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How To Improve Clay Soil And Improving Sandy Soil - Read This First

Gardeners ask how to improve clay soil more often than about improving sandy soil, but the reasons are generally the same, and the main reason has to do with water.

(I go into much more detail on how to improve both clay and sandy soil in my online gardening course).

In many gardens, clay doesn’t infiltrate and drain fast enough and sand drains too fast. One of the most commonly given pieces of advice on how to improve clay soil is to add sand. When it comes to improving sandy soil, the advice is often to add clay.

Both of these are poor organic gardening practices. Before we go into why, we need to take a quick look at how water moves through the soil.

Trust me, this is really good to know and quite interesting.

How Water Moves Through Clay And Sandy Soil

Water moves downward after rain or irrigation and upward to eventually evaporate from the soil surface.

This water flows through the open “pores” between soil particles. In any soil that is not dominated too much by sand, silt or clay, approximately half the soil volume is pore space. Water and air share this pore space.

When soil is entirely saturated with water, gravity forces the water to move very quickly through the big pores, but the rest of the time, gravity doesn’t play as big a role in how water moves through the soil.

The rest of the time, adhesion (how water molecules tend to stick to other surfaces) and cohesion (how water molecules tend to stick together) govern the movement of water in the soil. Interestingly it moves out in all directions fairly equally - up, down and horizontally. It moves downward only slightly more due to gravity.

How To Improve Clay Soil And Improving Sandy Soil
Adhesion and cohesion in action

So let’s say it’s a beautiful Saturday morning and you are doing some organic gardening. Let’s look at what happens when you have layers in your soil.

How To Improve Clay Soil

Let’s say you have a clay or silt loam soil that doesn’t infiltrate or drain well. What happens if you add 6 inches of a coarser soil such as a sandy loam on top of a the soil? When it rains, the water slows down when it hits that fine soil layer as you might imagine, although it does continue to move through.

Still, it slows down, which is the opposite of what you were going for. If you instead rototill the sand into the clay, it doesn’t create a nice soil texture like you would think. The sand just gets embedded in the clay and often forms a soil environment that is like concrete.

When deciding how to improve clay soil, adding sand is not the answer.

How To Improve Sandy Soil

This part is really interesting. Let’s reverse it and say you have a sandy soil that doesn’t hold water. What happens if you add 6 inches of a finer soil on top of a coarser soil below? This also may happen if the builder brought in some topsoil that was clay based and put it on top of your sandy subsoil.

When it rains, you might think the water would speed up when it hits the coarse sandy layer, but in fact, water movement stops until the soil becomes nearly saturated above.

Even more interesting, if the finer soil is on an extremely coarse sand or even gravel, the finer soil must become very wet before water will move down through the coarse layer. In this case, the overlying soil can hold two or three times as much as it normally would.

These same principles are often used when making golf greens. A layer of gravel is used underneath the sandy soil for the green in order to create a situation where water will stay in the upper layer of sandy soil and be available to the short roots of the grass on the green, rather than draining away.

But doing this in a home organic garden is dangerous because you may create the opposite problem, which is a very waterlogged soil, or you may make a soil that is like concrete if you rototill the coarse and fine soils together.

Improving Clay Soil And Improving Sandy Soil

The answer is the same for both: organic matter.

Compost is what I’m generally referring to. Amend soil with 6 inches of good compost. Work it right into the top of a clay soil and it will improve infiltration and will probably improve the amount of air and water available to your plants.

(I should mention that no amount of organic matter, rototilling and aeration will fix a serious drainage issue such as flooding. That needs to be addressed by installing drainage, or even better, work with nature and put in a pond.)

Back to compost. Improving sandy soil with 6 inches of good compost will drastically improve the water-holding capacity of the soil.

Amending Soil With Compost
Improving clay soil with sand won't help, but compost will

Other Organic Gardening Implications

Since we’ve been learning about how to improve clay soil and improving sandy soil and specifically about water moves through the soil, I’d like to mention a couple of other important organic gardening implications of what we’ve learned about amending soil.

Take patio pots for example. Gardeners will often put a layer of gravel in the bottom of a container to improve drainage, but as we have seen it actually does the opposite. The soil on top has to be very saturated before the water will drain through. This is not necessarily a bad thing with the right plants, but it is important to know how this works.

The other main implication is with regards to organic matter. If you amend soil by turning in (burying) coarse organic matter such as leaves or straw and they end up buried in a layer, or if you drop 6 inches of soil on top of a big layer of coarse material such as sticks or straw, you could be creating a coarse layer underneath a fine layer.

