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Crop Fertilization Based on North Carolina Soil Tests
Part I:  NCDA&CS's Approach to Soil Testing

Fertility is not the only factor that limits yield. Soil pH, soil moisture, planting dates, crop varieties, weeds, insects, diseases, nematodes, soil physical conditions and other variables also limit production. Therefore, the goal of Agronomic Division soil test recommendations is not to achieve a specific yield but to prevent fertility from being a yield-limiting factor. Additionally, soil test recommendations help curb excessive nutrient application, which is both economically and environmentally unsound.

Lime and fertilizer recommendations from the Agronomic Division are based on analysis of the soil, the cropping history provided by the grower, field studies and typical climatic conditions. The sample information form that a grower submits with soil samples appears in Figure 1.1. The soil test report generated from those samples appears in Figure 1.2 .


Volumetric Soil Testing

The Agronomic Division analyzes soil samples by volume, not by weight. Why? The answer is because plant roots obtain nutrients from a volume of soil regardless of the soil's weight per unit volume. Volume-based analyses are thus the most accurate means of assessing nutrient status in the field plow zone.

Volumetric testing maintains a constant ratio between soil volume and extracting solution, regardless of the soil's weight/volume ratio. This ratio can vary widely, as in sandy versus clay soils. Procedures based on soil weight use the same amount of extractant regardless of soil volume. Results based on such a process can produce a misleading picture of nutrient availability in the root zone. Having determined that volume is a more useful indicator than weight for determining fertilizer recommendations, the Agronomic Division maintains a fixed soil-volume/extractant ratio and allows the soil-weight/extractant ratio to fluctuate.


The Soil Analysis

NCDA&CS soil tests evaluate 22 factors. The first seven [soil class, HM%, W/V, CEC, BS%, Ac and pH] describe the soil and its degree of acidity. The other 15 [P-I, K-I, Ca%, Mg%, Mn-I, Mn-AI (1), Mn-AI (2), Zn-I, Zn-AI, Cu-I, S-I, SS-I, NO3-N, NH4-N and Na] indicate levels of plant nutrients or other fertility measurements. The nutrients are reported in one of two ways: as a standardized index [for phosphorus, potassium, manganese, zinc, copper and sulfur] or as a percentage of the cation exchange capacity [for calcium and magnesium]. These methods are explained in the next sections.

In most cases, nitrate nitrogen (NO3-N), ammonium nitrogen (NH4-N), boron (B), and soluble salt (SS) contents of the soil are not measured. The circumstances that might justify testing for these factors vary and are covered later in the section Special cases.


The first seven factors evaluated on the soil test report describe the soil and indicate the degree of acidity. These factors are soil class, percent humic matter (HM%), weight per volume (W/V), cation exchange capacity (CEC), base saturation as a percent of CEC (BS%), acidity (Ac) and pH. These data have a bearing on the amount of lime a crop needs.

Soils belong to one of three classes based on HM% and W/V: mineral (MIN), mineral organic (M-O) or organic (ORG). Plants can tolerate more acidity in organic soils than in mineral soils because organic soils contain less aluminum. Therefore, soil class exerts considerable influence on lime recommendations.

Percent humic matter (HM%) is a measure of the portion of organic matter that has decomposed to form humic and fulvic acids. HM% represents the portion of organic matter that is chemically reactive. This value affects determinations of lime and herbicide rates.

The weight-per-volume ratio (W/V) is a good indicator of soil texture. Very sandy soils often weigh more than 1.5 g/cm³, silt and clay loams near 1.0 g/cm³, organics as little as 0.4 g/cm³, and greenhouse media often even less.

Cation exchange capacity (CEC) indicates the extent to which a soil can hold and exchange basic cations such as calcium, magnesium and potassium as well as hydrogen, aluminium, iron and manganese. The CEC of North Carolina soils ranges from 40 or more meq/100 cm³; in clay and organic soils to 2 or less in some sandy soils. A high CEC is desirable because it makes leaching of fertilizer nutrients less likely and the maintenance of high reserve quantities more likely. The CEC will vary as pH changes and as organic matter fluctuates through addition or decomposition.

Percent base saturation (BS%) is the portion of the CEC that is occupied primarily by the nutrient cations calcium, magnesium and potassium. A high base saturation reduces soil acidity levels and increases the supply of other plant nutrients. Therefore, high BS% values are favorable for plant growth.

Exchangeable acidity (Ac) is a quantitative measurement of the portion of the CEC occupied by acidity factors, such as hydrogen and aluminum. The acidity in organic soils ranges from 4 to 8 meq whereas in mineral soils, it ranges from about 0.5 to 2.5 meq/100 cm³. The amount of acidity increases in both soils as pH decreases.

Hydrogen ion concentration (pH) is an index of the active acidity in a soil at an instant in time. Soil pH values range from 3 to 8 on a scale of 1 to 14. Weather, cultural practices and additions of lime and fertilizer cause pH to fluctuate. An extremely low pH interferes with plant nutrient uptake and can even cause root damage.


As explained on the soil test report, the Agronomic Division reports phosphorus (P), potassium (K), manganese (Mn), sulfur (S), zinc (Zn) and copper (Cu) levels as indices. The index scale used for fertilizer recommendation ranges from 0 to 100. The relationship between soil test index and fertilizer requirement is shown in Figure 1.3.

