S-deficient Soils Research Articles

Interactive effects of N and S fertilizers on canola yield and seed quality on S-deficient Gray Luvisol soils in northeastern Saskatchewan

On marginally S-deficient soils in the Parkland region, application of high rates of N and other fertilizers in combination with more frequent productionof canola (Brassica napus L. or B. rapa L.) causes rapid depletion of S and nutrient imbalance in soil, and S deficiency and yield reductionfor canola. Field experiments on Gray Luvisol (Boralfs) soils deficient in available N and S were conducted in northeastern Saskatchewan. Interactive effects of N (0, 50 100 and 150 kg N ha-1) and S (0, 10, 20 and 30 kg S ha-1) rates on yield, seed quality, and uptake of S and N in canola were determined. In the absence of S application, increasing N rate made the S deficiency symptoms more severe, and reduced yield, S concentration, oil concentration, S uptake and N uptake of seed, but there was a general response of no effect or some increase in protein concentration, S and N uptakein straw up to 50 or 100 kg N ha-1. When S was applied, canola yield, S concentration, S uptake and N uptake of seed as well as the yield and S uptake of straw increased with increasing N rate. Irrespective of S rate, fertilizer N reduced oil concentration and increased protein concentrationin canola seed. With S fertilization, yield, S uptake and N uptake of seed and straw, and total S concentration and oil concentration in seed were substantially increased, whereas there was no consistent change in protein concentration of seed. The response of these parameters to S application was generally greater at higher N rates. Sulphur and N uptake measured data indicated that significant N × S interaction effects were more frequent and pronounced for seed yield than for straw yield, indicating that the response to N rate was relatively more dependent on the S level for seed than for straw. In summary, the results suggest that an increased amount of S is needed on these S-deficient Gray Luvisol soils in northeastern Saskatchewan to adequately meet the S requirements at high N rates for optimum yield and quality of canola. Key words: Canola, N and S concentration and uptake, N and S fertilization, oil concentration, S-deficient soil, seed yield and quality

Crop Response to Elemental S and Sulfate-S Sources on S-Deficient Soils in the Parkland Region of Alberta and Saskatchewan

ABSTRACT Six field experiments were conducted on sulfur-(S) deficient Gray Luvisol (Typic Cryoboralf) and Dark Gray Chernozem (Typic Cryoborolls) soils at various locations in Alberta and Saskatchewan to evaluate the effectiveness of elemental S (ES-90, ES-95, Urea S, Fine S Powder), sulfate-S (ammonium sulfate, sodium sulfate), and elemental S + sulfate-S (Sulgro-68, Agrium Plus) fertilizers, and their rates and placement methods on seed yield and S uptake of canola and barley, and forage dry matter yield of pure bromegrass and bromegrass-alfalfa mixed stands. There was a substantial increase in yield and/or S uptake from sulfate-S application in most experiments. This indicated that the soils used in these experiments were deficient in plant-available S, especially for canola. In the initial year, the performance of elemental S fertilizers was usually inferior to sulfate-S fertilizers. Among the elemental S fertilizers, the ES-95 tended to produce greater yield and S uptake than ES-90. In general, there was no difference in seed yield between early (May 1) and late (May 23) spring applications for the elemental S (Urea S). The results indicated that from the second or third year of initiating annual applications the effectiveness of some elemental S fertilizers almost approached the level of sulfate-S fertilizers. Residual effect of S application lasted for at least three years. Overall, the findings suggest that correction of a severe S deficiency by elemental S alone may be risky in the short term, and thus addition of some sulfate-S in the initial 1 or 2 years to supplement the sulfate-S from elemental S is advisable. Increased exposure of elemental S fertilizers to oxidation either by incorporation and broadcast methods or by application ahead of plant use time tends to improve crop response.

