How to Read Your NC Soil Test Report
The soil test is free. Reading it is the hard part.
An NCDA&CS report arrives as a grid of abbreviations — HM%, W/V, CEC, BS%, Ac, P-I, K-I, Mn-I, Zn-I — and most people glance at the lime recommendation and file the rest. That is a shame, because two of those numbers decide whether every bag of fertiliser you buy is two-thirds wasted.
The structural key, in one sentence
NCDA&CS states it plainly, and it is the most useful sentence on their site: "The first seven values — soil class, HM%, W/V, CEC, BS%, Ac and pH — describe the soil and its degree of acidity. The remaining values are nutrient levels."
So the report has two halves. The first half describes the ground. The second half describes what is in it.
The index scale nobody defines for you
The nutrient values are indexes, not concentrations, and the scale runs 0 to 100. The Orange Book: "The critical quantitative value for each nutrient is assigned an index of 25." From there, "Values of 25 or below indicate low soil fertility… Values from 26 to 50 indicate medium fertility; those above 50, high fertility." Above 100 is excessive, and crops grown on such soils show no response to added nutrient.
But the line you actually read against is not the same for every nutrient, and no single NCDA&CS page states all of them together.
P-I (phosphorus) and K-I (potassium): sufficient at 50, with a published optimum band of 50 to 70.
S-I, Mn-I, Zn-I, Cu-I (sulfur and the micronutrients): "Indexes greater than 25 indicate that those elements are likely sufficient for plant growth."
Calcium and magnesium: not indexes at all. This is the trap in the middle of the row.
The magnesium rule you can actually run
The Orange Book states it directly: "Levels of calcium (Ca) and magnesium (Mg) are expressed as percentages of the CEC, not as an index." So Ca% and Mg% sit in the middle of a row of index values and are read by a completely different rule.
NCDA&CS gives one clean piece of arithmetic: "To evaluate the magnesium status of the soil, multiply Mg% by the CEC." And: "If this value is equal to or greater than 0.5 meq, no magnesium is needed."
Worked example. Mg% of 8 on a CEC of 7.5. Multiply: 0.08 x 7.5 = 0.6 meq. That is above 0.5, so no magnesium is needed — which also means you should buy calcitic lime rather than dolomitic. That single calculation is the whole basis of the lime-type decision.
There is no parallel numeric rule for calcium. NCDA&CS addresses calcium through the lime recommendation instead.
Where the lime number comes from
NCDA&CS publishes the actual equation, which is unusual and worth knowing:
tons lime per acre = Ac x [(target pH − current pH) ÷ (6.6 − current pH)] − RC
Ac is exchangeable acidity from your report and RC is residual credit from lime you applied previously. RC decays at 8 percent per month on mineral soils and 16 percent per month on mineral-organic and organic soils.
Notice what drives it: Ac, not pH alone. Two yards at the same pH can get very different lime recommendations because their exchangeable acidity differs. That is why the number is not something you can eyeball from a pH meter.
Target pH is set first by soil class — mineral 6.0, mineral-organic 5.5, organic 5.0 — then adjusted for what you grow. Azalea, rhododendron, camellia and mountain laurel want 5.0 to 5.5. Centipedegrass wants 5.5. Most shrubs and shade trees want 6.0. Fescue, bermudagrass, zoysia, berries and most vegetables want 6.0 to 6.5. Rose and tomato want 6.5.
The number that keeps you out of trouble. NCDA&CS Note 4, verbatim: "Do not surface-apply more than 50 lb of lime per 1000 ft² at any given time." If your report calls for more than that on an established lawn, it goes on in split applications, not all at once. That cap is written specifically for surface application; NCDA&CS publishes no cap for incorporated lime.
And lime is slow. NCDA&CS: "With adequate soil moisture, lime begins to react immediately, and pH changes should be evident within a couple of months. However, it takes about 6 to 12 months for the total benefit." Limed ground in September is not limed ground in October. Plan it ahead of the season you actually care about.
Turning the recommendation into a bag at the store
Your report tells you pounds of nutrient. The bag tells you a percentage. NCDA&CS publishes the rule: "The number of pounds of the nutrient you want to apply per unit area divided by the grade percentage gives the amount of fertilizer to apply."