The critical quantitative value for each nutrient is assigned an index of 25. Values of 25 or below indicate low soil fertility, a high fertilizer requirement and potentially dramatic yield increases in response to fertilization. Values from 26 to 50 indicate medium fertility; those above 50, high fertility. Values above 100 are considered excessive and show no response to fertilizer application. Certain micronutrient levels above a 250 index can be detrimental to crops.

The quantity of each nutrient required by different crops may vary greatly. The index, however, provides a common scale for judging nutrient supply and balance in the soil. The relationship between soil test index and fertilizer response is outlined in Table 1.1.

The phosphorus index (P-I) reflects the level of phosphorus found in the soil. Values less than 100 are of concern in predicting the need for phosphate fertilizer. The amount of phosphate recommended depends on soil test level and crop requirement. Actual P-I values are shown on the soil test report.

The potassium index (K-I) reflects the level of potassium found in the soil. Values greater than 100 are of little concern with regard to fertilizer recommendations. However, the total amount of potassium present has a bearing on CEC and BS% values.

Multiple manganese values appear on soil test reports (Figure 1.2).

The first value is the actual manganese index, Mn-I. Two manganese availability indices follow it: Mn-AI (1) applies to the first crop to be grown and Mn-AI (2) applies to the second crop. The Mn-AI is a function of Mn-I and pH based on crop sensitivity to Mn. Table 1.2 and Table 1.3 show grids of Mn-AI values and the formulas used for calculation.

The sulfur index (S-I) that appears on the soil test report may not accurately predict the amount of sulfur available to plants on sandy coastal plain soils. Sulfur leaches from sandy topsoils and accumulates in the subsoil. Hence, subsoil samples provide a more reliable assessment of plant-available sulfur. Prediction of plant-available sulfur is more reliable for clay soils than for sandy soils because they are less subject to leaching. Piedmont and mountain soils generally contain adequate sulfur for optimum yields. Plant tissue analysis in combination with soil testing provides the best means for determining when sulfur is deficient.

Two zinc values appear in the "Test Results" section on most soil test reports. The first is the zinc index (Zn-I); the second is an availability index (Zn-AI) based on soil class. The Zn-AI equals the Zn-I for mineral soils, is 1.25 times greater for mineral-organic soils and is 1.66 times greater for organic soils. The Zn-AI is not relevant for samples from homeowners and does not appear on those soil test reports.

The copper index (Cu-I) gauges sufficiency or toxicity of this element. Only one value appears. See Part II for information on how to interpret Cu-I.

Factors to convert any of these nutrient indices to more recognizable quantitative equivalents are listed in Table 1.4. Notice that some of these equivalents are weights per volume and other are weights per area. NCDA&CS soil tests are conducted on a volume of soil. Conversion to a specific unit per area is based on a depth of 20 cm (7.9 inches). See the footnote to Table 1.4 for further explanation of the volumetric rationale.


Levels of calcium (Ca) and magnesium (Mg) are expressed as percentages of the CEC, not as an index. The Ca determination is made to establish the CEC as well as the relationship between Ca and other cations. Since it is a component of lime, Ca is usually present in the soil in large quantities. The amounts of Ca and K, as well as the acid fraction of the CEC, have a bearing on the need for applying magnesium to the soil. Guidelines for evaluating magnesium are given in Part II. Lime and Fertilizer Requirements.


Predictive soil tests generally do not measure levels of nitrogen (N), boron (B) or soluble salts (SS) in a sample. However, recommendations for nitrogen and boron fertilization appear on the soil test report. These recommendations are based on specific crop requirements under known soil and climatic conditions.

Tests for nitrogen in soil samples from field crops are not routinely conducted because they have limited predictive value. Nitrogen often leaches out of the root zone before the crop is planted. Therefore, there is little point in measuring it.

Nitrate nitrogen (NO3-N) is evaluated routinely for greenhouse or nursery soils. Ammonium nitrogen (NH4-N) is measured under unique diagnostic situations. Test results for both types of nitrogen are expressed in mg/dm³ (ppm).

In the case of boron (B), no reliable soil test has been developed. Plant tissue analysis is the best method to confirm a boron deficiency. Plant analysis also provides a means to gauge the supply of boron in the soil.

Soluble salts (SS-I) are measured in greenhouse and diagnostic samples. The results are expressed in units of mhos×10-5. To convert this value to conductivity measured in deciSiemens per meter (dS/m), divide by 100. Guidelines for interpreting and managing soluble salts are shown in the crop management notes that accompany the soil test report (refer to explanatory tables in Note 9. Soil Analysis of Substrates for Greenhouse Crops).


Sodium (Na) is analyzed on all samples and is reported in meq/100 cm³. Sodium levels above 15% of the CEC can be detrimental to crop production. Such levels usually occur from salt water intrusion or application of waste products high in sodium.

 

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Hardy DH, Tucker MR, Stokes CE. 2014. Crop fertilization based on North Carolina soil tests. Raleigh (NC): North Carolina Department of Agriculture and Consumer Services, Agronomic Division. Circular No. 1.

Last Update January 10, 2014

 

 

NCDA&CS Agronomic Services Division, Colleen M. Hudak-Wise, Ph.D., Director
Mailing Address: 1040 Mail Service Center, Raleigh NC 27699-1040
Physical Address: 4300 Reedy Creek Road, Raleigh NC 27607-6465
Phone: (919) 733-2655; FAX: (919) 733-2837