Soil is one of the most fundamental aspects of any garden, and along with choosing appropriate plants for each sites sunlight and climate conditions, soil quality is THE key to the success of any garden plant and lawn. Depending on how deep you desire to delve into soils, the topic can be either really complicated or really simple, so for the sake of these blogs we will attempt to make soil simple enough for beginners to understand, while still being informative to allow readers to become competent keepers of soil themselves. Remember - soil is the key to plant health, so don't skimp out on the soil research, preparation, or maintenance.
Soil is made up of 'aggregates,' which are made up of primary particles of minerals such as sand, silt, and clay. Actually, these mineral particles only make up about 45% of soil, as about 50% of soil is empty pore space, filled with water and air in proportions constantly and dynamically changing, and ideally about 5% is made up of organic matter, but for convenience sake, remember that soil is made up of any combination of these various particles.
Each soil will have a different ratio of some or all of these particles, which join together to form aggregates. Clay is the smallest of these particles, silt is somewhere in the middle, and sand particles are the largest. The types of particles that makes up your soil are important for two main reasons: Drainage/Pore space, and 'Cation Exchange Capacity' (CEC), which has to do with the ability of the soil to hold onto water and nutrients at a molecular level.
In simpler terms, clay dominated soils hold onto nutrients better, but can become compacted and drain poorly, while sand dominated soils do not hold onto nutrients very well, but drain much more freely, which in some cases can lead to soil drying out too quickly or leaching essential nutrients.
As you can probably guess, different soil types have their strengths and their weaknesses. For most (but not all) garden plants you would ideally have a mixture of clay, silt, and sand in proportions of 20:40:40 respectively, in what is known as loam soil. Keep in mind that some plants prefer heavier soils and don't mind the poorer drainage, whilst others (like lawns) require the great drainage of sandier soils, and aren't too fussed about the comparative lack of fertility. Nevertheless, in many cases, we have to work with less than optimal soil conditions. Never fear, we can over time amend both heavy and light soils with the addition of organic matter as, as well as fixing soil instability, compaction, and pH imbalances with various additives. More on this later.
If you are working with a new site with either heavy clay or a sand dominated soil and don't wish to completely dig out all the existing soil and ship in copious amounts of premium black gold, it is still a great idea to amend soil structure, but should your budget be so tight that importing compost to dig into the soil is out of the question, it is important to choose plants that are suited to your soil type. Different plants will have different moisture requirements, but in the case of most garden plants we come across the term "well drained soil" frequently.
The terms 'well drained' and 'free draining' soil actually refer to the pore size in the soil structure. Typically, clay particles are so tiny that they can end up filling the smallest of gaps, and only 'micro-pores' are left. Conversely, sand particles are so large that they don't fit together as snugly and leave more numerous, larger 'macro-pores'. It isn't as simple in reality as most soils are made up of varying proportions of sand and clay (and silt), and in healthy, living, and well managed soils, organic matter, fungi, plant roots, and bacterial waste will glue particles together into larger aggregates, and these aggregates will have many macro-pores between them, but nevertheless these characteristics of sand and clay are helpful to remember.
The point is, pore size and their general frequency throughout the soil profile will determine the drainage of a soil. To better understand this, we need to understand how water moves through soil. When soil is irrigated either naturally or by man, the pore spaces in the soil that normally hold air are filled with water. Tighter spaces between smaller pores means the water takes much longer to infiltrate further down the soil profile and in extreme cases can be bogged down and take so long to drain that it causes anaerobic (without oxygen) bacteria to take over, and can damage many plant roots that require aerobic (with oxygen) environments and bacterial relationships. This phenomenon can also occur when free draining top soil is suddenly stopped by almost impenetrable hard-pans further down the soil profile - more on this later.
When the soil pores are all full of water so that there is no room for air, this is what is known as the soils 'Saturation Point.' Once the excess water has drained away by the workings of gravity, the remaining water that the soil is able to hold onto is known as the 'Field Capacity.' Field capacity is largely determined by the CEC of a soil because with limited CEC, limited water and nutrients will actually bind to the soil aggregates and most will just drain/leech away due to gravity, whereas with a higher CEC more water and nutrients will bind to the surface of the aggregates even as the excess water is drained by gravity, and the empty space once again fills with oxygen.
But some plant care labels call for "moist, but well drained soil." We know that clay has a high CEC and is therefore capable of remaining quite moist for a long time after irrigation, but has poorer drainage, whereas sand is very well draining but has a low CEC and does not retain much moisture content. How then can we have both well draining soil and moist soil, without using copious amounts of water on an incredibly frequent basis on sandy soils? Well a good loam provides a mixture of drainage and water/nutrient holding capabilities, but even loam soils can be a compromise between drainage and CEC, and having a great ratio of clay, sand, and silt isn't the only means of achieving a moist, fertile, and well drained soil. Enter organic matter.
