Container mix pH and the effect of Scotts controlled release fertilizers in altering pH increase

By Glen Lumis and Ligita Taurins, Department of Plant Agriculture, Horticultural Science Division, University of Guelph

Understanding pH
pH is a measure of acidity or alkalinity and is expressed on a scale from 1 to 14. A pH of 7 is neutral. Values below 7 indicate increasing acidity, while those above 7 indicate increasing alkalinity. For each unit of pH change, the acidity or alkalinity changes by a factor of 10. Thus, pH 5 is 10 times more acidic than pH 6.

     The optimum mix pH varies among genera. Ilex, Rhododendron and other ericaceous plants grow best in acid conditions (pH 4.5 to 5.5), while Buxus, Juniperus and many flowering shrubs do best at a higher pH (5.5 to 6.5) (12). The optimum pH can also vary within genera. Viburnum acerifolium and Quercus rubra grow best in more acid conditions (pH 3.9 to 4.8 and pH 4.7 to 5.5, respectively), while Viburnum plicatum var. tomentosum and Quercus macrocarpa will grow fine in more alkaline conditions (pH 6.4 to 7.4 and pH 7.7 to 8.5, respectively) (4).

Root medium pH affects the availability of essential nutrient elements as presented for mineral field soils by Troug (13), for organic field soils (muck peat) by Lucas and Davis (8) and for a commercial soilless potting mix by Peterson (11) (Fig. 1). These researchers found the availability of each macro and micro nutrient in the root medium changes as pH increases or decreases, and that the optimum pH range for availability of all nutrients in an organic field soil is pH 5.5 to 5.8 and a soilless potting mix is pH 5.2 to 5.5. These values are 1-1.5 units lower than the pH range established as favourable for plant growth in mineral field soils. Peterson (11) found that for a soilless potting mix, high pH levels resulted in reduced availability of phosphorous, iron, manganese, boron, copper and zinc, while low pH resulted in excessive availability of phosphorous and micro nutrients yet low availability of calcium and magnesium (Fig. 1).

     In the literature, most recommendations for the "ideal" pH range for organic (soilless) container mixes are based on the research findings of Lucas and Davis (8) or Peterson (11).

The following four "texts" list slightly different desirable pH ranges: Bunt (3)5.0 to 5.5 Davidson et al (4)approximately 4.5 to 6.5 Handrek and Black (6)5.0 to 6.0 Nelson (9)5.4 to 6.0

     These recommendations vary slightly since they are the author's interpretations of the range in which all essential elements are most available for root absorption. Outside the ideal range, as pH increases or decreases, many elements begin to become either chemically bound so they are insoluble and unavailable to plants or they become soluble and leach from the mix (5). Nursery production guideline's recommendations for the optimum pH range specifically target the kinds of container mixes used in the industry as well as the broad range of plant material grown. The Ontario Ministry of Agriculture, Food and Rural Affairs (10) and the British Columbia Ministry of Agriculture and Food (2) recommend a pH range of 5.5 to 7.0; the Southern Nurserymen's Association (12) lists pH 5.0 to 6.0 as suitable for most plants.

     Whitcomb (15) maintains that pH has little to do with nutrition of container-grown nursery stock grown in soilless (organic) container mixes, and that raising or lowering pH has little effect on micro nutrient availability within the range of pH 4.0 to 7.0. In unpublished data, Whitcomb and co-workers found that as long as a slow release source of calcium and magnesium was provided, azalea plants grew equally well between pH 3.0 and 8.0. Whitcomb stresses the important consideration in container plant production is not the mix pH but the quantity and proportions of the essential nutrients available to the plant. Handrek and Black (6) state that if nutrients are continuously supplied in adequate amounts, pH is far less important. Thus, a fairly wide pH range is tolerable in potting mixes containing slow release fertilizers or receiving a balance liquid fertilizer.

     In the production of container-grown plants, softwood (pine) bark and sphagnum peat moss are common ingredients for mixes since they provide good plant support, aeration, drainage and fairly good nutrient retention (4, 10). Despite the generally low initial pH of a pine bark and peat mix, the pH increases gradually over the growing season. Irrigation water alkalinity (its bicarbonate/carbonate content), not water pH has the greatest influence on mix pH. Water pH can merely serve to indicate the presence of bicarbonates (7). If water pH is below 7, it can be assumed that the water won't be a significant source of bicarbonates. However, if the pH is greater than 7, water will contain bicarbonate, but pH does not determine how much is present. Water from different sources with the same high pH can contain either very low or very high levels of bicarbonates. Bi­carbonates are bases, which when present in high enough quantities in irrigation water act as liming materials neutralizing the mix acidity. Mix pH increases gradually over time as the cumulative quantity of bicarbonate (plus carbonate if water pH is greater than 8.3) increases with repeated applications of water (9). The higher the bicarbonate alkalinity, the faster the mix pH will rise (1). An excessively high alkalinity level is one that causes the mix pH to rise to an unacceptably high level by the end of the crop production period (9). Irrigation water quality guidelines suggest that water bicarbonate alkalinity levels below 61 ppm will likely not affect container mix pH maintenance but levels between 61 to 214 ppm can cause increasing problems and levels greater than 214 ppm can cause severe problems (12). Proper irrigation management can minimize the effect of high bicarbonate water. Heavy or frequent irrigation of plants grown in container mixes with low water holding capacity and low buffering capacity can affect plant growth by excessively raising the mix pH (7). Thus, it is best to apply only enough water to slightly exceed the root zone water holding capacity. Higher volumes of alkaline water with heavy leaching will raise mix pH faster than lower volumes with less leaching.

