Agronomist hands holding soil for fertility inspection.

Getting To Know Your Soil With Testing.

Organic Matter And Material

f you have nice, dark, crumbly soil, you’ll have a lot of organic matter in it. And the organic matter is good—that’s why all gardeners know that a rich loam is a great fertile medium for growing all kinds of plants (well, almost all—cacti might not like it much). If you have thin, sandy soil, it might not have much organic matter at all.

Ah, but what is organic matter? It starts as organic material—that is, anything that was alive and has found its way into the soil, whether that’s a fallen leaf, your potato peelings, dead insects, tree roots, or manure that is produced by living creatures—and crucially, it’s organic material that has been decomposed into humus. This organic matter is rich in nutrients, and it retains a lot of moisture: it can hold up to 90% of its weight in moisture, almost like a sponge. In a forest, organic matter will naturally find its way into the soil. Fallen leaves, dead branches, twigs, bracken, and animal droppings will all eventually turn into organic matter. Some deserted gardens are surprisingly rich in organic matter, as all the nettles and weeds over the years have built up a rich sediment of humus.

However, in your well-looked-after garden, that’s not going to happen, so you will need to think about adding organic materials yourself. Straw, for instance, can be used as mulch and will eventually break down into the soil. You may be lucky enough to be near stables or farms where straw is used as bedding; it will then come together with a load of animal excreta which adds even more nutrients. But don’t forget that manure needs aging first, or else it can burn the plants. If you’re laying it down before planting, and you can wait a few months before you plant or sow, that’s not a problem of course.

Organic matter doesn’t have to be ‘natural’; it can also be man-made. For instance, these types of plant-based organic matter can all be added to your soil:

  • Coffee grounds
  • Cardboard
  • Sawdust
  • Wood chips
  • Paper
  • Tea bags

However, sawdust and wood chips will deplete the nitrogen in the soil, so either compost them with other organic materials first or use them as a top dressing or mulch. Kitchen waste can be composted in a traditional compost heap or in a wormery, as can leaf mold and grass clippings.

If you are composting organic matter, make sure it is covered up from the rain, otherwise, the nutrients will be leached out of it. That not only means you’ll be putting a nutrient-depleted mix onto your garden, but it also means this rich nutrient gravy is going to end up in the drains or, worse, in a stream or river, where it isn’t going to do any good. Liquid runoff can actually be quite harmful, encouraging organisms that shouldn’t grow in healthy water.

pH Of Soil

Now let’s think about whether your soil is acid or alkaline. The pH is a way of measuring this; it goes from one (very acidic) to fourteen (very alkaline), with seven as the neutral point. You’re unlikely to find that your soil is at either of the extremes; it’s more likely to be somewhere between five and nine. Higher rainfall areas tend to have more acidic soil.

pH Scale
pH Scale

The reason for testing your soil is that different plants prefer different pH levels. There’s no such thing as a ‘perfect’ pH, though most plants are able to cope with a pH of 6-8. However, some plants are choosy. Rhododendrons and blueberries like acidic soil and many vegetables prefer to be slightly on the acid side of neutral. Asparagus prefers alkaline soil, as does lavender. So you’ll want to test your soil before you start planning your plantings or raised beds.

You may also want to test the pH if your plants are not growing properly or the leaves go yellow: too high a pH can stop some plants from being able to access the nutrients in the soil, so their growth is inhibited.

You can now take action. Although your ground has a ‘natural’ pH, you can change it. In fact, natural processes come into play here as well as additions. When organic matter breaks down (whether in your yard or in the forest), the ground where this decomposition is taking place naturally tends to acidify.

Rainfall is naturally acidic, which is why higher-rainfall areas have lower pH soil (of course, you might also suffer from true ‘acid rain’ if you’re an urban gardener in a highly industrialized area). In a hard water area, where the water contains a lot of lime, just watering the garden will tend to make the ground more alkaline.

