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A few years ago, we used a clay type soil for filling holes in our garden. Later the soil became very hard and most of our plants and trees are not healthy. Only unwanted plants are growing.

Is there any way we can make the soil healthy for garden plants?

  • What is the pH of your soil? If it is low, you could add lime to flocculate the clay. – George of all trades Feb 17 '17 at 10:28
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    There are some good points below, but just a word of warning: avoid working heavy clay at all when it is wet. Even loamy soils can be seriously damaged by being worked when they are too wet. You may find that the problem you have stemmed from the fact that the infill soil may have been poorly handled/stored. – George of all trades Feb 17 '17 at 16:04
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You need to incorporate as much humus rich material into the soil as you possibly can, so that means things like composted animal manures, good garden compost you've made yourself, leaf mould, spent mushroom compost, anything organic like that. If its really solid clay, bad enough to make pots with, then the addition of plenty of horticultural sharp grit (note, preferably NOT sand) would also be helpful, but it would be best to add grit to particular areas rather than trying to alter the whole lot (see info here https://www.rhs.org.uk/advice/profile?PID=620, which also includes more information on how to manage clay soils). You'd need to keep adding humus material every year - whether you dig that in or simply apply it as a mulch is up to you, but if you do use grit, that's best turned into the soil and not just laid on top.

Not sure what you mean precisely when you say only 'unwanted plants' are growing - that might mean pernicious, very undesirable weeds or simply plants you don't like, so I can't comment on those. As for any trees and plants which are 'not healthy', clay soil becomes extremely hard when its very dry, so is it possible you don't water sufficiently when it needs it during summer? Not knowing where you live, its impossible to judge whether that would ever be necessary or how often it might be, so again, you'd need to clarify a bit to get a better answer.

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  • Might be of note that I heavily edited the question for readability. The original might have some context I missed, I don't know – J. Musser Feb 16 '17 at 23:27
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    Clearer post edit than it was before... I guess wells are holes, aren't they, but its possible something else was meant - not sure what that could be though. – Bamboo Feb 17 '17 at 0:17
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I'll add in to the fine answer above that the unwanted plants can do a rather good job at loosening up the soil, so don't look at them as enemies, but as limited term employees that will help in your efforts and eventually can be let go when the desired species are taking off.

Instead of fighting with these invaders, prune them, coppice them, use their leaves as mulch, use them as trellises to grow other things, etc.

Of course this approach will depend on what species are invading (ive never found a good use for English ivy), but the things that are growing first are typically pioneer species that are well adapted to these environments and can slowly but surely improve the conditions to the point that other species can thrive.

You can take this into your own hands by planting or sowing seeds of hardy, deep rooted species that can do a lot of work for you, and likely do a better job of it too!

Comfrey is a popular example, there are examples of using Diakon radish and docks and other deep rooted perennials. Leguminous species can add significant nitrogen to the soil. The specific options are dependent on your climate and soil.

Also I'll note that I've found better results, esp when working in hostile environments, in focusing my efforts on a smaller area and then growing outwards.

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    You'd want a sterile comfrey if you want to get rid of it again – Graham Chiu Feb 17 '17 at 3:50
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Clay is a type of soil that is composed of many very very tiny particles, and its nature is to retain water (poor water drainage, causing waterlogging and root rot) when wet, and to form large (hard to break up) clods when dry (hard for plants to expand new roots into). Some plants grow in clay soil just fine, but those plants are resistant to the root rot caused by clay's lack of drainage. Clay is normally rich in plant nutrients, but its tendency to compact and poor drainage cause many plants problems.

The quickest and easiest method to create an area for plants that do poorly in clay is to create a raised bed of non-clay soil on top of clay. In the case of the OP, he could remove the clay soil used to fill his holes and replace it with a non-clay soil filler.

A somewhat less quick and easy method is to convert existing clay soil to a non-clay form of soil. The disadvantage of this method is that non-raised garden beds in areas that are mostly clay soil is that water in the growing area is unable to easily escape through the surrounding clay. The solution to this lack of drainage involves digging drainage ditches/paths (and then possibly filling them with gravel and/or drainage pipes) through the surrounding clay to allow any water to escape. If your growing area is on a slope or hill, the slope may allow water to escape without digging drain paths.