     Limiting mix pH increase ("creep") may be accomplished by: 1) adding acid to the irrigation water (7, 14), 2) using acidifying soluble fertilizers (9, 10), and 3) applying sulfur (3, 7, 9). Acids [hydrochloric (muriatic), nitric, phosphoric or sulfuric] added to alkaline water supply hydrogen ions that neutralize much of the bicarbonate before it reaches the mix. Acids, however, are highly toxic and dangerous to handle. Addition of acid to the irrigation water also requires the added cost of acid resistant pumps and proportioners. If using acidifying soluble fertilizers, the higher the potential acidity (kg of calcium carbonate limestone required to neutralize the acidity caused by using one metric ton of a specified fertilizer) of the fertilizer, the faster the mix pH will be lowered (9). The form of nitrogen present in the fertilizer also affects mix pH. Ammonium nitrogen tends to lower mix pH while nitrate nitrogen tends to raise pH (3, 9). The increased use of controlled release fertilizers in nursery container production, however, makes the application of soluble fertilizers for pH control impractical for many growers. Sulfur or one of several sulfur compounds may be incorporated into the growing mix as a pre-plant amendment, all of which react in the mix to ultimately form sulfuric acid (3, 9). Sulfur compounds such as iron sulfate or aluminum sulfate are water soluble and can also be applied as a drench (9). Additions of sulfur, however, may result in lower than optimum mix pH early in the season with acidic ingredients such as bark and peat.

Research Trial
We undertook a trial to determine the effectiveness of Osmocote Plus 15-9-12, Osmocote Pro 19-5-8 and Scotts 35-0-0 in reducing mix pH increase ("creep") for three species of deciduous shrubs grown in containers.

     Osmocote Plus 15-9-12, Osmocote Pro 19-5-8 and Scotts 35-0-0 are controlled release fertilizers containing 2.3%, 8% and 18% sulfur, respectively. Osmocote Plus consists entirely of polymeric resin encapsulated prills each containing all nutrients. Of the total 15% nitrogen, 7% is ammonical nitrogen and 8% is nitrate nitrogen. Nutrient release is temperature dependent. Osmocote Pro is a blended product containing all nutrients and Poly-S nitrogen. Poly-S nitrogen is polymer-encapsulated sulfur-coated urea. Of the total 19% nitrogen, 6.6% is ammonical nitrogen, 5.6% is nitrate nitrogen and 6.8% is urea nitrogen. Nutrient release is dependent on temperature, moisture and microbiological activity. Scotts 35-0-0 contains Poly-S nitrogen and sulfur. Of the total 35% nitrogen, all is urea nitrogen. Nutrient release is dependent on moisture and micro­biological activity.

This trial was conducted at two locations: a com­mercial nursery [Connon Nurseries (Neil Vanderkruk Holdings Inc.)] and the University of Guelph, from mid May to early October 1998. Rooted cuttings of Hydrangea macrophylla 'Nikko Blue', Physocarpus opulifolius 'Dart's Gold', and Spiraea japonica 'Crispa' were potted on May 13 into #1 containers using a mix of 60% ground pine bark, 30% coarse sphagnum peat moss and 10% sand.

For each of three treatments (Trt), the following fertilizers were incorporated into the mix:
Trt 1. Osmocote Pro 19-5-8 (8-9 month) at 5.9 kg/m3, 1.1 kg N/m3 (8% sulfur)
Trt 2. Scotts 35-0-0 (4-6 month) at 1.2 kg/m3, 0.4 kg N/m3 plus Osmocote Plus 15-9-12 (8-9 month) at 4.7 kg/m3, 0.7 kg N/m3 (20.3% sulfur)
Trt 3. Osmocote Plus 15-9-12 (8-9 month) at 7.5 kg/m3, 1.1 kg N/m3 (2.3% sulfur)

     At both locations, irrigation water samples were collected monthly and sent to Agri-Food Laboratories, Guelph, Ontario, for total salts, pH, total hardness, total solids, bicarbonate, macro element, and micro element analyses. Mix pour-through solutions (using deionized water) were collected on several sample dates during the season and analysed for pH. At the nursery, all treatment plants were grown within the container production area under overhead sprinkler irrigation using pond water. Pour-through solutions were collected on May 15, July 3, August 14 and October 6.

