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Managing pH in Field Soils
James Altland, Ph.D. North Willamette Research and Extension Center Oregon State University Introduction We recently held a soil management workshop at the OSU North Willamette Research and Extension Center. One of the speakers was Dr. John Hart, an OSU soil scientist. Dr. Hart spoke about soil pH, the implications it has on plant growth, and practical applications for managing soil pH. This article is my best attempt at putting some of Dr. Hart’s comments down on paper to share with Digger readers. Other Digger articles I’ve written have addressed lime and topics related to pH (October, 2002). This article provides new information on collecting soil samples and modifying soil pH. The most critical aspect of soil testing is timing. Soil tests must be performed prior to planting. As discussed later in the article, correcting soil deficiencies or pH after planting is extremely difficult (maybe impossible). A major concept in this article is a 3-step process for ensuring proper soil pH/nutrition: conduct soil tests, correct deficiencies, then plant. Correlated photos and figures are at the bottom of the page. Determining soil pH Soil tests must be performed before the field is planted. Collect soil samples as soon as possible to give yourself the maximum amount of time to make corrections. After receiving soil test results, take one of three actions: add lime to raise soil pH, add sulfur to lower soil pH, or do nothing because soil pH is optimum for the intended crop. Raising pH with lime applications is a chemical reaction, while lowering soil pH with sulfur is a biological reaction. Lowering soil pH (discussed below) can take up to a year or longer. The earlier soil tests are completed, the more time there is to properly adjust field conditions. Soil pH affects solubility and availability of metallic micronutrients, especially manganese (Mn), zinc (Zn), and iron (Fe). Changes in soil pH due to lime or sulfur applications will change available concentrations of these nutrients. For example, as pH decreases Mn concentration increases. After amending soil with lime or sulfur, what will the resulting Mn concentration be? This is almost impossible to predict, and one can only be certain by first allowing soil pH adjustments to be made, then using another soil test to determine the resultant Mn concentrations. Consider the following example I encountered earlier this year. A local nurseryman wanted to grow ‘Red Sunset’ maple (Acer rubrum) in a field with pH of 6.5 and Mn concentration of 6 ppm. It is known from research that soils growing ‘Red Sunset’ need to have pH of 5.0 to 5.8, with Mn concentrations of 20 to 40 ppm. The planting date was close, and a decision to lower pH with elemental sulfur was made. It was understood that pH would be lowered slowly over the next year, but as long as the sulfur was incorporated prior to planting, pH would change in time to produce a high quality crop. Here was the question: how much Mn should be added to raise soil Mn concentration? This was difficult to answer. Lowering soil pH alone may result in sufficient available Mn so that no additional Mn need be applied. Too much Mn could cause nutrient toxicities and, at the very least, interfere with uptake of other mineral nutrients. However, too little Mn in the soil certainly causes problems for this species. The key is to collect soil samples as early as possible so there is time to make corrections. If necessary, retest to see the implications those pH changes have on other soil nutrients. An appropriate number of samples Nurseries collect on average one soil sample for every 10 acres of new planting. That is far too few. Soil conditions vary dramatically across fields. Using a single soil test to describe 10 acres of production is woefully imprecise. I recommend at least 1 soil test per acre of new planting. First identify regions of the field that might be unique. Unique areas could include low spots, poorly drained areas, areas with obvious differences in soil texture, areas with different cropping history, and any area that repeatedly produces crops of low quality or poor growth. Analyze these areas of the field separately. For large tracts of land that appear relatively uniform, break the field into roughly 1 acre sections by creating a grid. Collect samples from each grid section separately. Within each grid section, use a soil probe to collect 10 to 15 cores randomly throughout the area. Mix these cores together in a bucket, then take enough to fill up a sealable sandwich bag. Label the sandwich bag with a black pen so the sample can be identified by the area from which it was collected. Interpreting soil tests: pH vs. SMP pH Most soil tests report a value labeled as SMP pH (also SMP buffer or SMP). How does SMP pH differ from pH? Soil pH is determined by mixing soil with water, and measuring the pH of the slurry with a laboratory grade pH meter. SMP pH is measured by using a liquid solution called ‘SMP buffer’ instead of water. SMP stands for Shoemaker, McLean, and Pratt, the scientists who developed the buffer. Unlike the standard soil pH test, SMP also takes into account H+ bound to soil particles. When lime (calcium carbonate, CaCO3) is added to the soil, Ca2+ ions bump H+ off the soil particles and further decreases soil pH. This additional decrease in soil pH needs to be accounted