By Kristine Adel
Nine Pink Kangaroos?
New Pokadot Knickers?
Nope it’s a soil test: Nitrogen, Phosphorus, and Potassium. (I know, there are no ‘k’s in the word potassium. It’s just weirdness. Gotta keep moving.)
TESTING
Why test your soil?
If you’re not a commercial farmer, It’s tempting to just buy a bag whatever they recommend at the hardware store and hope for the best. Or, cover with lime because – that’s what everyone says to do?
This can be harmful for several reasons:
Too much of the wrong nutrients can damage or kill the plants you’re trying to encourage.
Your soil may have plenty of nutrients, but your plants can’t absorb them if the pH is off.
Adding lime or sulfur without knowing your soil conditions, or how much is needed, can create more problems rather than solving them. Up to 1/3 of garden soil samples are excessively alkaline due to the indiscriminate application of lime.
Your soil will probably be unable to store the excess nutrients, resulting in runoff that can pollute streams and other waterways with algae blooms and fish die-offs.
Or the excess may leach into the same underground water table that our wells draw from.
Fertilizer, lime and other supplements can be expensive.
A SOIL TEST IS EASY
The cost is $10 for a basic test, although you can add a test for soluble salts for $2 more, or get the percentage of organic matter for an additional $4.
Go to soiltest.vt.edu. Using the menu (the three stacked bars in the upper right hand corner) You can download forms and instructions. You can also call or visit the Patrick County VA Cooperative Extension in Stuart (https://patrick.ext.vt.edu) for forms, instructions, and lab sample boxes.
FORMS
Most non-commercial growers will use the form: Soil Sample Information Sheet for Home Lawns, Gardens, Fruits, and Ornamentals
The form requires the usual name, date, and address information at the top. Then it asks for a sample identification number. This is a number you make up to identify your sample. You will use the same number on the form and the sample box to make sure the sample is correctly identified and processed.
Sample identification numbers are especially useful if you are sending in more than one sample to test, for instance, separate samples (with a separate form for each) for the vegetable garden, the lawn, and the problem area out back where the grass won’t grow.
You can label your samples 1, 2 and 3. Or 15, 692, and 5041. The numbers don’t matter as long as the number on each form matches the number on its corresponding sample box, and – as long as you remember which one is for the garden, which one is for the lawn, etc. So be sure to write it down if you are getting multiple tests.
The form will also ask for a code indicating what you are growing, or intend to grow. A list with the codes to choose from is provided on the form. Example: code 210 Vegetable Garden.
Check the boxes for the tests you want, add up the payment.. and… Time to dig up some dirt.
THE SAMPLE
The ideal tool is a soil sampling probe, or soil sampling tube. If you don’t have one and don’t know where to borrow one, you can use an ordinary clean spade to dig and an ordinary clean bucket to mix.
You will want to take several ‘mini-samples’ from your sample area that will be combined and mixed to make up the actual test sample that goes into the lab sample box.
The ‘mini-samples’ will need to be spaced randomly to get a representation of the whole sample area. A large field may need as many as ten ‘mini-samples’. A vegetable garden may need 4. Remember to check the instructions for more details.
You will want to avoid sampling areas that may be uncharacteristic of your sample area like compost areas, manure piles, old road beds, etc.
Each ‘mini-sample’ plug will need to be the same size, digging straight down to a depth of approximately 6 inches. Again, check the instructions for more information on obtaining a good sample.
Be sure to remove any grass, leaves, bugs, sticks, roots, rocks, and such before you put each ‘mini-sample’ into the mixing bucket. Let it dry, and mix thoroughly.
The sample is ready to box, lable, and send to the lab with your forms and payment.
RESULTS
In a few weeks you will get an email detailing your results and recommendations for supplementing your soil in accordance to the needs of the plants you intend to grow.
The following is a brief explanation of what your results indicate.
