How Gypsum Improves Drainage
Question from Barbara:
Can you tell me whether adding gypsum will truly help break up and/or improve clay soil in the home garden? The information I find on line is conflicting and confusing. So glad you’re there (or here) to help us.
Answer from Pat:
Gypsum is a relatively inexpensive mineral mined from the earth and sold in bags. Gypsum is indeed an excellent way to combat problems with bad drainage in clay soils in areas where soils are alkaline and where the cause of bad drainage is the alkalinity of the clay. In other words, gypsum is an excellent additive for clay soils throughout the west and southwest or wherever clay soils are alkaline. If the lack of drainage is cause by physical factors such as compaction, gypsum won’t help that problem, but if the soil is not draining because of its alkalinity then gypsum will help it to drain.
What “compaction” means is illustrated by this example: If you have heavy clay soil and you have a horse trail on top of it, applying gypsum on top of the trail probably won’t help the puddles to drain away from the horse trail because the clay has been compacted by the weight of the horses and it has been “puddled” by the horses hoofs. In this case, a better solution would be to slope the trail slightly to drain off the water and then cover it with with a thick layer of mulch. No gardener should walk on his garden soil when it is wet, but particularly if it is clay soil since this increases compaction. (On lawns, combat compaction with aeration as explained on page 358 of my book, but also apply gypsum every year or two.)
In the west, where most likely you live, a frequent reason that clay soil does not drain (unless one has been driving a bulldozer over it) is probably mainly caused by its alkalinity. As explained in the box on page 23 of my book, clay soils are fine-grained earths containing a large percentage of tiny mineral particles that are less than 0.002 millimeters in size and these tiny particles have an electric charge that is negative. This negative charge makes positively charged minerals like iron, phosphorus, and potassium stick to them. This is a good thing since it makes clay soils, unlike sandy soils, highly nutritious, but when there is a lot of sodium (alkaline) in the clay, like we have in the west, then you have a problem because the sodium in clay acts almost like glue. As explained in my organic gardening book on page 21, the sodium in the soil pulls the tiny particles together and makes it hard for water to penetrate. Remember I said the particles have an electric charge so the action of sodium in clay is even more dramatic than glue, it’s as if there is a magnet pulling the particles together. You can see how this stops drainage.
So here comes the educated gardener. A wise gardener with clay soil, living in the west knows he or she needs to add gypsum and also knows to keep it up by adding the gypsum every year or every two years, since it only works for a couple of years. Organic gardeners believe in adding mined gypsum, but I have to tell you honestly that soluble gypsum works better. Soluble gypsum has been treated with chemicals so for a purist it’s not fully organic. The problem is it works. The chemical formula for gypsum is CaSOv. When you add it to clay soil, what results is a chemical exchange. The gypsum releases soluble calcium which takes the place of some of the sodium stuck to the clay particles and this loosens the soil by breaking apart the particles.
This is a long explanation, but the simple fact is that adding gypsum to clay soil can’t do any harm and often does a lot of good. If you have clay soil, always add about half a coffee can full of gypsum in the bottom of every planting hole and work it into the soil on the bottom of the hole before planting. This will increase drainage. Also add gypsum on top of the soil every year all over your garden in enough quantity to look as if a light snow has fallen. Follow instructions on the package. Water it into the ground or let the rains wash it in. See my book for more in-depth explanations.
And yes this works just as well in the home garden as anywhere else.
Pat, Thank you for this valuable advice. Bookmarking this!
Pat,
Thank you for a very informative article. I used gypsum to successfully improve drainage with two houses in Chevy Chase, Maryland and Chevy Chase, D.C.
Now I am looking at a yard in Bucksport Maine, and wondering if gypsum could help there. In one of the areas of poor drainage, there is moss growing on the ground, reflecting the moisture accumulation. Would Gypsum adversely affect the mosses? I would hesitate to hurt the mosses, except for as a side effect of improving the drainage, because the mosses are attractive.