     At the university, plants were grown at the container research facility and irrigated by hand, applying the same amount on each container at every watering. Pour-through solutions were collected on May 15, June 12, July 10, August 7, September 4 and October 1. On October 20, plant tops from each treatment were cut off, oven dried and weighed to determine final dry weight.

Results obtained from irrigation water analyses showed that during the growing season, the bicarbonate level was lower at the nursery (averaging 263 ppm) than at the university (averaging 345 ppm) (Fig. 2). Water pH values at the nursery and at the university ranged from 8.1 to 8.5 and 7.6 to 7.9, respectively, over the season (Fig. 2). Total hardness, which primarily indicates calcium and magnesium content, remained relatively constant during the season at the nursery, averaging 287 ppm. However, at the university, total hardness significantly increased from an average of 92 ppm on May and June sample dates to an average of 408 ppm on sample dates after July 22, when the university's water source was changed from softened city to well water (Fig. 2). Total soluble salts levels at the nursery tended to remain relatively constant over the season ranging from 0.6 to 0.8 mS/cm. At the university, levels tended to progressively increase very slightly (by 0.1 mS/cm) on consecutive sample dates following the change in the water source, levels ranging from 0.7 to 1.0 mS over the season (Fig. 2).

Results obtained from pour-through solutions collected on the first two sample dates, indicated that at the nursery from May 15 to July 3 and at the university from May 15 to June 12, for all species, differences among treatments in the rate of pH increase per day and in the initial pH of the mix were slight (data not shown). At both locations, for all species, the rate of pH increase per day differed by less than 0.006 units among treatments. On May 15, the initial pH differed by 0.1 units among treatments, averaging 4.9 at the nursery and the university. By July 3 at the nursery, and by June 12 at the university, for all species, pH values differed by less than 0.2 units among treatments, averaging 7.1 at the nursery and 6.8 at the university. The high initial increase in pH between the first two sampling dates was the result of applying water high in bicarbonates to the acidic mix. Once much of the initial acidity was neutralized, the effect of the bases in the irrigation water on raising mix pH was less on subsequent sample dates. At the nursery, from July 3 to October 6, for all treatments of Hydrangea and Spiraea, mix pH generally increased to a peak value and then declined except that for Hydrangea, Osmocote Pro 19-5-8 (Trt 1) values continued to increase (Fig. 3). The rise in pH for Osmocote Pro (Trt 1) appeared to be least rapid among treatments until mid- to late August, then became more rapid. From about mid-July to late September, pH appeared lowest for Osmocote Pro (Trt 1), however, values were less than 0.2 units (Hydrangea) and 0.4 units (Spiraea) lower than those for the other treatments. At the end of the season, pH values differed by less than 0.1 units among treatments. For all treatments of Physocarpus, the mix pH continued to increase over the season (Fig. 3). The rate of pH increase per day was lowest for Osmocote Pro (Trt 1). From about early August to early October, pH tended to be lowest for Osmocote Pro (Trt 1), but by less than 0.2 units. At the end of the season, pH values differed by less than 0.2 units among treatments. Generally, top and root growth for all species appeared visually similar among treatments or greater with Osmocote Plus (Trt 3).

     At the university, from June 12 to October 1, mix pH for Hydrangea and Spiraea, was affected in the same way, except that for Hydrangea, pH for each treatment continued to increase during the season, while for Spiraea, pH increased to a peak value and then levelled off (Fig. 4). For all treatments of Physocarpus, pH continued to increase during the season (Fig. 4). The increase in pH for Scotts 35-0-0 plus Osmocote Plus (Trt 2) appeared to be less rapid compared to other treatments until mid-August, then it became more rapid. From about mid-June to early October, pH tended to be lowest for Osmocote Pro (Trt 1), but by less than 0.2 units. At the end of the season, pH values differed by less than 0.1 units among treatments. Plant top dry weight for Hydrangea and Spiraea was greatest for Osmocote Plus (Trt 3), while for Physocarpus, dry weight was similar among treatments (data not shown).

At both locations, for all species, there were no to slight differences among treatments in their effect on mix pH over the growing season. The increasing amounts of sulphur contained in the fertilizer treatments applied in this trial: Osmocote Plus 15-9-12 (2.3% sulphur), Osmocote Pro 19-5-8 (8% sulphur), and Scotts 35-0-0 plus Osmocote Plus 15-9-12 (20.3% sulphur) appeared to have no biologically significant effects on mix pH "creep." The acidifying influence of the fertilizers was hindered by the highly alkaline water at both locations. The tendency for slightly lower pH values and the less rapid increase in pH during certain periods with Osmocote Pro (Trt 1) were most likely not due to the sulfur content, but rather to general differences among the fertilizers such as coating technologies and different nitrogen sources.

     Based on research reported in the literature, an adequate amount of all required nutrients in the mix is the best way to ensure good growth, regardless of pH.

We thank Connon Nurseries (Neil Vanderkruk Holdings Inc.), the Scotts Company of Marysville, OH. and the University of Guelph Plants Program for assistance and support.

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