Chapter 05 – Soils and Plant Nutrition

Learning Objectives

From reading and studying this chapter, you should be able to:

• Understand what soil is and list the components that make up a soil
• Describe the classification of soils by their physical characteristics
• Understand how physical properties of soil impact plant health
• Describe the basics of soil chemical properties
• Understand some of the differences between a soil and a container growing medium
• Define the elements that are essential plant nutrients and understand some of the important roles those nutrients play in plant health
• Understand the importance of pH in nutrient availability to plant roots
• Describe some major nutrient deficiency symptoms
• Understand basic types of fertilizers and their uses

Introductory Comments

Soil (or a container growing medium) is a basic aspect of almost all plant growing situations in the landscape, garden, nursery and greenhouse. The intent of this chapter is to provide you with a basic awareness of the important components and characteristics of soil that allow roots to acquire water and minerals essential for plant health. That foundation of knowledge coupled with an awareness of plant nutrients and fertilizers should prepare you for plant management activities that promote healthy soil and result in healthy plants.

What is Soil?

Soil is a naturally occurring part of the Earth’s surface. It is composed of the mineral particles that remain following millions of years of weathering of bedrock and the spaces (called pore spaces) between those particles. It also includes the decomposed remains of living organisms (organic matter) that have inhabited the soil or its surface. Furthermore, soil includes air and water (in ever-changing proportions) in the pore spaces between the particles. Finally, the spaces between the solid particles of soil are also occupied by currently-living organisms. An “ideal” soil is one in which the solid portion (minerals and organic matter) occupies about 50% of the volume of soil and the pore space between the solid particles occupies the other 50% (Fig. 1).

Figure 1. Components of an ideal soil.

Undisturbed soil occurs in distinct layers, called horizons (Fig. 2). The most weathered layer with the most organic matter, and the layer where most plant root growth takes place is at the surface. This is commonly called “topsoil.” However, be cautious about this term. Topsoil is a non-specific word. It does not necessarily mean it is a good soil for plant root growth. Below the surface layer are various “subsoil” layers. These are typically more firm with less organic matter. They act as the mineral “reservoir” for the surface soil. Below that is the “parent material” from which the soil developed.

Figure 2. Typical layers found in an undisturbed soil.

A growing medium (plural media) is any material in which plants are grown, usually in some kind of container or pot. A growing medium may contain soil, but often does not. It is usually an assemblage of low-cost materials, both mineral and organic, that provide the basic structural and chemical characteristics to support plant growth.

Physical Aspects of Soil

The texture of a soil is important in determining plant root access to air and water. It also impacts plant nutrient availability. Texture results from the size of the mineral particles that make-up the soil. Particle size may be large (sand), intermediate (silt) or small (clay). A “coarse” soil, often described as “gritty” to the touch even when moistened, contains a lot of sand. Such a soil has large spaces between the particles. A “fine” soil that feels “sticky” or “plastic” when wet is composed of mostly clay. It has primarily very small spaces between particles. Between sand and clay is silt. When moistened, silt feels “smooth” to the touch. Most soils are a mixture of various proportions of these three particle sizes. For example, a loam might be about 40% silt, 40% sand, and 20% clay particles.

The pore space between soil particles is generally filled with either air or water. Sandy soil drains water rapidly and allows quick air penetration because the spaces between solid particles are relatively large. Conversely, clay soil tends to hold water and drain slowly (excluding air) because the pore spaces between clay particles are very small. Silt is intermediate. Because air and water compete for the same pore space, “overwatering” as a cause of poor plant health is not really a case of too much water for the plant, but rather, one of too little air.

Soil texture is not a characteristic that is easily changed. Removal and replacement of soil is, for all practical purposes, the only way to alter texture.

Soil structure is an important physical property that can be more readily changed. Individual soil particles (sand, silt or clay) often clump together, or aggregate, to form larger units with larger pore spaces between them. A well aggregated soil is said to have a good structure. Under natural processes involving the activity of micro-organisms and organic matter, soil structure builds up very slowly. It can be quickly and easily destroyed. Excessive handling when soil is wet or pressure from heavy equipment cause the loss of soil structure most often. Growers and landscape managers can add amendments, particularly organic matter, to a soil to improve soil structure for plant health (Table 1). Amendments will not “reconstruct” a destroyed soil structure. However, they promote aggregation and “jump-start” the process of soil structure building.

