Dirt soils – are they the same? No! Are they close? No.
Soil has been one of my interests and loves since my college days. My background has been as a Park Ranger Naturalist and I did my Upper Division course work in soils and the ecology of soils. We talked about soils based on those mineral components and we divided our soils up between sand, silt and clay. The common misconception people have is that one’s good; one’s bad. There is no such thing. In nature, there is no such thing. In nature, there is no good and bad. It’s all trade-offs. Sands have benefits; sands have disadvantages. Clays have benefits; clays have disadvantages. Silts are a mixture – give me the best of a small world?
Sand Benefits of sand are they have great drainage; greater aeration; they don’t compact. Go out on the beach and jump up and down and come along with a pale and sand bucket and dig holes; the lifeguard can drive up and down the beach – sand doesn’t compact. Clays have advantages. They hold moisture; they hold nutrients; they have a high cat ion exchange capability. You apply nutrients in the form of fertiliser or other organic sources. They bind those and keep them available for plant uptake. They don’t leach through the soil. The disadvantages of sand is that they have good aeration and drainage – they dry out quickly; they need to be watered frequently; they don’t hold nutrients well so when you apply fertilisers they flush through the soil profile and they don’t bind the soil particles that remain available so you sands require fertilisers more frequently or you use a slow release form of fertiliser that remains in the root zone for a longer duration.
Clays have problems. They don’t drain well, they hold moisture too long and they compact easily. Probably the worst thing I have heard people do with sand and clay soils is they say “well, my clay holds moisture and my sand doesn’t and my sand has good aeration and my clay doesn’t, and my sand needs to hold more nutrients and my clay has good cat ion exchange ……., I’ll mix my clay and sand together or if I have clay soil, I’ll mix sand into it and it will improve the drainage, get better aeration, prevent compaction and …..wrong!” If any of you have any construction background, how do you mix concrete? Sand and cement. Cement is a clay so you take clay and sand and you add a little gravel and mix it but you didn’t get soil. Soils are a lot more than the mineral component. Soils ecology is the complex system of living, dead and dying organisms as well as mineral components and oxygen, water, bacteria, fungus, insects, roots and all of the interactions going on in that soil profile. It’s an eco system and similar to the eco system of the ocean, there are strata or layers of the eco system. Organisms that live in the surface few inches of soil cannot survive a foot beneath the surface. Organisms that live a foot beneath the surface of the soil cannot live on the surface and tilling your soil is not a good practice. Common for many decades we would plough the ground and you see advertisers who have cultivators and they tell you to go around your trees to control weeds and when you do that you’re disrupting the soil organism ecosystems causing problems for that soil. Now lets combine that fact with the way plants and trees grow.
On each tree, your root systems have 3 functions:
(1) is to anchor the plant and keep it upright.
(2) is to store food and nutrients to keep it through its dormant period.
(3) is to pick up water and nutrients.
The roots that pick up water and nutrients are cut out at the ends of the growing and developing roots are very fine delicate new growth called root hairs. These root hairs are extremely fragile and those are the roots that pick up water and nutrients – 70% of the root hairs – they are also known as feeder roots occur within the surface one foot of soil. 90% of the root hairs occur with the surface 3 feet of soil. We talk about deep root plants, tap roots, deep water and irrigating.
70% of feeder roots within the surface 1 ft of soil is where you want to focus your water and fertiliser programs. Deep watering your plants doesn’t mean putting probes and pipes into the soil and watering 3ft beneath the surface of the soil – there are no feeder roots there. Roots grow in the presence of water and air. Too much of either one or too little of either one and roots die. So in natural ground soils, in California we don’t have any ground water to speak of, the only ground water that’s available is what comes from the surface down. Also, oxygen exchange occurs from the surface down and when you have clay soils, the oxygen exchange occurs from the surface into the soil profile and it’s the lack of oxygen which often accounts for the lack of root development in the soil profile.
