How a soil test can determine suitability of soil for plants

By Kelli Belden Two or three small applications of soluble aluable information about soil qual- nutrients over the growing season will help Vity and its suitability for the plants you prevent loss of valuable fertilizer. want to grow can be gleaned from a soil Medium-textured soils are usually the test report. most favorable for plant growth. They gen- It can also provide information to help erally hold the most plant-available water determine how to manage the soil. Most and usually have adequate nutrient-holding reports provide a summary of the physical capacity. and chemical test results used to make the Heavy soils are often characterized by report’s recommendations. The report will slow water-infiltration rates and high po- state how much fertilizer is needed and may tential nutrient-holding capacity. They hold also provide how often watering is needed more water but generally do not release as or what amendments are appropriate. There much of it to the plants as a medium soil may also be warnings about properties of will release. Heavy soils may require more the soil unfavorable for good plant growth. phosphorus but are less prone to nutrient Understanding how to interpret soil leaching losses and require less frequent ir- test results will allow you to make the best rigation than lighter soils. use of the soil. Very light and very heavy soils usu- ally create management problems for the Possible Soil Tests grower. Soil texture is not easily modified, Texture: Soil texture describes the and soil texture problems are usually cor- amount of sand, silt, and clay in a soil. Soil rected by careful water management and textures can be divided into three broad building up soil organic matter. groups: light (sand, loamy sand, and sandy loam), medium (loam, silt loam, silt, and Organic matter: Organic matter (decayed sandy clay loam), and heavy (clay loam, plant material or composed manure) is silty clay loam, sandy clay, silty clay, and important in maintaining desirable soil clay). chemical and physical properties. Some Light soils drain quickly and need to be watered frequently to maintain enough water for good Why is this important? plant growth. Light soils allow Organic matter improves water-holding capacity soluble nutrients like nitrate (especially plant-available water), permeability, aeration, and sulfate to be leached and resistance to compaction. Organic matter increases (travel through and out of the the soil’s ability to absorb and hold plant nutrients and soil layer with water) quickly. will release nutrients as it decomposes.

spring 2012 13 herbicides are adsorbed (gathered Table 1. Relative crop tolerance to soil salinity. on a surface in a condensed layer) by Salt estimate at which organic matter and can cause their Relative crop salinity tolerance rating yield loss begins (dS/m) effectiveness to change. Consult the herbicide product label for specific Sensitive <1.3 information on possible interactions Moderately sensitive 1.3 – 3.0 with organic matter. Many Wyoming Moderately tolerant 3.0 – 6.0 soils contain less than 2 percent Tolerant 6.0 – 10.0 organic matter and will benefit from Unsuited for most crops >10.0 organic matter accumulation. (unless reduced yield is acceptable)

Lime estimate: Lime is a source of calcium, an essential plant nutri- between about 6.0 and 7.0, but many be corrected by improving soil drain- ent, but high-lime soils (lime content plants will tolerate a wide range in age and leaching. A plant’s sensitivity greater than 2 percent), which are soil pH. Perhaps the most common to salts can vary with growth stage. common in Wyoming, require more problem with soil-pH extremes is Many species are more sensitive to phosphorus for good plant growth. nutritional imbalance. Very acidic soils salts during germination and emer- Some plant nutritional imbalances can be improved by liming, but alka- gence than during the rest of their may be observed in high-lime soils line soils resulting from high lime are growth. Contact your UW Extension because of excess calcium or the not easily changed. A very high pH educator for specific crop salinity higher pH of the soil. (one over 8.5) may indicate the pres- tolerances. In general, if the salt ence of sodium. Soils high in sodium estimate is less than 2.0 dS/m (dS/m Why is this important? often have undesirable physical prop- – deciSiemens per meter – is the unit Excess lime can increase the erties, such as slow permeability and used to measure salt content. It mea- requirement for added phosphorous and poor aeration. Sodium-affected soils sures the electrical conductivity of the potassium. are not easily reclaimed but may ben- soil), salt should not be a significant efit from organic matter and adding problem. gypsum. Consult your UW Extension Soil Paste pH: pH is a measure of educator if sodium problems are Why is this important? the acidity or alkalinity of a soil. A pH suspected. You will need to request Salinity can be detrimental above 7.0 is alkaline; a pH below 7.0 additional tests to determine how to to plant growth and yield. is acidic. The pH of most Wyoming correct a sodium problem. Excess salts in the root zone soils is between 7.0 and 8.5. The hinder roots from withdrawing optimum pH for most crops will fall Salt estimate: Salt buildup, or soil water from soil. salinity, is common in arid and semi- Why is this important? arid regions. The soil may be naturally Soil pH controls many chemical salty or the salinity may be caused processes in the soil, especially the by poor-quality irrigation water, poor availability of nutrients to plants. irrigation management, and/or poor soil drainage. Soil salinity may usually

