You know how it is when you go to a meeting and want to write something down, or when someone calls on the phone and you can't find a piece of paper to write your note on, and you finally end up writing on the back of an envelope. Fortunately, we always seem to know where we put that envelope when we need it. Well, here is some information for the back of that envelope, information that might prove to be very useful and which you might wish you could find later, soil test information.
Several weeks ago a group of us were involved in a multi county cropping systems program for farm managers in the north central part of the state. My assignment was to provide information about critical soil test levels, primarily for dryland what production. In the process of preparing the materials for that program, I searched out some information that I thought might be useful and helpful as producers begin to make management decisions for this year's cropping program.
This first set of data is from booklet Fertilizer Guidelines for Montana, MSU Extension Service Bulletin EB104, by J. J. Lichthardt and J. S. Jacobsen and is a list of Critical Soil Test Levels. In the following list, the nutrient is listed, followed by the normal range we usually find in the soil and the critical level, i.e., ideally the soil test should be equal to or greater than this value.
|
Nitrate-N |
0-30 lbs/a |
(no critical level) |
|
Phosphorus (Olsen P) |
0-40 ppm |
18 ppm * |
|
Potassium |
0-500 ppm |
250 ppm |
|
Calcium |
0-1000 ppm |
400 ppm |
|
Magnesium |
0-100 ppm |
40 ppm |
|
Sulfate-S |
0-50 ppm |
10 ppm |
|
Zinc |
0-15 ppm |
0.5 ppm |
|
Manganese |
0-10 ppm |
1.0 ppm |
|
Copper |
0-1 ppm |
0.2 ppm |
|
Iron |
0-10 ppm |
2.5 ppm |
|
Boron |
0-10 ppm |
1.0 ppm |
|
Molybdenum |
0-2 ppm |
0.1 ppm |
|
Chloride |
0-50 ppm |
20 ppm |
*The level of a nutrient below which crop yield, quality or performance is unsatisfactory.
Generally, when someone calls MSU and starts asking about soil quality, soil testing, and critical values, soil fertility status is only one of the topics that might get addressed. There are some other parameters that can be measured which provide a good index of soil conditions. For instance:
*Organic matter: The expected range in organic matter in Montana soils is 0 to 4%; seldom do we see samples with values as high as 4%. In fact, most samples range between about 0.8% and 1.5%. Ideally, an agricultural soil should have an organic matter level greater that 2%. This comes about by: 1) intensifying cropping patterns, 2) reducing the amount of tillage and fallow, 3) incorporating green manures, 4) using diversified crop rotations.
*Electrical conductivity: Sometimes referred to as conductance, other times as specific conductance, the range that we usually find in Montana soils is 0 to 4 mmhos/cm (millimhos per centimeter). This is an index of the salt content of the soil and ideally this value should be less than 2.4. Most Montana soils range from about 0.8 to 1.6 mmhos/cm.
*Cation exchange capacity (CEC): This is a measure of the ability of the soil to hold, release, exchange the positive molecules (calcium, magnesium, other cations). In most soils this is ranges from 0 to 20 meq/100 g (milliequivalents per 100 grams of soil). This is an index value and I generally say that if the CEC is low, about all you can do is tread water, i.e., manage from year to year. There are some things you can do to increase the CEC including increase organic matter.
*Sodium absorption ratio: This is a ratio of sodium to calcium and magnesium in the soil (a little more complicated that a simple ratio) and ideally should be less than 10. The critical threshold value is 15, i.e., soils with SAR greater than 15 are generally considered 'sodic' or high in sodium.
Categories: Soil Testing, Twin Bridges
Date: 1998