Growing in the ground, it has long been tradition to amend or prepare the soils before planting your trees, shrubs etc. They would tell you “dig a hole twice as wide, twice as deep, amend the soil with 30%-50% of organic material and use that to fill to acclimatise the container grown plant to the native soil. This is wrong. This is not a good recommendation especially when we’re dealing with clay soils, compacted soils, poor quality soils. Why? Because what happens is that we amend the planting zone and you have a plant that is growing in the landscape, how far out should the root system develop on a mature established plant? Two to three times the height of the plant in radius. That means that the 10 ft tree in your front yard has a root system that is 30 – 60 ft in diameter and you’re amending a hole 3ft across!!! Where’s the benefit? To help it get established? That was the claim! The claim seemed to hold true because we take a tree and plant it in the amended hole and plant another in the native ground, the one in the amended hole seems to grow faster, quicker, bigger – more rapidly but what happens in the long term is the roots contact the native soil and especially where your soils are poor, the roots want to stay in that amended hole where life is good and the root system does not develop well into the native ground. Plants which were done as a test to determine which was the best soil amendment to use, studies were done initially out of Oklahoma, Davis California plus several other universities throughout the country to determine what was the best soil amendment to add to the planting holes when planting trees and shrubs and repeatedly the outcome was the group which were the control where no soil amendments were added, outgrew and outperformed the other trees in the long term. These trees developed root systems into the native soil without any interface or transition problem and became better established, healthier, stronger trees than those where soil amendments were used. Soil amendments in the planting holes also have some other problems. Amendments have traditionally been organic material. What happens to organic material over time? It decomposes and as it decomposes, we lose aeration and drainage and you add organic material into a planting hole and that organic material decomposes as the plant settles and now the roots at the bottom of the planting hole lack oxygen and they develop rootrot. If the plant settles below ground? and the soil fills in to cover higher on the trunk on the plant, the plant can develop crown rot. Furthermore, it is the physical property of water that as you change from one texture soil to another texture of soil, water will not move from the first into the second until the first is saturated so you can end up with what is called “perched water tables” and instead of improving drainage, you often cause poor drainage soils to be much worse. Organisations like International Society of Arboriculture no longer recommend soil amendments for planting trees and shrubs. The Nurserymans’ Association for decades argued that soil amendments are a big part of our sales and if we tell people not to use soil amendments, then we’ll lose sales but now even that association do not recommend soil amendments in the planting hole. Plant them in the native dirt but before you do, make sure that dirt drains adequately so that water and air move through the soil profile and as the water passes through the root zone of the plant, it pulls air in behind it – it fills the pore space in the soil. Drainage is imperative and adding soil amendments in planting holes does not improve drainage. Poor drainage soils are like bathtubs. If you dig a hole and fill it full of water and it doesn’t drain, it’s like a bathtub with a stopped up drain. If you have a bathtub with a stopped off drain and add soil amendments or organic material in your bathtub, what do you have? A swamp, mud, a peatbog. It didn’t fix the drainage. You just appear to because what people do is dig these big holes and water drains below the surface, you don’t see it so it and it sits on the bottom and the roots rot and that’s fine as long as the only water you get is from your irrigation because you can irrigate in moderation so that the water doesn’t fill up the hole from the top. Then you get a wet season and that hole fills up with water and the roots drown and die. We see that every wet winter in California. We pull out trees that have been growing for years because the root can’t get adequate amounts of air.
Mulching as opposed to amending the soil is the single best thing you can do for your soil. Think of the soils that we are all envious of. Those beautiful black soils of Illinois, Iowa. How did those soils develop naturally? Nobody added soil amendments into them. They developed by organic material falling from plants and trees that covered the soil and decomposed it from the surface downward. It was the action of the organisms within the soil; the earthworms, the bugs, the fungi, the bacteria that caused those organic materials to decompose and as they decomposed they produced humic acids that helped to dissolve excess salt, improve drainage, break up clay particles, improve aeration and to improve your soil, mulching is by far the single best thing you can do to your soil. Add to that the fact that it is feasible to amend the entire root zone of the plant. Can you amend an area 60 feet in diameter by 3 feet deep? You can with a backhoe and plenty of money but it’s not really practicable but you can mulch that same area 3 inches deep and mulching on a regular program keeping the mulch 2-3 inches over the surface of the soil, replenishing it as it decomposes will improve your soil in a number of years so that now instead of having poor soils, you have a rich growing area for the roots of your plants.
