released from Soil?
 

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.

Saxifraga
'Southside Seedling'

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

SO, WHAT HAPPENS TO THESE ELEMENTS IN THE SOIL WHEN IT RAINS?

These elements are leached down the layers of topsoil and subsoil, to the weathered bedrock and bedrock soil where the plant roots do not reach. These soil layers are called Soil Horizons:-

"Soil Horizons

So where to the leached ions go? Well the soil is formed in layers. The top layer (Horizon A - leaching zone. Organics here with most of plant roots) of soil is known as the topsoil. This is exposed directly to rain and generally leached of its nutrients by acid rain and the active growth of roots. The many living organisms and their side products and breakdown products make the topsoil very dark in color. Some would say it is black. It is certainly loaded with degrating organic particles. These help mitigate the leaching. So topsoil is sometimes considered "rich" but is really rich only if the pH has been kept corrected over a long period of time.

The next layer of soil (Horizon B) is where the minerals leached from topsoil accumulate. This layer is often called the subsoil. It is typically a red/brown color. It looks less rich, but in unmaintained soils is actually richer than the topsoil. When you are planting trees in a landscape we recommend digging a $50 hole for a $5 tree and mixing the recovered soil and sifting the gravel and rocks out of it, before filling in around the roots of the tree. This way you get the advantage of the organics in the topsoil along with the higher nutrient content of the subsoil.

The layer below the subsoil is called weathered bedrock (Horizon C). This is a very rocky layer and maybe packed or too deep for roots to penetrate. This is the source of new mineral ions however. So if the subsoil and topsoil are at correct pH, they can be replenished in minerals from Horizon C.

The lowest horizon (Horizon D) is the bedrock. Here in Eastern Connecticut this layer is granite. Our soil would be richer if our bedrock were limestone. In other parts of the country the bedrock is limestone and the crops thrive there!

Obviously if we want good crops over time, we need to take good care of the pH and nutrient levels in soil. But there is one other precaution we must take too. We need to control loss of farmland in order to keep feeding the world populations. A lot of our soil is eroded by wind or rain and is lost to us. Mark Twain wrote of the Mississippi River which drains much of the US farmlands. In his novels he called the river the "Big Muddy" because of all the clay and silt that is flowing down the river into the Gulf of Mexico. If you have seen this magnificent river you know what he is writing about. The Louisiana delta is where some of the horizon A and B soils end up. The organics decay in the submerged sediments and produce natural gas and oil. We are now mining this from there with offshore rigs. But the loss of farmlands is destroying our agriculturally based economy. Just like the Egyptians not realizing that the Nile flooding was the source of its agricultural productivity in ancient times, we are somehow missing the root of our national prominence. We are letting it go down the river.

Obviously as educated people, we must develop ways to do our agriculture without laying the soil bare to wind and rain for long periods of time. We need to cultivate the soil cautiously. We need to use mulches or ground covers more effectively. We also need to develop agricultural crops that are perennial rather than annual. Corn requires yearly plowing leading to erosion because it is an annual crop. Its closest wild relative is a perennial. If we can get the perenniation genes from Zea diploperennis into Zea mays, then we can have corn fields that do not need to be plowed. The corn would come up year after year and just need weed control and harvesting! This would save much soil as the tangle of perennial roots would hold the soil firmly against wind and rain. It would save on fuel costs for plowing and planting. It would reduce pollution of rivers. Science is at work on such progress and we need the society to understand the need to fund research at the USDA and NSF to accomplish these important goals." from Soil article by Plant Physiology Information website by Ross Koning.

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 in your first year as a top-up for most small gardens and then it would still be usefull to have a 3-4" deep mulch of organic material on the beds/hedges as well annually.

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 found and then its stockist locator can find a garden centre close to you in the UK.

The next section shows how the earthworm provides nutrients together with the nutrients in the manure from cows, pigs and horses to be included in the 3-4 inch (7.5-10cms) deep mulch of organic material together with grass mowings and shredded prunings.

The following is from "A land of Soil, Milk and Honey" by Bernard Jarman in Star & Furrow Issue 122 January 2015 - Journal of the Biodynamic Association;_

"Soil is created in the first place through the activity of countlesss micro-organisms, earthworms and especially the garden worm (Lumbricus terrestris). This species is noticeably active in the period immediately before and immediately after mid-winter. In December we find it (in the UK) drawing large numbers of autumn leaves down into the soil. Worms consume all kinds of plant material along with sand and mineral substances. In form, they live as a pure digestive tract. The worm casts excreted from their bodies form the basis of a well-structured soil with an increased level of available plant nutrients:-

• 5% more nitrogen,
• 7% more phosphorous and
• 11% more potasium than the surrounding topsoil.

Worms also burrow to great depths and open up the soil for air and water to penetrate, increasing the scope of a fertile soil.

After the earthworm, the most important helper of the biodynamic farmer is undoubetdly

• the cow. A cow's digestive system is designed to make use of roughage such as grass and hay. Cow manure is arguably the most effective and long lasting of all the fertilizing agents at the farmer's disposal and has been found to have a carry over effect of at least 4 years. It is also one of the most balanced and it contains no grass seeds, since they have been completely digested.
• Pig manure is rich in potassium, attractive to earthworms and beneficial on sandy soils.
• Horse manure increases soil activity and stimulates strong healthy growth, but it does contain grass seed and other seeds."