But just adding organic matter can change the pH. The exact nature of the change depends on both the nature of your soil and the kind of organic matter you are adding. Perfectly decomposed organic matter is almost always close to neutral, so if you are adding compost that has been really well composted, it will correct both too much acidity and too much alkalinity. However, if you add organic material which hasn’t been broken down, the process of decomposition will tend to acidify the soil.

Acid soilNeutralAlkaline soil
3.5-6.5 pH6.5-7.5 pH7.6-10 pH

Some mulches, such as pine needles or oak leaves, can also have a somewhat acidic effect, as can chemical fertilizers which are based on nitrogen or sulfur. You need to think about the pH preferences of your plants if you’re using these, though the acidic effect is somewhat reduced once the mulch has decomposed properly.

Testing Your Soil’s pH

So how do you test your soil?

First of all, look at what’s growing already. If your patch is full of rhododendrons, blueberries, and cranberries, or your area of the country is full of oaks and pines, those are signs of acidic soil.

Secondly, you might be able to see symptoms of problems if your soil has too low or too high a pH. A pH out of the central band will stop plants from being able to access some nutrients, so they will show signs of missing nutrients; if you recognize some of the symptoms in the table coming up shortly that’s why. The availability of nutrients is at its highest around the 6-7 pH range. This is also where earthworms and micro-organisms are most actively helping to keep your soil aerated and supplied with nutrients.

There is no single test that will, on its own, tell you what’s happening. For instance, both excessively acidic soil and soil that’s too alkaline will lead to blossom end rot, since both inhibit the absorption of calcium. However, the combination of a number of different symptoms can help you identify acid or alkali problems. If you have blossom end rot and you also have a lot of moss in the lawn and stunted growth, then you can diagnose acidity; if on the other hand, you see spotty and reddish leaves, alkaline soil is your problem. But it might be the blossom end rot that’s the single most pronounced symptom, which tips you off that something is out of balance.

Signs of soil being too acidic or alkaline

Too much acidToo much alkali
Yellow spots, yellowing leaves Yellowing of the lawn Wilting Blight, leaf necrosis or tip burn Weeds and moss in lawns Blossom end rot Stunted growth of plants and grass Poor soil drainageYellowing leaves and lawn Dead patches on lawn Poor stem development Brown spots on leaves Spots of leaf necrosis Tip death of new leaves Blossom end rot Red, purple, brown-tinged leaves Soil becomes hard and compact over time
Nutrient deficiencies: magnesium, calcium, potassiumNutrient deficiencies: iron, copper, calcium, phosphorus

Don’t forget that if the soil is out of balance, that will affect the various microorganisms in the soil, as well as your plants. For instance, bacteria can’t decompose organic materials in soil that has a pH lower than 4.7. That’s likely to lead to poor soil structure. As so often with soil, a number of different issues are likely to be connected, which means you need to get a holistic picture of what your soil needs. The good news is that the addition of organic matter will help to neutralize soil that’s too acidic and to remedy the poor structure of your soil.

Jam jar test

Back to testing. If you still don’t know, you need to take action. You could go to a laboratory to get your soil very comprehensively analyzed. You can get a simple soil pH test kit for $10 and upwards. Or you can do it for free by using basic kitchen ingredients. Grab a couple of jars and crumble a couple of tablespoons of soil into each. Add vinegar to the first. Moisten the other, then add baking soda. Which one fizzes? If vinegar makes it fizz, your soil is alkaline. If soda makes it fizz, your soil is acidic. And if there’s no fizz at all, congratulations!

Your soil pH is somewhere around the ideal range of neutral.

Cabbage test

You can also use the cabbage test to get a little (just a little) more precision. Grab a red cabbage. Red cabbage contains a pigment called flavin, and this is what makes the test work. You need two cups of chopped cabbage; bring a cup of distilled water to a boil in a pan, add the cabbage, and boil with the lid on the pan till the water is dark purple. Strain the water and let it cool.