Cultivar and fertilizer S rate interaction effects on canola yield, seed quality and S uptake

Since canola has a high requirement for S, and canola cultivars have differences in growth rate and yield potential, a differential response of canola cultivars to soil S deficiency and S fertilization can be expected. A 3 site-year field study was conducted on S-deficient soils in northeastern Saskatchewan to determine response of four canola cultivars to S deficiency and S fertilization in terms of yield (seed and straw), seed quality (oil, protein and S concentration), and S uptake (seed and straw). Two Brassica napus L. cultivars (Quantum and AC Excel) and two B. rapa L. cultivars (Maverick and AC Parkland) were compared at 0, 5, 10, and 15 kg S ha-1 rates. Both the actual values of seed and straw yield and seed S uptake and the responses to S fertilization were greater for the B. napus than B. rapa cultivars. Optimal yield response for all the four cultivars occurred at the 10 kg S ha-1 rate, though seed quality and S uptake responded up to the 15 kg S ha-1 rate. Sulphur fertilization response was quadratic for seed and straw yield, seed oil and protein concentration, and S uptake in seed, while the response for seed S concentration and straw S uptake was inconsistent. In summary, in spite of differences in magnitude of the response of tested cultivars to S fertilization, the similar nature of the response and optimal yield at the same S rate indicated th at specific S fertilization recommendations for individual canola cultivars are unnecessary. Key words: Canola cultivars, oil concentration, seed quality, S rates, S uptake, yield

Influence of formulation of elemental S fertilizer on yield, quality and S uptake of canola seed

The rate of S oxidation in elemental S fertilizers to plant-available sulphate-S depends on factors that affect microbial activity, such as soil moisture, temperature, aeration and size of fertilizer granules. Three field experiments were conducted on S-deficient Gray and Dark Gray Luvisol soils (Typic Cryoboralf) during 2000 and 2001 in Saskatchewan (exp. 1 at Porcupine Plain and exp. 3 at Canwood) and Alberta (exp. 2 at Legal) to determine the effects of formulation of elemental S fertilizers on yield, oil, protein and S concentration, S uptake, percent recovery of applied S and N uptake in seed of canola (Brassica napus L. ‘A4573’ at Porcupine Plain and ‘Invigor 2663’ at Legal and Canwood). In exp. 1 (2000 and 2001) and exp. 2 (2000), treatments included five elemental S fertilizers (granular ES-99, ES-95, ES-90 and Biosul-90; and Biosul-50 suspension), granular potassium sulphate and zero-S control. In exp. 3, treatments included eight elemental S fertilizers (granular ES-99, ES-95, ES-90 and Biosul-90; powder Lab fine ES-99.5, ES Settle-47 and ES SPB571-85.8; and Biosul-50 suspension), 21.7% elemental S plus 18.7% sulphate-S (Agrium Plus), blend of granular ES-90 and ammonium sulphate (1:1), granular ammonium sulphate and zero-S control. The S fertilizers were surface-broadcast or sprayed at 15 and 30 kg S ha-1 rates within 2–3 d after sowing in spring. Canola plants in the zero-S treatment showed S deficiency in the growing season, and seed yield increased with sulphate-S fertilizer by 21.8, 1.4, and 3.6 times in exps. 1, 2, and 3, respectively. There was little effect of S fertilizer on protein concentration in seed. Biosul-50 suspension in all experiments and powder forms of elemental S fertilizers, used only in exp. 3, were almost as effective as sulphate-S fertilizers. In all experiments, granular elemental S fertilizers had no or only a very small effect on yield, S uptake, percent recovery of applied S, N uptake, oil concentration, S concentration and protein concentration of canola seed in the first year of application. In the second year of exp. 1, granular elemental S fertilizers tended to increase yield, S uptake, N uptake, oil concentration, and S concentration of canola seed, but none of them produced seed yield or improved other parameters of canola seed comparable with the sulphate-S and elemental S powder or suspension fertilizers. In summary, granular elemental S fertilizers were not effective in increasing canola seed yield and S uptake on S-deficient soils in the first year of application. After the second annual application, most granular elemental S fertilizers tended to correct deficiency of S in canola plants, but improvements in yield and other parameters of seed were still less than the sulphate-S and suspension or powder forms of elemental S fertilizers. Key words: Canola, elemental and sulphate S fertilizers, granular, suspension, oil and protein content, seed yield, S uptake