This is the same situation where that soil will get very wet before water will drain out. If straw is incorporated very well, this will not occur, but if it is just turned under, it will.

If you need to bring soil into your yard in order to build your garden, you must make sure to rototill it in to the existing soil extremely well. If you just drop it on top, you are inevitably creating an interface that will slow drainage.

And try to find a soil that has a similar texture to your exiting soil (i.e. if your soil is clay, bring in more of a clay topsoil than a sandy topsoil). Also, use compost for perhaps 25% of the mix.

Is there anything organic matter can't do?

Garden Tilling Soil - What You Should Do Instead Of Tilling A Garden

Garden tilling is often done with a gas-powered rototiller that goes down perhaps 6 or so inches, or tilling soil can be done with a tool such as a pitch fork, too.

So what is tilling? However you do it, you're turning the soil over so some of the lower soil comes up and some of the upper soil goes down.

To till or not to till has been a hot topic for decades and continues to this day. I get into it in more detail in the Academy. I’ve researched this extensively and find that arguments on both sides make a lot of sense.

I've also found personally that by correctly tilling a garden or following a no-till method, excellent results can be achieved.

Garden Tilling Benefits
Rototilling a new garden area

The main reason tilling soil can be useful in the short term is to get organic matter incorporated into the soil of a new garden bed or a fallow vegetable garden. If you're preparing a new garden in a soil that is very low in organic matter, you might bring in 3-6 inches of compost and till it in as deep as a rototiller will let you.

You would do this knowing that it may take a few years for the soil structure to repair and produce a great crop, but it's often worthwhile if the organic matter was low to begin with. Garden tilling helps you get that organic matter down in there to create a deeper soil that encourages plant roots to go down. Double digging is another useful method to accomplish the same thing.

Research shows you can get more carbon and humus formation in a tilled soil. I could see this being the case because most of this research is done on conventional (non-organic) farms.

No-till farming often uses a lot of chemicals that decimate the soil food web, so organic matter left on the surface isn’t going to break down. However, in a soil with an abundant soil food web, earthworms, insects and fungi can get up on the surface and work on the organic matter.

Another argument for tilling a garden is that it gets the organic matter down into the soil where it can be broken down, whereas if it stays on top, more of the carbon is volatized into the air. This is true.

But other research shows that if no-till is used with sufficient existing biomass, both the supply of nutrients and good soil structure can be maintained. This is especially true if you have tilled in compost and green manures for the first couple of years to bring up that biomass.

In the long run, I am more likely to do just shallow garden tilling to a couple of inches deep, mostly by hand. The main reason you might do this shallow tilling is to lightly incorporate the organic matter from a cover crop or this year’s veggies or compost, as this does hasten decomposition and promotes more humus formation and less volatization of carbon into the air.

If you stay shallow, you won’t have as detrimental of an effect on soil structure, dormant weed seeds, microorganisms and earthworms. Even still, do this minimally and carefully in order to limit the disturbance.

More and more I am sheet mulching to prepare a new garden bed, which involves layering organic matter 12 or so inches high right on top of the grass/soil. It takes longer for the organic matter to get down into the soil, but you don’t cause the drastic soil structure damage associated with tilling soil.

People have had great success doing this, but I mostly prefer to do it when I know I have a decent organic matter content in the soil already.

Something else I sometimes do for fun is do half of a bed with the garden tilling or double digging method and half with the sheet mulching method and see which does better over the years.

The other main reasons gardeners may till the soil are to: make the soil look fluffy and nice, allow more air and water into the soil, loosen and warm up the soil in spring for planting/seeding, reduce weeds, and relieve compaction. Tilling soil is generally a short term solution for all of these.

Alternatives To Tilling Soil
Those autumn leaves make an ideal mulch

Reasons For Tilling Soil

1. One we can get rid of right away is "to make the soil look fluffy and nice," not that the goal isn’t achieved, but that the goal is arguably unreasonable. We have learned to think of bare, fluffy soil as being the most attractive look for our gardens.

I understand this - and tilling is a good fluffer-upper - but I also think leaf mulch and other mulches are attractive. That’s what you see in a forest, not bare soil or several inches of bark mulch for that matter.

As organic gardeners, one of our goals is working more with nature. It doesn't mean we need to have a messy garden, but we definitely aren't going for cleanliness like a living room floor. If you want to put a wood mulch on to make it look tidy, it would generally be better to not use bark, and preferably it would be from hardwoods, not softwoods such as cedar and fir.

2. Another reason a person might end up tilling a garden is to allow more air and water into the soil. This does happen in the short term, but the soil will eventually revert to its original structure because soil structure is a function of the soil texture, fertility and biology in the soil. It may even get worse if you burn up too much organic matter and kill all your fungi.