You need to have dried out the soil to do this test, which is easy to do simply by spreading some on a tray or saucer. Get a tablespoon of dry soil on a saucer and moisten it thoroughly with the purple water. Let it sit for a minute and then simply tip the saucer so you can see what color the water is.

Are you saying “It should be purple”? It might be. But it might not. You see, the interesting thing about flavin is that it’s a pigment that reacts to alkalinity by turning blue or green. On the other hand, if the soil is acidic, it will react by becoming red. If it’s still purple, it’s right in the middle.

Remember when you’re doing any of these tests that you don’t want to take soil from the top. You need to dig down about 8”, then take soil from the bottom of that hole. Some gardeners prefer to dig a number of holes around their patch, then mix the earth together so they get an average for the entire plot. On the other hand, if your garden seems to have two different types of soil, keep them apart and test them separately. When growing up, I had a house where one side used to be a farmyard and the other was always a vegetable garden—guess where the soil was best!

Changing The Ph Of Your Soil

Having tested your soil, you might find that you need to improve it. To improve acidic soil, you need to add lime to make it more alkaline. You can get that at a garden center; don’t confuse it with builders’ lime, which is used in mortar and lime plaster. Builders’ lime costs more and it’s dangerous caustic stuff. Garden center lime is just crushed-up limestone (sometimes called agricultural lime) and is completely harmless (though I wouldn’t advise trying to eat it, all the same).

You don’t need to add a lot of lime: 5lb is enough to treat a hundred-square-foot bed. You might need a bit more if you have very heavy soil. However, what you do need to have is plenty of time, as this won’t work overnight; it can take six months or even a year for the lime to dissolve completely into the soil. That’s why you might want to add it in the fall so that it’s already had several months to break down before you start planting in spring.

You can also add wood ashes. Again, don’t add too much. And remember that wood ashes contain potassium, so if you add ashes to make the soil less acidic, you don’t need to add more to increase potassium levels.

For alkaline soil, you’ll want to add acidifying materials. For instance, you could use pine needle mulch (it’s actually best to use fresh pine needles if you can—ask if a neighbor or friend is trimming a few branches—as they are much more acidic when green). You can also add powdered sulfur—as with lime, you don’t need to use much (a tablespoonful every square foot should be enough), and it will take a while to notice the benefits.

As I mentioned, whether your soil is acid or alkaline, adding heaps of well-rotted compost and other organic matter will really help to improve it and balance out the pH.

Once you’ve amended the soil, wait for two or three months and test again to see if it’s working. Keep testing, as things will likely continue to change over the months. Remember, the soil is a living organism, so besides the amendments you’ve made, plenty of other stuff is going on too. It’s also important to keep checking because adding soil amendments isn’t an exact science. It’s probably best to start with a relatively small amount of your chosen amendment and add more if, after a while, you’re not seeing any change.

You don’t always need to amend, as long as you’re prepared to accept limitations on which plants you can use. For instance, asparagus loves ‘sweet’ (alkaline) soil, while daffodils prefer soil on the acidic side (as do nasturtiums). On the other hand, brassicas will not tolerate acidic soil, and most prefer soil that’s on the sweet side. Either accept the limitations or if you want to grow a full variety of plants, you’ll need to get your soil as close to neutral as possible.

Plants that like acid soilPlants that like alkaline soil
Rhododendron, Holly, Ferns Begonia Hydrangea Magnolia Heathers Mountain ash Trillium Liriope muscari Ceanothus Bilberries, blueberries, cranberries, cowberries, blackberries, raspberries, elderberries, gooseberries Camellias Carrots Potatoes Sweet potatoes Japanese anemones Trees – beech, alder, cedar, dogwood, willow Azalea Bilberries Cucumber Sweetcorn Beans Pumpkins Turnips Tomatoes Squash Onions Radish Rhubarb Parsley Asparagus Leek Marigold Eggplant Lettuce Basil Chives Lemongrass Oregano Celery.Catmint Kniphofia (‘red hot poker’) Sage Lavender Borage Pole beans Spinach Garlic Black-eyed Susans Campanula Honeysuckle Trees – hawthorn, field maple, holm oak, blackthorn, yew, strawberry Peas Kale Cabbage Thyme Basil Rosemary Daylilies Delphinium Rosemary Nasturtiums Cauliflower Catmint Irises Daylilies Clematis Kiwi fruit Beets Sweet alyssum Goldenrod Broccoli
pH Of Different Types Of Soil

