7.2: Ideal Soil Test Numbers

One of the first steps in understanding how to read and interpret your own soil test report is to know what an “ideal” soil looks like. In this section, we will review averages for soil pH, Nitrogen (N), Organic Matter (OM), Phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), Cation Exchange Capacity (CEC), and percent base saturation. If your soils are a lot higher or a lot lower than the ideal numbers shown here, then you will probably consider taking corrective actions by amending the soil and applying appropriate fertilizers or amendments. Recommendations for action are presented in another section; here we concentrate on the ideal or average numbers you are looking for in a soil test report. In the remainder of this chapter,  we discuss in more detail testing methods, how to read the results, and what the numbers mean. Use this section as a more simple 'cheat sheet' for appropriate soil test values.

pH

This should generally be between 5.8 to 6.8. If your results are lower, your soils are probably too acidic and will require liming. Here is a list of the target pH for several crops important in the upper Midwest of the U.S. according to University of Wisconsin recommendations. More crops are listed in Publication A3030.

Table \(\PageIndex{1}\): Target pH for Crops in the Upper Midwest

Crop	Ideal pH	Crop	Ideal pH
Corn, silage, or grain	6.0	Pasture; grass or legume/ grass mix	6.0
Soybeans	6.3	Wheat	6.0
Most vegetable crops	5.8 - 6.0	Red Clover	6.3
Blueberry, cranberry	4.5	Alfalfa	6.8

Some soils (in northeastern Wisconsin for example) have a naturally occurring high pH due to underlying parent rock like dolomitic limestone. If you are in a high-pH soil area, do not add lime (calcium carbonate).

A typical university soil test report may show you the actual pH number, or it may just indicate if the soil pH is high, low, or just right in the context of the most pH-sensitive crop you submit on the information sheet.

Nitrogen (N)

Nitrogen is a very expensive input so farmers and gardeners should know what an ideal amount is in the soil, right? Well, it turns out that soil tests don’t measure nitrogen for a variety of reasons. The main reason is that nitrogen is a “changeling” – it exists in a variety of forms that are hard to test for and measure. Also, synthetic N fertilizers can’t be stored in the soil well; they may leach into the groundwater or volatilize or denitrify. Rather, the nitrogen numbers you see on most University soil test reports are simply suggested amounts based on likely crop responses to a standard amount of fertilizer.

Organic Matter

Organic Matter is another way to think about how much nitrogen and carbon an ideal soil should have. Biological and organic farmers should look at the percent organic matter (OM) on a soil test to get an indication of the nitrogen potentially available from within the soil itself. The higher the percent organic matter, the better. Though organic matter is a measure of soil carbon, it is also a reflection, generally, of how much soil life your fields or garden have. Soil life (soil livestock) is a source of slow-release nitrogen when their bodies die. Organic matter also increases other nutrient-holding capacity.

Organic matter amounts depend on farming practices and can be increased with good long rotations of hay, pasture, or cover crops and the addition of compost or manures. Negative influences on organic matter include tillage and salt-based synthetic fertilizers. Soils that have been row-cropped for a long time without much attention paid to returning organic matter to the soil will be in the 1.5%-2.0% range. Well-tended organic fields have OM in the 2.5% – 6.0% range. An ideal soil on a farm that has been farmed organically for a long time should have at least 3.5% organic matter. This level may not be possible at all on sandy soils or other similar soil types.

Phosphorus (P) and Potassium (K)

Phosphorus and potassium amounts are reported in parts per million (ppm) on soil tests. According to the University of Wisconsin, the amount of fertilizer needed to increase the ppm of P or K to the desired level is dependent on the subsoil fertility group. Including your soil name on the paperwork you submit with your sample is important for this reason. Most land grant university lab results are reported on a simplified scale of recommendations that tell you if, comparatively, the soil is very low (VL), low (L), optimum (O), high (H), very high (VH) or excessive (Ex).

Here is what optimum P and K levels look like for subsoil fertility group A.

Table \(\PageIndex{2}\): Optimum Phosphorus (P) and Potassium (K) Levels for Subsoil Fertility Group A

Subsoil Fertility Group A (naturally high in P, low in K)	Ideal P (ppm)		Ideal K (ppm)
Crop to be grown:	UW	Soil Bal.	UW	Soil Bal
Corn	11-15	50-100	81-100	100-350
Soybeans & low-demand field crops	6-10		81-100
Alfalfa & low-demand veg crops	16-23		91-120
Red clover & medium-demand field crops	16-20		71-100
High-demand vegetable crops	31-45		121-180
Potato	161-200		121-160

Remember, to find out your subsoil type you will need to know your soil name (see the last section). Laboratories that use the soil balancing approach tend to give a value for ideal P or K without regard to the subsoil fertility or the crop to be grown. The two labs I know of that use the “Soil Balancing” method are the Midwest BioAg lab and a Neal Kinsey-related lab – their average numbers are listed in the table.

