Sciency Answer: Too much compost? (Plus! Fun with lime!)

Leslie, who blogs over at Mountain Plover blogs.icta.net/plover, send me a great question.

My question is about adding compost... can you add too much? And if so, what harm does it

In my master gardener training, we were told to always recommend adding 4 inches of organic matter, dug in 8 - 12 inches. Colorado soils are notorious for their lack of humus. However, my brother-in-law (who lives near Denver) had a soil test (from an independent lab) come back telling him he had too much compost! (He has been growing veggies for a number of years, adding compost and shredded leaves to the soil every fall.) Apparently, this was throwing his other minerals out of balance. They actually recommended adding lime and gypsum, two things CSU told us to avoid. Now I'm totally confused.

"The compost and leaves have narrowed the calcium to phosphorous and the calcium to potassium ratio. Both should be 18 to 1. The reason a small amount of limestone is added is because calcium and potassium compete for space on the clay colloid. When limestone is applied it will push potassium off the clay colloid and then be taken up by plants or will leach below the root zone. This is a little trick to help reduce excessive potassium faster. The gypsum is applied because it will help correct a serious situation in your soil: excess sodium. Once applied, the calcium and sulfate in gypsum will disassociate. The sulfates combine with sodium to become sodium sulfate and is leached in the presence of adequate moisture. This will help reduce your sodium level. Having sodium at 166ppm is much worse by far than having calcium at 8000 lbs. per acre or even higher. High calcium levels are not that much of an issue. Calcium nitrate is applied to bump up the nitrates in the soil for quick, early growth in the spring. The amount of accumulated calcium will be negligible."

Whew! What a lot of great questions! And a lot of excessively technical language from the soil lab. I'll try and break this down in bits and see if we can make sense of it all.

Fertilizer Vrs. Soil Amendment

When thinking about adding stuff to soil, I like to mentally break things down in to two categories: Soil amendments and fertilizer. I think of things like peat moss, charcoal, sand, as soil amendments. Their primary purpose is to change the texture and structure of your soil so it is a better space for plant roots and beneficial microorganisms to grow in. Fertilizers, on the other hand, are plant vitamin pills, providing specific chemicals they need to supplement their main diet of carbohydrates they make from water, sunlight, and air. It is pretty hard to overdose on most soil amendments. You can't really have too much organic matter in your soil – after all, peat or coir based potting mixes are essentially 100% organic matter. Fertilizers, on the other hand, are usually damaging to plants at high levels, just as you can overdose on almost any vitamin.

Compost is, of course, both a fertilizer AND a soil amendment. Compost has a lot of organic matter which improves soil texture, and it also has varying amounts of nitrogen, phosphorous, etc. The exact amounts will vary widely depending on that your compost is made from, but certainly you can add too much fertilizer by adding too much compost. The most common compost over-dose, in my experience, is adding way too much nitrogen when using composted manure, but you can overdo other elements as well.

To lime or not to lime?

The rest of Leslie's question is interesting because it make, once again, the point that all gardening is local. The soil testing lab recommended adding lime and gypsum – but her Master Gardening training at Colorado State said to avoid both of those! What gives?

The usual reason to add lime or gypsum to soil is to correct a calcium deficiency, and/or to make a soil less acidic. Where I live, we get around 30-40 inches of precipitation a year and all the water leaches out the alkaline chemicals, usually producing an acidic, calcium deficient soil. Around here, adding lime is pretty much standard practice. Leslie gardens with only 16 inches of precipitation a year, and that lack of water lets calcium build up in the soil, so adding more is just making a problem worse. There are exceptions to the rainfall-soil pH connection of course... lucky people like Gail who, thanks to limestone bedrock, have alkaline soil despite a wet climate. I gardened briefly on limestone when I was at Ohio State... it was a pain. Suddenly rhododendrons went from over-planted and boring to difficult and desirable. Because wet, alkaline soils are rare, it is hard to find plants adapted to those conditions.

All of which is a bit beside the point in this case. The soil lab is recommending lime and gypsum here for a different reason. Leslie's brother has too much potassium and sodium in his soil, and adding lime and gypsum is a kind of chemistry trick to let those chemicals leach out of the soil faster. It will increase the amount of calcium in his soil, which isn't a good thing, but having lots of sodium is a much WORSE thing, so is the best option for the healthiest soil.

Get Tested

The bottom line here is: get your soil tested periodically by a qualified lab in your area. Soils vary radically depending on your climate, they type of bed rock you have, and how you manage it. Compost is a catch-all term, and composted cow manure is going to be very different than composted yard waste or composted coffee grounds. Find out what your soil is actually like now, so you can make the right choice on what to add to it in the future.

Have a question? Get a sciency answer! Just e-mail me (engeizuki at gmail dot com) or ask one in the comments!