Kelli Belden is manager of the Laramie Research and Extension Center Greenhouse Complex in the University of Wyoming College of Agriculture and Natural Resources. She can be reached at (307) 766-5045 or at [email protected].

14 BARNYARDS & BACKYARDS Nutrient analyses provide pulse of soil health

aboratories use many methods to plants. It functions as one of Lfor estimating nutrient availability. the major factors in photosynthe- Read the explanation supplied by the sis, nutrient transport, and energy laboratory for its methods to deter- transfer. Phosphate application is mine if the nutrient content in your often recommended when soil P soil is low, medium, or high. has not been built up by fertilization. There are 16 essential nutrients Phosphorus is considered immobile (oxygen, carbon, hydrogen, nitrogen, in soil, does not leach readily in the phosphorous, potassium, calcium, soil, and does not usually consti- magnesium, sulfur, iron, zinc, man- tute an environmental hazard when ganese, copper, nickel, chlorine, and soil erosion is prevented. Soil-test boron). Oxygen, carbon, and hydro- phosphate is an index of phosphorus gen are supplied by the air and water. availability and should not be consid- For the others, we usually only see a ered a measure of the actual amount plant nutrient response to the addi- available to plants. Different labs use tion (when lacking) of nitrogen, phos- different methods for determining an phorous, potassium, iron, and zinc on available phosphorous index so be our native Wyoming soils. sure to only compare results between laboratories if they have used the Nitrate: Nitrogen (N) fertilizer is the same method. most frequently applied form of nutri- ent. Most nitrogen fertilizer will be Potassium: Potassium (K) is usu- transformed to the nitrate form if wa- ally required by plants in relatively ter and oxygen are present in the soil. large amounts. Potassium is consid- Micronutrients important in Because nitrate is easily leached from ered immobile in soils and does not Wyoming: Iron (Fe) and zinc (Zn) are soil, excess nitrate can end up in the usually constitute an environmental often abundant in Wyoming soils but ground water and become an envi- hazard. Most Wyoming soils have are in forms unavailable to the plant. ronmental hazard (high levels in water large K reserves, and K fertilizer ap- Deficiencies of these nutrients may can cause methemoglobinemia, or plications are usually not necessary. be observed in susceptible plants blue baby syndrome). Excess ni- However, some K deficiencies have growing in high-lime soils. Zinc may trogen can also be toxic to plants. been observed in soils that have been be applied to soil, but soil applica- Follow application recommendations under production for a long time in tions of Fe are often ineffective. carefully and do not exceed the rec- areas where the soil is light in texture. When Fe and/or Zn deficiencies are ommended rate. Potassium analysis is recommended confirmed, foliar treatments may be if the soil has not been tested for K beneficial. Phosphate: Phosphorus (P) is essen- recently. Potassium assists in photo- tial for all plants and is often applied synthesis, stronger stalks and stems, To learn more about soil prop- as fertilizer because much of the and movement of water, nutrients, erties, consider joining one of the P in the soil is in forms unavailable and carbohydrates in plant tissue. Master Gardener programs offered in many Wyoming counties. Contact a local University of Wyoming Extension office for more information.

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