Influence of four successive annual applications of elemental S and sulphate-S fertilizers on yield, S uptake and seed quality of canola

The effectiveness of elemental S fertilizers in increasing crop yield in S-deficient soils depends on the rate at which S is oxidized to sulphate in soil for plant uptake. A 4-yr field experiment (1999 to 2002) was conducted with canola (Brassica napus L. or Brassica rapa L.) on a S-deficient loamy sand Gray Luvisol soil near Tisdale in northeastern Saskatchewan to determine the effectiveness of elemental S and sulphate-S fertilizers applied annually at different times and S rates on seed and straw yield, oil and protein concentration in seed, concentration and uptake of S in seed and straw, amount of residual sulphate-S in soil, and recovery of applied S in plant, soil and plant + soil. The S fertilizer treatments included granular types of two bentonite -elemental S fertilizers (ES-90 and ES-95), one sulphate-S (ammonium sulphate), one containing elemental S and sulphate-S (Agrium Plus), and a zero-S control. The S fertilizers were surface-broadcast at 10 or 20 kg S ha-1 rates in the previous autumn or in spring, and incorporated into the soil a few days prior to seeding in May. Canola plants in the zero-S control showed S deficiency in the growing season. Seed yield, straw yield, oil concentration in seed, and S concentration and uptake in seed and straw increased with the sulphate-S fertilizer in all years. There was no significant increase in seed yield from the elemental S fertilizers in 1999, though autumn application tended to give slightly greater seed yield than the spring application. Elemental S fertilizers usually increased seed yield and S uptake significantly over the zero-S control, but yield and S uptake were less than the sulphate-S fertilizer in most cases in 2000, in many cases in 2001 and in some cases in 2002, especially when the S fertilizers were applied in spring. Autumn-applied elemental S usually had greater seed yield and S uptake than the spring-applied elemental S in 2000, 2001 and 2002. On the other hand, autumn-applied ammonium sulphate produced or tended to produce lower seed yield and S uptake than spring-applied ammonium sulphate in some cases. There was little effect of S fertilizer application on protein concentration in seed. In summary, the results indicated that elemental S fertilizers were not effective in increasing canola yield and S uptake on the S-deficient soil in the first year of application. The elemental S fertilizers became effective in the second to fourth year, but seed yield and S uptake were still less than those obtained with sulphate-S fertilizer, particularly when the S fertilizers were applied in spring. The findings suggest the need for research on best management practices to improve effectiveness of elemental S fertilizers. Key words: Canola, elemental and sulphate S fertilizers, oil and protein concentration, seed yield, sulphur uptake

Effect of Timing of Sulfur Fertilizer Application on Growth and Yield of Rapeseed

There is usually a positive yield response when sulfur (S) is applied to rapeseed (Brassica rapa L.) plants grown on S-deficient soils. Recommendations include application of some of or the entire amount of S at planting, but the plant requirement as well as availability of S to rapeseed during its various phenological stages is not well documented. In a field trial, 40 kg S ha−1 was applied as gypsum (CaSO4·2H2O) at planting (T1), during vegetative (T2, 30 days after sowing), flowering (T3, 50 days after sowing), and pod-filling (T4, 65 days after sowing) stages, and in split doses (T5). Biomass accumulation, leaf-area index, and leaf photosynthetic rate were studied at various growth stages. Seed S (sulfate and organic S) and yield parameters were studied at harvest. Sulfur (40 kg S ha− 1) applied in three split doses during different growth stages caused maximum increase in these parameters, followed by T2 and T1. However, no significant discernable difference was observed for the application at planting (T1) and during vegetative stage (T2). No significant response was seen following the application of T3 and T4 when compared with the control (T0). On the basis of these results, we concluded that application of S fertilizer in split doses during growth stages is better than application of the entire amount of S at any stage for obtaining optimum yield of rapeseed.