Organic matter is a broad term, but in soils it typically refers to the partially decayed remains of plant matter, animal matter, and the matter of microbes such as bacteria and fungi, and is collectively known as humus. Humus levels in soils ideally should be around 5%, however, in agricultural land and urban development areas where we typically have gardens, much of the topsoil that contains this humus has been degraded largely by our poor management practises like over-tilling and the grading/cutting into the topography for construction purposes.
Humus has incredibly CEC, even more so than clay, and holds water and nutrients very well. In fact, a soil with high levels of organic matter will both hold onto excess water for much longer in periods of drought, while also displacing excess water in periods of excess irrigation, making it a real buffer against higher or lower than average irrigation periods (particularly important in water conscious gardens). Therefore, by adding organic matter to overly sandy soils, we can drastically add CEC and thus improve the fertility and moisture holding ability of well draining but otherwise dry and infertile soils. But Humus has other tricks up it's sleeve...
Humus is habitat to billions of beneficial bacteria, fungi, and nematodes - each with their own unique roll to play in soil health, the recycling of dead organic material, maintaining and buffering of pH levels, and making nutrients present in the soil actually available to plants - and we would love to talk in more detail about all of these topics (nutrition & pH, and the roles of these various microbes in soil and plant health), but that will come another time. In this post we want to focus on just one of the beneficial byproducts of many good bacteria and fungi: Glomulin.
Glomulin is soil super glue, and is the reason we always recommend organic soil management practices that preserve and promote beneficial microorganisms in soils. Glomulin will glue particles of soil together to form micro-aggregates, and then macro-aggregates from the micro aggregates. Catching on yet? By binding small clay particles together into larger aggregates, glomulin can take fertile and moist clay soils, and add that elusive free draining element to clay soils that many gardeners just aren't aware is possible.
So now that we understand the long term solution to poor soil structure, how do we actually incorporate this organic matter in our soils? There are a few ways, and some work quicker than others, but the faster methods aren't necessarily the most sustainable.
Beneficial fungi is a great addition to soils
We can till/cultivate compost into the soil, which basically means breaking up and digging the soil, turning it over and breaking up the clumps while mixing compost into the mix. This can add air back to compacted soil and make it lighter temporarily, and will instantly incorporate rich organic matter further down into the soil profile, but tilling has a dark side too. It destroys the soil profile and breaks up the aggregates, which leads to smaller pore size and greater degrees of compaction over time from water and traffic. Furthermore (more on this in a future plant nutrition post), it provides extreme amounts of air to the organic matter and the microbes breaking it down - which creates a very quick 'burn' of this organic matter - resulting in a lot of nutrients being made available quickly, but not sustainably. It's like using a blast furnace to cook a lamb shank, when you really want a slow cooker.
The other methods of incorporating organic matter into soil are quite the bit slower, but in the long run more sustainable. Top dressing with compost will provide surface level food for microbes and worms, which will eat the organic matter (while aerating the soil for you), and their nutritious poo will ideally be pooped further down the soil profile, and thus incorporating the organic matter into the soil profile via microbial action. A top dressing of organic mulch like straw or wood chips will protect and insulate soil while slowly breaking down and being incorporated through the soil profile by microbes, and is usually a great practise for managing soils. Another method that works a bit faster is 'cover cropping.'
Worms incorporate organic matter deeper into the soil profile Typically an agricultural technique when managing fields in between the harvest of one annual crop and the growing of another, a crop of a suitable plant (often times rye-grass, buckwheat, or clover) will be grown to keep the soil covered (like a living mulch, similar to ground cover plants) and also to develop roots into the soil profile. When the plants die (of either natural or man-made causes), the roots break down adding organic matter to the soil, providing food for microbes, and leaving large pores and cavities, thus, improving soil structure. This can be applied to poor soil locations by the patient gardener who doesn't mind spending a year or more tending to 'coloniser crops' that aren't part of the final design. While topic for another post in more detail, cover crop species can be chosen for the actions of their root system as one consideration. Some root systems are great at breaking through compacted ground, some are great and holding together loose soil, and some have specialised nodules that house beneficial bacteria that convert atmospheric nitrogen into plant available ammonia and nitrates, adding fertility to soils.
I personally recommend only digging a bed over if it is poor soil, and adding compost at this stage. Afterwards, simply focus on mulching and top dressing the soil with compost every year or two to keep the organic matter up. Cover crops are a great idea in production gardens, whilst cutting down and composting spend annuals rather than pulling them up, followed by immediately replacing them with the next seasons crop will keep the soil food web going in a similar fashion. When planting in reasonable soil and wanting a boost, mix the native soil from the hole you've dug with some compost, and that way organic matter has been incorporated into the soil around the plant, but the rest of the soil profile has not been dug over. Be careful not to add too much as you don't want to spoil plants and discourage them to spread their roots out in source of more water and nutrients, but feel free to seize the opportunity add a little goodness to the soil when planting.
