Sulfur (S) is found in certain amino acids and is a necessary component of certain proteins, vitamins, and oils. It is considered a secondary nutrient and is taken up in the sulfate form. Plants do not require as much S as they do nitrogen, phosphorus, and potassium (N, P, and K); but S is still needed in greater quantities than are the micronutrients boron, copper, iron, manganese, zinc, and chloride. Plant tissue should contain 1 part sulfur for every 15 to 20 parts nitrogen for optimum growth and quality. Deficiencies typically occur in deep, coarse-textured soils where sulfate leaches easily. Today, S deficiency may be more of a concern due to several factors that farmers may not have considered: (1) tighter air quality standards for atmospheric emissions mean less sulfur falls onto the landscape, (2) past use of single superphosphate fertilizers (12% S) led to S accumulation, but the fertilizer industry switched decades ago to triple superphosphates (1% S), and (3) heavy rainfall leaches more S downward in the soil profile. Sulfur deficiencies are being correctly diagnosed more often due to plant tissue analysis, while in years past they were often mistaken for nitrogen deficiency.
Sulfur is an immobile element, and as a result S-deficient plants have green lower leaves and yellow upper leaves (Figure 1).
This is in contrast with N, a mobile element. N-deficient plants have yellow lower leaves and green upper leaves. S deficiency is most likely to occur during early growth, before extensive root development. Crops deficient in S show little or no response to N fertilizer until the S deficiency is corrected.
Factors Influencing Crop Responses to S Fertilizer
Corn, cotton, and wheat, when planted on S-deficient soils, have shown yield increases in response to the addition of S fertilizer (Figure 2). Crop species are not all equally sensitive to soil S levels. Sulfur deficiency is rare in soybean, even though it is grown in rotation with corn, which often responds to S fertilizer. Soybean is a tap-rooted crop that may acquire S from deeper depths than more shallow-rooted grass crops such as corn. Peanuts are typically fertilized with gypsum, which also supplies S (1,000 lb CaSO4 contains 190 lb S), so additional S should not be necessary for other crops in this rotation.
Since not all crops respond to S fertilizer, awareness of crop S uptake and removal will help decide if the additional expense of S fertilizer is necessary. Table 1 lists nutrient removal rates for different crops at typical yield levels. Crops vary widely in S removal rates. Silage corn (24 lb S/ac/yr in grain + stover) and burley tobacco (18 lb S/ac/yr) remove more S than other agronomic crops; while wheat (3 lb S/ac/yr) and cotton (5 lb S/ac/yr) remove relatively little S. Orchardgrass removes the most S among forages, even with biomass yields lower than those of coastal bermuda. Among vegetables, cabbage (and probably other cole crops such as collard greens and broccoli) removes more S (44 lb S/ac/yr) at typical yield levels than any other North Carolina crop. Producers may need to supply more S fertilizer when growing high S-demanding crops, especially in high-yielding environments.
|Crop||Component||Yield Level||S Removal (lb S/ac)|
|Cotton||seed + lint||2,600 lb||5|
|Tobacco, flue-cured||leaves||3,000 lb||12|
|Tobacco, burley||leaves||3,000 lb||18|
|Red clover||2.5 tons||7|
|Coastal bermuda||8 tons||32|
|Sweet potato||root||300 bu||6|
|Sweet corn||ears||9,000 lb||11|
|Source: SoilFacts: Nutrient Removal by Crops in North Carolina, Zublena, 1991, AG 439-16.|
Responses occur most often when soil test S levels are low in both the topsoil and subsoil, typically in deep sandy soils. Sulfate, the plant-available form of S, can leach through sandy surface soils, but usually accumulates in clay subsoil. The negatively charged sulfate ion is attracted to soil clay particle surfaces. Even subtle changes in clay content, such as a transition from a loamy sand topsoil to a sandy loam subsoil can result in S accumulation. Unfortunately, most producers submit only topsoil samples for soil test analyses, when sufficient S may be present in the subsoil. Typical topsoil samples for fertilizer recommendations are collected from a 6- to 8-inch deep layer (4-inch if continuous no-till). It is possible for a soil to have very little S in this upper surface layer, but sufficient S present at a lower depth to support plant growth. Response to S fertilizer is more likely if the depth to increased clay content is greater than 18 inches below the soil surface. Natural Resource Conservation Service (NRCS) county soil surveys and field observations should be used to identify problematic fields. When a loamy sand topsoil with low S content overlays a sandy loam subsoil, significant S accumulations may occur (Figure 3). Sulfur previously applied as gypsum or other materials leaches downward in the profile, but may still be within reach of roots.
