How to improve your soil for gardening - Fork turn organic matter
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How to Improve Your Soil for Gardening

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How to improve your soil for gardening is the 3rd and last post in the soil post series. In my previous posts how to find out my soil type and pH testing of soil, we found out how to determine your ‘gardens soil structure’ and how to test and find out your soil’s pH.

Also, our buyers guide post looked at some of the best pH testers currently on the UK market; which can help you determine the pH of your soil quickly.

Hopefully, you have undertaken these ‘soil tasks’ and ‘pH tests’ and we can now move on and get down to and know ‘how to improve your garden soil’.  Let us explore the best options to correct your soils pH and or soil structure should it be required.

Most gardens have soil that provides something less than the ideal growing conditions for many garden plants. Perhaps its rocky or there is too much clay in your soil; maybe too much ‘sand’ to suit the plants you want to grow.

 

How best then to change your soil structure to improve your growing potential?

How to improve your soil for gardening - Gardner-mulching
Add Organic Matter to Improve Your Soil Structure

Changing a Soils Structure

Although it can be difficult to improve a ‘soils structure’ the best way to do so irrespective of whether your soil is predominantly ‘clay’ or ‘sandy’, is to add organic matter. Yes, it is as ‘simple’ as that.

What is Organic Matter

Organic matter or alternative names like soil improvers and soil conditioners are what is added to most soils to improve its structure and is derived from living things, but in chemistry, ‘organic’ means carbon-containing compounds.

Most organic matter is best rotted or perhaps a better term would be ‘composted’ before application, to ensure a good carbon/nitrogen ratio.

Many gardeners use homemade ‘garden’ compost made from both kitchen and garden waste as the ‘organic matter, which is an excellent soil improver, it’s both cost effective and environmentally good.

Other good alternative examples of organic matter beside garden compost to improve soil structure could be:-

  • Well-rotted animal manures from farms and or stables.
  • Composted cardboard
  • Fallen Leaves (Best composted first)
  • Lawn Clippings (Best composted first)
  • Shredded Paper Waste

There are others. Sawdust or woody garden waste and straw, deplete nitrogen from the soil as they rot, so it’s best that these types of organic waste are composted first, with nitrogen rich materials such as grass clippings.

Organic matter is best described by gardeners as a ‘cure-all’ in the garden. You cannot go horribly wrong with ‘organic matter’ however it’s added to your garden. Organic matter will generally improve any soil type when ‘worked’ into the ‘topsoil’ to a depth of around 30cm (1ft).

Organic matter can also act as a good ‘mulch’ when spread over the soil’s surface. A layer of approximately 2-3 in (50-75mm) deep around plants, shrubs and trees will help with water retention and aid in the fight against weeds.

How much organic matter should I add?

To improve your soil structure, it is recommended that you ‘work’ into your soil 5 -10kg per square metre (11-22 lbs per yard) of ‘organic matter/soil improver’, which is generally about half to one 15L (3gallon) bucket full. The slow release of nutrients is also a characteristic of manures, composts and other bulky organic matter and is a desirable horticultural feature that leads to healthier and higher quality plant growth.

Clay Soil Improver

By adding organic matter to clay soils, drainage will be improved, due to the fact the ‘organic matter’ forces the tightly packed clay particles apart, making it easier for plant roots to penetrate.

Improve Sandy Soil

By adding organic matter to sandy soils, organic matter enters the large spaces found in sand and acts as a sponge, slowing down drainage so the soil stays moist for longer.

What is the Best Time to Add Organic Matter?

It is best to work in any ‘soil improvers ‘before you plant’, especially if it is a new garden. In the UK generally for most soils Spring is the best time around March or April to work in the organic matter before the growing season.

 

However, with ‘sandy soils’ it is best worked with a soil improver in late winter. Avoid applying improvers in late summer as the warmer soil can lead to significant nutrient loss as the organic matter will breakdown quickly to produce ‘soluble nutrients’ which are easily washed out (leached) by our winter rains.

What About pH?

If you have followed my earlier posts, you should have by this stage some idea of the current pH (pondus Hydrogenii) levels of your soil, prior to the addition of any organic matter.

It should be noted that the introduction of ‘organic matter’ into your soil will likely increase the pH of your soil in the first few weeks of introduction.

