Iron and Clay Soils
by Paulette Mouchet
Originally published in "The Rose Garden" newsletter, June and August 2003. Updated November 2006.
© 2001 by the ARS
According to the American Rose Society's Consulting Rosarian Manual, "Iron aids chlorophyll formation, and [in forming] sugar burning enzymes [that] activate nitrogen fixation." Iron is required for healthy, vigorous plants with dark green leaves. An iron deficiency usually affects younger plant leaves first with a general lightening of the leaf color. The veins stay dark green but the interveinal areas will turn light green to yellow.
Soils that contain a lot of clay usually have plenty of iron. Unfortunately, your roses can't get to it because clay soils are alkaline (pH above 7) and the high pH causes the iron to be held in an insoluble form.
So how do you know if you have enough iron and if it's available to your plants? The only way to know for sure is to test your soil. The simplest approach is to buy a pH meter for about $6 from your local gardening shop. These meters have a metal probe that you stick into the ground and then you wait a minute or two for the reading. They are not the most accurate in the world but they will give you an idea of your soil's pH.
Another approach is to have your soil tested by a lab that specializes in doing so. In addition to testing for pH, the lab can run other nutrient tests that will help you know the overall health of your soil. While testing from a lab can run $80-$90, you can save money in the long run by using the test results to tailor your fertilizing program.
Labs can do several types of testing. Water-extraction nutrient testing (a.k.a. saturation-extract method) will tell you what nutrients are immediately available to your plants at the moment. But, according to Otto Lund, a Consulting Rosarian works at the Soil and Plant Lab, Inc. in Santa Clara, CA, the saturation-extract method does not tell you what nutrients are being held in reserve on cation exchange sites and could become available if the plant puts a little energy into the system.
Cation Exchange Capacity (CEC)
- A cation is a positively charged nutrient such as calcium, magnesium, or potassium
- The cation exchange capacity (CEC) is the amount of cations that can be held by a specific volume of soil
- The higher a soil's CEC, the more nutrients that can be held in reserve for later use by the plant. Soils with a low CEC cannot store nutrients and they are lost to leaching.
- A cation exchange site is the place on a soil particle where a cation is being held
- Nutrients being held on cation exchange sites are safe from the effects of leaching and can easily become available to the plant when needed.
Otto explains that your soil has both a checking account (nutrients in solution in the soil) and a savings account (nutrients held on CEC sites). Nutrients easily flow out of the checking account into the plant, and then the checking account needs to be replenished by nutrients from the savings account. If the soil has a high CEC, there are a lot of nutrients in the savings account that can replenish the checking account. If the soil has a low CEC, then the checking account may overdraft because there are not enough nutrients in the savings account to replenish it. When the checking account overdrafts, there are no nutrients available for the plant and the plant suffers.
This checking-saving system is always trying to balance itself. When nutrients go out of the checking account, the savings account tries to replace them. Adding organic matter to the soil is an excellent way to improve your soil's CEC and prevent the loss of valuable nutrients.
When choosing a soil test lab, be sure they do tests for both immediately available nutrients (saturation-extract testing) and nutrients that could become available (cation exchange capacity testing).
A good soil test should include:
- macronutrients (in order of importance to your rose)—nitrogen, potassium, calcium, magnesium, phosphorous, sulfur
- micronutrients—copper, iron, manganese, boron, molybdenum, and zinc
- evaluation of your soil texture (optional)
Evaluation of your soil texture—the percentages of clay, silt, and sand particles—is optional because short of bringing in an earthmover to remove several feet of your existing soil, there's nothing you can do with the basic texture of it.
If you have clay-based soil that contains enough iron but is too alkaline, the long term solution to making it available to your roses is to lower the soil pH by applying LOTS of organic matter. As the organic material is broken down and utilized by soil organisms, the pH of your soil will gradually come to neutral (pH = 7). When the pH is between about 6.5 and 7.5, all nutrients, including iron, become freely available to plants.
There are several benefits to using organic matter to bring down your pH including
- compost contains, feeds, and supports a highly diverse collection of beneficial soil organisms
- composts contains almost all the nutrients plants need
- manure-based composts contain humic and fulvic acids that help improve soil porosity (structure), normalize pH, and provide food for beneficial soil organisms
It may take some time to normalize your soil pH, so what do you do meanwhile to make sure you plants have enough iron?
First, make sure your plants actually need supplemental iron by having the soil tested. You might be surprised at the results. If you do need to supplement, a good choice is chelated iron.
Plant cells have a net negative charge. Trace metals such as iron that are positively charged (anions) get stuck at the door and have a hard time entering the plant. Chelation is a process where a carrier molecule is attached to the iron making the new chelated molecule neutral or slightly negatively charged. This chelated molecule can then pass easily into the plant and across cell membranes.
In healthy soil, beneficial bacteria and fungi provide and chelate proteins or amino acids to anions so they can enter the plant! If your soil is not up to snuff and you need to supplement iron, use man-created chelated iron to insure the most iron utilization by your roses.
In man-created products, there are several chelating agents that can be used as the carrier molecule including EDTA (ethylene-diamine-tetra-acetic acid) and lignins. EDTA is a synthetic material and generally cannot be Certified Organic under the National Organic Program (NOP).
Lignins are "glues" that bind together cellulose in nearly all woody plants, and they have strong chelating properties. For the most part, lignin chelates can be Certified Organic. Cascade Organics Iron and Fert-All Liquid Iron Chelate, available from Peaceful Valley Farms, use lignin.
Chelated iron is usually applied as a foliar spray when plants require a quick iron fix. It also can be applied in powder form to the soil.