Soil variability complicates fertilizer management decisions. Note that for each region of the state, certain soils have deeper and/or sandier surfaces, and are thus more likely candidates for S leaching and fertilizer S response (Table 2). In the mountain floodplains, leaching and S depletion are more likely in coarser soils such as Biltmore and Dellwood than in finer soils such as Rosman. Many upland piedmont soils such as Cecil and Vance are eroded and have significant clay near the surface, while floodplain soils, such as Chewacla and Congaree, have more sandy surface layers due to deposition and therefore are more prone to deep leaching of S through surface layers.
|Region||Soil Series||Topsoil Texture||Subsoil Texture||Typical Depth in Inches to Clay|
|Dellwood||Cobbly sandy loam||Loamy sand/sand||50+|
|Piedmont||Cecil||Sandy loam||Sandy clay loam||7 (less if eroded)|
|Chewacla||Silt loam||Loamy fine sand/silt loam||36+|
|Congaree||Sandy loam / loam||Silty clay loam||38|
|Upper/Middle Coastal Plain||Candor||Sand||Loamy sand||25|
|Norfolk||Loamy sand||Sandy loam||15|
|Rains||Sandy loam||Silty clay loam||25|
|Lower Coastal Plain||Conetoe||Loamy sand||Sandy loam||25|
|Craven||Silt loam||Silty clay loam||10|
|Goldsboro||Loamy sand||Sandy loam||13|
|Roanoke||Silt loam||Silty clay loam||25|
|Bojac||Loamy fine sand||Fine sandy loam||8|
|Newholland||Mucky loamy sand||Sandy loam||27|
|Portsmouth||Fine sandy loam||Sandy clay loam||24|
|Tomotley||Fine sandy loam||Sandy clay loam||15|
Of the coastal plain soils listed, the Candor and Conetoe are most prone to deep S leaching. Most crops grown on tidewater region soils are less likely to suffer S deficiency. Some soils have mucky surfaces that can supply S as organic matter decomposes (Belhaven, Newholland). Sulfur deficiencies on these organic soils are more likely in the winter months due to slower decomposition rates with cooler temperatures. Other tidewater soils have either finer-textured surfaces (Portsmouth, Tomotley) or a coarse-textured surface layer that is relatively thin (Bojac), so less leaching is expected.
Heavy rains can leach S to lower profile depths, while cold weather and/or high water tables can reduce crop rooting depth and residue decomposition rates, thus reducing sulfur supply.
Long-term no-till cultivation is increasingly popular in North Carolina. Without cultivation, crop residue remains on the soil surface; a subsurface network of dead roots and animal burrows persists; and surface-applied lime and fertilizer may concentrate near the surface. While the surface cover and subsurface channels can reduce overland water flow and soil erosion, more water may percolate and leach soluble nutrients deeper in the profile. Producers should consider the possibility that soil stratification and slower residue decomposition may reduce S availability, especially when low subsoil pH or compaction limits rooting. Occasionally, the producer should sample the subsoil to detect chemical limitation, and probe soils and observe roots to detect physical limitation.
Diagnostic Laboratory Data
Although visual nutrient deficiency symptoms can be very distinct, there are cases where S deficiency looks similar to N, manganese (Mn), or other nutrient deficiencies. Waiting until you see visual symptoms is often too late to recover from crop economic loss. Laboratory analysis of soil and plant tissue samples will confirm the diagnosis.
