One of the most common annual and ongoing requests that comes to a consultant/agronomist is to assess a crop (at various stages) and then provide recommendations for fertility adjustments. At the same time, there are always a host of other mysterious ailments that need sorting out, some of which have roots in the soil (if you will excuse the pun).
All plants have different nutrient needs. All plants require nutrients to reach their full potential in terms of plant health, yield, and productivity. Some plants need more nutrients to meet their continuous or ongoing growth than crops that have a set or finite growth pattern. Most perennial plants require annual supplementation of soil nutrient levels to offset the continual, annual withdrawal by the harvested crop.
It is possible to make general fertility recommendations based solely on estimated plant growth demands in an average year. However, lately an average year is somewhat rarer than a new Canadian penny or the Easter Bunny. There are many changing and variable factors that affect both a plant’s ability to take up available nutrients, as well as the available pool of nutrients in the soil that can be drawn from. It is difficult to get it right without the correct information. The solution is to gather soil information through focused and periodic soil testing.
What to Sample
The information that you acquire from a soil test is very much dependent on the nature of the sample that is collected. It is important to submit a sample that is a good representation of the soil in the area that you want information on. That area could be as small as a few square feet, or it could be from a field spanning dozens of acres. The area you select depends on how confident you are about the uniformity of the soil and how precise you want to be in your management decisions. If you include samples from locations within the overall area that contain oddities or anomalies, such as excess nutrients, organic matter/plant debris, or other atypical factors, your overall average will be off and the accuracy and usefulness of the results will be reduced.
Collect soil samples from the same depth from a pattern of sites within the overall area. You can follow a zigzag pattern, or just sample randomly across the field, ensuring that you capture the “essence” of the site. Bulk or combine all the similar samples that you collect (e.g. same depth) from across the site. Once you have a combined sample, mix it up and then take your soil test sample from that homogenized mix. If you want to compare different depths, keep those apart and test them separately.
If you think that there are differences between fields, it is a good idea to test those separately. You will find that some fields are sandier, some are heavier, some have more organic matter, some have less. Having that sort of information will help you in your future management of that field. You might only do this sort of separated testing initially, until you know what the differences are (if there are some).
What Tests are Needed for Which Crops
I think that the more information that you have at the start of your crop, the better. Baseline data is important no matter what crop you are working with, whether annual or perennial. I prefer to see sample data from the main soil growing profile depth for the main crop or crops (e.g. potatoes or mixed vegetables), plus deeper levels as well.
For example, if you will have a crop that will mostly be drawing from the 0-6-inch profile, test that, plus the 6-12 depth or deeper. Subsequent testing can look at whatever depth you think makes sense for the crop. Most annuals are not likely going to need to look much beyond 12 inches unless you are looking specifically for something.
For big woody perennial crops, such as fruit orchards, you are not going to have another chance to do a whole lot of testing right where the crop is planted, so upfront baseline data collection will be even more critical. Additionally, most orchards are not going to be fertilizing for the first few years, with the new plants drawing upon the soil nutrient bank for a time. You must know what is present, to be able to build it up before you plant.
For an orchard, I would get samples from the 0-12-inch profile depth, plus a 12-24-inch depth, for this first test. In subsequent years, you can rely on a 0-12-inch test for maintenance and adjustments, but you need to know what the deeper levels contain up front. It will help you to pinpoint any deeper drainage issues, fertility reserves, etc.
Frequency of Soil Testing
Personally, I think that a soil test is not necessarily required for everyone, every year. I absolutely think that a test is critical at the start of any project, or when you are starting off in a new growing area. Testing is necessary when you lack information. When you are starting out, you should probably test the year or two before you start, just to give you at least one year to gather information and make any adjustments that might be needed.
Once you have the baseline data, maintenance testing should be done periodically, to allow any changes to be noted, and adjustments to be made. This will likely be every few years, at a minimum, but in sandier soils that are prone to more leaching, you might drop that interval down by a year. If you are rotating between crops regularly, with certain crops having different needs, you might increase your frequency of testing. If you are planning on growing a crop that is known to have higher nutrient demands, you might test prior to growing it.
If you are seeing yields or growth drop off, or if you note some sort of issue in your crops, or experience a particularly unusual growing season (e.g. a wet year), soil testing can be useful to flag any nutrient or soil chemistry-related issues, or major changes in the soil.
When to soil sample
Soil sampling is generally recommended to be carried out when soils are cooler, so typically in the fall, or in the early spring. If you need information on a soil, sampling can occur at any time. The idea is to capture information from a stable soil, rather than one that is actively changing.
What Does a Soil Test Tell You?
Even though soils are fairly consistent and do not change rapidly (without outside influence), soil tests still only represent a snapshot in time of the soil that you are sampling. Depending on what tests are run on the sample that is provided, you can learn about the soil texture, organic matter content, some of the soil chemistry components (e.g. electrical conductivity, pH, etc.), as well as get a sense of the available soil nutrients (both macronutrients and micronutrients).
For your baseline test, I would include all the soil physical and chemical characteristics (texture, pH, EC, OM, etc.), as well as the major nutrients. Later tests can just focus on things that are more likely to change, or that you can have some influence on in your management. This includes things like macronutrients, organic matter, and salts, or any other thing that might be of concern. Unless you have reason to suspect a problem, you will not likely need to test for contaminants.
Most soil test reports will give you a series of suggestions for what sorts of nutrients need to be added. Often, these are based on unrelated crops (usually field crops), so those can be disregarded for the most part, other than getting a general sense of things.
I typically scan a soil test report for the basic information about a soil, such as the depth of the sample, soil texture, pH, organic matter, as well as looking for any anomalies and oddities, such as highlighted deficiencies, high salinity, or anything that seems high or low. I will also look for general levels of macronutrients and compare those to established recommended requirements for the crop.
Problem solving
You can learn a lot from a soil test, but not every problem that you encounter is due to nutrient deficiencies or excesses, so not every problem you encounter in a crop can be solved by throwing fertilizer at it. That being said, when issues that are likely linked to nutrients are identified, a soil test is a good place to start gathering information and formulating solutions.