Grassy cover crops with fibrous root systems can stabilise loose soils and add copious amounts of organic matter
"What about Gypsum?" you may ask. Many people are under the impression that gypsum (Calcium Sulphate) will take away compaction problems in clay soils as well as fixing unstable soils, but seeing gypsum as a one size fits all approach to fixing clay soils and unstable soils is potentially harmful to your garden. To understand why, we first need to understand how gypsum works. Gypsum is made up of Calcium and Sulphur. The Calcium binds to clay particles with a strong chemical bond, and this can help bind clay particles into larger aggregates and therefore increase pore size, thus improving drainage.
Calcium will also replace any Sodium ions bound to the clay particles, which have a weaker bond with the clay that is prone to breaking up when wet, causing what is known as 'dispersion' where clay particles disperse from the aggregates, and can go on to settle together in tight, impenetrable crusts and hard-pans. Soils with excess Sodium are known as 'sodic' soils. Some readers may be realising that another source of Calcium, namely, garden lime (Calcium Carbonate) could also work to alleviate compaction in clay, as well as treating sodic soils, and they would be correct. Lime should be used in place of gypsum when soils are acidic as they will raise the pH of the soil, whereas the Sulphur present in gypsum usually counteracts this alkalising effect and is useful when you wish to make as minimal change to the soil pH as possible. But there are a few problems with jumping to, and relying on gypsum or lime as a fix for sodic and compacted clay soils. First things first, this chemical reaction is not permanent, and therefore a long term solution needs to be introduced (such as increasing soil organic matter to bind clay particles into larger aggregates and to be broken down by microbes to provide nutrients such as Calcium). Adding gypsum or lime will only temporarily alleviate compacted clay soils, usually for a few months. "This reaction is not permanent, and a long term solution needs to be introduced" Secondly, is that jumping to Calcium additions to soils already rich in Calcium can create problems (such as decreased Magnesium or Potassium availability). Often times soils that are very acidic are so because of the presence of either way too much acidic nutrients such as Nitrogen or Sulphur, or the absence of other alkaline nutrients such as Potassium or Magnesium. Therefore relying on lime to amend acidic soils without first determining what is causing soil acidity is shortsighted and can lead to further issues. Thirdly, relying on the Calcium in lime and gypsum to replace Sodium and amend unstable soils without first determining that soils are indeed sodic and dispersive, rather than what is known as 'slaking' soils (where unstable soil aggregates break up when wet due to lack of glomulin from organic matter holding them together, rather than the presence of Sodium) can lead to completely ineffective and wasted applications of gypsum or lime.
Finally, adding lime to soils with a balanced pH, or gypsum to soils with an acidic pH, can be both ineffective at a chemical level, and also worsen or at least fail to amend the soil pH, which leads to its own set of problems. It is for these reasons that the use of lime or gypsum as treatments for various soil problems, such as compaction, lack of stability, and excess acidity without first conducting soil tests and determining what the causes of these problems in your soil are, is shortsighted and potentially going to do more harm than good to your garden. We would always recommend soil testing before investing in gypsum and lime to amend these common soil issues. pH and select nutrient test kits are inexpensive from garden supply stores, soil flocculation tests are simple and free to carry out and can be learned by watching a simple Youtube video, and a knowledge of plant nutrient deficiency and toxicity symptoms can assist in determining which nutrients are actually deficient (or present in toxic amounts) in the soil without having to pay for soil nutrient tests. A final note regarding soil pH issues. Many people rely on the Calcium in garden lime to continuously amend soil acidity, and similarly rely on Sulphur products. So long as gardeners are aware of the toxicity we can add to soils with Sulphur, and so long as gardeners make sure to determine that Calcium is indeed deficient (rather than Magnesium or Potassium) and is the likely cause of soil acidity, these additives will have good affect in the short term, but it is important to note that soil lacking adequate organic matter (and soils that deplete organic matter quicker than organic matter is added) will continue to have many of these issues. Adequate organic matter will act as a buffer to pH issues and help stabilise your soil at a good balance (between 6 and 7 on a pH scale), as well as preventing salinity, sodicity, and nutrient deficiencies in the long run, provided that the beneficial microbes in the organic matter are encouraged and looked after.
Ultimately, the takeaways we wish to impart on readers new to soils are:
Soil makeup Soils are made of sand, silt, and clay. Sandier soils drain better, but hold less water and nutrients. Clay heavy soils drain poorly, but hold more water and nutrients at field capacity. Organic matter improves clay drainage and sand water and nutrient holding capacity. Long term soil amendment Add organic matter! Silver bullet and elixir of soil life! Compost mixed in or top dressed. Avoid tilling unless necessary. Utilise mulch, cover crops, and ground covers. Promote good microbes - feed them more organic matter! Buffer pH long term. Lime and gypsum Potentially harmful or inefficient. Must first determine the cause of problems to determine suitability of gypsum/lime. Effective when used correctly, but only temporarily.