Current Issues, Jorge, Plants

Unlike genetically modified crops, mutation breeding goes largely under the radar, but has been ongoing since at least 1942 when scientists Freisleben and Lenn induced mildew resistance in barley through the use of X-rays. The same scientists coined the term in 1944, defining it as “the utilisation of induced mutations in crop improvement”. Mutations are the “sudden heritable change in an organism” and crop improvement is induced “desirable changes in the genetic constitution of plants” and improved “performance of a cultivated variety” whether that be increased drought resistance or early flowering (and hence fruiting).

Standing at over 30 billion dollars, the seed market is a huge industry with such firms as the maligned Monsanto, which has run into public disdain and increasingly legislative hurdles as it tries to introduce new GM varieties into the world’s markets. A large chunk of this is mutation breeding that has no such regulation and offers an opportunity for companies to circumvent anti-GM laws and public scrutiny, while introducing new patented strains of seeds.

Before delving into the science and the question of whether foodstuffs derived mutagenesis are dangerous, it will be first worthwhile telling the fascinating history of mutation breeding.  

Mutation breeding was first proposed at the turn of century when Hugo de Vries suggested using radiation to induce mutations in plants and animals. By 1927 his ideas were confirmed when scientists Gager and Blakeslee carried out radium ray treatment of a Datura stramonium, inducing mutations. It was however Hermann J. Muller’s work in the 1910s and 1920s that provided the chief principles of spontaneous gene mutation, which eventually won him the Nobel Prize in Physiology and Medicine in 1946.

Mutation breeding achieved popularity in the 1950s, when it became part of the atoms for peace movement – a movement dedicated to the use of atomic energy for peaceful ends. The movement was kickstarted by the United States government that funded both research into peaceful applications of the technology and the construction of nuclear power plants around the world. The program was seen as a way to resolve the atomic dilemma as summarised in Dwight D.Eisenhower’s 1953 speech to the U.N. General Assembly that the “miraculous inventiveness of man shall not be dedicated to his death, but consecrated to his life”. This speech was followed by multiple conferences in the 50s that sought to bring together scientists from both East and West and reduce animosity between the two blocs.

The atoms for peace symbol, used during the 1955 Atoms for peace conference.

As part of the research into the application of atomic technology, mutation breeding was funded with the establishment of gamma gardens, in which crops were arranged in concentric circles around around a radiation source – usually a cobalt-60. The experiments were crude with crops near the source simply dying, and the ones further away riddled with growth abnormalities. It was the ones further away apparently healthy, but with alterations that were of interest.

Some experiments proved fruitful and gave us varieties that overcame limitations and now dominate as a percentage of production. Peppermint for example was extremely susceptible to Verticillium wilt, a fungal disease and cause of plant death, and it was experiments at the Brookhaven National Laboratory that led to the release of the ‘Todd’s Mitcham’ cultivar. A variety which underpins the $930 million global mint oil industry, which is used in everything from chewing gum to toothpaste. Another resultant variety from such experiments is the ‘Rio Star’ grapefruit, which is more red in colour and produces more flesh and juice. The variety accounts for 75% of grapefruit production in Texas.

Atoms for peace inspired certain sections of the public to conduct their own experiments such as Muriel Howorth in the United Kingdom and C.J. Speas in the United States, part of the atomic gardening movement.

Muriel, a laywoman, was extraordinarily passionate about the technology and promoted all things nuclear: publishing books (including Atomic Gardening for the Layman) and journals, forming multiple societies (including the Atomic Gardening society) and even staging a “Radioactivity Jubilee”. She was a maverick, who at the time was the only person speaking to women about the new science, founding the Ladies Atomic Energy Club. In 1959, she was the host of a dinner party of the Royal Commonwealth Society and decided to surprise her guests with irradiated peanuts as big as almonds. To her disappointment, they did not take off. Unruffled, she planted the peanuts in her greenhouse, which upon growing rapidly to two feet, she phoned the press to make the best out of a bad situation.

Holworth presenting her two-foot peanut plant to Beverley Nichols, a popular garden writer at the time.

C.J. Speas, another enthusiast, managed to obtain a license from the Atomic Energy Commission for a cobalt-60 source, which he encased in a cinderblocks in his back garden. From this he irradiated trays of seeds of which he reportedly sent millions (of seeds) to the Atomic Gardening Society, who distributed them to nearly a thousand members. He used to give tours of his cinderblock bunker to tourists and school groups. Separately, as pictures from Life magazine document, ‘super atomic energized seeds’ and ‘atom blasted seeds’ were sold at store and fairs in the late 50s and early 60s.

Atom-blasted seeds on sale in 1958. Photo by Grey Villet for Life.
Speas giving a tour of his bunker. Photo by Grey Villet for Life.