A review of sulphur fertilizer management for optimum yield and quality of canola in the Canadian Great Plains

In the Parkland region of the Canadian prairies, Canola (Brassica napus L. or Brassica rapa L.) is an important cash crop. Canola has a high requirement for sulphur (S). However, many soils in this region are deficient or potentially deficient in plant-available S for optimum canola seed yield. Application of sulphate-S at about 15–30 kg S ha-1 is usually sufficient to prevent S deficiency in canola on most of the S-deficient soils. Application of sulphate-S to canola at seeding time gives the highest increase in yield and S uptake. Deficiencies of S in canola plants can be prevented and/or corrected and seed yield improved with the use of sulphate-S fertilizers in the growing season. Application of sulphate-S at bolting can substantially restore seed yield, while an application at early flowering can moderately correct S deficiency damage. Side-banding is the most effective way to apply sulphate-S fertilizers to produce maximum seed yield and to prevent any damage to canola seedlings from seed-row placement. In relatively moist areas, broadcast-incorporation methods can produce seed yield similar to side-banding in most years. Elemental S fertilizers were not effective in increasing seed yield in the year of application, and were generally less effective than sulphate-S fertilizer even after multiyear annual applications, especially when applied in spring. Autumn-applied elemental S was more effective than spring-applied elemental S. Banding delayed availability of elemental S as compared to broadcast application. Use of granular elemental S products is not reliable for optimum seed yield of canola under Canadian prairie conditions on S-deficient soils, particularly in the initial year and with spring application or band placement. Elemental S fertilizers may have a role to maintain or build-up sulphate-S levels in soils marginally low in S where residual benefits are desirable, but management decisions should consider both immediate and long-term effects of S fertilizer on seed yield, seed quality and economics. The findings suggest the need of future research to increase dispersion and distribution of S particles from granules for faster oxidation of elemental S in soil, and to develop elemental S fertilizer products/formulations that can be used on a commercial scale to prevent and/or correct S deficiency in the growing season to optimize seed yield and quality of canola. Research is also required to determine the long-term effects of balanced application of S with other nutrients on soil quality, accumulation and distribution of nitrate-N, sulphate-S and other nutrients in the soil profile, efficiency of nutrient, water and energy use, and crop diseases. More research should be conducted in relation to soil/plant tissue testing issues for optimum seed yield and quality of canola. Key words: Balanced fertilization, canola, elemental S fertilizers, method of application, rate of S, seed quality, seed yield, sulphate-S fertilizers

Flue Gas Desulfurization Products as Sulfur Sources for Alfalfa and Soybean

Sulfur deficiencies in soil are expected to increase due to growth of high‐yielding crop varieties, use of S‐free fertilizers, and removal of S from industrial emissions. Flue gas desulfurization (FGD) products, created when coal is burned and SO2 is removed from the flue gases, may serve as efficient S sources. However, there are few reports on their use for the enhancement of crop growth. Agricultural gypsum and two types of FGD products, that contain either vermiculite or perlite, were applied at 0, 16, and 67 kg S ha−1 to an agricultural soil (Wooster silt loam, Typic Fragiudalf). Dry weight of a new planting of alfalfa (Medicago sativa L.) was increased up to 40% by the treatments of FGD products or gypsum compared with the untreated control. Gypsum and FGD products were also applied at 0, 8, 16, and 24 kg S ha−1 to five established alfalfa stands in different Ohio regions. Mean alfalfa yield was significantly (P ≤ 0.05 ) increased by approximately 5.0% in 2001 and 6.0% in 2002 with the S treatments of FGD products or gypsum compared with the untreated control. Alfalfa yields for FGD products and gypsum treatments were similar. A slight positive yield response was observed for soybean (Glycine max L.) when soils were treated with S‐containing materials. Soil and plant analyses were made to assess potential adverse environmental impacts and none were observed. Thus, these FGD products can be safely applied to agricultural soils as S sources and can improve alfalfa yields in S‐deficient soils.