To quickly convert clay to a less clay like soil, you may use one or both of two methods: 1) Mix in any amount of organic matter, such as compost, into the clay or 2) Mix sufficient amounts of sand into the clay (at least as much sand as clay by volume). In both of these cases what you are trying to do is "uncompact" the clay, or in other words, allow air pockets to exist in the clay soil. If you use the "add sand" method, mixing insufficient amounts of sand (less than 50% by volume) will actually make the situation worse, since the soil will become heavier (sand is heavier than clay, making the soil compact faster). Adding insufficient amounts of organic matter, on the other hand, may not help 'fix' the clay sufficiently, but will not make soil compacting worse.

The air pockets you create in the soil will do two things: allow growing roots to penetrate the soil more quickly (allowing faster plant growth), and allow water to drain through the soil faster, removing the tendency of clay to cause waterlogging and root-rot. Ploughed or rototilling clay soil (or clay soil with insufficient added sand) will quickly revert to a solid clump (without air pockets) when wet, while organic matter or sufficient amounts of sand mixed with clay resist compaction of the needed air pockets.

When selecting the type of organic material to add to your clay, remember that un-composted (raw or green) organic matter such as wood chips, lawn clippings, or fresh manure cause changes in the soil chemistry and pH that may create problems for growing plants (see the rules for judging soil type below).

The slowest but easiest way to convert clay to a less clay-like soil is to encourage the growth of any plant that is willing to grow in the clay soil, and then occasionally rototill or plough under those plants into the clay soil. Those roots of those clay loving plants break up the clay and add organic matter when rototilled into it, so this method can be considered a slower method of adding compost to the clay. When using this method, adding a layer of an inch or two of sand or gravel on top of the clay will allow plants that cannot normally grow in clay to thrive. This is because some or most of their root system is above the clay soil, but the tips of the roots can still pull nutrients and water out of the clay.

Rules for judging the soil type you have:

1) Physical composition: How easily the soil becomes compacted. Affects soil water retention, water drainage, and ability of roots to easily penetrate the soil. Clay is at one end of this scale (few air pockets, roots penetrate with difficulty, retains water, and drains poorly), sand is at the other end (high water drainage, lots of air pockets, roots penetrate easily), and loam (ideal soil) is in the middle. different plants are happiest with different soil types; some like sand, some like clay, but most are grow best in loam.

2) Nutrient levels: Adding organics (mulch) or fertilizer to soil tend to also add plant nutrients to that soil. There are exceptions to this; for example wood chips reduce nitrogen when breaking down, which is why they can be used as a ground cover to inhibit weed growth. Almost all plants require Nitrogen, Potassium, and phosphorus, but there are dozens of other micronutrients plants also need from the soil in lesser amounts. Organics also tend to improve water drainage (they retain water in sandy soil and allow better drainage in clay soil). Although few plants have problems with too many nutrients in soil, adding the 'wrong' organics or fertilizer can cause pH problems (see below). Lots of organics in soil that have broken down completely tend to create the 'black' soil that you often see as potting soil.

3) pH of soil: this is the acidity or alkalinity of the soil. Adding raw manure or green mulch (grass clippings, leaves, etc.) will cause soil to become more acid; the process of organic breakdown creates acid. Adding lime or limestone to soil will cause it to become more alkaline (reducing acid as the lime breaks down). Any plant has a "preferred" pH level that it really likes best and adding lime or organics to adjust the pH allows you to tailor soil acidity to match the plant you want to encourage. Adding raw or green organics that have not broken down (composted or aged) sufficiently will tend to 'acid burn' plants until that raw or green organic has broken down.