Today, mutagenesis is practiced by chemical companies and conglomerates such as BASF and DuPont. (It is important to mention that mutagenesis can be instigated by three classes of agents – biological, chemical and physical mutagens, so radiation is not necessarily involved.) Although, the legacy of Atoms for peace lives on in the work of the International Atomic Energy Agency, which is commemorating its sixtieth birthday, and the Food and Agriculture Organization of the United Nations, who through their technical cooperation programme contribute to the UN sustainable development goals through providing scientific support to member states.

One fascinating example of mutagenesis was carried out by the RIKEN Nishina Center for Accelerator-Based Science, Japan, who used heavy ion beams to induce mutations in a cherry tree, creating a new cherry blossom that blooms in all four seasons. The tree is unique in that it does not need a period of cold weather to trigger growth in spring and ostensibly produces three times more flowers than standard trees and stays in bloom for twice as long when blooming in April.

Interestingly, mutagenesis has proved highly profitable for Japan with the country investing $69 million on mutant breeds from 1959-2001, which have yielded $62 billion worth of goods in the same period. Hence, bringing new cultivars to market through mutation breeding is significantly cheaper than through GM, with Monsanto spending up to $200 million to launch a single GM product. And as things stand, this offers a huge incentive for firms to abandon GM methods and switch to mutation breeding.

How does mutation breeding work?

Mutation breeding is a two stage process involving mutation induction and detection. It is extremely effective, increasing the natural mutation rate by a thousand to a million fold. Mutation induction works by damaging an organism’s cellular structure, causing a change in the DNA, which when not repaired by the cell’s repair mechanism, lives on as a heritable mutation. These mutations are induced through two classes of mutagens – chemical and physical with the latter generating 70% of released mutant variables.

Physical mutagens are primarily induced through ionising radiation from gamma and x rays. These rays form part of the electromagnetic spectrum, just like visible and infrared light, except are extremely high energy. Chemical mutagens work differently involving chemical reactions within the genome, which alter a section of the DNA. Unlike physical mutagens, chemical mutagens are varied, with a number of agents, altering DNA through different causal chains.

With physical mutagens, mutations can be induced through a number of methods such as the aforementioned gamma gardens or fields. Alternatively, seeds or plant propagules can be placed within a gamma cell with a Cobalt-60 source (similar to Speas) or simply irradiated with an x ray machine. More recently, ion beam technology has been used to introduce mutations.

Plants arranged in concentric rings around a Cobalt 60 source. C.1959 at the Brookhaven National Laboratory.

Usually, scientists set upon finding the optimal dose that will be high enough to cause mutations, without putting a halt to germination or growth. And with most methods, scientists will go through thousands of plants before a mutation imparts a desirable characteristic. In addition, as many mutations are recessive, these characteristics are not revealed till subsequent generations.

The true art of mutation breeding lies in the mutation detection stage that has long been a bottleneck in plant breeding due to the reliance on phenotypic screening. Put simply, genotypes and phenotypes are used to distinguish between a plant’s hereditary information and an organism’s observed properties. As these observed properties are influenced by both the environment and a plant’s genetic code, scientists can’t be sure an observed trait originates from genetics. Rather a plant’s ostensible disease resistance may originate from an absence of a pathogen, as opposed to an inbuilt resistance to disease.

More recently, the introduction of genotypic screening has allowed scientists to distinguish between putative mutants and true mutants, by identifying variations that are inherited and linked to a trait of interest. By identifying a variation in the DNA, populations can be then assayed, leading to the identification of molecular markers that allows breeders to introduce mutant traits into different cultivars for improvement. Next, putative mutants are evaluated under a set stringent conditions, leading to mutant confirmation.  

Are foodstuffs derived from mutants dangerous?

As previously mentioned, unlike GMO, mutagenesis is unregulated and to some hasn’t received the attention it deserves. Accordingly, the National Academy of Sciences has stated the risks of creating unintended genetic consequences from mutation breeding is higher than any other techniques due to the imprecise nature of the method and the random alteration of DNA. However, they also state that the risks are small relative to the incidence of other foodborne illnesses. Unsurprisingly, BASF, states that the crops are safe with the technique being used for many decades without concern.

In line with this, mutant breeds are relatively widespread, especially in Asia where countries such as China, India and Japan produce over 10% of their produce from such varieties. According to the UN, there are over 3200 mutant varieties released for commercial use in more than 210 plant species for use in more than 70 countries. Furthermore, there may be many more varieties with mutant genetic code that we have simply forgotten about due to the long history of mutant breeding. So, it is probable such foodstuffs have already entered our food supply.

Ultimately, mutation breeding has proven a vital tool to increase crop yields in our increasingly hungry world. Due to the work of the UN, mutant strains are widely used throughout the developing world and have done much to alleviate hunger. Certainly, neither GM, nor mutagenesis derived varieties should receive a blanket ban, but be assessed on a case-by-case bases. As with many ethical dilemmas, the truth lies hidden in the details.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

Garden Edging, Infographics, Jorge, New Products

At Primrose we are always looking to bring you innovative products, and are proud to introduce our new range of recycled rubber garden products.