Sulphur fertilizer and tillage management of canola and wheat in western Canada

A 3-yr field study in Manitoba, Saskatchewan and Alberta evaluated immediate and residual effects of source, timing and placement of sulphur fertilizers on canola (Brassica napus L.) and wheat (Triticum aestivumL.) under conventional (CT) and reduced tillage (RT). Sulphur fertilization did not increase wheat yield, even when soil sulphate was low. Ammonium sulphate (AS) and ammonium thiosulphate (ATS) increased canola yield in 2 of 3 yr at the Saskatchewan site while elemental S and Tiger 90™ did not. Residual S from AS and ATS increased canola yield on S-deficient soils in the second and third crops after application. Canola yield on the S-deficient soil was lower where broadcast Tiger 90 rather than AS or ATS was applied the previous year, indicating that broadcast Tiger 90 oxidation was insufficient to support canola yield in the second crop year after application. Banding or seed-placing Tiger 90 delayed oxidation further. In the third crop after application, canola yield was similar with broadcast Tiger 90, AS or ATS but was lower with seed-placed or banded Tiger 90 than AS or broadcast Tiger 90. Small particle size and broadcast application of elemental S are needed to hasten oxidation of S to sulphate-S. Key words: Elemental S, Bentonite S, zero tillage, reduced tillage

Effectiveness of sulphate-S fertilization at different growth stages for yield, seed quality and S uptake of canola

Canola ( Brassica napus L. and B. rapa L.) is an important cash crop in the Prairie Provinces of Canada. The majority of canola is grown in the Parkland zone where many soils are deficient or potentially deficient in plant-available S for optimum seed yield of canola. Because canola has high S requirements and S is immobile in plants, its deficiency can occur any time during the growing season and drastically reduce canola seed yield, particularly on soils fertilized well with N and P. The objective of this study was to assess the effectiveness of sulphate-S fertilizer (potassium sulphate) applied using various application methods at seeding and during the growing season for yield, seed quality and S uptake of canola on S-deficient soils. Six field experiments (two sites per year from 1998 to 2000) were conducted on Grey Luvisol soils in north-eastern Saskatchewan. Fertilizer treatments were no fertilizer, N (120 kg N ha-1) alone at seeding, S (30 kg S ha-1) alone at seeding, and N (120 kg N ha-1) at seeding + S (15 and 30 kg S ha-1) at seeding, bolting and flowering stages of canola. Methods of S application were surface broadcast and incorporated into the soil, sidebanded and seedrow placed at seeding, and topdressed and foliar sprayed at bolting and early flowering. Canola at all experimental sites showed severe S deficiency symptoms. Compared to N alone, N + S fertilization increased yield, oil content and S uptake of seed in all cases, increased yield and S uptake of seed + straw in most cases, while protein content of seed and S index (ratio of S uptake in seed to S uptake in seed + straw) were not influenced consistently. A trend of seeding > bolting > flowering time of S applications was generally shown by yield and S uptake of seed and seed + straw, and by S index. There were no noticeable differences among treatments applied at seeding. Topdressing at bolting and flowering tended to produce less seed yield than foliar-applied S in few cases and method of S application had no consistent effect on other parameters. Increasing the rate from 15 to 30 kg S ha-1 generally increased yield and S uptake of seed and seed + straw, but had no consistent effect on oil and protein contents in seed and S index. Fertilizer S alone tended to increase yield, oil content and S uptake of seed but had no effect on seed protein content. However, application of N alone tended to reduce yield, oil content and S uptake of seed while it had an inconsistent effect on yield and S uptake of seed + straw and S index. In conclusion, application of sulphate-S fertilizer on S-deficient soils improved yield, seed quality and S uptake of canola, with applications at seeding generally more effective than at bolting and early flowering stages. The results suggest that application of sulphate-S can correct S deficiency in canola if it occurs in the growing season, and restore seed yield substantially at bolting and moderately at early flowering. Key words: Application times, canola, placement methods, seed quality, S index, S rates

Effects of nitrogen and sulphur fertilization on oats yield, quality and digestibility and nitrogen and sulphur metabolism of sheep in the Inner Mongolia Steppes of China

