8 Managing Soil PH And Crop Nutrients

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8Managing Soil pHand Crop NutrientsFabián G. FernándezDepartment of Crop Sciencesfernande@illinois.eduRobert G. HoeftDepartment of Crop Sciencesrhoeft@illinois.eduThe inherent complexity of crop production systemsrequires integrating many factors to ensure maximum crop yields with the least risk to the environment.Assessing present- and reserved-nutrient status of the soil,understanding its nutrient-release and nutrient-holdingcapacity, and knowing the plant and environmental factorsthat impact nutrient availability are necessary to guidefertilization rates, sources, and method of application ofadditional nutrients. The information here is intended toprovide fundamental principles to help the reader understand what to do, and why, when making managementdecisions related to phosphorus (P), potassium (K), secondary macronutrients (calcium [Ca], magnesium [Mg],and sulfur [S]), micronutrients (boron [B], chlorine [Cl],copper [Cu], iron [Fe], manganese [Mn], molybdenum[Mo], and zinc [Zn]), and pH.Factors Impacting Plant-NutrientAvailabilityNutrient availability can be impacted by soil chemicaland physical properties, including parent material andnaturally occurring minerals; amount of organic matter;depth to bedrock, sand, or gravel; and permeability, waterholding capacity, and drainage. In addition, environmental conditions and crop characteristics have an importantimpact on nutrient availability. It is not unusual for cropsin fields or portions of fields to show nutrient deficienciesduring periods of the growing season, even where an adequate nutrient management plan is followed. The fact thatnutrients are applied does not necessarily mean they areManaging Soil pH and Crop Nutrientsavailable. Plants obtain most of their nutrients and waterfrom the soil through their root system. Any factor that restricts root growth and activity has the potential to restrictnutrient availability. This is not because nutrients are notplant-available in the soil, but because the ability of thecrop to take up those nutrients is restricted. Understandinghow these factors can cause nutrient deficiency in crops isimportant to avoiding excessive concern about the need foradditional fertilization when a sound nutrient program isalready in place.Soil compaction can limit or completely restrict root penetration and effectively reduce the volume of soil, includingnutrients and water, which can be accessed by the plant.To limit soil compaction, avoid entering fields that are toowet, and minimize the weight per axle by decreasing loadweight and/or increasing tire surface area in contact withthe soil. Planting when soils are wet can create a compacted wall next to the seed that will prevent the seedlingfrom developing an adequate root system. Tilling wet soilswill result in clods that become hard and dry out quicklyon the surface, preventing roots from accessing resourcesinside the clod.Soil water content is critical not only to supply the waterneeds of the crop but also to dissolve nutrients and makethem available to the plant. Excess water in the soil, however, depletes oxygen (O2) and builds up carbon dioxide(CO2) levels. While O2 is needed by roots to grow and takeup nutrients, high CO2 levels are toxic.Temperature is important in regulating the speed of soilchemical processes that make nutrients available. Undercool soil temperatures, chemical reactions and root activ-91

ity decrease, rendering nutrients less available to the crop.Portions of the plant nutrients are taken up as roots extractsoil water to replenish water lost through the leaves. Coolair temperatures can lower evapotranspiration and reducethe convective flow of water and nutrients from the soil tothe root.Light intensity is low on cloudy days. Low light intensityreduces photosynthetic rates and nutrient uptake by thecrop. Since low light intensity sometimes occurs whensoils are waterlogged or temperatures are cool, cloud covercan exacerbate the capacity of the crop to take nutrients.Diseases and pests can have an important impact oncrop-nutrient uptake by competing for nutrients, affectingphysiological capacity (such as reduction in photosynthesisrates), and diminishing root parameters through root pruning or tissue death.Estimating Nutrient AvailabilitySoil AnalysisSoil tests are not perfect, so a soil test value should beconsidered not a single value, but rather a value within arange. There are multiple reasons why soil tests are notperfect: a soil test represents a measurement at one pointin time, while a crop takes nutrients through an extendedperiod, and typically under very different soil-water andtemperature conditions than at the time of sampling; theinformation generated typically comes from a sample fromthe plow layer, but the crop roots extract nutrients belowthat layer; laboratory precision is typically within 5% to10% of the true value. Despite these imperfections, soiltesting is the most important guide to profitable application of phosphorus, potassium, and lime because it provides a framework for determining the fertility status ofa field. In contrast, plant tissue analysis is typically morereliable than soil testing for secondary macronutrients andmicronutrients. Since crop yield response to applicationof these nutrients has been very limited in Illinois, thereis not a large enough database to correlate and calibratesoil-test procedures. Ratings in Table 8.1 can provide aperspective on the reliability, usefulness, and cost effectiveness of soil tests as a basis for planning a soil fertilityand liming program for Illinois field crops.Traditionally, soil testing has been used to decide howmuch lime and fertilizer to apply to a field. With increasedemphasis on precision agriculture, economics, and the environment, soil tests are also a logical tool to determine areaswhere adequate or excessive fertilization has taken place. Inaddition, they are used to monitor the impact of past fertilitypractices on changes in a field’s nutrient status. Of course aTable 8.1. Ratings of soiltests.TestWater pHRatinga100Salt pH30Buffer pH30Exchangeable H10Phosphorus85Potassium60Boron: alfalfa60Boron: corn andsoybeans10Iron: pH 7.530Iron: pH 7.510Organic y60Sulfur40Zinc45soil test report can onlybe as accurate as thesample sent for analysis. In fact, the spatialvariability of available nutrients in a fieldmakes soil samplingthe most common andgreatest source of errorin a soil test. To collectsamples that provide atrue measurement ofthe fertility of an area,one must determine thesampling distribution;collect samples to theproper depth; collectsamples from preciselythe same areas of thefield that were sampledin the past; and collectsamples at the propertime.Field soil. A soil probeis the best implementManganese: pH 7.510for taking soil samples.Copper: organic soils20An auger or a spadeCopper: mineral soils5can also be used asaOn a scale of 0 to 100, 100 indilong as care is taken tocates a very reliable, useful, andcollect an exact depthcost-effective test, and 0 indicatesa test of little value.with a constant slicethickness (Figure 8.1).A soil sample, or sampling point in the field, should bea composite of at least five soil cores taken with a probefrom within a 10-foot radius around the sampling point.Composite samples should be placed in bags with labelsidentifying the places where the samples were collected.Manganese: pH 7.540Sampling distribution. The number of soil samples takenfrom a field is a compromise between what should be done(information) and what can be done (cost). The most common mistake is taking too few samples to represent a fieldadequately. Shortcuts in sampling may produce unreliableresults and lead to higher fertilizer costs, lower returns, orboth. Determine a soil sampling strategy by first evaluating cost, equipment to be used, past fertilization practicesused, and the potential response to fertilizer application.Possible strategies include sampling for the following:uniform fertilizer applications. For this approach, sampling at the rate of one composite from each2-1/2-acre area is suggested (see Figure 8.2, diagram a,for sampling directions).lW hole-field92Illinois Agronomy Handbook