Browse our range of recycled rubber products including planters, edging, deck tiles and stepping stones.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

Allotment, Composting, Gardening, Grow Your Own, Infographics, Jorge, Plants

soil science

Soil type, texture, structure, pH, nutrients and organisms are often bounded about in the gardening matrix but what do they all mean and why are they important? In this comprehensive article, we try to explain each of these one at a time without dumbing it down; and with the ultimate aim of producing the go-to article for improving crop yields and plant health. As the article is very long our findings and recommendations are summarised in the conclusion, but I’m sure the reader will be interested in the full explanations in the body of the text.

Mineral nutrients

Plants require three main nutrients: nitrogen (N), phosphorus (P) and potassium (K) that are collectively known as NPK. Deficiencies in such elements will significantly reduce plant growth. Also important to plants are calcium (Ca), magnesium (Mg) and sulfur (S). These are collectively known as macronutrients and make up 3.5% of dry plant weight.

Plants need a number of elements in minute quantities known as trace elements or micronutrients. They make up 0.04% of dry plant weight and include chlorine (Cl), iron (Fe), manganese (Mn), zinc (Zn), boron (B), copper (Cu) and molybdenum (Mo), although nickel (Ni), silicon (Si) and cobalt (Co) are sometimes included.

A plant will continue to grow until restricted by the supply of an essential nutrient. A deficiency of any nutrient cannot be corrected by the addition of other nutrients. Thus plant growth is limited by the nutrient in the shortest supply. This is known as the “Law of the Minimum”. The first limiting nutrient and most important is nitrogen.

In general, plants absorb essential nutrients in soluble, inorganic forms, although some metals can be absorbed as organic complexes. In order for nutrients to be absorbed they must come into contact with the root’s surface, which occurs through three main mechanisms: root interception, mass flow and diffusion.

  • Root interception occurs when roots grow through the soil and incidentally come into contact with nutrients. It makes up a small portion of total nutrient uptake.
  • Mass flow occurs when dissolved nutrients move with water and come into contact with root surfaces where they are absorbed. It makes up a dominant portion of total nutrient uptake and often results in excess nutrient uptake. As mass flow depends on flows of water, dry conditions and lower temperatures reduce nutrient uptake. It is through this mechanism that plants absorb most of their nitrogen.
  • Most of a plant’s potassium and phosphorus uptake occurs through diffusion, whereby nutrients spread from areas of high concentration to areas of low concentration. As roots absorb nutrients from a soil solution the concentration of nutrients surrounding the root drops. A result of this is nutrients in areas of higher concentration migrating towards the root.

Nutrients in the soil go through a continuous process of cycling that involves gains, losses and transformations in pools in the soil. With nitrogen, for example, seven forms are involved in the N cycle that each exist in different pools. These pools can be highly soluble or insoluble and strongly bound.

A simplified version of the nitrogen cycle. In fact, the nitrogen cycle is a bit of a misnomer as it is really a maze.

Plants can only directly utilise two soluble forms of nitrogen (NH4+ and NO3-) and depend on microorganisms to transform plant matter into such forms. This process is known as mineralisation and is dependent on the carbon-to-nitrogen ratio of the plant residues. When microorganisms break down organic matter, they utilise some of the resultant nutrients (such as carbon and nitrogen) for sustenance and growth but leave excess nutrients available for uptake by other organisms. Other microorganisms can easily access the excess nutrients, while plants cannot. Thus when there is a deficiency in nitrogen, plants sometimes miss out.

Eventually, these microorganisms will die and the immobilised nitrogen will be released back into the soil. But in the short term, nitrogen will be unavailable for uptake by plants, possibly leaving your plants nitrogen deficient. Nitrogen deficiency can be indicated by pale green leaves due to a reduction in chlorophyll – the nitrogen based pigment responsible for photosynthesis. And as nitrogen is an essential component of amino acids – the building blocks for proteins – nitrogen deficiency can also be indicated by stunted growth, particularly with dormant lateral buds.

A green bean plant suffering from a deficiency of nitrogen as indicated by the pale green leaves.  Picture credit: Rasbak (2009) licensed under CC BY-SA 3.0.

Now, it is probable that you wish to correct such a deficiency. As a long term fix, you want to add both compost and organic fertiliser as well as inorganic fertiliser. The latter, already in mineral form, will be immediately available for uptake by plants, and quickly correct the deficiency. The former however will correct the underlying problem by providing adequate feed for the soil’s microorganisms. And as organic fertilisers require organisms to transform the nitrogen into mineral forms, they provide a slow release of nutrients, helping to maintain healthy nitrogen levels.

It is important to note that the compost applied must be the correct carbon-to-nitrogen ratio, or microorganisms will continue to immobilise nitrogen at the expense of plants. In general, you want less than 30 parts carbon to 1 part nitrogen (C:N;30:1) to meet the nitrogen needs of decomposing organisms. The carbon to nitrogen ratio of various compost inputs can be found in the table at the bottom of this page.