for when making lime recommendations, hence the SMP buffer. Interpreting pH from a soil test is a two-step process. Use the pH reading to determine if corrective action is needed (i.e., should soil pH be raised or lowered). If pH needs to be raised, use the SMP value to determine the quantity of lime to add to the soil. To know whether pH needs to be changed requires you to have a target in mind. Research tells us that ‘Red Sunset’ maple should grow in pH of 5.0 to 5.8. An experienced grower stated that rhododendron need to grow in pH of 5.2 to 5.4. What is the optimal pH of the crops you grow? We don’t have standards for most plants. Experience (as in the rhododendron example) provides most the answers. Communicate with other nurseries growing similar crops. In time, research may be able to provide more information on this topic, but until then, experience and knowledge of how crops responded to various soil pH conditions in the past provides the only information we have to work from. Do not underestimate the knowledge of experienced and observant growers. Seasonal fluctuations in soil pH Soil pH will change throughout the year, even with no intervention by man. Soil pH can differ by as much as 0.5 from early spring to late fall. Soil pH tends to decrease form spring through summer, but then increases again from late summer through spring. These fluctuations should be considered when evaluating soil tests. Clay particles in soil are negatively charged. Similar to a magnet, negatively charges soil particles attract positively charged nutrients. Because H+ is positively charged, it is also attracted by soil particles. There is a pecking order governing which nutrients are bound to soil particles and which ones are easily displaced. Because of its atomic radius and charge, H+ is the ‘weakest’ charged ion. It is easily replaced by all the other positively charged cations (K+, Ca2+, Mg2+, etc.). When large amounts of H+ are bound to soil particles, they are effectively removed from the soil solution. When H+ is removed from soil solution, pH goes up. If H+ is released from soil particles back into soil solution, pH goes down. Oregon’s winter rains flush the soil, pushing salts and fertilizers deeper into the soil profile. By leaching away nutrients, many of the sites on clay particles are vacated by nutrient cations and replaced by H+ ions. As H+ binds to soil particles and leaves the soil solution, pH of the soil solution increases. Therefore, winter rains tend to raise soil pH. During the spring and early summer, rains taper off and the soil begins to dry. Water evaporates from the soil surface, and water from deep in the soil profile moves up to the surface via capillary action. As water moves up through the soil profile it brings with it many of the nutrients and salts that were leached away over the winter. These nutrient cations easily replace H+ ions attached to soil particles. As the H+ concentration increases in the soil solution, pH is lowered. Adjusting pH with lime and sulfur Lime reactions with soil are purely chemical reactions, they do not require biological organisms. Lime thoroughly mixed with soil will react and change pH almost immediately. Lime particle size can affect the speed of this reaction. Most agricultural lime is pulverized to a consistency similar to cooking flour. Granulated or pelletized lime will react more slowly because larger particles must first disintegrate or dissolve before reacting with soil. Lime must be incorporated into the soil. Surface applied lime will have very little effect on the bulk soil where most roots grow. Applications of granular sulfur (don’t use powdered sulfur) will lower soil pH. Sulfur is oxidized to sulfuric acid by soil bacteria (Thiobacillus), which in turn lowers soil pH. Soil temperatures need to be above 50º F for this reaction to occur, and temperatures are optimum between 70 and 80º F. It typically takes one year or more for all the sulfur to be oxidized and complete the reaction. Applications can be made any time of the year, but changes to soil pH will not occur until soil temperatures have warmed. Elemental sulfur is not soluble in water, so it will not leach away with winter rains if applied in late fall. Because of the long time needed for the sulfur-mediated pH change to occur, it is best to incorporate the product well in advance of planting (6 to 12 months). Sulfur must be incorporated into the soil profile to be effective. Topdressing sulfur on the soil surface is not a viable option. Tables for lime and sulfur amounts are too large for reprinting in this article. Visit my website listed below to access these tables printed in OSU Extension publications. Management guidelines
Summary Soil pH cannot be successfully managed by guessing. The only way to know which amendments should be applied is to have a detailed knowledge of field conditions. Soil tests, when properly collected and interpreted, are the best assurance that field nutritional properties have been maximized. Dr. James Altland is a nursery crop extension agent at the North Willamette Research and Extension Center. He can be reached by email at James.Altland@oregonstate.edu or calling (503) 678-1264. Find more information on this and other nursery related topics at his website: http://oregonstate.edu/dept/nursery-weeds/ Captions for photos (Click picture to enlarge)
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