Installment 8
Should I take a Soil Test?
Our thoughts and recommendations on soil testing
Whether soil testing is a smart thing to do depends on how you approach it. Testing soils requires planning, labor, and money; getting an economic return from soil testing requires a program for implementing and recording the resulting recommendations. Carrying all this information forward so that it can be easily reviewed in years to come adds value to the testing as well. There are as many ways to approach soil testing as there are growers. Below is a general description of 4 broad categories of soil testing regimens. We consider all of them to be valid, depending on the circumstances.
No Soil Testing
It’s too much trouble, costs too much money, and they recommend too much fertilizer! I would go broke following “their” recommendations!
(And anyway, I’ve been doing this long enough – I know what my plants need)
Minimal Soil Testing
Soil testing only to find out the major characteristics of the soil is a common approach. This level of testing might be done on each soil type one time or on an intermittent basis and will still yield some useful information. Because you are only looking for very general information, your sampling technique doesn’t have to be as precise or comprehensive as it would if you were looking for more specific information. So, it doesn’t tell you a lot about the soil, but it still gives some guidance without a lot of cost or effort.
“Moderate” Soil Testing
A soil test on each soil type once a year can be more useful, providing general information about soil fertility and trend information about how the soil is changing over time. Repeating tests in the same area each year also gives you more assurance that your results are a reliable representation of what is in the field. However, since more money and time are being spent, your sampling technique, record keeping, and implementation plan should be correspondingly better.
Note: When testing annually (or more often), it is best practice to sample the same specific area of the field each year to reduce the possibility of introducing additional variability into the results.
Intensive testing
In order to justify spending significant time and money on soil testing, your planning and recordkeeping will be very important. Where moderate testing might include yearly tests only on different soil types, in a more intense program a grower might test any block that is fertilized or irrigated differently. A block might be tested more than once a year if it is being double cropped. A biological test might be performed from time to time. And if a grower is doing intensive soil testing, they are probably doing at least some plant testing and irrigation water testing as well. Lots of information is being generated.
Recap of important points
Soil tests must be taken (sampled) properly for the information to be useful. At least three vertical soil slices of the desired soil depth should be combined into a single ‘sample’.
If the results of a soil test seem suspect, have the lab retest it. Most labs hold back half of your sample for a month or so in case it needs to be retested. Mistakes can happen!
The grower must perform ‘due diligence’ to choose the right lab for the job. The selected lab should use appropriate testing parameters for the grower’s soil type and location and have good quality control measures in place.
A comparison of soil tests is usually much more useful than an individual soil test. At least 2 tests in the same field (at the same time) and/or tests from the same area in successive years yield the best information.
The interpretation of a soil test depends on the relevancy of the information that is used to ‘calibrate’ it and the skill of the interpreter. For best results the interpreter should have familiarity with the soils, irrigation water, and climate. Both the grower and the lab need to compare test results with actual yield and quality results to get the most out of their testing.
The grower should have a plan to ‘do something’ with the test interpretation. Keeping records of tests over the years (and looking at them!) shows trends and can identify unexpected results from fertility programs (and irrigation water quality issues).