How Closely Is Potassium Mass Balance Related to Soil Test Changes?

The exchangeable fraction of soil potassium (K) has been viewed as the most important source of plant-available K, with other sources playing smaller roles that do not influence the predictive value of a soil test. Thus, as K mass balance changes, the soil test should change correspondingly to be associated with greater or reduced plant availability. However, soil test changes and the availability of K to plants are influenced by many other factors. This chapter reviews research on soil test K changes and the relation to crop uptake and yield. A mass-balance relationship is rarely achieved from the measurement of exchangeable K because of the potential for buffering of K removal from structural K in feldspars and from interlayer K in primary and secondary layer silicates. Similarly, surplus K additions can be fixed in interlayer positions in secondary layer silicates, or potentially sequestered in sparingly soluble neoformed secondary minerals, neither of which is measured as exchangeable K. In addition, soil moisture, temporal differences in exchangeable K with K uptake by crops, K leaching from residues, clay type, organic matter contribution to the soil CEC, and type of K amendment confound attempts to relate K additions and losses with an exchangeable K soil test. Research is needed to create regionally specific K soil test procedures that can predict crop response for a subset of clays and K-bearing minerals within specific cropping systems.

Application of seed-row potash to spring wheat grown on soils with high available potassium levels

Karamanos, R., Flore, N. A. and Harapiak, J. T. 2013. Application of seed-row potash to spring wheat grown on soils with high available potassium levels. Can. J. Plant Sci. 93: 271–277. Two experiments were conducted at numerous locations across western Canada from 1990 to 1994 to ascertain the response of hard red spring (CWRS) wheat (Triticum aestivum L.) to seed-row applied K fertilizers. Soil test K levels at all sites (location×year combinations) exceeded the critical level for western Canada of 125 mg NH4OAc-extractable K kg−1, the concentration below which the probability of a K deficiency is high. In the first experiment, days to maturity and yield for wheat were statistically similar whether or not KCl was applied in the seed row. When the cultivar Roblin was grown significant (P<0.05) yield reductions as a result of seedrow placing low rates of potassium fertilizers were obtained. The second experiment showed that maturity and yield did not respond to treatments including KCl, K2SO4, or CaCl2 fertilizer applied in seed row. Furthermore, spring wheat responses for any of the preceding treatments were not different relative to the control (no K fertilizer applied). These findings bring into question the benefit of seed-row K fertilizer application to hard red spring (CWRS) wheat production on K-sufficient soils in western Canada.

Soil potassium—crop response calibration relationships and criteria for field crops grown in Australia

The Better Fertiliser Decision for Crops (BFDC) National Database holds historic data for 356 potassium (K) fertiliser rate experiments (431 treatment series) for different rain-fed grain crops and soil types across Australia. Bicarbonate-extractable K (Colwell soil-test K) is the most extensively used soil test reported in the database. Data are available for several crop species grown on a range of soil types from all states except Tasmania. Species represented and number of treatment series in the database are: wheat (Triticum aestivum L.), 254; barley (Hordeum vulgare L.), 5; canola (Brassica napus L.), 130; lupin (Lupinus angustifolius L.), 32; sunflower (Helianthus annuus L.), 10; sorghum (Sorghum bicolor L.), 5; and faba bean (Vicia faba L.), 2. About 77% of the available soil-test K (STK) data on wheat, canola, and lupin are from Western Australia. The usual sampling depth of 0–10 cm is recorded for all treatment series within the database, while 68% of experiments have STK information from other soil horizons down the profile, usually in 10-cm increments. The BFDC Interrogator, a comprehensive data search and calibration support tool developed for use with the BFDC National Database, was used to examine STK–yield relationships for each crop across Australia, with more detailed analysis by state/region and then by soil type if data were available. The BFDC Interrogator was used to determine a critical STK concentration to achieve 90% of the maximum relative yield (90%RY) for each crop species, with a critical range (determined by the 70% confidence limit for the 90%RY) also reported. The STK for 90%RY for wheat was 40–41 mg/kg on Tenosols and Chromosols, ~49 mg/kg on Kandosols, and ~64 mg/kg on Brown Ferrosols. There was some evidence of critical values increasing with increasing crop yield and on soils with no acidity constraints to root growth, with effects presumably driven by increased crop K demand. The STK for 90%RY for canola, grown mainly on Tenosols, was similar to that for wheat, ranging from 43 to 46 mg K/kg, but for lupin, also grown mainly on Tenosols, the STK for 90%RY was a relatively low ~25 mg K/kg. Data for sunflower were limited and the STK for 90%RY was poorly defined. A comparison of critical STK concentrations for different crops grown on Tenosols suggested that critical ranges for 90%RY of lupin (22–27 mg K/kg) were significantly lower than that for wheat (32–52 mg K/kg) and canola (44–49 mg K/kg). Critical STK values were not determined for sorghum and faba bean.