The best soils for plant root growth are those that have the capacity to hold adequate water for plant uptake and at the same time drain water away rapidly enough to provide adequate air to roots for respiration. This means the best soil is one with a mix of particle and pore space sizes. The larger pore spaces allow air in. The smaller ones hold water best. The need for this combination is why a well-aggregated loam soil is considered optimum (Fig. 3.)

Figure 3. The U.S.D.A. soil texture triangle.

Soil Fertility

Soil chemistry that impacts plant growth is called soil fertility. This is because the soil is the source of 14 essential mineral nutrients for plants. The other essential elements, carbon (C), oxygen (O) and hydrogen (H) come from air and water. Nutrient elements from the soil that are required by plants in large amounts are called macro-nutrients. They are Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg) and Sulfur (S). Elements needed in very small quantities, micro-nutrients, are also essential for plant growth. They are Iron (Fe), Manganese (Mn), Copper (Cu), Zinc (Zn), Molybdenum (Mo), Boron (B), Chlorine (Cl) and Nickel (Ni). Table 2 lists some of the important metabolic roles nutrients play in plant growth.

The absence of adequate amounts of a particular element in a soil, or conditions that prevent a plant from taking up adequate amounts of any nutrient result in a nutrient deficiency. Table 3 lists common symptoms associated with some mineral nutrient deficiencies. The use of fertilizers (the addition of the deficient element) is a common plant management activity. Under certain circumstances (especially plants growing in containers with a soilless growing medium) any essential element may be deficient. Most commonly, especially in the field or landscape, the macro-nutrients are the ones that limit plant growth. Of the macro-nutrients, nitrogen, is most often supplemented through fertilization, followed by phosphorus and potassium.

Nutrients are taken up from the soil by plants, not as neutral atoms, but in the form of ions. Ions are positively (+) or negatively (-) charged particles. An ion has an attraction for the opposite charge. This provides for chemical reactivity of the nutrient ions. Table 2 illustrates some of the ionic forms of important plant nutrients. A soil property that influences nutrient availability based on the ionic form of the nutrient is cation exchange capacity. In order to be available to plant roots, ions must be soluble in water. That is, they must be dissolved in the water that occupies some of the pore space in the soil (called the soil solution). A soil characteristic that affects nutrient uptake based on each ion’s tendency to dissolve in water is soil pH.

Figure 4. Essential plant nutrient availability in relation to soil pH. Wider bands indicate greatest availability, narrow bands represent limited availability.

A soil’s pH (potential Hydrogen) is a measure of its hydrogen ion activity. Commonly, this is referred to as the soil’s acidity (lots of hydrogen ion activity) or alkalinity (low hydrogen ion activity). The pH scale goes from 0 to 14. A pH of 7 is neutral. A soil is acidic below pH 7; it is alkaline above pH 7. Soil pH controls the solubility (and thus availability to plants) of mineral ions in the soil solution (Fig. 4.) Many plants tolerate a relatively wide range of pH levels near neutral (6.0 – 7.5). However, some important landscape ornamentals require specific pH conditions (e.g. rhododendrons need pH near 5.0.). pH is a logarithmic function, and moving from one number to the next on the pH scale increases the acidity or alkalinity tenfold. A pH of 5 is ten times more acidic than a pH of 6 and one hundred (10 x 10) more acidic than a pH of 7.

Soil pH can be raised (made more alkaline) by incorporation of limestone (ag lime) or lowered (made more acidic) with sulfur (flowers of sulfur). In either case, it is a temporary change. Other characteristics of the soil, such as the minerals from which the soil was formed, will determine how long the pH alteration will last. A few to several years may be a realistic time period before a repeat of the ag lime or sulfur application is needed. Soil testing is required to monitor soil pH.

Cation Exchange Capacity (CEC) is a measure of a soil’s capacity to hold nutrient ions so they are available to plants. It defines the soil’s ability to prevent nutrient loss due to leaching (being carried away by water that moves through the soil). A cation is a positively charged ion. Several of the most important plant nutrient ions are cations (Table 2). Soil particles are usually negatively charged. Thus, they attract the nutrient cations and hold them until a plant root comes along. The degree to which a soil has such negatively charged particles is its CEC. Clay particles and organic matter are the primary locations of CEC. Note, however, that one important nutrient, nitrogen, is most often available to plants as NO2- or NO3-. Because these are negatively charged ions (not cations), nitrogen is not retained by a soil even with high CEC.