In general, green coloured compost inputs (fresh organic garden waste) are high in nitrogen while brown coloured compost inputs (old rotten leaves) are low in nitrogen. As a rule of thumb, adding 2 parts green to 1 part brown will produce compost with the desired 30:1 carbon to nitrogen ratio.

Soil organisms

Soil organisms play a key role in the soil by breaking up organic matter into mineral nutrients, available for uptake by plants. Soil organisms are both abundant and highly varied, ranging in size from microscopic bacteria to the 1 meter long giant tunnelling earthworm. Like plants, they require certain conditions to survive and are suited to aerated, moist soil. This explains why 75% are located within the top 5cm of soil.

Significant chemical and biological activity takes place in the zone of soil surrounding a plant’s root – an area known as the rhizosphere. It is in this area that plants engage with a host microorganisms, both pathogenic and mutualistic, and act to shape a soil’s characteristics. To do this, plants release exudates – water and compounds such as carbohydrates – that stimulate biological and physical interactions between roots and organisms.

Important (and somewhat famous) mutualistic organisms that form symbiotic relationships with plants include that of mycorrhizal fungae and rhizobia bacteria. The former, ubiquitous in the soil, provides nutrients in return for carbohydrates and helps increase the surface area of a plant’s roots, significantly boosting plant growth. The latter fixes nitrogen from the atmosphere in return for carbohydrates and is one of the only environmentally-friendly effective methods of restoring a soil’s nitrogen content.

Promoting soil organism abundance is simple as organisms are suited to the same conditions as plants requiring organic matter as a source of food as well as aerated, moist soil. As many form symbiotic relationships with, or prey upon plants, plant life is crucial and will act to promote moisture retention and protect organisms from the sun rays. Use of chemicals will reduce the incidence and diversity of microorganisms in the soil and in some cases may wipe out certain species. Promoting microorganism diversity can help keep plants healthy as soils with high biodiversity can help suppress soil-borne fungal diseases.

Organic fertilisers provide a source of nutrients for organisms and allow beneficial microorganisms to carry out their natural function, transforming nutrients into mineral form. Inorganic fertiliser also provides a source of nutrients but should only be used in tandem with an organic fertiliser, which bolsters a soil’s health over the long term. Organism diversity can be promoted through crop rotation or mixed borders as different root types promote different organisms.

Some actions can promote microorganisms detrimental to plant growth. For example, compaction of the soil can lead to the emergence of anaerobic bacteria that produces toxic compounds. Excessive use of nitrogen fertilisers can promote fungal with pathogenic traits. Henceforth, excessive use of fertilisers should be avoided.

Soil pH

pH is a measure of acidity and alkalinity, ranging from 0 (most acidic) to 14 (most alkaline). A pH of 7 is neutral. Technically speaking, pH is the negative log of hydrogen ion concentration in a water-based solution, hence the equation pH = -log[H+]. It is a logarithmic scale and a whole pH below (6) is ten times more acidic than the higher value (7) and the hydrogen ion concentration increases by ten times. Put simply, a soil with a high concentration of hydrogen ions (H+) is acidic.

The pH of soil is important as excessively acidic or alkali soils will result in key nutrients becoming unavailable for uptake by plants. For example, at low pH phosphorus and calcium become less available, while others such as aluminium and manganese become available to such an extent that they are toxic to plants.

Different plants are each suited to different pHs, although 5.2 to 8 is acceptable to most. Some plants are sensitive to small changes in the pH, while others can tolerate a wide range of pHs. Soil organisms are also suited to different pHs, but most the activity occurs in the pHs 5 to 7. Changes in the pH will influence the species mix and functions of microbes in the rhizome.

Acidification of the soil occurs through various human activities such as the emission of air pollutants (leading to acid rain), use of agricultural fertilisers (usually ammonium-based), harvesting of crops (causing the removal of the slightly alkali plant matter) and mining. When pH levels drop below 4.5, there is a large increase in soluble aluminium, leading to soil toxicity. Acidification leads to leaching of nutrients such as calcium, magnesium and potassium to soil horizons out of the reach of plants, and severely decreases the microorganisms in the soil.

Excessively acidic soils’ pHs can be raised through liming; this usually involves dumping large quantities of pulverised limestone (calcium carbonate) on agricultural land. Sometimes the soil is ploughed to increase penetration. As calcium carbonate (CaCO3) dissolves in the soil solution, it reacts with hydrogen (H+) to form carbonic acid (H2CO3) or water (H2O). Thus liming acts to remove hydrogen ions (H+) from the soil, raising the pH. The detrimental effects of acidic soils can be partially alleviated through the creation and introduction of acid tolerant varieties.

You can measure a soil’s pH through purchasing a soil pH kit, although a lab test will provide the most accurate measurement. You can also estimate a soil’s pH by analysing the plants that naturally grow in your soil and judging how well certain plants grow. Stunting of a pH sensitive plant may indicate inappropriate pH. The morning glory variety of the Ipomea genus, for example, is very sensitive to changes in pH and is suited to slightly akalki soils. Weeds can be used as a rough estimation of certain pHs with very acidic soil producing sorrel and plantain but no charlock or poppy. Neutral pH soils, on the other hand, tend to promote chickweeds.