A NOTE ABOUT pH
It is important to keep in mind that pH is crucial to plants being able to use the nutrients in the soil. A pH between 6.5, slightly acidic, and 7.5, slightly alkaline, is the range that best enables plants to uptake all of the necessary nutrients available in the soil. Too high or too low can result in important nutrient deficiencies.
How much difference does it make? The pH scale is logarithmic. Each number on the scale increases by a factor of 10. So a pH of 8 is ten times more alkaline than a pH of 7 (the neutral midpoint), and a pH of 9 is 100 times more alkaline than a pH of 7, and so on.
NITROGEN
You will notice that there is no test result for nitrogen.
N – nitrogen is highly mobile in the system. Plants in the legume family might be getting it out of the air and fixing it in the soil, but most nitrogen is coming from microbial activity breaking down organic material into forms that plants can use – or the application of commercial fertilizers.
At the same time, however, nitrogen is constantly being leached out of the soil by rainfall and / or converted back into forms that can release to the atmosphere.
The amount of nitrogen available in a form that plants can use at any particular time is difficult to pin down. The recommendation for nitrogen supplementation will therefore, be based on the needs of the crops you intend to grow, rather than a measurement from the soil.
There are many ways to reduce the need for supplemental nitrogen. Maintaining a good percentage of organic material 3% – 6% can help. Practices like no-till, low-till, mulching with organic materials, leaving plant residues in the field, and maintaining a mix of cover crops like legumes and cruciferous plants are excellent ways to naturally maintain a slow release of nitrogen in your soil.
None the less, harvesting your crop will still remove substantial amounts of nutrients and supplementation may be necessary to maintain annual yields.
MACRO-NUTRIENTS
The first 4 results are rated from Low to Very High. These are the macro-nutrients: phosphorus, potassium, calcium, and magnesium.
Recommendations for supplementation are only given if the rating is Low to Medium. These results will be measured in pounds per acre.
*Please note that a chart of conversion factors will be included at the end of this article to aid in reading results and recommendations.
P – Phosphorus is needed by the plant to store energy created from photosynthesis and carbohydrate metabolism to be used for growth and reproduction. Phosphorus is not as naturally abundant in the soil as other macro-nutrients and it is relatively immobile.
Even so, over-supplementation will almost always lead to leaching and runoff as the soil has limits to how many nutrients it can hold. This limit is measured in the Cation Exchange Capacity, explained in more detail further down.
K – Potassium is necessary for many plant biological processes. Most soils contain potassium in large quantities, although it is not always available due to a range of factors like moisture, temperature, and microbial activity.
Ca – calcium helps regulate soil pH and enhances the uptake of nitrogen and many other nutrients. Calcium is an abundant mineral in most soils. Deficiency can occur in acidic soils with a pH less than 5.
Mg – magnesium is essential for photosynthesis. Deficiency is not common in the United States, though it can occur in highly weathered, acidic, and coarse soils.
Supplementing soil with agricultural lime to raise the pH will also add calcium, as agricultural lime is composed of calcium carbonate. Supplementing soil with dolomite lime to raise the pH will add both calcium and magnesium as both minerals are present in the dolomite lime – calcium magnesium carbonate. Most studies concur that an appropriate ratio of calcium to magnesium in the soil for balance is at least 2:1, but can be greater.
S – not reported – Sulfur is also considered a macro-nutrient, but like nitrogen, it is highly mobile and difficult to access. Therefore it is not measured on the test. Deficiency, if it occurs, is most likely to occur in high yield commercial crops. When it is suspected, tissue sampling of plants is the most reliable testing method.
MICRO-NUTRIENTS
The next results will be for 5 important micro-nutrients: Z – zinc, Mn – manganese, Cu – copper, Fe – iron, and B – boron. These are measured in parts per million and rated as either sufficient or insufficient. Supplements are rarely recommended for micro-nutrients since the most common issue is inhibited uptake due to a pH imbalance. Good results can often be obtained by adjusting pH.
