|The amount of lime and fertilizer needed for optimum crop growth depends on the specific crop requirement, soil type and current fertility status of the soil. A soil test determines aspects of fertility such as pH, acidity and nutrient levels. This information is prerequisite to calculation of lime and fertilizer requirements.
Lime recommendations are a function of soil class, target pH, current pH, level of acidity (Ac) and residual credit (RC):
- tons/acre lime = Ac × [(target pH –- current pH) ÷ (6.6 –- current pH)] – RC
- (tons/acre) 46 = lb/1000 ft²
- RC = lime rate –- (lime rate × months since application × reduction percentage).
The precalculated factors listed in Table 2.1 make this determination easier. The table provides values for the factor in brackets above, based on current and target pH values.
To calculate the lime recommendation, you need the following information about the variables. Target pH is different for each soil class [Table 2.2 and Figure 2.1]. Current soil pH and Ac values appear on the soil report. RC is the amount of any recently applied lime (tons/acre), reduced by a certain percentage for each month elapsed since application: 8% for mineral (MIN) soils and 16% for mineral-organic (M-O) and organic (ORG) soils. The RC decreases faster for M-O and ORG soils because lime reacts rapidly to the higher levels of acidity in these soils.
Lime recommendations are in units of tenths of a ton per acre. When calculations are less than 0.3 ton/acre, the soil report indicates that no lime is needed. However, the current soil pH may still be slightly lower than the target pH. In this case, application of a standard rate of 0.3 ton/acre or 15 lb/1000 ft² is appropriate.
A special situation occurs when the second crop listed on a soil sample information form requires more lime than the first crop. In this case, the higher lime rate appears as the recommendation for the first crop. No lime recommendation appears for the second crop.
Two types of limestone are available for agricultural purposes: calcitic and dolomitic. Calcitic limestone contains calcium carbonate (CaCO3) but no magnesium (Mg). Dolomitic limestone is a mixture of calcium and magnesium carbonates [CaMg(CO3)2] and contains at least 120 lb Mg/ton. On soils where Mg is difficult to maintain, application of dolomitic lime solves two problems in one treatment.
Although the soil report does not generally indicate soil nitrogen levels, it does provide recommendations for nitrogen fertilization (Table 2.3), depending on the crop. Years of field research and observations have led to these recommendations. The crop notes (see Part III) suggest ways to vary these rates. Plant tissue analysis during the vegetative growth stage is a good indicator of nitrogen sufficiency.
Soil-report-recommended rates for P2O5 and K2O (based on soil-test-index values and the equations in Table 2.4) are shown in Table 2.5. The P-I or K-I value is inserted into the equation, and the results are rounded to the nearest 10 lb for field crops and to the nearest 0.5 lb for greenhouse, golf tees and golf greens. In 2012, NCDA&CS began reporting field crop recommendations for P2O5 and K2O as specific rates instead of ranges (e.g., 40 lb/acre instead of 30–50 lb/acre). The current recommendation is the midpoint of the range previously reported.
The soil report does not contain a space for a calcium recommendation. Lime, which contains calcium carbonate (CaCO3), provides the crop with this nutrient. Because calcium is supplied during the liming process, supplemental calcium is rarely needed.
If a soil needs magnesium, this nutrient can be supplied through liming in much the same way that calcium can. Dolomitic limestone is a mixture of calcium and magnesium carbonates [CaMg(CO)2] and contains at least 120 lb Mg per ton. On soils where magnesium levels are difficult to maintain, application of dolomitic lime should provide an adequate amount.
To evaluate the magnesium status of the soil, multiply Mg% by the CEC.
The $ Note in Part III of this booklet gives additional information on fertilizing with magnesium.
- If this value is equal to or greater than 0.5 meq, no magnesium is needed.
- If this value is less than 0.5 meq and less than 10% of the CEC, 25 (lb/acre) will appear in the Mg column of the Recommendations section to signify that Mg is low and the crop will benefit from application. Approximately 20 to 30 lb/acre of Mg can be supplied by lime, if needed, or by fertilizer. For samples that list a 2nd crop, the presence of a $ symbol in the Mg column is a reminder that Mg was low for the 1st crop and may still be low for the 2nd crop. Refer to the $ Note: Secondary Nutrients and Micronutrients hyperlink on the last page of the soil report for more information.
- If this value is less than 0.5 meq but greater than 0.25 meq and greater than 10% of the CEC, no magnesium is needed.
- If magnesium is less than 0.25 meq, 25 will appear in the Mg column on the report, regardless of the Mg%.
Ten pounds of manganese per acre is recommended when the manganese availability index (Mn-AI) is less than 25 and the crop is known to respond well to manganese (Table 1.2). For crops where response to manganese is less certain (Table 1.3), a $ refers the grower to the $ Note flyer entitled Secondary Nutrients and Micronutrients. When the Mn-AI is less than 25 and the soil pH is 6.2 or greater, $pH appears in the Mn column. This designation indicates that high pH is responsible for the manganese deficiency and refers the grower back to the $ Note for several treatment options.
When the zinc availability index (Zn-AI) is greater than 25, no addition of zinc is necessary. A rate of 6 lb/acre is appropriate if the Zn-AI is 25 or less and the crop is known to be responsive to zinc. When crop response to zinc is less certain, the recommendation will be flagged with a $, referring the grower to the $ Note.
The need for copper fertilization depends on the copper index (Cu-I), the crop and the soil type. A Cu-I of 25 or less, for a crop known to respond to copper, indicates that copper is needed. Recommended rates depend on soil type: 2 lb of copper per acre for mineral soils, 4 for mineral-organic soils and 6 for organic soils.
If a $ notation appears in the Cu column of the Recommendations section for a first crop, it indicates that the crop's response to the addition of copper is uncertain. If this notation is for a second crop, it indicates that copper may be needed if it was not applied to the first crop as recommended. The crops listed in Table 2.3 have an R in the Cu column if they respond well to copper fertilization. The $ Note in Part III of this booklet gives additional information on fertilizing with copper.
Even though there is no test to assess boron levels in soil, soil reports may include boron recommendations for certain crops. Rates given are based on field observations and experiments. Crop response to these levels has been proven. However, too much boron can be toxic to plants.
As mentioned in Part I, sulfur leaches readily on sandy soils and accumulates in the subsoil. Recommendations for this nutrient vary with soil type and rainfall. If the S-I of deep sandy soils is less than 25, fertilization with sulfur is probably necessary. The general rate for sulfur application is 20 to 25 lb/acre. Sulfur is rarely lacking in piedmont or mountain soils due to their high clay content. Under normal growing conditions, sulfur is generally sufficient on organic soils as well.
Because sulfur is so mobile in the soil, plant tissue analysis is the best way to gauge sufficiency. The deficiency symptoms for sulfur and nitrogen are very similar. Likewise, soil conditions that cause nitrogen to leach also have the same effect on sulfur.
Sulfur deficiency commonly occurs on deep sandy coastal plain soils. These soils are often low in organic matter and have a clay layer 15 inches or more below the surface. Sulfur deficiency can occur when the clay layer is closer to the surface on shallow-rooted crops such as small grains. Corn, coastal bermudagrass and tobacco are also particularly susceptible to sulfur deficiency. Most dry-blend tobacco fertilizers supply adequate sulfur for the current growing season.