Their worked example on 10-10-10: to apply 1 lb of nitrogen per 1,000 ft², 1.0 ÷ 0.10 = 10 lb of product per 1,000 ft².
NC State's worked example on 24-6-6, scaled to a whole lawn: "To find out how much total product it takes to apply 1 pound of nitrogen, divide 1 pound by 0.24. 1 ÷ 0.24 = 4.17. If your lawn is 5,000 square feet, multiply 4 pounds by 5. The result is 20."
Those two together are the whole method. Pounds of nutrient wanted, divided by the decimal on the bag, gives pounds of product per 1,000 square feet; multiply by your area in thousands of square feet.
Standard nitrogen rate from the homeowner page and Note 4: 1.0 lb of nitrogen per 1,000 sq ft, except centipedegrass at 0.5 lb.
The line that saves people the most money
The NCDA&CS homeowner page states that when P and K levels exceed 50, only nitrogen is recommended.
A great many Piedmont yards that have been fertilised for years have P-I and K-I comfortably above 50. If yours does, every bag of 10-10-10 you buy is two-thirds wasted — and the phosphorus you do not need is the part most likely to end up in a creek.
Read those two indexes before you buy a balanced fertiliser. It is one line on a free report.
Taking the sample, and the December trap
A report is only as good as the sample, and NCDA&CS publishes sampling instructions in two places that do not agree. The web page says 8 to 10 random locations per sample and 6 to 8 inches for a vegetable garden. The homeowner PDF says 12 to 15 locations and 6 inches. Both say 4 inches for established lawns.
Where they disagree, take the higher core count and the deeper garden sample — 12 to 15 cores at 6 to 8 inches. Both figures sit inside published NC guidance, and more cores is simply a better composite.
Combine the cores in a clean plastic bucket, break up the clumps, mix thoroughly, and fill the box to the red line. Avoid brass, bronze or galvanised tools — galvanised metal is zinc, and Zn-I is a field on your report.
One sample per distinct area: front lawn and back lawn separately, one for the vegetable garden, and separate samples for beds that look or perform differently. Averaging a good bed with a bad one gives a recommendation that is wrong for both.
And a rule with real teeth on Piedmont clay: "Do not collect soil when it is too wet. If the soil is too wet to till or plant, it is too wet to sample."
The peak-season fee, which catches people every December. Soil tests are free to North Carolina residents from 1 April through late November. Samples arriving at the Agronomic Division lab in Raleigh on or after 1 December carry a peak-season fee of $5 per sample, and reports are not released until paid. To get the free test, the sample must be physically present at the lab by 5:00 pm on 30 November — no exceptions.
The fee and its start date change annually. Confirm at ncagr.gov before you post the box, and if you can, sample in October.
One more limit worth knowing: heavy metal testing is not available to homeowners or community garden growers. If you are worried about lead in an old urban lot, this is not the test that answers it.
A method note
NCDA&CS analyses P, K, Ca, Mg, S, Na, Mn, Cu and Zn by Mehlich-3 extractant using ICAP, and determines CEC by summation of basic cations. If a nursery or a lawn company quotes you a soil number from a different extraction method, it is not comparable to this report.
The full guide
This post covers the index scale, the lime equation and the sampling rules. The complete How to Read Your NCDA&CS Soil Test Report walks every field on the form in order — soil class, HM%, W/V, CEC, BS%, Ac, pH and all nine nutrient values — with the NCDA&CS wording each interpretation rests on, the full target-pH table, the calcitic-versus-dolomitic decision, incorporation depths, and the places NC's own documents disagree.
When the number is not the problem
Plenty of thin lawns and failing beds in Waxhaw, Weddington, Marvin and south Charlotte test perfectly fine. They fail because the soil is compacted, or because water stands where roots need air, and no amount of lime fixes either one.
If your report comes back unremarkable and the plants still struggle, that is usually a grading and drainage question. That is the work we do. You can request a consultation here.
Alex Purdy, Carolina Terrain. NC Landscape Contractor License CL.1872. Compiled from NCDA&CS Agronomic Division and NC State Extension publications, written for the North Carolina Piedmont. Soil test indexes and units are state-specific — nothing here transfers to another state's lab report. I am a licensed contractor, not a professional engineer and not a registered landscape architect.
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