There finally seems to be a glimmer of hope in the air that Spring might actually be around the proverbial corner. I don’t know about you but I’m ready! We have looked at a number of interesting topics in the past several newsletters. Hopefully, they have been of some interest and help to you. With Spring beginning to tease us with thoughts of flowers blooming and vegetables to harvest, I thought that I would spend a little bit of time in the next several newsletters looking at soil. OK, I suppose you could say that I will attempt to give you the “dirt” on soil.
As growers and gardeners, we all know that the Big Three of successful plant production are – sun, water and soil (with all its nutrients etc). But what would you say if I asked you “What is soil and why is it so important?” I could start this series on soil in several different ways. Like all the other topics I have written about, the field (no pun intended!) of soil science is very complex and the focus of many varied PhD programs. Therefore, as always, I will proceed with the caveat that I am going to greatly over simplify and generalize.
I thought that I might break the topic up into several different areas. One topic being the structure of soil and its effect on plant growth and productivity. Another topic might be the area of soil nutrients and their action and importance in plant growth. In short, these two issues are WHAT IS IN SOIL? So the next obvious topic might be – how does WHAT’S IN SOIL get into the plant itself?
I will try to deal with some of the important aspects of this last topic in this article. If you have been to one of my talks or read almost any gardening handbook, you are constantly urged to get the pH of your soil tested. It is very important! However, do you really know what pH is and why it is important to you and your plants?
In chemistry, pH is defined as the logarithm of 1/cH or hydrogen ion (H+) concentration. Don’t worry about understanding the math or chemistry part here. In simple terms, pH is a way to express the relative ACIDITY or ALKALINITY of a material or substrate. The scale of expression goes from 0 to 14 with 7 being neutral. A number below 7 indicates that the material is “acid”. The lower the number the stronger the acidity. Obviously, a number above 7 indicates the material is alkaline, the higher the number the more alkaline the sample. Therefore, something that tests at a pH of 6 is slightly acid while something that tests at a pH of 8 is slightly alkaline. Interesting, you say, but “so what?”
Soil is a very complex melting pot of decayed material, minerals, bacteria, fungi, disease pathogens, salt compounds etc. For a plant to grow, mature and produce properly, all these elements must be in a proper balance in relation to the particular plant. How does the plant get its nutrients out of the soil? One part of the answer lies in the complex root system of plants and the root hairs that can “take in” the nutrients that are available in the soil. BUT, a critical component of the plant’s ability to properly take up nutrients is determined by the pH of the soil. Why? Well, ponder this. Take some sulfuric acid (example being the old acid based drain cleaner). Read the long and bold list of warnings on the back of the bottle. If you spill some on your hands or clothing what happens? It starts to eat your skin or clothes because of the “acid” action (the material that the acid lands on is “eaten” or broken down). Battery acid in the eyes? It will eat the eye tissue. BUT, take baking soda – which is alkaline. Hold it in your hands. Put some on your clothes. It will basically (pun intended) just sit there. I am taking a lot of liberty here because a strong alkaline substance can cause an “alkaline burn” due to its own chemical reactions but this is just an analogy. I am over simplifying again but think of ACID soil and ALKALINE soil in the same way. An acid condition in the soil (say, a pH of 6 or 6.5) will tend to “dissolve” or “free up” some of the minerals in the soil matrix. This will make them more “available” to the roots of the plant and thus more able to cross the root hair barrier and enter the plant as a useable nutrient. Conversely, a slightly ALKALINE soil (say, a pH of 
will not “dissolve” the minerals and thus the nutrients will tend to be “locked up” and not as available to the plant. The whole process that happens at the cellular barrier between the root hairs and the soil is critically dependant on some very precise chemical reactions and the presence of essential minerals and micronutrients. Without the correct pH level, reactions necessary to facilitate the crossing of nutrients from the soil into the roots may be inhibited or prevented. In addition, some of those essential micronutrients may not be “free” or conversely there may be too much free nutrient load. The result can be inability of nutrient transfer from the soil into the roots (causing deficiency problems) or too much nutrient availability and uptake (causing toxicity problems).
For a commercial grower (and you for that matter) this becomes critically important. When the pH of the soil RISES above 6 in a commercial house, micronutrients such as iron, boron, zinc and manganese begin to get “locked up” and unavailable to some plants. The result is visual signs of deficiency such as chlorosis and leaf discoloration on pansies along with improper plant development. This translates to loss of money! Conversely, when the pH FALLS below 6 for some plants like geraniums, too many nutrients can be freed up and the plants can become “over fertilized”, showing up as leaf margins that yellow and become covered with brown spots.
If you look carefully at the last paragraph, you will notice something very important in this discussion of pH. We saw that the same pH – around 6 – can have a dramatically different effect on different plants! This is perhaps the most important thing to remember about this article. DIFFERENT PLANTS REQUIRE DIFFERENT pH LEVELS TO OPTIMIZE GROWTH AND PRODUCTIVITY! A healthy pH for one plant may be toxic to another!
This brings up the interesting question of how are we supposed to regulate soil pH so that it is proper for all the different plants in the garden? This seems impossible! Fortunately, most vegetable (and flower for that matter) plants that we seem to enjoy the most are pretty tolerant of soil pH levels that vary a little – as long as the pH stays within range limits. Those limits vary from source to source but in general, if your soil is testing anywhere from 5.5-6.8 you are probably dealing with an “acceptable” pH range for many of the plants in your garden. Ideally, you’d like to see the pH in the 5.8-6.4 range for the “average” garden growing a variety of plants. Go on line or get your garden guide out and copy the pH ranges for the veggies you want to grow. You’ll see that most are in the range we just talked about. Some plants like Blueberry, Azalea, Rhododendron, Hydrangea actually NEED an acid soil around a pH of 4.5-5.0 to grow. So check recommended pH range for any new bushes or ornamentals you plan on adding to your collection. The closer you can come to the “ideal” pH for each individual plant the better. But for the general garden, that 5.8-6.4 range should work quite well.