How does the pH of soil relate to the type of soil? This is quite interesting, as apart from silty soils, most soils sit somewhere specific on the pH spectrum. Of course, your soil isn’t likely to be pure clay or pure sand; it’s going to be a mix, so probably you’ll not get a reading as extreme as 3.7 (highly acidic) or 10 (very alkaline).

Soil typepH range
Peat3-4.5 (very acidic)
SandAround 7
Testing The Composition Of Your Soil

How do you identify what composition your soil has?

Testing the composition of soil

’Get your hands dirty’ test

The first test is simple: pick it up in your hands and feel it. If it’s gritty and won’t stick together, you have sandy soil. If it smears and becomes sticky when you wet it, and if you can roll it into a sausage, you’ve got heavy clay; lighter clay won’t hold its shape quite so well. You’re unlikely to have pure silt, but if you do, you’ll find it has a slightly soapy texture, and won’t stick together easily.

Soil squeeze test

Squeeze the soil. It will hold its shape, and when you give it a light poke, it crumbles. Lucky you—this means you have a luxurious loam!

It will hold its shape, and, when poked, sits stubbornly in your hand. This means you have clay soil.

It will fall apart as soon as you open your hand. This means you have sandy soil.

Soil test kit

If you want to go further into the examination of your soil, then get a full soil test kit. That will let you assess the NPK nutrients as well as the pH level.

Jam jar test

There’s the jam jar test I already mentioned, which involves putting soil in a jar with water and shaking it up, then leaving it to settle. You’ll get three distinct bands—sand at the bottom, then silt, then clay at the top—so you can see which of these make up the biggest proportion of your soil.

Puddle test

Drainage is important too. A good quick test for drainage is the puddle test. Dig a hole about 6” wide and 1’ deep. Fill it with water and let the water soak in completely. Now refill it, and time how long it takes to drain completely. If it takes more than a few hours, you have a drainage problem. Adding compost and mulch will help to address that, or you might add sand or grit. If the soil is very clayey, just adding a bit of sand can also help with drainage.

Worm test

The worm test is a good check on the general health of your soil. It’s very simple to carry out. Grab a tarp or a big piece of cardboard (an old cardboard box squashed flat will do), and dig a hole a foot square and a foot deep. Put all the dug-out soil on the tarp or cardboard. Find all the worms! The easiest way to do this is simply to put the earth back in the hole in handfuls, counting the worms as you go.

You should find at least ten earthworms. If not, then your soil isn’t as healthy as it should be. There may not be enough organic matter in the soil, or it may be too acidic or too alkaline—the other tests will help you work out the exact problem.

Wire test

A wire test helps you assess whether your soil is too compacted. Buy a wire flag —the kind that surveyors use—they’re easy to find online. Push the flag into the soil as far as you can. You’re looking to push it down a foot, at least. If you can’t push it that far without it bending, you have a problem with compaction. You’ll need to add organic material, and you’ll also want to aerate the soil (just use a gardening fork to loosen it a little; you don’t need to dig).

It’s worth writing the results of your tests down and even keeping a small soil sample. You can look back at those in the future to see just what effect your various actions have had on the soil.

Before I finish this chapter, now you know what kind of soil you have, I ought to tell you what you’re aiming for. The ideal soil for most gardens is a loam that has a good mix of sand, silt, clay, and organic matter, and that is neither so dense that it’s practically solid nor so crumbly that it won’t hold water. As mentioned earlier, 40% sand, 40% silt, and 20% clay, with a neutral pH, is what you should aim for.

