Composting, George, How To

At its heart, gardening is rooted in sustainability, from growing your own food to enhancing the local ecosystem. Most gardeners are used to finding thrifty solutions to dilemmas, patching things up with what’s lying around. But there’s always room to find new ways to make your garden greener. With that in mind, here are our tips for recycling in the garden.

recycling in the garden corks

1 – Corks

Break up old corks and use them to help drainage in plant pots. What better way to justify your drinking habit?

2 – Plastic bottles

Cut the ends off plastic drinks bottles and use them as cloches to protect your tender plants. Bottles come in plenty of sizes to fit all your flora.

3 – CDs

A classic grandmother’s trick! Hang up old CDs around your vegetable patch, so the reflecting sunlight will scare off birds.

fish tank

4 – Fish tank water

If you have an aquarium, save the water when you’re cleaning it out. It’s full of nutrients, so perfect for watering your plants.

5 – Compost bags

When you’ve emptied out a fresh load of compost, don’t throw away the bag! Reuse it as a sturdy container for transporting debris around the garden.

egg boxes

6 – Egg boxes

As well as being compostable, egg boxes are the perfect containers for chitting potatoes. Simply pop your potatoes in with the eyes upright.

7 – Lollipop sticks

Forget what you’ve planted where? Take a Sharpie to your used lollipop sticks and give them new life as plant markers.

8 – Windows

If you’re about to throw out unwanted window panes, consider repurposing them as lids for homemade cold frames.

recycling cardboard

9 – Cardboard

Delivery boxes, kids’ art projects… any bit of old cardboard will do for recycling. They make great insulation for plants or even compost.

10 – Tyres

The classic upcycling project – turn worn out car tyres into planters by stacking them up and filling with soil. Paint them for a colourful touch.

11 – Toilet roll tubes

These little cardboard tubes are perfect for seeding vegetables like carrots and peas. Fill and when they’re ready, transplant the tube into the ground, where it will gradually decompose.

seedlings in tubes

Hopefully these ideas will help you see what you can reuse, reduce and recycle in your garden. If you have any tips for the green-fingered community, let us know!

George at PrimroseGeorge works in the Primrose marketing team. As a lover of all things filmic, he also gets involved with our TV ads and web videos.

George’s idea of the perfect time in the garden is a long afternoon sitting in the shade with a good book. A cool breeze, peace and quiet… But of course, he’s usually disturbed by his energetic wire fox terrier, Poppy!

He writes about his misadventures in repotting plants and new discoveries about cat repellers.

See all of George’s posts.

Composting, Garden Design, Garden Edging, Gardening, How To, Insects, Liam, Make over, Planting, Trees

In this step-by-step guide we’ll not only show you how to mulch but explain the different kinds and what will work best for your plants and garden. Mulches are a thin layer of organic or inorganic material placed over a bed or the soil surrounding plants. The more attractive ones may grab your attention and look like a great addition to formal landscaping, but the practical uses are vast. 

Mulches are used primarily to improve the soil around plants, reduce weeds, increase fertility, help the retention of moisture and during winter can protect the roots of the plant from damaging frosts. Using the right mulch for your plants can help eliminate the need for chemical pesticides and fertilisers which is fantastic for your garden’s biodiversity. This all contributes to a healthy, great looking garden you can be proud of.

Now that Autumn is approaching it is the perfect time to start planning!

The Types:

You can roughly separate the different types of mulch into two categories; organic and inorganic.

Organic mulches are best for improving the fertility and overall structure of the soil. Over time the mulch will degrade and replenish the soils nutrients including nitrogen, potassium and phosphorus. Organic material also promotes biodiversity and encourages insects such as worms and spiders which will actually keep pests at bay while further enriching the quality of your soil.

For this reason an organic mulch is fantastic for plants try to establish themselves or are just generally hungry. Roses for example love a good organic mulch of well-rotted manure. More on this to follow.

Inorganic mulches are used to protect the soil around the plant and can also have an aesthetic edge to them. The benefits include locking in water to the soil, keeping weeds at bay and unlike some organic mulches it won’t wash away which is brilliant if you are planting on a slope.

