For Those Who Utilize Soil Tests

The second in a series of articles aimed at helping growers improve both crop and livestock nutrition by improving productivity in the soil

In a survey conducted some years ago, farmers were asked whether they believed that soil testing was something that should be used for raising better crops and improving soil fertility levels. 82% answered "yes". When these same farmers were asked if they used soil tests, only 28% said" yes"!

In agriculture today, anecdotal evidence suggests the percentage has improved. To remain profitable in agriculture under present conditions, every farmer and grower should consider that fertility levels must be measured. These measurements can then be used to manage soil fertility to more precisely achieve top production and quality, while still keeping costs at the minimum necessary to meet the goal.

The following information is especially meant for those who already believe that using soil tests is important. It is particularly intended for those who count on such tests to guide them in terms of providing better soil and plant nutrition, and is an attempt to provide additional "food for thought" to those who really work at managing fertility, using soil testing to measure what they have, from farm to farm, field to field, and perhaps even from area to area within a field.

Test your soil tester! There are hundreds of soil testing laboratories, and many different methods used for determining the levels of nutrients in the soil. Which laboratory or which methods are best to use is not debated here. If you have tests that work well for you, use those tests. If you are not sure then test your soil tester! - as we encourage everyone to do, in Hands On Agronomy and in our material Taking A Good Soil Sample.

For those who use soil analysis to manage soil fertility, there are several questions that ought to be addressed:

  • How often should you test to maximize benefits?
  • How many samples are needed to properly represent a field or farm?
  • How deep should a soil sample be taken?

It is surprising to see how many different answers are given to each of these questions. And not all of those answers are correct! Each question will be expanded and discussed below.

How Often Should You Soil Test?

Rather than one general answer to this question, the focus should be on other factors that have to do with the land use, the crops to be grown, and the needs of the operation.

A soil sample every 3 or 4 years will only make possible a somewhat general indication of fertility (the stated purpose for most soil tests that only check for pH, phosphate and potassium) without providing the very detailed analysis required for specific management decisions. This is not an approach we would recommend.

We recommend sampling every year. On the other hand, at times when initial soil test recommendations have not been accomplished sufficiently and further basic work needs to be done as indicated by previous samples, taking another set of tests would not generally be advised. For example, if a test accurately shows a need for lime and it has not yet been applied, spend the time and money on liming, not taking more soil samples. Or, possibly the amount of principal materials required would be more than has been budgeted for fertilizer and it would take 3 or 4 years to accomplish the most needed applications before being feasible to proceed.

Then, too, some people may not believe it is possible to determine the full effects of lime or other materials being applied without waiting several years to be sure it will show up. It does require up to 3 years to measure the full effects from an agricultural limestone application, but if the calcium and magnesium content and the fineness of grind are both correctly determined, it can be pro-rated beneficially over all 3 years, so long as accurate spreading records are kept. (Especially keep records of the source of the lime, the year, the month or season, and the tonnage applied.)

What are the economic benefits? If you are taking soil samples, and have the capabilities to follow through on what is shown to be needed, don't just take soil samples every 3 or 4 years. Instead, stop and analyze the situation. How valuable is the crop? Those who are growing seed crops, or crops of higher value, should test before every crop. One client who owned a seed farm, for example, from the time sampling first began, would sample twice within a 12 month period, when raising a seed corn and a seed wheat crop. We also have commercial vegetable and berry growers who sample to fertilize for their principal crop, and again prior to whatever will be used in the rotation, even if it will be cover crops. Some of our largest clients grow tree fruits and nuts, and they correctly take soil sample every year. Even cotton, corn, soybeans, and wheat producers who are achieving high yields should do the same thing, and many who are clients of ours do just that. One alfalfa grower put it this way, "If I soil test properly, it costs me less than 1 or 2 small square bales of hay per acre. But if I don't test, it can cost far more than that buying fertilizer I really don't need, or through loss of yield for failing to apply something the soil did need"

As long as it is possible to purchase and apply what is required for the crops to be grown, soils should be tested before every major crop. And for plants and crops that are grown over a period of years, soil tests should be taken prior to any major fertilization program that involves materials that are used to build up the soil fertility levels.

Know the soil fertility of your land. Always manage your soil testing work to correctly measure what you are doing, including progress or regression in terms of soil fertility. One of the greatest mistakes in agriculture today is failure to sufficiently use soil tests to properly measure, and correctly mete out, the fertilizers and soil amendments needed for the land.

How Many Samples Are Needed To Properly Represent A Field or Farm?