Sulfur (S)

This is another element that, like nitrogen, is “slippery” and moves out of the soil profile as well as changing forms. Sulfur enters the system in applied manure, and (in previous years) as free fertilizer in the form of acid rain. It had been released into the atmosphere as a contaminant from coal-fired power plants--it was free--except it was killing lakes in the northeast U.S. because it was too acidic. Sulfur is present in the subsoil and is also released by organic matter in the soil. All these available forms should total something like 3-40 ppm in an ideal soil according to the UW. Soil balancing proponents suggest 50 ppm as an ideal. Most farmers in Wisconsin do not add S fertilizers. Many organic farmers feel that they do get a crop response so they add some sulfur fertilizers.

In general, if you as a farmer or grower, feel like you get enough of a yield increase to cover the cost of adding a fertilizer, then go ahead and do so. Just be aware that with N and P, if you go over the recommendations, you may inadvertently be adding to environmental pollution.

Calcium (Ca)

This is where the UW ideal and the soil balancing ideal numbers part company. There are conflicting theories about the interactions and relationships between soil calcium, potassium, and magnesium; this influences how much Ca is deemed “adequate” by the two different schools of thought. Suffice it to say for now that the UW staff indicate 400-600 ppm Ca is about right for sandy soils while a value of 600-1000 ppm Ca is ideal on loams, silts, clays, organic soils, and “red” soils. Interestingly, until just recently, the University faculty argued that Ca was almost never a limiting factor in plant growth and therefore was not considered an important nutrient for which to test. Since flue-desulfurized gypsum (a waste product from the coal burning plants’ chimney scrubbing) has become available, more and more farmers are applying that product to boost Ca. It is not allowed in organics.

The Soil Balancing professionals, on the other hand, look for ideal Ca numbers in a wide range between 700-2700 ppm, depending on which lab is checked. Some soil balancers are not as concerned with the actual ppm of Ca as they are with the amount of calcium occupying or saturating the cation exchange sites. Remember that this term is called “base saturation”. For soil balancers, the percentage of cation sites that should ideally be occupied by calcium is 70-75%. Anything less than that likely means, in their theory, that not enough calcium will be plant available.

Magnesium (Mg)

The ideal amount of Mg, according to the University, is 50-250 ppm on sandy soils and 100-500 ppm Mg on loams, silts, clays, organic soils, and “red” soils. One soil balancing source gives a range of 110-190 ppm as ideal. Once again, balancing advocates prefer to suggest that an ideal % base saturation is 12-15%.

Boron (B), Manganese (Mn), Zinc (Zn), Copper (Cu) and Iron (Fe)

These are all considered micronutrients, and most farmers and gardeners don’t test the soil for them. Some organic farmers do test for B, Mn, and Zn, and Cu. The University publication A3030 states that presently there are no soil tests for Cu and Fe calibrated for Wisconsin because deficiencies are just so rare. In any case, here are the “ideal” numbers I found listed in the literature and important to soil-balancing advocates:

Table \(\PageIndex{2}\): Ideal Numbers

B	Mn	Zn	Cu	Fe
1-2 ppm	11-20 ppm on low pH soils	3-20 ppm	2 ppm	20 ppm

Cation Exchange Capacity (CEC)

This is a function of the parent material of your soil, just like texture. There is not a lot that one can do to increase the CEC of soil, except perhaps to add organic matter to sandy soils. I’m not sure if it’s possible or desirable to decrease the CEC of soil. In general, expect the following CEC levels. The units are milliequivalents (mEq)/100 grams of soil.

Table \(\PageIndex{3}\): CEC Levels to Expect

Muck to high OM soil	Clay to clay loam	Silt loam to loam	Sandy loam to sand
200 - 30	25 - 20	15 - 10	10 - 5 - 1

Percent Base Saturation

As mentioned earlier, base saturation is a poor name. It is confusing. What it means is “how many cation exchange sites are occupied and by what kind of cations.” It is expressed as a percent of the occupied sites. It must total 100%. It is a calculation, not an actual test result number. The University does not report it. Soil balancing theorists suggest the following ideal “occupation”:

Table \(\PageIndex{4}\): Ideal Percent Base Saturation

% K	% Ca	% Mg	% H	% Na (sodium)
3-5	70-75	12-15	10	1

If you use a soil testing lab that uses the soil balancing approach, these are your ideal % base saturation numbers.

Summary

Using the values found in this section, you should be able to compare your soil test reports with the numerical values you get from the lab. You’ll be able to see if you are too low, or too high. Then, you’ll be better equipped to make choices about what fertilizers or amendments to put on your crops. You can work with your crop consultant or agronomist to make sure you both understand your numerical results. The more you know, the smarter you’ll be about adding inputs and adding to your bottom line.