There are some things you can do to your soils that are beneficial in the way of chemical treatment. Southern Californian soils are typically desert soils and desert soils are typically high in pH – they have a fair amount of salt because of the water quality that we get from the Colorado River. pH of soils in southern California average about 7.3-7.8 which is somewhat alkaline. Most plants prefer a pH of soil to grow in which is somewhere in the neighbourhood of 6.5 and they like it slightly acid. Tropical and sub-tropical plants like even a little more acid than that 6 to 6.5 are usually ideal for most of your sub-tropicals so how can we modify our soils to lower the pH and alleviate some of these salts? With mulching techniques and the addition of chemical treatment. Chemical is not necessarily a bad word. Chemicals can simply be inorganic materials – Gypsum is a naturally occurring mineral. Soil sulphur in the form of sulphate anions is often used to alter the pH of your soil. It doesn’t do it overnight. Gypsum with 12-18% sulphate can change the pH of your soil but it doesn’t do it overnight. It relies on a bacteria in your soil to convert the sulphate to a water soluble form that changes the pH of your soil. The addition of things like gypsum is beneficial but you need the association of the bacteria and it takes time so it will be 3-6 months before you see any change. Gypsum has some other benefits – it can help to replace sodium in your soil with calcium. Sodium is detrimental to plants and it will allow the sodium ions to leach more freely through the soil profile. Calcium is a secondary nutrient that’s valuable for plant health so adding things like gypsum can be beneficial. Also gypsum in some clays and it’s not a magic pill, can cause clay particles to bind together into clumps called aggregates and as it binds together, it opens up larger pore space between the aggregate, allowing water to pass through. Adding things like gypsum to our soils can be beneficial on a yearly program. Gypsum is largely water insoluble so taking gypsum and spreading it out on the surface of your soil has limited benefit. There are more water soluble products that are better at improving the soil structure chemically when applied to the surface and watered in. One product is called Soil Buster, another is called Liquid Gypsum and those have some real improvements. Mulch your soil with organic material. Use whatever you can get. It’s just like what you eat. So if you use a mulch that has more components, you’re going to get more benefits but anything is better than nothing and the old idea that eucalyptus mulch is bad, are unfounded. It has been studied quite extensively used as a seed media, used as a potting media, used as a mulch and it has been shown has no detrimental effects on your plants. Be careful of Oak mulch which may have oak root fungus.
Compost worm castings provide humic acid which helps improve drainage. It decomposes quickly but it should be viewed more as fertilisers than soil modifications. A lot of research has been done by Garry Bender recently on the benefit of compost and preventing Avocado rootrot and the use of mulches on your soil are antagonistic to phytophthora which is the rootrot causing fungi that kills avocadoes. It doesn’t kill the phytophthora but it improves the root system of your avocado so that the symptoms of root rot are not so severe and avocados will often grow faster and produce more roots more quickly than the phytophthora can kill so producing a healthier eco system and healthier plants. In your soil there are harmful organics like phytophthora but there are also beneficial organisms like mycorrhiza and this has been the subject of much research done in the past 20 years. Mycorrhiza are beneficial fungi that are naturally occurring in most soils and have a very important association with root systems of plants. They greatly expand the surface area and absorbent capability of root systems and are incredibly beneficial in enhancing the ability of the roots of plants to absorb phosphorous from the soil; it improves the drought tolerance and improves the health of the plants’ roots. Most native soils have mycorrhiza but if you have a soil that was stripped you may need to reintroduce it to your soil and that’s also true of container soils.
Container Soils were formulated as soil material for growing plants in containers. There is no dirt in container soil and almost without doubt, anytime people put dirt from their garden and adding it to a pot, it will fail dismally. Why? Because you are introducing a lot of pathogens at that time and because the media compacts and when this happens you lose aeration and drainage, the mineral component of dirt is such that it’s not uniform and particles fit together like jigsaw puzzles impeding drainage, and without aeration and drainage, water and air, roots will die. Container soils are made out of products to give roots of plants good aeration, good drainage and water holding abilities and nutrient availability but container soils were never intended to be long term. If Garry were here, he would talk to you about soils and tell you that most container soils on the market today are … in a bag and he would also tell you that most container soils will lead to the death of your plant because he will argue that all container soils today are made principally of 75-80% organic material mostly in the form of wooden bark product and that those wooden bark product decompose and as they decompose, the soil compacts which then impedes air movement and water drainage and roots rot and plants die. Container soils were never meant to be long term. They were meant as a growing media from the time the nurseryman produced the plant until you put it in the ground at home and it not only takes most container soils a couple of years before that organic component breaks down to a point where it causes problems. I’m sure that most of you have had potted plants where you start out with a nice full potted plant and over time, it’s only half full of dirt. The dirt didn’t wash out; it decomposed and compacted and that’s the problem with most potting soils. Most commercial potting soils are pretty good; but there are also bad ones. There have been a number of studies on commercial potting soils: Super Soil, Patio Plus, Dr. Earth, EB Stone, Uncle Malcolm etc. are