The influence of nitrogen (N) and sulphur (S) fertilizer on yield and quality of spring oats and the N and S metabolism of sheep was determined. Eighteen crossbred wether lambs (about 30 kg) were fed oats fertilized with N (0, 138 kg N/ha as of urea) and S (0, 30 and 60 kg S/ha as gypsum) assigned randomly to six treatments in an experiment of 2×3 factorial design. On S-deficient soils, S and N fertilization increased dry matter (DM) by 10.5 and 71.9%, and crude protein (CP) yield of oats by 11–13 and 94.5%, but there was no interaction between N and S fertilizer. S fertilization increased S content of oats from 0.14 to 0.29% and significantly reduced the N:S ratio from 14.4 to 7.22. There was a significant interaction between N and S on N content. N and S fertilization significantly increased apparent DM digestibility of oats by about 4%, but did not affect intake (g/kg BW0.75), DM intake being almost 60 g/kg BW0.75 for all treatments. N increased average daily gain of lambs by 62.4–78.6% if S was supplied, being twice the gain without S supply but no interaction between N and S was observed. N retention was affected by interaction between N and S fertilization. N retention and N utilization were positively correlated with S intake. N:S ratio in oats was positively correlated with N metabolism. S fertilization increased N utilization efficiency and vice versa. The critical levels of total S in oats were 0.20 and 0.23%, and the critical levels of N:S ratio were 17.0 and 15.7 for maximum DM and CP yield, respectively. The critical levels of S content and N:S ratio for maximum average daily gain of lambs were 0.26% and 10.5, respectively. Therefore, in the diagnosis of S status in soil–forage–animal systems, the use of total S and N:S ratio in the forage tops for the maximum CP yield is preferable to that for the maximum DM yield because the values are more closely related to the requirements for maximum daily gain of lambs; similarly, the use of a combination of total S and N:S ratio is preferable to the use of N:S ratio alone.

Microbial biomass, S mineralization and S uptake by African millet from soil amended with various composts

Microbial biomass growth, S mineralization after compost amendment (plant seeding) and S uptake by African millet at d 30, 60 and 120 (first, second, and third cutting, respectively) were monitored in an S-deficient soil amended with cattle manure compost (CMC), saw dust compost (SDC) or rice husk compost (RHC) at the rate of 20 t ha −1 in the presence or absence of growing African millet. A chemical fertilizer (CF) treatment at the rate of 30 μg g −1 soil along with a control (CT) was included for comparison. CMC produced a significantly larger microbial biomass-C and -S than SDC or RHC. In the planted soil, during rapid growth of African millet, microbial biomass-S decreased more rapidly than in unplanted soil. Both biomass-C and biomass-S then showed a significant flush particularly at d 60–120 in all the treatments. CMC, RHC and SDC released 20, 10, and 8 μg CaCl 2 extractable S g −1 soil, respectively, by d 5. Microbial biomass showed a marked increase in C-to-S ratio across the treatments which eventually reached 154 in the unplanted soil and 291 in the planted soil from an initial value of 64. Substantial mineralization of soil organic-S in all the treatments was observed during the period of greatest plant growth, but not in the absence of plants. Total S uptake was 37, 81 and 76% lower in the CMC, SDC and RHC amendment, respectively, than that of CF. CMC improved the S supplying potential of the soil, but addition of SDC or RHC (high C-to-S ratio) resulted in severe S deficiency of plant due to S immobilization in soil.

Effects of sulphur nutrition on growth and nitrogen fixation of pea (Pisum sativum L.)

A S-deficient soil was used in pot experiments to investigate the effects of S addition on growth and N2-fixation in pea (Pisum sativum L.). Addition of 100 mg S pot−1 increased seed yield by more than 2-fold. Numbers of pods formed were the most sensitive yield component affected by S deficiency. Sulphur addition also increased the concentration of N in leaves and stems, and the total content of N in the shoots. The amounts of N fixed by pea were determined at four growth stages from stem elongation to maturity, using the 15N dilution technique. Sulphur addition doubled the amount of N fixed at all growth stages. In contrast, leaf chlorophyll content and shoot dry weight were increased significantly by S addition only after the flowering and pod fill stage, respectively. Pea roots were found to have high concentrations of S, reaching approximately 10 mg g−1 dry weight and being 2.6–4.4 times the S concentration in the shoots under S-sufficient conditions. These results suggest that roots/nodules of pea have a high demand for S, and that N2-fixation is very sensitive to S deficiency. The effects of S deficiency on pea growth were likely to be caused by the shortage of N, due to decreased N2-fixation.