The drainage does really need to be improved though, because it is one of the causes of a very damp basement.
Charlie
I am not sure if gypsum will harm moss but I would not advise applying it on top of moss because there would be no point in doing so. Moss likes areas of poor drainage and moss is well adapted to growing on top of acid soils. As I’m sure you know from your own observation, moss often grows well in shady places where drainage is poor. But I also doubt gypsum will do anything to correct drainage if spread on or worked into the soils of Maine. The soils in Maine are extremely complex and result from the glacial history of the state. In many coastal areas of Maine, the glacier did not actually reach the existing land, but it leaned down heavily on Maine, submerging much of the state, and where the rocky promontories were higher, rocky islands protruded. Later the coast lifted and this history resulted in the jagged coast with many fingerlike inlets and rocky islands of Maine. The reason I am somewhat familiar with all this is that my family has a wonderful family island off the coast of Maine and I learned as a child about the geology that led to its formation. The last glacier also created and left behind it alluvial deposits on top of the rocky substrate of Maine and these alluvial deposits resulted in gravelly and rocky soil in many areas and in general a soil that usually drains well but is infertile and needs a lot of organic matter to be added to it in order for the soil to produce good crops. In most cases, also the soil is acid—the cranberry and blueberry bogs being just one good example—so unfortunately gypsum will not help. Gypsum only works to correct drainage where the soil is alkaline. It corrects the drainage through a chemical change that takes place when it is combined with alkaline clay soils. In some areas of Maine the soil is shallow due to a hidden rock face beneath the surface of the ground. Buried sloping rock faces can cause water to move underground in a sideways fashion. Also, when bulldozing has been done or when one digs to put in a cellar the soil horizons might have been disturbed and this too can effect drainage. Or if your house is on low ground at the bottom or part way down a hillside, this situation could cause a damp cellar. I think you should employ a drainage specialist. It may be possible to dig ditches and install exterior sumps and drains to catch the water before it can seep through the walls of your cellar. I had to do this years ago behind my house right here in California. We had a deep trench dug and waterproofed the wall on the outside. Waterproof membranes are now available to manage this treatment far better than the methods that were available when we corrected our drainage problem. We installed drainage pipes on the bottom of the trench to carry the water around the house where it could drain out lower on the property. Then we filled the trench with gravel and covered it with concrete. In our unique situation we were also able to cover the bank with concrete, which cut off one of the main causes of the problem. Luckily it was an area that was hidden from view. Here is an important caution regarding the proper installation of drainage pipes: The pipe needs to be laid so that the holes are on the side and on the bottom of the drainage pipe, never on the top! If workmen wrongly install the pipe with holes on top, silt can and will sift down into the drainage pipe and clog it, thus stopping it altogether from working. This is a tragic mistake that is often done by folks who don’t understand the process.