A similar reason for garden tilling is to loosen and warm up the soil for spring planting and seeding. This can be done lightly with a hoe or garden fork if you want, but there’s no need to slide and dice everything.

The long term solution for improving air and water is balancing the nutrient ratios, increasing organic matter and improving the soil food web. The details of how to do this are too long for one article. The process can take a few years, so tilling soil during the transition may be worthwhile. Even better might be to do it manually with a garden fork to avoid the violent soil movement from a rototiller.

3. Yet another reason to till is to kill weeds. Again, this is a short term measure because the weeds are killed and new ones come in. In fact, while annual weeds will have been killed, many perennial weeds may have been cut into pieces that all come back as new weeds. While your vegetable seeds now have perfect conditions in which to germinate, so do all of the weed seeds that were lying dormant lower down in the soil.

Farmers have developed various plows that are effective at knocking down weeds on the surface, but a rototiller is more muscle that is wanted for this purpose. The gardener’s version of the farmer’s plow is a hoe. The long term weed management strategy is the same as up above - balancing the nutrient ratios, increasing organic matter and improving the soil food web - plus mulch and cover crops.

4. The last reason for garden tilling is to relieve compaction. As stated above, soil structure is not only a mechanical problem. It’s a chemistry, biology and physics problem. We need to establish a healthy, diverse population of microorganisms and earthworms, build our humus and balance those nutrient ratios in the soil.

A balanced soil acts like a sponge. You can drive across it with a heavy tractor and it won’t compact. If the calcium:magnesium ratio is less than 7:1 and/or there is more than 70 ppm sodium, the soil will compact, but if we can get those numbers in line along with the phosphate:potassium ratio, compaction is gone. How to balance those nutrient ratios is a big topic.

So you can see how in the short term the above organic gardening goals are often satisfied, but other than the important goal of getting organic matter incorporated, tilling soil can sometimes cause more problems than benefits, especially if done often and if done too deeply.

The Problems Of Garden Tilling

The main disadvantage is the effect on beneficial microorganisms and earthworms, both of which are absolutely essential to the health of the soil. Upon deep tilling a garden, some of the microbes that need oxygen are buried, killing many of them. Some of the microbes that can’t live with too much oxygen are brought to the surface, killing many of them.

Tilling soil causes the miles and miles of beneficial fungi to be sliced into pieces. Those fungi provided important nutrients to the plants, so that is no longer happening after tilling. Earthworms are also killed and their tunnels destroyed.

All of these critters had taken a long time to find the perfect spot for themselves in the soil. They worked day and night to build themselves little homes and communities. It can take years for this to happen and garden tilling destroys that all very quickly.

Problems Of Tilling A Garden
Garden tilling and earthworms don't get along

Perennial flowers, shrubs and trees prefer fungal-dominated soil, so if we’re trying to establish a shrub garden or grow trees, it makes sense to leave the mulch on the surface rather than tilling it in. If we're tilling our gardens, we cut these fungi all to pieces, resulting in a bacterial-dominated soil. This is more conducive to growing annual vegetables, but we still want some fungi in these soils, too.

Another big problem is that soil structure can be decimated if the soil is too wet, especially clay soil. Tilling soil when it is wet causes long term structural damage to the soil that can last for years. It’s much better to wait a couple of weeks and plant late, rather than working wet soil.

While the initial influx of air and water after tilling soil breaks down the organic matter more quickly, and releases nutrients to allow microbes and plants to flourish for a short time, that organic matter is oxidized faster than it is replenished.

Annual deep garden tilling without adding more organic matter can cause a gradual decrease in organic matter in the soil. This decreases soil fertility, nutrient-holding capacity, water-holding capacity and hurts soil structure. If the soil is left bare, it can crust over so that water runs off and causes erosion instead of infiltrating.

So what is tilling good for? We can see that tilling a garden has advantages and disadvantages. Tilling can be successful in vegetable gardens over the long term if organic matter is brought in every time. No-till and sheet mulching can be successful over the long term, especially if the soil had some humus to start with.

I think the main reasons people may believe strictly in one of these camps may be because the theory makes the most sense to them, or because they have had success using that method in their garden.

The bottom line is that one of the methods will work best in your garden, and a combination of the two may even be optimal. I may till in some compost in the beginning if my soil is very low in organic matter, and no-till and sheet mulch in future years. I definitely opt for no-till long term in order to establish a healthy soil food web. You may want to experiment to see what works for you.

Interesting, right? Let me know your thoughts below.

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