Science Answers: tap water versus rain water

Another great question from Annie of Annie's Annuals:

Is there something special about rain water ? I mean beyond "hydration"? As we are a Mediterranean climate here in the Bay Area, we have to use city water all Summer .  Then, after the first rain everything goes bonkers and all my garden plants seem to grow quite a bit overnight and vibrate with inner happiness.  No, I'm not currently on drugs and I swear this is true. I notice it every year. I know its not that they get watered deeply for the first time in a while- because our waterers  at the nursery water everything  every day whether I like it or not!  Can you solve my mystery?

I think I can solve your mystery. Let me start by telling you about another mystery.

Columbus Ohio has a lovely public conservatory which had a marvelous collection of palms. These palms had been happily, healthily growing for decades, then started mysteriously wasting away and dying. The workers at the conservatory hadn't started doing anything new, the plants weren't diseased or infested with insects. At a loss, they enlisting help from horticulture professors at Ohio State who determined the soil pH was WAY too high. Which was an answer, but lead to another question. They had been maintaining these palms the same way for decades, and everything had been fine. Why would the soil acidity suddenly get all out of wack?

Water.

The city, it turns out, gets water from three different treatment plants, and each uses water from different reservoirs and/or wells, making the water from each plant chemically different. As the city grows and changes, they sometimes switch a neighborhood from water from one plant to another, and they had changed the source for the conservatory's tap water -- but didn't mention it to them. After all, water is water, right? Nope. The new water source was radically more alkaline than the old one and suddenly all the plants that had been vibrating with inner happiness were stressed and dying. The conservatory started pH testing and treating their water with acid, and the remaining palms were saved though many beautiful trees had already died.

(a moment of silence for the palms)

The sad story of the palms goes to show that sources of water can be pretty radically different and have a big effect on plants. Truly pure water essentially doesn't exist. Water is sometimes called the universal solvent because almost anything can, and does, dissolve into it. Even ultra-super-extra-triple distilled water won't stay perfectly pure for long. As soon as it is exposed to air, carbon dioxide will dissolve into it and make it slightly acidic. So don't be fooled by "pure" bottled waters. If it was truly pure, it would cost a lot more (ultrapure, research-grade water costs about $25 a liter), and besides, extremely pure water tastes unpleasant, and isn't good for you.

City water comes from various sources like wells, lakes, rivers and treated sewage (yum...) and picks up all sorts of different minerals, salts, and gasses along the way. Water treatment plants remove some of those compounds, add others, and sterilize it to make it safe for people to drink. The emphasis here is on people, and some water treatments aren't all that good for plants. Some plants are sensitive to chlorine, and water softeners take out excess dissolved minerals in water (which are mostly fine for people or plants) and by replacing them with dissolved salts (which are very bad for plants). Rain water generally has less stuff dissolved in it than tap water, but it isn't pure either, not by a long shot. Clouds contain not just water, but also various dust, gasses, and (unfortunately) industrial air pollution, making rain water chemically different than tap water.

So how might the difference between rain water and tap water be effecting your plants? As demonstrated by the palm story I started with, pH could be a big one. Rain water is slightly acidic (or not so slightly, if you've got acid rain) whereas most tap water is alkaline due to dissolved minerals. It may be that over the summer your soil pH slowly goes up, out of the plant comfort zone, and then gets brought back down with the arrival of the fall rains. You could test this theory pretty easily be measuring your pH before and after the rains start.

Another option would be mineral and salt build up. Most tap water contains a fair amount of dissolved minerals, and fertilizers (of any sort -- organic or synthetic) are salts. As water evaporates from the soil, it leaves the minerals and salts behind, which over time can affect the health of the plants. In extreme cases, like house plant that have been around forever, you might even see a crusty white layer on the surface of the soil. Rain water is purer than most tap water, and coming in large amounts could flush out the excess minerals and salts letting plants grow more happily. You could sort of test this one by comparing your soil's salt concentration before and after the rains with an EC (electical condictivity) meter.

It could also be nitrogen. I didn't realize this until I started researching this answer, but rain water can have significant amounts of nitrogen in it (but, then again, so can tap water, especially in agricultural areas). The papers I've looked at on the subject find the amount to be pretty variable by region and season, so your local fall rains may or may not be giving your plants a fertilizer boost along with hydration.

Finally, as I mentioned above, the water treatment process adds chemicals like chlorine to tap water, which some plants are sensitive to. These may be slowly building up in the soil over the watering season gradually hurting the health of the plant.

In short, rain water is very different from most tap water, and the "inner happiness" your plant exhibit with the arrival of fall rains isn't a figment of your imagination nor a drug-addled hallucination, though it is hard to say for sure which factor or combination of factors are responsible for their joy.

Have a question? Get a sciency answer! Just e-mail me: engeizuki at gmail dot com.