FUN FACT
One acre of dirt to a depth of 6 inches (an acre furrow slice) weighs about 2 million pounds. Therefore, if your measurement for, say, the micro-nutrient zinc, comes back as 1 part per million, your acre contains approximately 2 lbs. of zinc. Not a lot for such a big area.
SOIL pH AND THE BUFFER INDEX
As mentioned before, pH affects the availability of all nutrients to the plants and is of primary concern for plant health and productivity. The pH of acidic soils can be raised with agricultural lime or dolomite lime depending on the recommendation. Alkaline soils can be treated with elemental sulfur.
But there is more to determining the quantity of treatment needed than a simple pH measurement. The Buffer Index is a measurement of how resistant your soil is to these treatments. The lower your buffer index is, the more resistant the soil will be to change, and the more treatment it will require. Moreover, change in pH is a slow process that can take 3 months or more. You will want to plan ahead, using your results, on the type of treatment, the recommended amount, and the timing of your application if a pH imbalance needs correction. It may be best to wait until fall and treat for the next coming year.
CEC – CATION EXCHANGE CAPACITY
CEC is a measurement of how many nutrients your soil can hold before they begin to “stockpile” with risks of leaching and runoff polluting the surrounding water systems. A CEC above 10 milliequivalents per 100 grams (10 meq/100 g) is considered adequate.
ORGANIC MATTER – OPTIONAL TESTING
OM – Organic matter affects many biological, chemical, and physical properties that influence soil nutrient availability. A general guideline is to reduce N recommendations by 20 lb/acre for soils with >3% OM and increase N recommendations for soils with <1% OM.
SOLUBLE SALTS – OPTIONAL TESTING
High soluble salt content can cause water stress and other issues especially in seedlings. Sampling should be considered if high rainfall and / or irrigation have increased the risks of leaching.
RECOMMENDATIONS
The numbers on the fertilizer labels represent the percentage of the 3 standard nutrients in the product. They are always presented in the order of N, P, K.
For example: if the label says 18 – 46 – 0, it contains 18% nitrogen, 46% phosphorus, and no potassium. Or, put differently, 100 pounds of that product would contain 18 pounds of nitrogen, 46 pounds of phosphorus, and no potassium.
Converting your recommendations, which are given in pounds per 1000 square feet, into a determination of how many pounds of a particular product you will need for your area may require a calculator and a measuring tape.
The Conversion Factor Table below should help. If you’re not sure, or you just want confirmation, you can call or visit the Patrick County VA Cooperative Extension in Stuart (https://patrick.ext.vt.edu) for assistance.
Don’t forget to tune into WHEO Tuesday morning 8:30 a.m. with Patrick County Master Gardeners for more on SOIL TESTING.
Mark your calendar for the PCMGA Annual Spring Plant Sale on Saturday, April 25, 8 a.m. – noon, at the main Rotary Building, 264 Woodlawn Dr., Stuart.
Find us on Facebook or contact through https://patrickmastergardeners.org/
CONVERSION FACTORS
*Some Values are Approximate
1 acre = 43,560 square feet
1 pound of 5-10-5, 5-10-10 or 10-10-10 fertilizer = 2 cups
1 pound of ground limestone or ground dolomitic limestone = 1.5 cups
1 pound of aluminum sulfate or magnesium sulfate = 2.5 cups
1 pound of sulfur = 3.3 cups 1 quart = 2 pints = 4 cups
1 pint = 2 cups = 32 tablespoons
1 tablespoon = 3 teaspoons
1 bushel = 35.24 liters = 1.25 cubic feet
Pounds per 100 square feet x 0.54 = pounds per cubic yard
100 square feet = 5 feet x 20 feet or 10 feet x 10 feet or 2 feet x 50 feet
1,000 square feet = 50 feet x 20 feet or 10 feet x 100 feet or 25 feet x 40 feet
Pounds per 100 square feet x 436 = pounds per acre
Pounds per 1,000 square feet x 43.6 = pounds per acre
Pounds per acre x 0.0023 = pounds per 100 square feet
Pounds per acre x 0.023 = pounds per 1,000 square feet