Soil slice1/2'' thick165 ft165 ftS 1330 ftSoil probeAugerSpadeS 2lC onservationtillage fields with fertilizer band applications. There is not presently enough research data todefine an accurate method for sampling these fields, sothe following methods are given as suggestions. Whenthe location of the band is known, collect the regular7-inch depth sample 6 inches off the side of the band.Another approach would be to multiply a factor (0.67)by the distance (in inches) between bands to determinehow many cores need to be collected from outside theband for each sample collected in the band. For example,in a 30-inch band distance, collect 20 cores from outsidethe band for each sample collected in the band. If thelocation of the band is not known, the best approach isto increase the number of samples (20 to 30) and to varysampling position in relation to the row so the band doesnot bias test results.Sampling depth. The proper sampling depth for pH,phosphorus, and potassium is 7 inches. This is because thefertilizer recommendation system in Illinois is based oncrop response to fertility levels in the top 7 inches of thesoil. For fields where conservation tillage has been used,Managing Soil pH and Crop NutrientsS 4110 ftS 1S 12S 6b5 SS 12330 ft330 ft13 SS 10S 9220 ftS 13220 ftS 11220 ftS 4330 ft220 ft110 ft220 ftS 314 SOne sample per 2.5 acres220 ftS 2330 ft11 S330 ft330 fta330 ft6 S330 ft330 ftS 15330 ft3 S16 S330 ftS 10330 ftlS ite-specific9 S330 ftS 7Figure 8.1. How to take soil samples with a soil probe, anauger, and a spade.applications for fields where large variations in test values over a short distance are suspected.Under these conditions, collecting one sample from each1.1-acre area (Figure 8.2, diagram b) will provide abetter representation of the actual field variability. Thegreater sampling intensity will increase cost of the baseinformation but allows for more complete use of technology in mapping soil fertility patterns and thus moreappropriate fertilizer application rates.lZ ones with common characteristics. This is a directedsampling approach that is also known as “smart” orzone sampling. This method integrates informationincluding such details as yield maps, crop canopydata, soil type or other characteristics, past manage ment history, and the like. It defines sampling zoneswith common characteristics that may influencecrop productivity and nutrient and water supplies.The size of such zones varies depending on fieldcharacteristics, but it seldom exceeds 10 acres.330 ftS 8220 ftS 14220 ftS 16220 ft220 ft220 ftS 35220 ftS 27S 21S 36S 26220 ftS 22220 ftS 25220 ftS 23220 ftS 15220 ftS 24220 ftS 34220 ftS 28220 ftS 33220 ft220 ft220 ft220 ft220 ft220 ft5 S8 S17 S20 S29 S32 S220 ft220 ft220 ft220 ft220 ft220 ftS 7220 ftS 18S 19S 30220 ftS 31220 ftOne sample per 1.1 acresFigure 8.2. How to collect soil samples from a 40-acrefield. Each sample (diagram a) should consist of five soilcores, 1 inch in diameter, collected to a 7-inch depth fromwithin a 10-foot radius around each point. Higher frequencysampling (diagram b) is suggested for those who can usecomputerized spreading techniques on fields suspected ofhaving large variations in test values over short distances.accurate sampling depth is especially important, as suchtillage results in less thorough mixing of lime and fertilizers than a tillage system that includes a moldboard plow.This stratification has not adversely affected crop yield,but misleading soil tes

information generated typically comes from a sample from the plow layer, but the crop roots extract nutrients below that layer; laboratory precision is typically within 5% to 10% of the true value. Despite these imperfections, soil testing is the most important guide to profitable applica-tion of phosphorus, potassium, and lime because it pro-

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