Soil formation

Soil formation is influenced by five soil forming factors: CLimate, Organisms, Relief, Parent Material and Time (CLORPT), although the key factor is climate. If the temperature is too low, organic material will not decompose. If there is little precipitation or wind, the rate of physical weathering may be insufficient to break up the parent material. Thus, the perfect climate for agriculture is humid and warm as it both supports and decomposes large quantities of organic matter and weathers the parent material.

Like plants, insects are also highly sensitive to temperature and are found in an abundance in warm climates.

Parent material is important as it affects the rate of weathering and the types of minerals and nutrients in the soil. Rocks are composed of different minerals that each possess different susceptibilities to weathering. For example, granite is primarily composed both of quartz and feldspar. The former mineral is highly resistant to weathering, producing coarse sand particles, while the latter weathers quickly turning into fine clay particles. Limestone on the other hand is composed of calcium carbonate that is highly susceptible to weathering in humid climates.

The weathering of the parent material breaks down rock into smaller and smaller pieces, eventually forming sand, silt, and clay particles. While the weathering process produces many different sized particles, soil particles can either be categorised as sand (.05-2mm in diameter), silt (.002-.05 in diameter) or clay (<.002mm in diameter). The size of the particles is important as it affects how quickly water moves through soil. As such, the larger the particle, the quicker it drains water. This explains why sandy soils are known to drain quickly, and clay soils slowly.

The above categories – sand, silt and clay – are known as the fine earth fraction, while soil particles greater than 2mm (i.e. partially weathered rocky fragments) are known as the coarse fraction. Such rocky fragments include boulders, stones, gravels and coarse sands.

Organisms function to continue the weathering process and add organic material to the soil, improving the soil’s structure further. Soils are improved slowly and pioneering plants prepare the ground for larger organisms. Over time, organisms will radically alter the soil, producing new soil horizons as their roots grow deeper with the soil in the upper horizons ending up highly granular.

Organisms (vegetation) can heavily modify a soil’s chemistry. Trees can alter a soil’s pH depending on the amount of calcium found in its leaves. (Remember calcium is used to raise the pH of acidic soils.) Pine trees, for example, create acidic soils that acts to strip soluble nutrients from the soil. Broadleafs, on the other hand, tend to raise a soil’s pH, although there are exceptions in both groups.

Relief plays an important role in soil development with soils at the bottom of a slope different from soils at the top and soils upstream different from soils downstream. A soil’s position on a slope affects its development as both runoff and water velocity increases lower down a slope. A possible result of this is high levels of erosion at the base of slope that can strip soil, producing weakly developed soils. A soil’s position on a slope and the direction the slope faces can affect evaporation with soils in direct sunlight for different periods. Relief also helps determine a soil’s texture, but more on this below.

Soil texture

Picture credit: Mikenorton (2011) licensed under CC BY-SA 3.0.

The most arable soils are comprised of 40% sand, 40% silt and 20% clay and are known as loam soils. The soil is fertile, easy to work with and drains well, although will still need mulching as with all soil types. With clay, silt and sand soils as the size of the average soil particle increases, the soil’s nutrients falls but drainage increases. So clay soils are rich in nutrients, but drain poorly, while sand is low in nutrients, but drains well.

As clay is so clumpy, the soil will need to be broken up and organic matter added to improve its aeration and drainage. Silt retains moisture, drains well and is fertile, but is vulnerable to compaction and will need mulching to improve its structure. As sandy soils are low in nutrients and do not hold moisture well, extensive use of mulching and application of fertiliser is necessary to improve yields. Chalk soils can be made of many different particles, but are notable for being alkaline and henceforth only suitable for certain plants. Peat, uncommon in gardens, are high in nutrients and moisture, but are often acidic.

Relief plays an important role in determining a soil’s texture. As a river empties from a mountain stream and enters its middle course its velocity decreases and particles drop out of suspension, the largest first. Thus coarser soils are found near the base of the mountain and the fine textured soils further downstream.

A soil’s texture can be ascertained through two simple tests:

  • The bottle method: place a cup of dry soil in a 500ml water bottle, fill it with water and then shake thoroughly for a few minutes. Stand the bottle upright and watch the particles settle with the largest at the bottom, which should take no longer than five minutes. The resultant of layers will give you an estimate of your soil type as indicated below. (Although, it should be noted that some aggregates will resist disintegration and clay particles may take ages to settle.)

  • The knead method: take a small handful of soil and break up the aggregates, removing large particles such as gravel or leaves. Then add water a drop at a time and mould a 4cm ball in your hands. Stop adding water when the ball starts to stick to your hands and knead for another 30 seconds. Now press the ball between your fingers. It will either feel gritty, silky or sticky and the textures indicate sand, silt and clay respectively.