Carbon substrate mineralization and sulphur limitation

The addition of large quantities of carbon substrate to S-deficient soils will cause S immobilization and may bring about S limitation of the microbial biomass, thus depressing the rate of C mineralization. Glucose, cellulose or starch were added to eight arable soils from the northeast of Scotland having a range of phosphate extractable-S values (1–12 μg S g −1 dry weight soil). All nutrients were added in excess except C or S. Carbon dioxide emission was monitored over 3 weeks during incubation at 25°C. In the soil with the lowest extractable-S, the rate of C mineralization was depressed as compared to the same soil amended with excess S. This effect decreased with increasing extractable-S and was undetectable in the soil with the highest extractable-S. The effect also tended to decrease with increasing total soil carbon. S demand during cellulose decomposition was approximately twice that during glucose decomposition, with that for starch being intermediate. The critical C-to-S ratios, the ratios above which C mineralization would be limited by S deficiency, were estimated to be 1110, <720 and 490 for glucose, starch and cellulose, respectively. The relationship between the response of cellulose mineralization to S additions and a test to assess the S status of soils is discussed.

Partitioning of ryegrass residue sulphur between the soil microbial biomass, other soil sulphur pools and ryegrass (Lolium perenne L.)

Ryegrass shoot residues, labelled with 35S, were added to an S-deficient soil. The transfer of S to the microbial biomass, to the soil S pool extractable by NaHCO3 and to growing ryegrass when present was followed over 34 weeks. After 2 weeks 16% and 15% of the S residue was found in the biomass and in the extractable S pool, respectively. Where plants were grown, they became S-deficient (shoot S <0.2%) simultaneously with the biomass showing a marked increase in C:S ratio. This eventually reached 262 from an initial value of 59. Concurrently, the extractable S pool, which included some labile organic S, decreased to <0.2 μg g−1 soil. After 34 weeks 27% of the S residue was found in the growing plant, 7% in the biomass and 2% in the extractable S pool. Some mineralization of unlabelled soil organic S was observed during the period of greatest plant growth (8–14 weeks), but not in the absence of plants. A second phase of mineralization occurred between weeks 22 and 34, concurrent with a rise in mean temperature, which was unaffected by the presence of plants or by the size of the microbial biomass. This may have been due to “biochemical” mineralization of ester sulphate. The amount of unlabelled soil S involved in active cycling was estimated to be 11%–13% of the total soil S.

An evaluation of carbon disulphide as a sulphur fertilizer and as a nitrification inhibitor

There was little release of extractable SO4-S during four weeks from CS2 applied by injecting into two S-deficient soils. In this incubation experiment, the rate of CS2 was 30 μg S g−, placement was injection at 9 cm depth, soil temperature was 20°C, and soil moisture tension was 33 kPa. The yield of barley forage after seven weeks in the greenhouse showed only small increases from 10 or 30 μg S g−1 of CS2 as compared to Na2SO4, on the two soils.

SOURCES OF SULFUR FOR RAPESEED

Rapeseed grown in pots on a S-deficient soil showed little response to elemental S fertilizers applied at rates of up to 150 ppm S. The dry matter yields, percent S and N/S ratios of the aboveground material indicated severe S deficiency in the crop. A readily available S source (ammonium sulfate) at rates of 75 and 30 ppm S increased yield by three to five times over that produced by the highest rates of elemental S.

Nodulation, nitrogen fixation, leaf area, and sugars content inLablab purpureus as affected by sulfur nutrition

In order to explore interrelations between S nutrition, soluble sugars, leaf area, nodulation and N2 fixation, greenhouse experiments were done with several levels of S added to perlite-sand cultures or to a moderately S-deficient soil. Sulfur had indirect effects on nodulation and N2 fixation, possibly by improving sugars supply and N metabolism.