This soil mix is ideal because, for optimal plant growth, water, nutrients, and oxygen must all be available to the roots at the same time. A neutral pH allows plants to access the full range of nutrients they need and the texture of the soil allows water and oxygen to pass freely. That also helps the bacteria and other micro-organisms that grow in the soil and help to keep it healthy.

Agronomist hands holding soil for fertility inspection.

If you have a small garden, you’ll probably be able to change your soil quite comprehensively over the whole garden. I once lived near a keen gardener who had an incredibly fertile yard, while the rest of us in that street found the heavy clay soil very difficult to cope with. He told me why: his mother used to keep chickens and the manure, over the years, had been dug into the soil, introducing organic material and opening up the texture.

If you have a larger garden, to some extent you may have to live with what you’re given. But using raised beds is an excellent way to change the soil just where you need it. For instance, if you have clayey soil but want to grow asparagus, make an asparagus bed where you can introduce a lot of sand and lime to change both the texture and the pH of the soil. Or if you want to grow camellias, which love acid, lime-free soil, but you live in a chalky soil area, you can scoop out a bed and fill it with erinaceous compost to give your showy flowers just what they need.

I’ve included a list of plants that do well with specific soil types, so if you can’t change your soil, you can live with it!

Type of soilPlants for the soil
ChalkMaple Horse chestnut Ginkgo Birch tree Buddhleia Ceanothus Forsythia Pyracantha Hawthorn tree Beech tree Geranium Apple and crab apple trees Common oak tree Kiwi Honeysuckle Spinach Beets Corn Cabbage
ClayMaple Apple and crab apple trees Citrus trees Figs Plum Pear Hazel Ash trees Pine tree Juniper tree Ash trees Elder tree Rowan Yew Fir Berberis Viburnum Forsythia Pyracantha Hedera Potentilla Apricots Lettuce Cauliflower Broccoli Squash Pumpkin Cabbage Kale Peas Corn
Peaty soils (lime-free)Camellia Magnolia Rhododendron Azalea Hamamelis (witch hazel) Erica Potatoes Beets Celery Onions Carrots Lettuce Tomatoes Strawberries Blueberries
Sandy soilsEucalyptus tree Birch tree Holm oak tree Black locust tree Berberis potentilla Cotoneaster Salvia Helichrysum Carrots Parsnips Beets Radish Aromatic herbs Onion Garlic Potatoes Asparagus Turnip Strawberries
Wet or swampy sitesWillow tree Persimmon tree Poplar Black or water birch Aronia berry Rowan Medlar Tulip tree Cypress Hydrangea Rhododendron Lobelia Primula Hosta Gunnera Trollius (globeflower) Ligularia (leopard plants) Asparagus Pear Apple tree High bush and lowbush cranberry Mint Garden pea Celery Watercress Cabbage American groundnut Taro Argula

Earthworms—your tiny helpers

Earthworms are incredibly important in ensuring the health of your soil. They are the engineers who create your underground infrastructure. They improve soil structure by tunneling through it, loosening it, mixing it up, and oxygenating it. They also run your recycling network, eating dead plants and fallen leaves and breaking them down into rich organic matter (they each eat two tons a year!). The little worm casts they leave above ground are full of nutrients, especially nitrogen and phosphorous, and contain 1,000 times more bacteria (good ones, naturally) than the soil.

They even help prevent water runoff, making the soil vastly more effective at soaking up water when it rains. That helps stop erosion and flooding. In fact, where the soil has been contaminated, they can even help to clean it up: they help distribute micro-organisms that specialize in bioremediation.

So you should set out to encourage worms. You can start by reducing the amount of tilling you do to your soil. No-dig gardening techniques are best for this. Keep the soil moist and cool by using mulch to protect the bare earth. Leave plenty of organic matter on the surface for them to get hold of; you can even poke your orange peels underneath the mulch on your flower bed, for instance. Protein such as cornstalks or wheat straw is also good for them. Add plenty of manure and compost to the soil, to make the texture good for them to tunnel through. And use as little artificial fertilizer and pesticide as possible.