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    Mixing sand with clay can be problematic (more or less turning it into concrete). – George of all trades Feb 17 '17 at 16:02
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    @Georgeofalltrades: concrete is made by mixing powdered slaked lime (CaO) with sand and gravel. The slaked lime combines with carbon dioxide from the air to turn back into "manmade limestone" (CaCO3), which acts as the glue that binds together the concrete. Clay does gets very hard when it dries out, but concrete does not set up because of the water drying out (as most people believe), but instead it is a chemical process similar to epoxy glue setting up. If there is enough sand mixed in with clay, that sand will prevent the ultra-hardening you can see in pure clay when it dries out. – Mark Ripley Feb 17 '17 at 16:22
  • It depends on the proportion of sand, that's why I find this answer valuable and I'm going to upvote it. gardening.stackexchange.com/questions/30560/… – Alina Feb 17 '17 at 17:02
  • @Alina Adding sand is a really, really bad idea, does sweet FA for clay soil and may cause extra problems, as I learned years ago when I made that mistake - see here patwelsh.com/soils/never-add-clay-to-sand-or-sand-to-clay – Bamboo Feb 17 '17 at 18:12
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    @Bamboo Even if it's a bad idea, both G. Catenazzi in the above link and M. Ripley on this answer claim that they have a way of doing it right. IMHO this is useful info even if it's not the best thing that can be done to improve soil. I wouldn't have mentioned this if I didn't notice that this answer was downvoted, which I considered to be unfair. I think it is my duty to help balance facts and experience on one side, and personal preference on the other. – Alina Feb 17 '17 at 19:16
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Really would like to see what you've done and perhaps more detail what your planting procedure entailed.

Clay is GREAT SOIL. All soils are great soils IF one knows how to MANAGE the soil they have. Now listen to my words here, ALL soils can only be improved ONE WAY and one way only; the addition of DECOMPOSED ORGANIC MATTER. Decomposed is the key word...otherwise it could possibly take years for the micro and macro organisms to be able to eat it after the decomposers do their thing (using up lots of Nitrogen in the process).

Adding anything else DOES NOT WORK. There is no recipe for amending soil other than adding decomposed organic matter.

I now have pumice with low organic matter for soil, now. All my other gardens have been almost pure clay (blue). Clay is the tiniest particles of the soil matrix and they are FLAT as well giving them electrostatic properties that hold water and chemicals very well. Very good trait...less water and fertilizer is needed.

Concrete is made from: Clay, gravel, sand, water, lime, gypsum (depending on conditions or type of project) and ROTATION. The more agitation, rotating, rototilling, manipulation the more clay is able to bond...to become concrete or brick. Simple.

When I first start a garden and double dig my rows (3' wide minimum) I will add DECOMPOSED organic matter and fertilizer (depending on my soil test results) and mix sorta kinda. Without using anything else this makes raised beds. Sometimes there is a lot of moisture in the soil but it will look dry to you. My beds for plants are always ALWAYS raised with trenches at the bottom of the slopes to take excess water away to where ever I dictate. No matter what kind of soil I have, I make these raised beds. They start out a couple of feet high with the fluffy soil and then they are formed and compacted to get rid of large air pockets. They end up a good foot off the ground. I dig at least 12 to 18" deep.

And that is the last time I will mess with the soil of my beds expending lots of calories on my part. Listen to this; just by adding DECOMPOSED ORGANIC MATTER to the top of the bed of soil, the micro and macro organisms come up through the soil profile, EAT this stuff, go back down into the soil to poop it out mixing the organic matter into the soil FOR YOU. You always have to replace this 'mulch' as it does feed the fauna in your soil and gets depleted as they eat, take it back into the soil to poop it out. Also, by dumping a couple of inches on top of your beds, whether veggie garden or landscape ornamental beds, is the best way to get rid of weeds. Gardening made easy!! I am the laziest gardener of all time. I can't stand to take extra steps, do things that are unnecessary.

Understanding your soil and creating proper beds FIRST will save you so much trouble, headaches you will love being a gardener!!

One big glaring question is; what kind of soil do you have outside those holes you dug for your plants? It must be clay? Take a small shovelful of your soil, put it in a quart mason jar and fill with water. Shake. Allow to settle. You will get a good idea what types of soil and percentages by doing this. (take a pic and send).

Most plants grown at the nursery that come balled and burlapped are grown in heavy clay. This ensures an intact root ball. When a clay root ball is planted in a sand amended back fill, the water will roll right off the root ball and into the amended back fill, starving your plant of water.

Clay root balls should be planted in clay soil. We need to know what your original soil texture is first. And, did you dig further down in other words deeper than the height of your root ball? Leaving the hole only the depth of the height of your root ball to sit on undisturbed subsoil is CRITICAL. Otherwise your plant will subside and for some plants this would be the kiss of death especially for trees and woody perennials.

To alleviate this condition as well as planting on slopes, we inserted perforated pvc pipe into the hole as close to the root ball as possible. Watering for at least 6 months INTO that pipe will get water to the roots where it is needed. Careful to not overdo! Pictures please. Our answers are fairly generic until we are able to SEE what you are seeing.

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