How do you test your pH? There are several ways. Before I tell them to you, understand this – nutrient delivery to the plant is more complex than just taking a teaspoon of soil and testing it for pH range. Soils usually have a stratified pH gradient. The soil near the top that has been leeched by rainwater and covered with decaying organic matter like pine needles is more acid than the soil below the surface. Thus, to properly test the garden’s pH, samples should be taken from multiple sites and different depths. The soil may have a good “nutrient load” in the form of minerals in the soil, but different forms of those minerals are easier or more difficult to break down. Rainwater leeching can deplete Calcium (Ca) and Magnesium (Mg) levels and replace these micronutrients with Aluminum (Al) and Iron (Fe). This will cause the soil pH to drop (make more acid) and may create nutrient deficiency symptoms in the plants. There are complex relationships between the pH and what are known as soluble salts. Fertilizers are salts. You measure soluble salts by another test called Electrical Conductivity or EC. Just accept the fact that too much soluble salt in the soil can cause visual damage, growth damage such as stunting and plant death. Every time you water, you are adding soluble salts because water from your tap or well contains salts. Every time you fertilize, you are adding salts. You can probably see that sandy soils or heavy clay soils will present different scenarios for the whole issue of soluble salt concentrations in your soil. I’m not trying to depress you! Just pointing out how complex this whole issue of nutrient load, pH, EC and alkalinity buffering can be.
So, back to testing. You can get one of the test kits at your local garden center. These can be the type that you mix soil and water in a vial and add a chemical agent to get a color you can compare to a pH scale. BUT, keep in mind that the pH of the WATER you use to test the pH of your soil will effect the outcome! You can use litmus paper testing. Or you can use an inexpensive pH meter that uses electric conductivity. The results you get will be an indicator of your soil’s pH range. Accurate testing requires expensive instruments, multiple site and depth sampling, use of distilled water and about 30 minutes. As a side note, we plan on offering such a service this coming season. If your simple testing shows a potential problem, have your soil tested by a lab. The University of New Hampshire offers high quality testing for a very reasonable price. In fact, it is not a bad idea to also have UNH test your nutrient profile if you have never had it done before.
So you’ve tested your own soil or had it tested and the pH is high or low, what do you do? First, never add pH adjusters to your soil unless you have a reason to. In general, if you are following good organic practices, amend and build your soil on a continual basis, your soil is probably within virtually every nutrient and pH range that you would like to see. If you do need to adjust the pH, here are a few BASIC guidelines. To RAISE the ph of your soil (make it more alkaline or “sweeter”) use AGRICULTURAL LIME .You can add it at any time of year, it is slow acting thus long lasting and it is gentle. Apply at an average rate of about 1-2 lbs per square yard. Quick Lime or Slaked Lime – don’t use it. Gypsum? Won’t do too much for pH so go back to agricultural lime instead. To LOWER the pH of your soil (make it more acid) you can use a variety of Sulfur compounds – sulfur, iron sulphate, aluminum sulphate – urea products and ammonium nitrate. The organic grower can use decayed vegetable matter, compost and stable manure.
Want a couple more reasons to take pH seriously? We all know the importance of beneficial bacteria in the soil for soil health and thus plant health. Well, most beneficial bacteria seem to prefer a soil pH of somewhere between 6.3 – 6.8 (sound like a familiar range?). Plant pathogens that cause all those dreaded diseases that we are all so worried about each have preferred pH ranges. For example, when the soil that brassicas are planted in (broccoli etc) has a pH that is too low (acid), the destructive condition Club Foot can take hold and wipe the crop out. When the soil pH is too high (alkaline) for lettuce, chlorosis or a pale green/yellowing of the leaves occurs. Too high a pH for potatoes can cause scab. And the list can go on and on and on….Ever wonder how some of the organic fungicides work? You are basically spraying on a substance like potassium bicarbonate (or baking soda for that mater). Both are alkaline. What do you think it does to the fungus spore that is the pathogen? It appears to drastically alter the pH of the spore surface and disrupts its chemical balance to the point of killing the spore.
Oh yes, are you one of those people who put wood ashes from the stove on your garden? Well, on one hand, wood ashes provide a good source of “available” nutrients such as potassium. On the other hand, wood ashes are VERY alkaline (will drive your soil’s pH UP). So NEVER put wood ashes on your garden without knowing what the soil pH is FIRST. If your soil has a pH of 6.5 or higher – DON’T ADD WOOD ASHES. Many people will advise that you never add more than 25 lbs of wood ashes per 1000 square feet per year. If you add ANY, test your soil pH again. (See Mother Earth News, December 2009).
Bottom line? Have your soil tested as a base line number for you to reference. Know the suggested pH levels for the plants you plan to use this season. You can group many of the same pH range plants in the same area. Only add acidifying or alkalizing agents for a reason. Practice good organic techniques for simplicity and overall health of your soil. If you don’t compost, start a compost pile. Compost is the gardener’s gold. Consider spreading a good organic fertilizer on your garden yearly or side dress as the season progresses. This will help ensure that any of those essential micronutrients that may have been leeched out by rainwater are replaced. It is your garden’s multivitamin! The more you practice good organic techniques and soil building methods, the more “self sustaining” your soil will become. Nature has been doing her thing for millennia! Soil can “self regulate” if we give it the tools to do so.
Relax and work WITH nature!