Effect of rate and timing of potassium chloride application on the yield and quality of potato (Solanum tuberosum L. ‘Russet Burbank’)

Mohr, R. M. and Tomasiewicz, D. J. 2012. Effect of rate and timing of potassium chloride application on the yield and quality of potato ( Solanum tuberosum L. ‘Russet Burbank’). Can. J. Plant Sci. 92: 783–794. Potassium is frequently applied to irrigated potato in Manitoba. Field experiments were conducted at two sites in each of 2006, 2007 and 2008 to assess effects of rate and timing of potassium chloride (KCl) application on the yield, quality, and nutrient status of irrigated potato (Solanum tuberosum ‘Russet Burbank’) in southern Manitoba. Preplant application of KCl increased total and marketable yield at one site, and tended (0.05<P ≤ 0.10) to increase total and marketable yield at three additional sites. At three of the four K-responsive sites, soil test K levels were <200 mg NH4OAc-extractable K kg−1, the level below which K fertilizer is recommended based on existing guidelines. Effects of timing of KCl application on total and marketable yield were limited although, averaged across sites, KCl applied at hilling reduced the yield of small tubers (<85 g) and increased the proportion of larger tubers (170 to 340 g) compared with preplant application. Averaged across sites, KCl applied preplant or at hilling reduced specific gravity compared with the 0 KCl treatments. Improvements in fry colour with KCl application were evident at only one site. Petiole and tuber K and Cl− concentration, K and Cl− removal in harvested tubers, and post-harvest soil test K concentration increased with KCl application. However, petiole K concentration measured 82 to 85 d after planting predicted only 24% of the variability in relative marketable yield for sites containing between 164 and 632 mg NH4OAc-extractable K kg−1 to 15 cm. Results demonstrate the potential for yield increases and specific gravity declines with KCl application under Manitoba conditions, but suggest that further research will be required to better predict the potential for yield responses using soil and petiole testing.

Foliar Potassium Fertilizer Additives Affect Soybean Response and Weed Control with Glyphosate

Research in 2004 and 2005 determined the effects of foliar-applied K-fertilizer sources (0-0-62-0 (%N-%P2O5-%K2O-%S), 0-0-25-17, 3-18-18-0, and 5-0-20-13) and additive rates (2.2, 8.8, and 17.6 kg K ha−1) on glyphosate-resistant soybean response and weed control. Field experiments were conducted at Novelty and Portageville with high soil test K and weed populations and at Malden with low soil test K and weed populations. At Novelty, grain yield increased with fertilizer additives at 8.8 kg K ha−1in a high-yield, weed-free environment in 2004, but fertilizer additives reduced yield up to 470 kg ha−1in a low-yield year (2005) depending on the K source and rate. At Portageville, K-fertilizer additives increased grain yield from 700 to 1160 kg ha−1compared to diammonium sulfate, depending on the K source and rate. At Malden, there was no yield response to K sources. Differences in leaf tissue K(P=0.03), S(P=0.03), B(P=0.0001), and Cu(P=0.008)concentrations among treatments were detected 14 d after treatment at Novelty and Malden. Tank mixtures of K-fertilizer additives with glyphosate may provide an option for foliar K applications.

