Q: What exactly is meant by “soil texture”?
A: Soil texture is the relative proportions of sand, silt, and clay mineral particles in a given soil. It is one of the most important physical properties affecting plant growth because it determines a soil’s fitness as a medium for growing plants (known as its tilth) as well as nutrient and water-holding capacities. A sandy, coarse-textured soil is often called a light soil, whereas a clay or fine-textured soil is referred to as a heavy soil. The terms reflect the relative ease of working the two soil types. Twelve basic soil textural classes are recognized, based on the USDA classification system.
Q: What is “soil structure”?
A: Except for sand grains, soil particles typically do not exist as single particles but rather coalesce into groups of particles known as aggregates. Soil structure refers to the way in which these aggregates are arranged into various sized crumbs and clods. Plant growth is strongly influenced by soil structure because it affects moisture availability to plants (water movement), fertility, aeration, porosity, heat transfer, bulk density, and mechanical resistance to root growth. Plants require air, water, and nutrients from the soil, and the air-water relationship depends largely on soil structure. A soil with good structure will have good water infiltration, drainage, aeration, and overall tilth.
Q: What does the soil pH refer to?
A: The pH of a garden soil is a measure of its relative acidity or alkalinity. Soil pH is important because it influences numerous chemical reactions that occur in the soil, including the availability of the 17 nutrient elements essential for plant growth and development. The soil pH is a function of the hydrogen ion (H+) concentration in the soil. The pH scale consists of 14 divisions from 1 to 14, with the value of 7 being neutral, 1 being most acidic, and 14 being most alkaline or basic. Like the scale for measuring earthquakes, the pH scale is logarithmic, not linear. A pH of 5.0 is 10 times more acidic than a pH of 6.0 and 100 times more acidic than a pH of 7.0. A pH of 7.0 means that alkalinity and acidity are equally balanced in the soil.
Q: Why are soil pH, acidity, and alkalinity levels important?
A: Most crops do best when the pH is slightly acidic to neutral (pH of 5.5 to 7.5). Soils in California typically range from pH 5 to 8.5, but a pH near neutral or slightly higher (alkaline) is most common. Measuring soil pH is done most easily with the soil test kits sold at garden centers. They are relatively cheap and reliable.
Q: What does it mean when a soil is high in salt or it has high salinity?
A: Soluble mineral salts dissolve easily in water and can accumulate in soil. In moist climates they are naturally washed out by rainfall, but in arid and Mediterranean climates they naturally accumulate. Fertilizers contribute plant-essential nutrients in the form of soluble salt compounds. Irrigation water also contains soluble salts, with the salt concentration in water varying throughout the state.
Too much salinity can damage susceptible plants. Many ornamental, fruit, and vegetable plants are very susceptible to salinity. Saline soils should be irrigated with water beyond the amount required by the plants in order to wash away the excess salts from the root zone.
Soil salt concentration is measured as electrical conductivity (EC). in units known as deci-Siemens per meter (dS/m). A salt level of 2.0 dS/m or greater can affect sensitive plants, but many plants tolerate an EC level of about 4.0 dS/m. Levels of two to four times this amount are usually necessary before easily diagnosed foliar symptoms (leaf burn or scorch, leaf drop) are expressed. Measuring salinity or electrical conductivity requires technical knowledge.
Q: What nutrients are required by plants?
A: Currently, 17 nutrient elements are known to be essential for plant growth and development. Of the 17 essential nutrients, 3 are taken from the air and water, and the other 14 are absorbed from the soil by plant roots. As the source of the majority of necessary plant nutrients, soil solids are critical to plant health. All 17 elements are equally essential for plant growth and development, but the 14 derived from the soil are divided into three categories based on the relative amounts required: primary nutrients, secondary nutrients, and micronutrients.
Is fertilizer always needed when planting in the garden or landscape?
A: Garden and landscape soils are rarely fertile enough to supply all of the nutrients required for the best growth of plants, particularly while they are becoming established. It is equally rare, however, for a soil to be deficient in several of the mineral nutrients that plants need. Because California soils contain most of the elements known to be essential to plants, it is necessary to add only the ones that are deficient in a particular soil. In general, gardeners have a tendency either to underfertilize or overfertilize. Too little fertilization results in poor plant growth and appearance. Too much fertilization, regardless of the source, is unnecessarily expensive and may cause plant injury or contribute to nitrate pollution in the environment.
What plant nutrients are most commonly needed for fertilizers to supply?
A: Because nitrogen is naturally low in almost all California soils, additional amounts are needed to ensure optimal plant growth. Nitrogen may not be the only supplement necessary, however. Phosphorus may be low in some highly weathered California soils, which are often reddish and have hardpan or claypan layers in the subsoil. Other soils may be low in available phosphorus because of long-term cropping or because of alkalinity, which makes soil phosphorus unavailable. Phosphorus is relatively immobile in soil. Potassium is not commonly needed in most gardens because California soils naturally contain this element. Iron deficiency is common when acid-loving plants are grown in alkaline soils. Iron deficiency can be corrected by acidifying the soil or using iron fertilizers (iron chelate or iron sulfate) according to package directions. Zinc deficiency may occur in gardens where the surface soil has been removed during building and leveling operations. Soil analysis may help to diagnose a deficiency. Fertilizers containing zinc, such as zinc sulfate, zinc ammonium nitrate, or zinc chelate, can correct the problem.
Q: Are there regulations on fertilizer products in California?