Soil structure

Soil is formed when weathered rock mixes with decomposed organic matter, known as humus. Tiny particles of sand and silt are bound together by clay and humus, forming peds (aggregates). Peds have distinct boundaries and well-defined planes of weakness and can range in size from 1-300mm. Soils can contain multiple types of peds. The smallest peds are in the soil horizons (layers) near the surface and size of peds increases with depth.

The different types (blocky, columnar, granular, lenticular, platy and prismatic) are formed by different forces, although the only type you need to know is granular. Granular peds are usually less than 0.5cm in diameter and are commonly found in the uppermost soil horizons where plants’ roots have been growing; they function as an indicator of good soil structure.

Some soils are apedal and either have no peds or are not composed significantly of peds. Apedal soils can be divided into single grain and massive soils. Single grain soils have no adhesives to bind the grains together and do not aggregate into peds. Such soils are usually very sandy soils. Massive soils are a coherent, solid mass that do not separate into peds and are usually clay. Both soils are unsuitable for plants as with single grain soil the permeability is rapid, and with massive soils the permeability slow.

Soil structure refers to how these peds fit together. Good soil structure will have adequate pores (spaces), allowing for water and air to enter the soil and to drain easily and hold enough moisture for plant growth. Poor soil structure will have few, large aggregates and few pores that will both retard root growth and restrict access to air and water, which is essential for plant growth.

The structure of a soil can be graded by how distinct and stable the peds are. The different grades being structureless, weak, moderate and strong. At the lower end with structureless there is either no observable aggregation (single grain soils) or no orderly arrangement of natural lines of weakness (massive soils). And at strong, peds are distinct in undisturbed soil, and remain durable when disturbed.

Soil structure can be measured by calculating three metrics: bulk density (mass per unit bulk volume of soil dried to a constant weight at 105oC), particle density (mass per unit of volume of soil particles) and soil porosity (percentage of soil that is pore space or voids).

Bulk density is easy to calculate and can be used as a measure of compaction. In general, bulk densities range from 0.5 (organic soils) to 1.8g/cm3 (compacted clay soils). Bulk densities beyond 1.8g/cm3 are highly detrimental to plant growth. Particle density is relatively constant, ranging between 2.55 to 2.7g/cm3 and is often assumed as 2.65g/cm3. The average soil has about 50% porosity and sand has larger pores than clay, but clay has more pore space.

A worked example calculating the bulk density, particle density and porosity of a cube of soil.

Soil permeability is slightly different than porosity as it is the ease that air, water, or plant roots penetrate and pass through soil. Soils with large, connected pores, such as sandy soils, are more permeable than soils with small pores, such as clays, even though clays have greater total porosity.

Soil strength is the amount of force required to rearrange soil particles and affected by three factors: moisture content, soil texture and bulk density. Moisture content is the most important factor as dry soils are extremely difficult to work with; henceforth the drier the soil, the greater the soil strength. Soil texture is important as the strength of aggregated soils increases as clay content increases. Poorly aggregated or single grain soils (sandy soils) have the weakest soil strength. And finally, as when bulk density increases, the amount of pore space decreases, soil strength increases with bulk density.

Aggregate stability refers to the ability of soil aggregates to resist disintegration by disruptive forces whether from human activities (tilling) or weathering (precipitation and wind). Unsurprisingly, poorly aggregated soils have low aggregate stability and are vulnerable to disintegration in rainstorms. Once dispersed soil particles fill surface crusts, producing a layer of hard physical crust once dried. This layer can prevent the emergence of seedlings and reduces infiltration, leading to increased runoff and water erosion.

A soil’s aggregate stability can be worsened by human activity. Chiefly this occurs when soils are left bare without living plant organisms that improve structure and protect from weathering. Also detrimental, is the removal of decomposing organic matter, which function to aggregate soil particles into larger aggregates. Aggregate stability can be improved through increasing a soil’s organic matter content, which furthers biological activity, both microorganisms and plant life.

Summary of Findings

Mineral Nutrients:

  • Plants require three main nutrients: nitrogen, phosphorus and potassium, along with many others in smaller quantities. These nutrients are important as plant growth is limited by the nutrient in the shortest supply. This is known as the law of the minimum.
  • Plants can only absorb nutrients in inorganic forms and are dependent on microorganisms to break down organic matter into inorganic mineral forms, a process known as mineralisation. As plants are poor at absorbing nutrients they are sometimes crowded out by other organisms, leaving them nutrient deficient.
  • Nutrient deficiencies can be corrected by both inorganic and organic fertilisers. Inorganic fertilisers will quickly correct the deficiency as they are in soluble forms, immediately available for uptake by plants. Organic fertilisers, on the other hand, will first need microorganism to break down the nutrients into mineral form. This results in a slow release of nutrients and as such it can be stated that both types of fertiliser complement each other.
  • Organic fertilisers have an additional advantage: the potential to solve the underlying problem behind a soil’s dearth of nitrogen – a lack of food for soil organisms, providing it is of the correct carbon-to-nitrogen ratio.