Quantifying pasture dry matter responses to applications of potassium fertiliser for an intensively grazed, rain-fed dairy pasture in south-western Australia with or without adequate nitrogen fertiliser

Rain-fed dairy pastures on sandy soils common in the high rainfall (>800 mm annual average) Mediterranean-type climate of south-western Australia comprise the annual species subterranean clover (Trifolium subterraneum L.) and annual and Italian ryegrass (Lolium rigidum Gaud. and L. multiflorum Lam.). In wet years, clover becomes potassium (K) deficient and shows large dry matter (DM) responses to applied fertiliser K due to leaching of K in soil by rainfall. In contrast, ryegrass rarely shows DM responses to applied K. Many dairy pastures in the region are now intensively grazed to maximise pasture use for milk production, and nitrogen (N) fertiliser is applied after each grazing. It is not known if frequent applications of fertiliser N to these pastures changes pasture DM responses to applied K. Therefore, a long-term (2002–07) field experiment was undertaken on an intensively grazed dairy pasture in the region to quantify pasture DM responses to applied fertiliser K with or without applications of adequate fertiliser N (141–200 kg N/ha per year). Soil samples (top 10 cm of soil) were collected from each plot of the experiment each February to measure soil test K by the standard Colwell sodium bicarbonate procedure used for both K and phosphorus soil testing in the region. When no N was applied, pasture comprised ~70% (dry weight basis) clover and 25% ryegrass, compared with ~70% ryegrass and 25% clover when adequate N was applied. Significant linear responses of pasture DM to applied K occurred in 3 of the 6 years of the experiment only when no N was applied and clover dominated the pasture. The largest response varied from ~1.7 to 2.0 t/ha DM consumed by dairy cows at all grazings in each year, giving a K response efficiency of between 8 and 10 kg DM/ha per kg K/ha applied. Significant pasture DM responses to applied N occurred at all grazings in each year, with ~2–3 t/ha extra DM consumed by dairy cows at all grazings in each year being produced when a total of 141–200 kg N/ha was applied per year, giving an N response efficiency of ~7–19 kg DM/ha per kg N/ha applied. Soil test K values were very variable, attributed to varying proportions of soil samples per plot collected between and within cow urine patches, containing much K, arbitrarily deposited on experimental plots during grazing. Soil test K values were not significantly affected by the rates of K applied per year. A re-evaluation of results from the major soil K test study conducted for pastures in the region confirm that ryegrass rarely showed DM responses to applied K, and that for clover, soil K testing poorly predicted the likelihood of K deficiency in the next growing season.

Soil and tissue tests to predict the potassium requirements of canola in south-western Australia

The predominantly sandy soils of south-western Australia have become potassium (K) deficient for spring wheat (Triticum aestivum L.) production due to the removal of K from soil in grain and hay. The K requirements of canola (rape, Brassica napus L.) grown in rotation with wheat on these soils are not known and were determined in the study reported here. Seed (grain) yield increases (responses) of canola to applications of fertiliser K occurred at sites where Colwell soil test K values (top 10 cm of soil) were <60 mg/kg soil. Grain yield responses to applied K occurred when concentrations of K in dried shoots were <45 g/kg for young plants 7 and 10 weeks after sowing and <35 g/kg for 18 weeks after sowing. Application of fertiliser K had no significant effects on either oil or K concentrations in grain.

538 Understanding Why Horticultural Crops in the Desert Rarely Respond to K Fertilization

Vegetable and fruit crops produced in the desert southwestern United States generally do not respond to K fertilization. Even when pre-plant soil test K levels are low and crop K accumulations are high, responses are infrequent. We have performed a number of evaluations aimed at understanding why crops produced in this region fail to respond to K fertilization. First, data show the potential for substantial K inputs through irrigation. For example, Colorado River water, which is widely used for irrigation in this region, contains ≈5 ppm K, resulting in potential K inputs of 30 to 60 kg K/ha. Second, many of the soils used for crop production have a clay content and mineralogy making a response to K unlikely. Studies evaluating the kinetics of K release from the mineral fraction of soils in the region has shown that many soils used for crop production have a high capacity to replenish K to the soil solution and exchange sites following crop uptake. Finally, the observation that Na can partially substitute for the K requirement of many fast-growing leafy vegetables may also be a contributing factor for the infrequent K fertilizer responses for these commodities.