very good potting soils and they’re formulated out of components that are intended for good root development but they all decompose over time. Some also add organic fertiliser. Are they better than those that don’t? No, it’s just another way of fertilising. Some also add mycorrhiza. Because most potting soils are made up of sterile material, there are no mycorrhiza in most potting soils so a potted plant does not have the beneficial association with the fungi so we need to inoculate the soil in order to get that development of beneficial mycorrhiza. Mycorrhiza is amazing – not only does it improve the drought tolerance in the plant, water absorption capabilities, the abilities to take up fertiliser and also ecto-mycorrhiza which live outside of the roots protect it from many harmful fungi and bacteria because this fungi coats the outside of the root when fungi that normally attack roots of plants come and see roots which are covered in beneficial mycorrhiza they don’t see the root of the plant, they see the beneficial mycorrhiza and they don’t attack the root because they don’t know it’s there so the presence of the mycorrhiza on the roots are protecting that plant from harmful fungi by association. They have also just recently shown some amazing film footage using an electron microscope where mycorrhiza is not just a passage as we thought it once was. Mycorrhiza is a fungus and fungus rely on plants and animals for their nutrients. Most mycorrhiza will invade the roots of the plants and take nutrient from the plants to survive giving the plant back benefits but they find that mycorrhiza also in certain cases are actively killing soil organisms like nematodes. Mycorrhiza can produce a network of filaments that entangle nematodes and noose-like snares and strangle these nematodes killing them and as the nematodes decompose in the soil and die they become nutrient sources for these mycorrhiza – pretty amazing stuff! Adding mycorrhiza to your potting soil can be hugely beneficial and the time to add it is at planting time because mycorrhiza lays in a dormant stage until it comes into contact with the root and once it comes into contact with the root, it can grow and invade and colonise that root. The plant only needs to be inoculated once. You can buy mycorrhiza alone and inoculate the plant or you can buy soils that have the mycorrhiza in it and you can buy other additives.
If you have really bad drainage and want to put in raised beds, what do you do? A raised bed is a container. For long term growing in containers or in raised beds, you want to use a media that does not decompose or decomposes very slowly. The best potting soil or media in which to grow long term is one that does not break down and decompose causing root rot issues. That means we have to use a media that is either primarily inorganic material or is organic material that is either already highly decomposed. Pumice, decomposed granite and quartz sand for drainage. The problem is that all these things are heavy. Garry Matsuoka has a formula that he uses for most of his nursery plants and it’s 40% pumice, 40% peat moss and 20% perlite. The problem is that it drains really fast and it is really heavy but it doesn’t decompose so is satisfactory for long term container growing in raised beds. I would use something along those lines. You can mix in different proportions but peat moss does not decompose much because it is already decomposed. Rice hulls take up to 10 years to decompose and one potting mix I’m familiar with called Patio Plus by Kelloggs has the primary organic component of rice hulls v wooden bark chips which decompose in 3 years. If you’re going to grow long term in containers and use these types of fast draining potting soils with little organic material like Garry’s 40% pumice 40% peat and 20% perlite, you’re going to have to water at least every other day and sometimes every day and they do require mulch on the top of the soil.
I wouldn’t use Perlite in the ground as it tends to float – pumice is better.
Drainage is imperative Dig a hole 2ft deep and fill it full of water twice. If it drains in 12 hours, or less, you’re fine. If it takes more than 12 hours to drain, you need to go to raised beds or containers or put in drains.
Rock Dust takes a very long time to break down and add any nutrient value.
Note from Sheryl: One of the guest speakers failed to turn up at the California Rare Fruit Growers Conference so Tom stepped in and this talk was totally impromptu!
TOM DEL HOTAL, a nurseryman for 37 years, holds 4 advanced certifications from the California Association of Nursery Professionals. He is also a certified Arborist, a certified pesticide applicator, and a member of the International Plant Propagators Society. He currently works full time at the Home Depot in Lemon Grove as a sales associate and as district trainer for the garden department. He is also a part-time horticulture instructor in the Landscape and Nursery Department at Southwestern College in Chula Vista. In addition, Tom has also restarted his own business / nursery “Fantasia Gardens”. Tom has been a member of the California Rare Fruit Growers (CRFG) for the past 26 years and an avid grower of a great variety of fruiting plants on his property in Lemon Grove. He is a past president of the San Diego Chapter of the CRFG and worked for Pacific Tree Farms as the manager and propagator for more than 12 years. Tom is an experienced lecturer and speaker and gives many presentations to gardening organizations as well as at fairs, seminars and special events.
Australian Soil Resource Information System – www.asris.csiro.au
ASRIS provides online access to the best publicly available information on soil and land resources in a consistent format across Australia. It provides information at seven different scales (view animation).
The upper-three scales provide general descriptions of soil types, landforms and regolith across the continent. The lower scales provide more detailed information in regions where mapping is complete. Information relates to soil depth, water storage, permeability, fertility, carbon and erodibility. Most soil information is recorded at five depths. The lowest scale consists of a soil profile database with fully characterised sites that are known to be representative of significant areas and environments.
Article compiled by Sheryl Backhouse