Soil organisms:

  • Soil organisms are both ubiquitous and highly varied. They play a key role by converting organic matter into mineral nutrients, available for uptake by plants.
  • Many form relationships with plants, which can be mutualistic or pathogenic. Such mutualistic organisms include mycorrihizal fungae and rhizobia bacteria. The former acts to increase a plant’s root area increasing the uptake of nutrients while the former fixes nitrogen from the atmosphere, restoring a soil’s nitrogen content.
  • Soils organism abundance can be promoted through reducing the use of chemicals and inorganic fertilisers, preventing compaction, and maintaining moisture and plant covering. Thus it can be said that organisms thrive in the same conditions as plants.

Soil pH:

  • pH affects the uptake of nutrients by plants. For example, excessive acidity can render nutrients unavailable, while excessive alkalinity can increase nutrient availability as so it is toxic.
  • Different species of soil organisms and plants are suited to different pHs although most activity occurs between 5.2 and 8 for plants and 5 and 7 for soil organisms.
  • PH can be estimated by the plants that naturally take up root and how well certain plants grow. It can be measured through a soil pH kit.
  • Acidic soils’ pH can be raised through liming – the addition of calcium carbonate to soil.

Soil formation:

  • There are five soil forming factors: CLimate, Organisms, Relief, Parent Material and Time (CLORPT), although climate is the most important. Inadequate temperatures or precipitation may be insufficient to weather rock and be unable to support plant life.
  • Organisms function to continue the weathering process, breaking up the parent material (rock) to form horizons (layers) with the upper horizons ending up highly granular.
  • Weathered rock is eventually transformed into small particles of sand, silt and clay, which are part of the fine earth fraction. Sand, silt and clay are categories of particle size measured in diameter with sand the largest, clay the smallest and silt in between.

Soil texture:

  • Different compositions of particles produce different soil types such as loam, clay, silt and sand, each with different properties.
  • As particle size increases drainage increases and thus sandy soils drain quickly and clay slowly. Conversely, as particle size increases a soil’s nutrient capacity falls and henceforth clay soils are nutrient rich.
  • Of the four main soil types listed above, each will need mulching, but sand, silt and clay may need extra work. Clay will need to be broken up as it is clumpy, while sand will need fertiliser to improve its nutrient capacity. With silt it is important to avoid compaction.
  • Other soil types not related to particle size include peat and chalk, the former alkaline and the latter acidic, but high in nutrients.
  • A soil’s texture can be gauged through two methods: the knead method and jar method, which are described in detail above.

Soil structure:

  • Soil is composed of weathered rock and decomposed organic matter. Most soils are composed of aggregates known as peds – these soils are known as structured soils.
  • Peds are formed when clay and humus (organic matter) bound particles of sand and silt together. Granular, stable peds function as an indicator of good soil structure as they create adequate pore space for air and water to flow through the soil, while draining easily and holding enough moisture for plant growth.
  • Soils without peds are known as structureless soils. They come in two types: single grain and massive. The former is usually sand and possesses no adhesive to bind the particles together while the latter, usually clay, form a coherent solid mass.
  • Soil structure can be measured by calculating bulk density, particle density, and soil porosity. Other component factors of soil structure include soil permeability, soil strength and aggregate stability.
  • Like soil organisms, soil structure is benefited by maintaining plant life and adding organic matter to the soil, which helps support the development of stable, granular peds.

Conclusion

For healthy soil, plant life should be maintained to promote microorganism abundance. Plants do this by shielding microorganisms from the sun’s rays as well as providing a host. Plant life also acts to break up large aggregates, creating small, stable peds. Mulching and the application of organic fertilisers are both recommended as to provide nutrients for both microorganisms and plants. Mulching also protects microorganisms from the sun’s rays and should cover the ground where they is no plant life. Compaction should be avoided at all costs as it acts to reduce flows of air and water through the soil. Likewise, bare soil should be avoided as it leaves soil vulnerable to heat and extreme weather events that can dry out or sweep away layers of soil respectively.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

Gardening, Jorge, Planting, Plants, Trees

For many, fast growing trees are a no brainer as they allow you to quickly achieve your dream garden or block out annoying eyesores. Although, it is not so simple as selecting a fast growing variety that you like the look of. It is important you choose a variety that is disease and pest free and suitable for your soil type and hardiness zone. You also need to consider whether you want an evergreen tree or a tree that will shed its leaves in winter. Often unconsidered factors include weak wood, invasive roots, short life spans, a tree’s width and interference – the effects of competition and allelopathy.

As with all trees, you will need to clip in order to keep it the size/shape you want. Growth can be staggering and certain varieties will be impossible to manage after a certain point. Hence, it is important to carry out formative pruning while the tree is still young. Fast-growing trees will need to be clipped at more often than their slower growing cousins with extra clipping in the warmer seasons.