The next thing to consider is the question of just how many soil samples should be taken from an area, field, or farm. Above all, do not let the number of different fields you have determine the number of samples you will take. That is, do not assume you are doing the job correctly if you take one composite sample per field. Sometimes that may be the case, but too often, it is not.

Where to pull the soil samples. To begin, consider pulling a few samples from good, productive land, some from average land, and some from poor production areas. Then after you see what is involved, depending on the fertilizer budget and timing, additional samples could be taken where the need is evident.

At the beginning decide what should be the smallest area worth taking the time to fertilize separately. Then pull samples on those fields selected for testing, taking samples from any areas which are that large or larger, where visible differences are detectable.

When farmers or growers are only willing to treat whole fields, yet there are observable differences in that field, (in soil types, yield differences, weed populations, large problem areas, etc.), that operation is losing top potential for the field in question. This is another big mistake that is continuously made by farmers, growers, and fertilizer dealer field men. Be sure to pull sufficient soil samples for optimizing soil improvement and yield.

Yield monitoring One of the most revealing new developments in high-tech agriculture for some growers has been computerized yield monitoring on harvesting equipment. Suddenly, producers can see the vast differences from the best to the worst in the same field. Using a detailed soil analysis, this method can then show why these differences are so great - with fertility as the key. Whether in good or poor yielding areas, fertility is generally always a major key, even when other factors are also involved.

Use of the information provided by yield monitors has helped farmers and growers see the importance of doing a more complete job when it comes to taking soil tests and supplying the fertilizer needed for each area.

The longer each person works with the Albrecht Model of soil testing, the more they will realize the cost of losses to those who skimp on properly testing the land. This means those producers are also skimping to some degree in evaluating actual fertilizer needs, and yields will suffer as a result. Again - proper measurement is required to adequately manage for top yields and top quality.

How Deep To Sample?

The proper depth needed for an accurate soil sample will depend on several factors. If the soil will not be worked at least 2 inches deep, sample only the top 4 inches. This would be true for pastures, hay meadows, no-till crops, orchards, vineyards, lawns, golf courses, etc. Of course, many will disagree when it comes to hay meadows, and even more so for orchards or vineyards! But the determining factor here should be the depth that materials can be expected to saturate between soil samplings. This is one big reason why we advocate sampling every year. Four inches is best in all these cases, even if sampling is not done every year.

This depth avoids the chance of applying excessive amounts of materials in any given area where the plant will be taking up nutrients, thus ensuring that exceeding the "law of the maximum" (which means robbing plants of certain nutrients due to the inhibiting effect of excessive levels of others) will not happen.

For soils that will be worked or tilled, the proper sampling depth is as deep as a fence post rots. Generally this is around 6 1/2 - 7 inches deep. We use 6 3/4 inches as an average, since an acre of soil to that depth weighs approximately 2 million pounds.

Soil will usually tend to drop in terms of fertility the deeper you go. So samples taken too deep make it appear you need more fertilization than is truly the case. But in cases that matter, consider taking subsoil samples in at least some of the most important areas, to accurately measure the general levels of nutrients deeper down in the soil

When to sample deeper There are times when samples need to reflect what is below a 4 inch or 6 1/2 inch depth. For example, iron may be deficient in the top few inches, but present in sufficient amounts below that level. In such cases, take the top 4 inches as one sample and then a subsoil sample to the depth desired below that level. For example, some areas that will be planted to trees or wine grapes will be sampled as the top 6 3/4 inches, then 6 3/4 - 12 inches, 12 - 18 inches, 18 - 24 inches, 24 - 30 inches, and 30 - 36 inches.

This is especially important when soils will be worked unusually deep, or roots are expected to draw nutrients from depths below the topsoil layer. In addition, be sure to sample down to the depth of complete nutrient incorporation capabilities. This should be figured at half the depth to which the soil will be worked. For example, if an offset disk will be going 36 inches deep, figure the top 18 inches will need to be sampled and properly corrected for an adequate "balance" to that depth to be maintained, and for maximization of biological activity.

Again, keep in mind that based on extensive work over the years, it is true that soils tend to be lower in overall fertility the deeper you go. But there are also soils that have higher nutrient levels below the portion normally tested. The only way to be certain is to take subsoil samples, at least in key areas, to determine the actual levels.

In Conclusion:

We recommend that under normal circumstances farmers and growers should sample every year. Pull a separate sample from each area of a farm or field which shows differences of any type, so long as the area is large enough to fertilize separately. And sample to a proper depth, depending on whether the soil will be worked, and to what depth.

For additional information on taking soil samples see Taking Soil Samples also our book Hands On Agronomy.

- Neal Kinsey