Soil Testing

The only way to be certain about the nutrient status of soil is through soil testing. The best time to test soil is prior to planting because it is more difficult to correct soil deficiencies afterwards. Even after planting, it is wise to re-test soil every few years to determine current conditions. This is especially true if significant modifications to pH have been made.

To prepare a soil sample for submission to a testing lab, collect several separate samples from the area you expect to fertilize uniformly at the same time (a field, part of a landscape, etc.). Blend the separate samples into one combined sample. This will tend to average the variations within the area and provide appropriate information for a single, uniform fertilizer application to the area. Sampling depth should be 6 – 8 inches in turf areas and 10 – 12 inches in woody plant root zones. Dry and thoroughly mix the samples. For other particulars, follow the instructions of your testing lab.

Soil test results usually include nutrient levels for phosphorus, potassium, calcium and magnesium, soil pH, and the cation exchange capacity. Nutrient test results are usually stated in pounds per acre (lb./A.) or in parts per million (ppm). Recommendations for levels of fertilizers to apply are generally provided by the testing lab. Table 4 illustrates commonly desirable test levels. The lower ends of the ranges are generally most suited to landscape situations while the higher levels are appropriate for nursery production.

*To convert  lb./A. to parts per million (ppm), divide by 2 (800 lb./A. ÷ 2 = 400 ppm)

Nitrogen is not part of a typical soil test. This is because N levels change rapidly given that nitrogen is readily leached from soil. Nitrogen fertilizer recommendations are totally based on the requirements of the crop. It is assumed that very little residual nitrogen persists in any soil so it all must be added by fertilization.

Organic matter (OM) is not part of a typical soil test, but an OM test is usually available on request. OM can be important as a nutrient source, it helps “glue” soil particles together into aggregates, and it is a contributor to CEC, helping hold other nutrient ions. A minimum level of 5% OM is desirable.

Growing Media for Container Plants

Plants grown in containers (whether in a nursery, greenhouse, or in the landscape) require the same structural support and opportunity for root uptake of water and minerals as do plants growing in the ground. Thus, many of the same functional characteristics just discussed for soil must be provided by a container growing medium.

Modern container media seldom actually contain soil. They are generally composed of organic matter (OM) and quickly draining minerals. The OM portion is often milled tree bark or sphagnum moss peat, or may be a low cost, locally available by-product such as rice or cottonseed hulls. Minerals such as sand, vermiculite or perlite are used to promote drainage (aeration) and to alter media weight. Sand is heavy while vermiculite and perlite are very light weight.

Growing mixes vary greatly. Some growers mix their own with different formulas for specific crops. Uniform, defined products are commercially available.

A growing medium must include a range of pore sizes to facilitate both root aeration and water retention. The weight (per unit volume) of the medium should be appropriate to the use. In an outdoor nursery, containers must be heavy enough to avoid blow-over in wind, but lightweight enough to allow ease of shipping and handling. In landscape planter applications and for indoor use, a lightweight medium will facilitate ease of handling. It is desirable, too, that a growing medium not be subject to excessive shrinkage.

The fertility characteristics of container media are somewhat different than soil. Nutrient holding capacity is usually sacrificed in favor of maximum root-zone aeration. This is compensated for in two ways. At the pre-plant stage, slow release fertilizer products are typically incorporated into the mix. These, in effect, substitute for the nutrient holding ability found in soil. During plant management, frequent fertilization is typical. Often this is accomplished via liquid fertilizer in the irrigation water.

Two characteristics of a growing medium that are critical to successful fertility management are pH and soluble salts. Depending on the components of the medium, initial pH may be very different than what is optimum for the plants to be grown. Adjustment of pH by incorporation of ag lime (to raise) or sulfur (to lower) should be done during medium preparation.

Soluble salts is a measure of total nutrient availability in a growing medium. Regular monitoring of the medium’s electrical conductivity (as an indicator of soluble salt level) is helpful to avoid both under- and over-fertilization. Low levels of nutrients may result in small, slow growing plants. The real danger, however, lies in levels of soluble salts that are too high. Excessive salts can injure and even kill plant roots.