Sulfur is reported in the routine North Carolina Department of Agriculture & Consumer Services soil testing procedure, and a sulfur index of 25 or less is indicative of a need for fertilizer. Actually, the need for fertilizer S depends not only on laboratory results from a topsoil sample, but also an awareness of the nature of the soil profile as mentioned previously in the soil discussion. Periodically take a deep sample to measure the amount of subsoil S. When S is needed, 15 to 25 lb S per acre should be adequate; the lower rate is recommended on soils with higher levels of organic matter.
Plant Tissue Analysis
Both the percent S and the ratio of N to S (N:S ratio) are used to evaluate crop S status. Find more information at NCDA&CS's Plant Tissue Analysis Section website.
S Fertilizer Materials, Costs, and Typical Application Scenarios
Several fertilizers are available to North Carolina farmers that can be applied in preplant granular blends or included in N solution applications (Table 3).
|Ammonium sulfate 21-0-0-24 (also ammonium sulfate liquid, 8-0-0-9S; or industrial by-product liquor, 7-0-0-9S)||(NH4)2SO4||24 (or diluted)|
|Gypsum (calcium sulfate)||CaSO4·2H2O)||19|
|Potassium magnesium sulfate (Sul-Po-Mag or K-Mag)||K2SO4-MgSO4||18-23|
|Normal superphosphate||Ca(H2PO4)2·H2O + CaSO4||12|
|N-S solutions (24-0-0-S)||UAN mixed with ammonium+ sulfate, polysulfide, bisulfite, or thiosulfate||3-5|
|Elemental Sulfur (powder or 50% suspension in water)||S||90-100|
The most economical applications of S are in combination with other fertilizer materials. Some convenient options for North Carolina crops are:
- 24-0-0-S to supply 12.5 - 21 lb S with each 100 lb N. This could be at planting, or as a sidedress/topdress. Based on a 2006 survey, this should cost approximately $0.15/lb of S, as compared with applying N alone. The S adds about 10% to the cost of N fertilizer alone, so it is important to document cases when the extra expense is justified.
- Ammonium sulfate (21-0-0-24) to supply 20 lb S with each 17.5 lb N. This could be at planting, or as an early sidedress/topdress. Based on a 2006 survey, this should cost approximately $0.20/lb of S, as compared with applying N alone.
- Sul-Po-Mag (0-0-22-23% S) broadcast to supply 1 lb S with each 1 lb K2O. This would usually be at planting, unless also correcting a mid-season K or Mg deficiency. Based on a 2006 survey, this should cost approximately $0.33/lb of S, as compared with applying K alone as muriate of potash (0-0-60).
- Sulfate of potash (0-0-50-17% S) broadcast to supply 1 lb S with each 3 lb K2O. This would usually be at planting, unless also correcting a mid-season K deficiency. Based on a 2007 survey, this should cost approximately $0.56/lb of S, as compared with applying K alone as muriate of potash (0-0-60).
- Manure broadcast preplant. See approximate composition values in Table 4.
Incidental S inputs from rainfall, dry atmospheric S deposition, P fertilizer, and especially animal wastes (Table 4) can represent substantial amounts of S. The balance between these inputs and crop S removal rates indicates likely fertilizer needs.
|Source||Material application rate (lb/ac/yr)||Rate (lb S/ac/yr)|
|Dry atmospheric deposition||6|
|Triple superphosphate fertilizer||75 lb P2O5||0.75|
|Swine lagoon liquid||100 lb plant-available N||8|
|Dairy lagoon liquid||100 lb plant-available N||26|
|Stockpiled broiler litter||100 lb plant-available N||42|
Adequate S is necessary for high crop yields and profitable farming. However, one size does not fit all, and not all crops benefit from S fertilizers. The best management plan considers S removal and incidental S inputs for the entire crop rotation, soil type and profile depth layers, and soil and plant analysis results. Consult the Natural Resources Conservation Service county soil survey and look at your own soils to determine the likelihood of S leaching or accumulation in your fields. Send soil and plant tissue samples to Agronomic Services Division, North Carolina Department of Agriculture & Consumer Services to confirm S fertilizer needs.
Publication date: May 7, 2014
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