Bark and Wood-Chippings

Here is a mulch which is attractive but also helps improve your soil’s nutrients and structure as it rots down. It also allows water to flow through it without binding throughout the year and really is a fantastic for pretty much all plants and circumstances. The only issue with it is that it is difficult to move or work around and so is best for around trees where you won’t be doing any more planting. Bark and wood-chippings will last you through the year and maybe even two depending on the grade, see how far it has broken down and replace if necessary.

Wood Chippings Mulch

Leaf-Mould

Leaf-mould is arguably the most nutritious and nature-friendly mulch you can apply. Pretty much every plant loves it and what’s more it can be completely free! It may not look like the most attractive mulch but apply in Autumn and by spring it will have blended in with and really enriched your soil. The only major drawback is that leaves do take some while to decompose and if you plan to DIY this is something you plan for a year in advance.

Collect as many leaves as possible in black bags and cut some small holes to let the air in. Ensure the leaves are thoroughly wet as leaves break down through fungi. Come next Autumn you’ll have some of the finest and richest mulch money can buy… not that you have to spend a penny! Of course, leaf mould is available to purchase in fairly substantial bulks.

Compost

There are two main reasons why compost can make a great mulch: 1) It is packed full of nutrients ready to leach down into the soil and 2) It is something you can make yourself free of charge. Additionally it helps with keeping the soil moist and fending off weeds. One thing to look out for however is that no weeds have made their way into the compost as these will simply sprout up from the compost and steal your plants nutrients.

Manure

As I’ve briefly mentioned before, when it comes to roses and other phosphate hungry plants nothing compares to some well-rotted manure. Like a compost that has gone through a far more strenuous decomposition process it is packed full of nutrients and its dense texture protects the roots and keeps the water locked in. It is also a really great mulch for trees and shrubs although to prevent waterlogging it may be worth mixing with some sand to allow for greater drainage.

Manure – As is Comes From a Stable or Farm

Gravel, Slate and Stone Chippings

There really isn’t a great difference here between them as you will want roughly the same thickness of layers. Stone mulches are fantastic for drainage and keep the soil underneath moist. It is also brilliant for retaining heat and so should be used for plants that are used to very hot conditions and can be worked into a Mediterranean themed garden well. Overall many stone mulches look fantastic and can maintain a pristine look for formal garden structures. They do not however add any nutrients to the soil and can become too hot during summer for more tender plants and young trees.

Rainbow Foras Tumbles Coloured Pebbles

When to Mulch

The best time to apply a mulch is in Autumn, as you come into bare-root season, and spring. You will need to apply the mulch when the ground is relatively warm and moist, avoid periods when it is frozen or waterlogged. When the ground is good to dig and plant, it will be good to mulch which is very handy!

How to Mulch

  • Before you apply your mulch first you have to prepare the soil. Clear the ground of any weeds and give it a watering if the soil appears too dry.
  • If you are reapplying a mulch now is a good time to break up any old layers which may have matted to allow better water penetration.
  • Then cover the ground in a layer of mulch roughly 2 inches thick. Avoid mulching right up to the stems of plants and trees as this can cause them to become soft and rot.
  • Level out with a rake to an even finish. This is imperative, some people mulch little mounds, especially around trees. This will cause the bottom of the trunk to grow soft and rot while also drawing water away from the roots.
  • If you noticed that your mulch has matted over the year and become a hard layer, simply break  and fluff up a bit.

You can apply a fertiliser on top of the mulch through the year if you wish. Follow these rules and you should be all set!

Liam at PrimroseLiam works in the buying team at Primrose. He is passionate about studying other cultures, especially their history. A lover of sports his favourite pass-time is football, either playing or watching it! In the garden Liam is particularly interested in growing your own food.

See all of Liam’s posts.

Composting, Flowers, Garden Design, Gardening, Grow Your Own, Liam, Pest Advice, Planting, Wildlife

For hundreds of years farmers have used companion planting as a method to help improve their yields and get the most out of their fruit trees. This organic solution does far more than simply prevent pests from eating your fruit. Certain plant combinations serve a whole host of benefits including increased pollination, weed prevention and improved soil nutrition. Additionally it is a great way to cover the space under a fruit tree offering more colour and variety to your garden!