This initial increase can be caused by ‘high nitrogen plant residue’ as soil microbes further break down the plant material to ammonium, which temporarily increases pH. Slowly the ammonium gets converted to ‘nitrate’ which causes pH to go down. If the ‘nitrate’ however is lost to leaching, then pH drops even more. Generally, in the long term, microbial decomposition decreases pH.  

The net effect of ‘organic matter’ addition on soil pH depends on the rate at which all these processes above occur and what happens with the ‘nitrogen’ produced (e.g., nitrate plant uptake vs leaching loss), the quality and quantity of plant material, and your initial soil pH.

Periodic pH Testing

Allow your soil to stabilise over a period after the introduction of ‘organic matter’ and undertake pH testing periodically over several months to see where your pH levels lie, determine if your soil is stabilising towards acidity or alkalinity.

 

Once you see some sort of stability in the pH of the soil, then you can perhaps adjust the pH of the soil further if needed, but try to avoid correcting the pH unless it is necessary. It is important to monitor how the pH changes over a longer period.

 

If the pH value drops below pH 5 or rises above pH 8 then it is advisable to gradually start adjusting levels.

 

Adjustments to pH should be based on the different cultivars of plants and their optimum growing conditions you are looking to plant. Certain plants prefer more alkaline soil, while others prefer a more acidic type soil.

 

Generally, though most things will grow happily in a pH of between 5.5 – 7.5.

Alkaline Soil (pH 7.5 – 14)

In many areas, you will come across high pH alkaline soils, as these areas are found throughout the UK.

If after testing your original soil, and prior to adding any ‘organic matter’ or other soil improvers, soil testing results show that your soil has a pH of 7.5 and above, then you know your soil is largely made up of ‘calcium carbonate’ (limestone) and is likely very alkaline.

Identifying Chalky Soil

It is well known amongst gardeners that it is often more difficult to reduce high soil pH (reduce alkalinity) in chalky/limestone soils than it is to ‘improve’ acidic soils with a lower pH.

Alkaline soils are primarily caused by ‘calcium carbonate’ rich ‘parent’ materials that contain chalk and limestone to excess. There are some other tell tale signs,  that may lead you to also conclude your soil maybe alkaline.

Alkaline chalky/limestone soils are often very stony, less water retentive and very dry in summer. Alkaline soils are also very often poor in nutrients like manganese and iron. Poor growth and yellowing leaves (chlorosis) are a result of plants not being able to absorb sufficient iron and manganese through their roots.

How to Improve Alkaline Soil - Acidify

To counteract the slight effects of alkaline soils (pH7.5) the first step would be to dig in plenty of ‘organic matter’, in the top 25cm (10in) this will assist greatly with ‘water retention’ and create ‘humus‘ which is just ‘organic matter’ in the soil and derived from the micro decomposition of plant and other animal materials.

In addition to adding organic matter, you should also apply good fertilisers rich in both iron and manganese. 

For more severe cases, where pH is higher than pH7.5 then large doses of ‘sulphur powder’ may be required to be added to your soil to a depth of around 30cm (12in).

Elemental sulphur is the ‘method’ most use when lowering high alkaline pH of  soil. The soils bacteria change’s the sulphur to sulfuric acid, and thus ‘acidifying’ or lowering the soil pH. 

Care should be taken though with the addition of high doses of sulpher and making the soil too acidic as this method can be very damaging to plants.  I suggest just adding small amounts of sulpher over time. Better to do this, than risk a bigger dose all in one go.

Be prepared to monitor this method of acidification over many many months, as sulpher added to soil does not take immediate effect.

 

How to Apply Sulpher

Sprinkle sulphur over the soil to be treated at the rate required (see table below).

Do this in still weather as the dust is very fine and drifts easily. Gloves, goggles and dust-mask are sensible precautions if treating larger areas.

Sulphur is best incorporated, by cultivation, into the soil in advance of planting so it has plenty of time to take effect. 

Applied to the surface it can take years for the acidity to be changed at the root depth. If deep-rooted trees and shrubs are to be planted it may be necessary to dig half the dose into the soil and cultivate the rest into the surface by hoeing, raking and cultivating. A rotovator is ideal, where available.

Swipe the table!