With planting the usual advice still stands but for optimal growth a tree will need to be accustomed to the hardiness zone and fit the drainage profile of your soil type. Native trees are always a good bet, as well as trees from colder climates. Sandy soils tend to drain quickly, while clay soils hold moisture well, so thirsty trees in sandy soils will need frequent watering in the hottest months.

Often ignored, but important is the effects of interference – the effects of nearby plants – on tree growth. Unlike how tree roots are traditionally represented, most absorbing roots are in the upper few feet of soil and root systems tend to spread horizontally, often extending well beyond a tree’s circumference. It is here that a tree’s roots will come into contact with other plants roots, where they both will compete for nutrients and moisture.

Especially detrimental to trees is grass that is known to retard root growth. Hence, it is important to remove grass and mulch 4 inches deep 1 foot beyond the root ball when planting. (Make sure the mulch does not directly contact the tree trunk to prevent disease/pest problems.) Expand the radius of mulching 1-2 feet per year for 3 years that will allow the tree to establish its root system.

Now, without further ado, here are some fast growing tree suitable to the UK’s climate.

Eucalyptus gunnii

The Eucalyptus gunnii or cider tree is a beautiful tree with with peeling cream and brown bark and elliptic grey-green foliage. Originating from Tasmania, the gunnii is suitable to grow throughout the UK, being able to withstand temperatures of -15 Celsius for significant periods. Growing at a whopping 1.5-2m per year, the tree tends to take a columnar shape, reaching a maximum height of 25m. The tree is highly versatile and will flourish in all soil types.

Did you know? The gunnii produces a sugar-rich sweet sap that the aborigines ostensibly fermented into an alcoholic drink – the first of its kind in Australia.

Weeping Willow (Salix babylonica)


As a genus Salix trees (mostly willows) are all fast growing, but the weeping willow species is the most famous, known for its grace and often found on the banks of lakes and rivers. The tree is distinctive for its low, sweeping branches that droop to form a canopy. Commonly used as a shade tree, the babylonica will grow 1.2-2.4m per year, reaching about 15m high. Suitable for most soils, the tree will flourish in waterlogged soil and will even absorb standing water. Modern hybrids include the highly popular Golden Weeping Willow (Salix x sepulcralis), known for its ability to create luxurious curtains of gold light.

Saule pleureur (weeping willow) by Claude Monet (1918).

Did you know? Despite its name, the babylonica actually originates from China. It received its scientific alias from the botanist Carolus Linnaeus, who incorrectly believed it was the tree described in the bible in the opening of Psalm 137. In fact, the trees growing along the rivers of Babylon were the Euphrates poplar.

Lombardy Poplar (Populus nigra ‘Italica’)

Like the smaller Italian cypress, the Lombardy poplar is known for its dramatic upright form and is often planted in rows to form a screen and can be sometimes viewed lining roads. The Italica grows extremely quickly with reported yearly growth rates of 3.6m, although one can expect 2.4/3m. Reaching a maximum height of 20m, the tree is deciduous and is identifiable by its catkins that come in two forms: crimson red (male) and cottony white (female). Although, most trees sold will be male clones. It must be noted the plant is short lived (15 years) and susceptible to disease.

Did you know? While the Italica first spread around Europe in the 18th century, it exploded in popularity in the 19th. This was to the concern of Scottish horticulturist John Claudius Loudon who deemed it “a most dangerous tree”, due to its capacity to destroy the harmony of the landscape when left in the hands of the amateur landscaper.

Silver Birch (Betula pendula)

Native to the UK, the silver birch is instantly recognisable by its white peeling bark, triangular shaped leaves and various catkins. (The tree is monoecious and thus possesses male and female catkins with the former distinguishable by their greater length and appearance in clusters.) The tree is extremely hardy and grows as far north as Lapland, reaching a maximum height of 30m, although 15m can be expected. Paling in comparison to the previous trees, but certainly no slouch, the pendula will grow at an average rate of 40cm per year.  

A Silver Birch’s female catkins.

Did you know? While birch bark has long been used in construction, it actually contains substances that make it an extremely useful material. The compound betulin, for example, possesses fungicidal properties can help preserve food, making it perfect for storage containers. Currently, scientists are researching the bark’s various substances that may be of use in medicine.

Golden False Acacia (Robinia pseudoacacia ‘Frisia’)

A favourite at Primrose, the Frisia produces fantastic colour throughout the year with its pinnate leaves and gorgeous white flowers. Bushy in appearance, its leaves emerge golden-yellow in Spring, before turning greenish-yellow in Summer and orange-yellow in Autumn. Wonderfully fragrant, the tree will grow between 30-45cm per year and is suitable for most soil types. Extremely hardy, the Frisia can expect to grow to a maximum height of 25m, although 15m can be expected.

Did you know? Due to the high concentration of flavonoid pigments in the heartwood, the False Acacia’s wood can endure for up to a hundred years, and is extremely resistant to rot. It is also extremely hard, making it perfect household furniture and flooring.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

Share!