The Basics

As fellow gardeners I’m sure you recognise it is important to try and keep a natural balance, even in your garden. A key premise to companion planting is trying to avoid monocultures by planting a variety of different plants together. Among other things, you make it difficult for pests to find their desired food and spread amongst your crop.

For the Love of Fruit

Many people believe that it is difficult to grow anything under a tree. However, there are a great variety of plants which naturally thrive in this space. With that being said it is important to remember that if your fruit tree is trying to establish itself it is important to water it regularly, especially if you plan on planting more plants around it.

Fruit trees constantly come under attack from various pests because of their delicious fruit. They also require extra levels of potassium to help stimulate bud and fruit growth. If you want to avoid using chemical fertilisers or pesticides here is an essential list of companion plants for your fruit tree:

  • Chives – The scent of chives provides a strong deterrent to pests including deer and rabbits as well as insects and yet is attractive to the more beneficial pollinators. Additionally chives have been known to prevent apple scrab which is a notorious scrounge of apple fruit. A cautionary note is that chives are aggressive growers and so they will require maintenance to stop them invading the entire bed.
  • Nasturtium – A real favourite in the world of companion planting. This is a great plant to lure away aphids and particular caterpillars from your trees. It is a sacrificial crop. Nasturtium requires minimal nutrients, sun or water and so is brilliant for diverting pests while keeping your fruit tree strong. It has also been known to repel codling moth, a particular scrounge of apples.
Companion Planting - Nasturtium
Nasturtium in bloom
  • Fennel – This plant is fantastic for attracting pollinating and predatory insects. Hoverflies, ladybirds and parasitic wasps all love fennel and they love aphids and caterpillars even more. Plant this in your garden to help wage a natural war against these pests. Fennel can of course also be used for cooking and has been known to carry medicinal properties.
  • Dill – Very similar advantages as fennel; it attracts a host of predatory and pollinating insects… and it can also be used in cooking. Win win!
Companion Planting - Dill
A Hoverfly resting on a Dill plant.
  • Comfrey – Not only has this plant been used medicinally by people for nearly 2,500 years it is an amazing miner of soil providing nutrients for your tree! Being a deep-rooted plant it draws nutrients from the soil and then can be cut back and the clippings used as an organic mulch. Comfrey is drought, frost and pest resistant and grows well in partial shade so is perfect for the space under your tree. I would recommend trying to plant the ‘Bocking 14’ variety developed by organic pioneer Lawrence Hills. ‘Bocking 14’ being sterile won’t self-pollinate and spread all over your garden.
  • Chamomile – This beautiful flower deters pests with its strong scent while drawing in pollinators. Being drought and frost resistant and also not afraid of a little shade makes it perfect to plant around a tree. If suffering from a pest infestation a triple strength chamomile tea can be brewed and used as a spray for the affected area.

    Companion Planting - Chamomile
    Chamomile
  • Daffodils – Flowering early in the season daffodils are perfect for bringing in and supporting those pollinating insects. For a splash of spring colour plant in a circle around your tree at around 1ft from the base.
  • Lavender – Truly a favourite amongst all pollinating insects, including and especially bees; it’s strong scent also confuses pests. Lavender not only looks great in your garden but can be used for various DIY product such as soaps or teas. Or you can simply pick it and put it into a bowl for around the home to create a calming aroma.
Companion Planting - Lavender Flowers
Some bees thoroughly enjoying the pollen rich Lavender flowers

Understandably when it comes to food, especially food you’ve devoted labour and love to, you are cautious about spraying it with potentially harmful pesticides or even using fertilisers. Companion planting therefore offers an age-old organic method to ensuring healthy fruit trees while adding a touch of vibrancy and colour to your garden. You may also end up with some extra herbs to liven up your dishes!

Jorge at PrimroseLiam works in the buying team at Primrose. He is passionate about studying other cultures, especially their history. A lover of sports his favourite pass-time is football, either playing or watching it! In the garden Liam is particularly interested in growing your own food.

See all of Liam’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.

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