Sulpher Table:

Aproximate Sulpher to apply to soil - Grams per sq m (Ounces per sq yard)

Original pH of Soil

Clay Soils

Sandy Soils

Loam Soils

8.5 - 6.5

270g (9.6 oz)

180g (6.4 oz)

220g (8.0 oz)

8.0 - 6.5

180g (6.4 oz)

90g (3.2 oz)

140g (5.0 oz)

7.5- 6.5

90g (3.2 oz)

40g (1.44 oz)

80g (3.0 oz)

7.0 - 6.5

20g (1.0 oz)

20g (0.16 oz)

10g (0.5 oz)

Acidic Soil (pH 0 – 6.0)

If you find that your soil has a high acidic pH reading, then to take the pH levels up towards pH neutral 7.0 (dependant on your growing requirements) the ‘addition of garden lime’ to the soil is commonly used by gardeners. The active ingredient in the lime being ‘calcium carbonate’.

Widely offered in garden centres also ground limestone is easy to spread, using a garden spreader or evenly by hand, please ensure your wearing gloves and safety goggles at all times when spreading by hand. No matter how you spread the lime, make sure that its worked well into the top soil afterwards to a depth of about 20cm (8in). 

 

When is the Best Time to Lime

There is no optimal time to lime, however, it makes sense to add lime to your soil at the beginning of the winter for any annual crops you wish to plant for the following year and just before you ‘turnover’ the soil. This will allow for the ‘lime’ to take effect over the winter months and thus avoiding any damage to young growth come the spring. 

Due to the time ‘lime’ takes to take effect (it could be years) when simply applied to the surface of the soil, if you want to plant perennials like lawns, shrubs, fruit or trees in a ‘new garden area’ its best to adjust the pH before sowing/planting perennials or laying of turfs.

How Much Lime to Apply?

The quantity of lime to apply to your garden soil will depend on the area and structure of your soil. For example, soils with high ‘clay’ content would require more lime added due to what is called the ‘buffering capacity’ compared sandy soils.

In addition, quantities used will also depend on what ‘lime’ you use. Lime has a calcium oxide neutralising value (NV) so more lime will need to be added to the soil if the type of lime used has a smaller (NV) value than another type of lime with a larger (NV) value. 

There are 3 main types of lime.

Ground limestone is the recommended lime to use for gardeners. However, Dolomite Limestone can also be used on soils that lack ‘magnesium’ as it is rich in magnesium carbonate as well as calcium carbonate. Hydrated lime (calcium hydroxide), sold for use by builders, can also be used. It is a fine powder, quick-acting but caution should be used when handling it as it can cause skin and eye irritations if not handled safely (See above recommendations).

The quantity recommendations below are based on the use of ground garden lime (calcium carbonate) based on the Department for Environment, Food and Rural Affairs (DEFRA) recommendations. If you are using other limes as mention previously you should adjust quantities accordingly.

The lime should be worked into the top 20cm (8in) of topsoil. These recommendations are enough to raise the soil pH to pH6.5. Which is considered the best all-around pH for the majority of garden plants.

 

Swipe the table!

Ground Lime Table:

Ground Garden Lime to apply to soil - Kilogrames per sq m (Pounds per sq yard)

Original pH of Soil

Clay Soils

Sandy Soils

Loam Soils

4.0 - 6.5

2.2kg (4.0 lbs)

1.6kg (2.9 lbs)

1.9kg (3.4 lbs)

4.5 - 6.5

1.8kg (3.3 lbs)

1.3kg (2.3 lbs)

1.5kg (2.7 lbs)

5.0 - 6.5

1.4kg (2.5 lbs)

1kg (1.8 lbs)

1.2kg (2.2 lbs)

5.5 - 6.5

1kg (1.8 lbs)

0.7kg (1.2 lbs)

0.8kg (1.4 lbs)

6.0 - 6.5

0.6kg (1.1 lbs)

0.4kg (0.7 lbs)

0.5kg (1.0 lbs)

Conclusion

How to improve your soil for gardening, concludes our series of posts on soils. We have covered ‘soil structure’ and how to find out what your own soil type is, soil pH and how to test for it and finally how to improve your own soil based on the findings of the other 2 elements.

These 3 very important elements are critical in the preparation and growing process for any types of plants and I urge you to follow along carefully if you want to reach your growing potential.

You may also look at our FAQ section to seek answers concerning other aspects of your garden and gardening.

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