Container growing media testing techniques are somewhat different than those used for soil testing. For meaningful results, be sure the lab where you send media samples offers “container media testing,” that is different than “soil testing.”

Fertilizers

Substances that provide nutrients for plant growth are called fertilizers. There are two basic types, organic and inorganic.

Organic fertilizers come originally from a living plant or animal. They may be waste products (manure, sewage sludge) or remains from the organism itself (bone meal, blood meal, seaweed.) Nutrients are released slowly from organic fertilizers because decomposition must take place. The amount of nutrient per pound of fertilizer is small. Organic fertilizers often act as soil structural amendments promoting soil particle aggregation, in addition to their fertilizer effect.

Inorganic fertilizers are produced through mining and manufacturing. The amount of nutrient per pound of fertilizer is comparatively large. Inorganic fertilizers are formulated in one of two ways. The nutrients may be rapidly available to plants (fast release.) Conversely, availability may be restricted to a limited amount at any one time (slow release.) Inorganic fertilizers do not typically contribute to improvement of soil structure.

Available fertilizer forms include liquids, soluble dry powders, granules or compressed granules (tablets, spikes.) Liquid fertilizers and soluble powders that readily dissolve in water are both applied in water solutions. They may be sprayed on the soil surface, sprayed on to plant tissue (leaves), or injected into the soil. Fertilizer burn is a risk with liquids if they are applied at too high a concentration. Liquids do offer the opportunity for nutrients to rapidly enter plant tissue when they are applied to leaves. Foliar application also permits the by-passing of root uptake of nutrients. This can be useful when micro-nutrient deficiencies resulting from soil conditions are being treated.

Granular fertilizers are the most commonly used form. Loose granular fertilizers are the form with the lowest cost per unit of nutrient. They are generally satisfactory for applications of nitrogen and may be suitable for many other nutrients. The granules can be manufactured so the nutrients are released in a prescribed manner, either immediately (fast release) or over a specified period of time (slow release.) Slow release products cost more per unit of nutrient, but save labor costs through a reduced frequency of application.

Tablets or spikes are a fertilizer form of convenience. They are a costly form per unit of nutrient, but are easy to use and require no equipment calibration. For example, one can easily place a pre-determined number of tablets in a planting hole. Because they are granules “glued” together, tablets and spikes typically release nutrients somewhat slowly reducing the risk of fertilizer burn on roots.

Fertilizer analysis is a numerical statement of the amount and kind of macro-nutrient found in the fertilizer. It is used primarily for fertilizers that contain nitrogen, phosphorus, and/or potassium. By law, it must appear on the fertilizer package. The analysis is presented as three numbers, such as 10-16-8. The first number indicates the percentage, by weight, of nitrogen in the fertilizer. Thus, the example above contains 10% nitrogen. In a 50 lb. bag of fertilizer, there would be 5 pounds of nitrogen. The second number is the percentage, by weight, of phosphorus in the mix. The number actually represents a compound of phosphorus, P₂O₅, which is the accepted way of stating P content. In the example, there is 16% P₂O₅ in the formulation or 8 lb. in a 50 lb. bag. Potassium is the last number, presented as potash, K₂O. Eight percent, by weight, of the example fertilizer is potash, or 4 lb. in a 50 lb. bag. The rest of the weight of a fertilizer (33 pounds in our example), is an inert carrier. It is important for even distribution of the nutrients.

Fertilizer formulations can be described based on their contents. A complete fertilizer contains some of each of the nutrients, N, P₂O₅, and K₂O. A balanced fertilizer is one in which the amount of each nutrient is approximately equal, such as 12-12-12. A single-nutrient fertilizer contains only one of the elements (i.e. 46-0-0.) Table 5 illustrates a range of fertilizer analyses and their relative speed of nutrient release.

Fertilizers that contain nutrients other than N, P₂O₅, and K₂O list their contents on the label, too. It is done in the same way, as a percent of content, by weight.

The use of fertilizers must be done with care and precision to assure the desired effect without causing harm to plants. This often includes careful calibration of application equipment to apply fertilizer materials at the proper rates. Specific techniques of application are discussed in other sections of this manual. Help with equipment calibration can be found on some of the websites listed below.