I am not sure if gypsum will harm moss but I would not advise applying it on top of moss because there would be no point in doing so. Moss likes areas of poor drainage and moss is well adapted to growing on top of acid soils. As I’m sure you know from your own observation, moss often grows well in shady places where drainage is poor. But I also doubt gypsum will do anything to correct drainage if spread on or worked into the soils of Maine. The soils in Maine are extremely complex and result from the glacial history of the state. In many coastal areas of Maine, the glacier did not actually reach the existing land, but it leaned down heavily on Maine, submerging much of the state, and where the rocky promontories were higher, rocky islands protruded. Later the coast lifted and this history resulted in the jagged coast with many fingerlike inlets and rocky islands of Maine. The reason I am somewhat familiar with all this is that my family has a wonderful family island off the coast of Maine and I learned as a child about the geology that led to its formation. The last glacier also created and left behind it alluvial deposits on top of the rocky substrate of Maine and these alluvial deposits resulted in gravelly and rocky soil in many areas and in general a soil that usually drains well but is infertile and needs a lot of organic matter to be added to it in order for the soil to produce good crops. In most cases, also the soil is acid—the cranberry and blueberry bogs being just one good example—so unfortunately gypsum will not help. Gypsum only works to correct drainage where the soil is alkaline. It corrects the drainage through a chemical change that takes place when it is combined with alkaline clay soils. In some areas of Maine the soil is shallow due to a hidden rock face beneath the surface of the ground. Buried sloping rock faces can cause water to move underground in a sideways fashion. Also, when bulldozing has been done or when one digs to put in a cellar the soil horizons might have been disturbed and this too can effect drainage. Or if your house is on low ground at the bottom or part way down a hillside, this situation could cause a damp cellar. I think you should employ a drainage specialist. It may be possible to dig ditches and install exterior sumps and drains to catch the water before it can seep through the walls of your cellar. I had to do this years ago behind my house right here in California. We had a deep trench dug and waterproofed the wall on the outside. Waterproof membranes are now available to manage this treatment far better than the methods that were available when we corrected our drainage problem. We installed drainage pipes on the bottom of the trench to carry the water around the house where it could drain out lower on the property. Then we filled the trench with gravel and covered it with concrete. In our unique situation we were also able to cover the bank with concrete, which cut off one of the main causes of the problem. Luckily it was an area that was hidden from view. Here is an important caution regarding the proper installation of drainage pipes: The pipe needs to be laid so that the holes are on the side and on the bottom of the drainage pipe, never on the top! If workmen wrongly install the pipe with holes on top, silt can and will sift down into the drainage pipe and clog it, thus stopping it altogether from working. This is a tragic mistake that is often done by folks who don’t understand the process.
While enjoying moderate Mediterranean climate, sitting around a fire place is the best way to spend an evening if you are in a hotel room with your family in Malta. It is also a beautiful addition to a luxury hotel room. Though there are modern facilities for room heating, most tourists prefer to sit around this old fashioned hearth in the traditional way. If you are also one who could enjoy sitting in front of a fireplace in Malta its the country you must visit. Visit Gypsum Malta for more details.
Your comment appears to be an advertisement from Malta for gypsum used for a purpose other than gardening. Though this use for does not pertain to our subject of plants and gardening, it is certainly true that your other topics: fireplaces and the Mediterranean climate have some application. Outdoor patio fireplaces are compatible with Mediterranean gardening. An outdoor fireplace can add to the ambience of one’s patio and garden and increase enjoyment of outdoor entertaining. I have visited Malta and enjoyed it enormously. Towards the end of his life, my father, an Englishman, and his American wife built a small limestone villa on Malta and he lived and painted there until his death from heart failure at age 90. My husband and our family stayed with them there on several occasions. Most of the plants my Dad grew on Malta were the same plants we have here, such as roses and oleander to name only two, but Dad found gardening a bit difficult due to the rocky, highly alkaline soil. He often wrote to me that he had just finished painting in his studio and was about to go outdoors and work in his “rock pile”—his humorous name for his garden. I particularly enjoyed the hiking on Malta with its expansive views of the Mediterranean, the chariot ruts left from ancient times, the terraced farms and fascinating watering systems made of small channels cut out of limestone and fitted together by the farmers. I never visited the island of Gozo but hear that is also charming.
If you decide to use gypsum, I would recommend getting a good brand and try to check where it’s from. I’ve heard reports some of the made in china stuff have toxic slag in it, similar stuff the gypsum wall board had a few years ago from china that ruined many homes. I prefer the pelletized gypsum than the powerdery form because less of a mess, especially on a windy day.
Thank you so much for a good warning. All the bagged gypsum in my area is now pelletized. None is sold in powdered form any more. The big difference here is between rock gypsum and soluble gypsum. The soluble gypsum does more good, but organic gardening purists prefer rock gypsum since it is dug from the ground and has not been chemically treated to render it soluble. I also advocate liquid gypsum (Soil Logic) for ease of application especially on lawns.