How Can Soil Tests Reflect the “Right Chemistry” to Improve the Productivity of a Farm?
Soil Productivity is largely affected by its physical properties, its biological properties and its chemical properties. A soil test mostly evaluates the chemical properties of a soil however, chemical properties do affect both the physical properties and the biological properties of the soil. In this blog, I will describe some of the soil test factors that can improve the physical and biological aspects of the soil and make a soil more productive.
Soil Test and CEC
The first part of the soil test I usually look at is the Cation Exchange Capacity (CEC). The CEC is an indicator of how heavy the soil is or how much clay content it has. The higher the CEC the heavier the soil. Below are some ranges that you can use:
CEC Soil Texture
5-11 Sandy loam
12-18 Silt Loam
19-24 Clay Loam
The most productive soils usually fall into the 12-18 range since they have a good infiltration rates and good water holding capacity, yet have good drainage to allow air back into the soil. They normally have a good mix of sand, silt and clay particles. Having adequate air and water within the pore space will also improve biological activity within this soil. If soils are too sandy, they will not have adequate water holding capacity for adequate growth. If irrigation is available these soils too can be very productive.
Soils with a high clay content usually have more limitations in productivity since air and water management of these soils are difficult. If these soils have a high water table, installing tile drainage can greatly improve the productivity by insuring greater air and water management. Root development can also be limited within these soils because of the physical nature and less aeration.
I next look at the organic matter (OM) test relative to the CEC. Higher CEC soils should have higher OM levels. If this isn’t the trend, it will usually indicate a soil that is more eroded. Eroded soils are less productive and will have poorer physical and biological properties. If a lower CEC has a relatively high organic matter reading, it usually indicates a very productive soil and may be one which hasn’t been farmed very long. Keep in mind that many labs now test OM by using the “Loss on Ignition” test. This is different than the “digestion” tests that used to be done. If a soil has a high amount of “free lime” or limestone present, it will be reflected as OM in the soil test which will actually be an elevated reading not reflective of the true OM level.
The next test I would look at is the pH test. The most productive soils and the soils that have the best physical and biological activity will be those that have a pH in the range of 6.0 to 6.8. When soils fall outside of this range you will find the soils to be less productive for most agricultural crops. Availability of most nutrients are best within this range as well. High pH soils often (not always) reflect a soil with a high water table. If this is true of your soil, then installing tile drainage would be helpful in both lowering the pH and making the soil more productive. Low pH can be caused by roots growing through the soil, nitrogen fertilizers, leaching of cation by rainwater, etc. Applying lime is most likely your most economical solution to low pH.
% Base Saturation
After pH, I will then observe the % Base Saturation numbers since they are related somewhat to pH. It is also a very good indicator of air and water management and soil structural differences. Rules of thumb to consider are to have the % calcium about 70-75%, % magnesium about 10-15%, % potassium at about 4-5% and % hydrogen at about 5-10%. If your soil has % magnesium levels >15% the soil will have less potential pore space which will both limit biological and physical properties of the soil.
Levels of potassium greater than 8% and sodium levels greater than 2% will also limit soil productivity by limiting pore space and air and water management. Having adequate calcium is key to better soil structure. If your soils’ pH is in the optimum range as mentioned above then gypsum would be your best source of calcium. If your soil’s pH is less than 6.0 calcitic lime would be a good source of calcium.
Another consideration in evaluating a soil’s physical condition is studying the level of mobile nutrients such as sulfur and boron. If these nutrient levels are high or very high this is an indicator of poor internal drainage or possibly a compacted layer or a high water table. Being mobile, these nutrients should leach with water. If not, then something is limiting their mobility.
All of these soil tests mentioned are helpful in evaluating a soil’s productivity and we really haven’t discussed soil fertility levels as such and what is considered optimum for nutrient availability. These tests do reflect factors that will affect nutrient availability. Often these particular tests are either not run on a soil test or not referred to by the crop advisor. Don’t be short changed. Be certain that you get these tests included on your soil test. They truly do help you know that you have the “right chemistry” in your soils.
About the Author:
This post was written by Bob Hecht. Bob was an agronomist for Midwest Laboratories, Inc for 21 years before becoming a partner in Soil Solutions in 2001. Advising agricultural producers to make them most profitable has been the cornerstone of their business. Helping farmers with cropping decisions, understanding soil fertility, air and water management, and the benefits of soil amendments such as gypsum and lime are key components to a producers profitability. You can learn more at www.soilsolutions.net