A: Yes, state law contains specific requirements for fertilizer products. By law, the guaranteed content of the fertilizer, expressed as a percentage of each plant nutrient supplied, must be stated on the bag or container. Under this labeling method, the first number shown is the percentage of nitrogen (N); the second is the percentage of phosphorus (P), expressed as P2O5 (phosphoric acid); and the third is the percentage of potassium (K), expressed as K2O (potash). A 100-pound (45.4-kg) bag of a 12-12-12 grade fertilizer contains 12 pounds (5.45 kg) each of N (nitrogen), P2O5 (phosphorous), and K2O (potassium). The other 64 pounds in the bag is filler, which facilitates even spreading of fertilizer. If any other plant nutrients have been added to the fertilizer, then they must be listed below the N-P-K content with their guaranteed percentage.
Q: Is there any difference between a “fertilizer” and a “soil amendment”?
A: Whether a material is considered a soil amendment or a fertilizer is usually determined by its effect on plant growth. Fertilizers affect plant growth directly by improving the supply of available nutrients in the soil. Amendments, on the other hand, influence plant growth indirectly by improving the soil’s physical properties (e.g., soil tilth, water infiltration, etc.) or chemical properties (e.g. pH, salinity, etc.). The distinction between these two concepts is clear when you compare materials such as ammonium nitrate (a fertilizer) and gypsum (an amendment). It is more difficult to distinguish between amendments and fertilizers when evaluating natural or organic products. Animal manure, for example, easily falls into either category, depending on your reasons for applying it. Manure can be a source of readily available nutrients, but it can also supply significant quantities of organic matter, which decreases soil bulk density, improves soil aeration, water retention, water infiltration, and nutrient-holding capacities.
Q: Is it better to use organic fertilizers?
A: The general belief that organic materials are better sources of plant nutrients than inorganic forms is not correct. Plants respond equally well to organic or inorganic (chemical) forms of fertilizer. For both sources to be beneficial to plants, their nutrients must be changed into chemical forms that plants can absorb and use. Many successful gardeners use a combination of inorganic and organic fertilizers. Regardless of which fertilizers you apply, use them efficiently to reduce cost, prevent plant injury, and to prevent unwanted changes in the environment, such as leaching of nitrogen.
Q: What are the differences between organic fertilizers and other fertilizers?
A: By state law, organic fertilizers are defined as materials derived from either plant or animal products containing one or more nutrients other than carbon, hydrogen, and oxygen, which are essential for plant growth; which may be subjected to biological degradation processes under normal conditions of aging, rainfall, sun-curing, air drying, composting, rotting, enzymatic or anaerobic/aerobic bacterial action, or any combination of these; which shall not be mixed with synthetic materials or changed in any physical or chemical manner from their initial state except by physical manipulations such as drying, cooking, chopping, grinding, shredding, or pelleting. These fertilizers are typically derived from various manures or concentrated organic materials like blood meal, fish meal, and bone meal.
Other fertilizers, sometimes called synthetic, chemical or inorganic fertilizers, are comprised of mined minerals and man-made chemical compounds that provide combinations of plant-essential nutrients. Common examples are urea, ammonium nitrate, ammonium sulfate, superphosphate, and muriate of potash.
Q: What are the advantages and disadvantages of using organic versus inorganic or synthetic fertilizers?
A: Organics typically cost more per unit of nutrient than synthetics, vary widely in their nutrient content, but often contain a large assortment of plant nutrients in small amounts. Also it is usually assumed that organic fertilizers are slowly released. They are more slowly released and made available to plants than many inorganic fertilizers, but release varies among organic sources. In warm soil conditions found in much of California, the nitrogen release can be completed after as little as six to eight weeks. The principal limitations of organic fertilizers are their bulk, availability, odor, potential salt and weed seed hazards, and expense per pound of nutrient. Organic concentrates—such as bonemeal, cottonseed, and fish emulsion — contain a variety of nutrients, but their cost per pound of nutrients is high relative to inorganic fertilizers. The value of manures and organic concentrates does not lie solely in their nutritional value, however. Some organic fertilizer materials also improve the soil’s physical properties, in which case they are considered soil amendments.
Inorganic fertilizers are characteristically fast-acting and relatively low in cost per pound of actual nutrient. Some inorganic fertilizers, such as ammonium sulfate based fertilizers, can acidify the soil (lower the pH) with long-term use, while others such as calcium nitrate will tend to raise pH. Because inorganic fertilizers are salts, they dissolve and release nutrients readily. Key disadvantages are their potential to (a) contaminate the environment through runoff or leaching of irrigation and rain, and (b) burn crops if over-applied or mismanaged. Slow-release nitrogen fertilizers are available at much higher cost, and their release rates are governed by environmental factors such as soil moisture content and temperature. Slow-release inorganic nitrogen is sometimes called water-insoluble nitrogen (WIN). Use of these products reduces the chances of environmental contamination and crop injury because relatively small amounts of nitrogen are present in a soluble form at any time. When used properly, plants receive a consistent supply of nitrogen, and the number of applications can be reduced versus using traditional fertilizer.
Q: How should I apply and use organic fertilizers to get the most benefit for my plants?
A: Until they decompose, manures supply plant nutrients in carbon-based, organic forms that are largely unavailable to plants. Apply manure by mixing it into the soil at least one month before preparing the soil for planting. Advance application allows time for partial rotting or decomposition of the manure and leaching of excess mineral salts from the root zone before planting. Because manures may contain undesirable weed seeds and relatively high amounts of salts, they should be used cautiously where weeds or salts could create problems.
Q: How do I decide which fertilizer product to buy, besides the price?
A: Aside from the price and the different brands of the same formulations, there is first the issue of whether you want to use a bulky organic fertilizer product, such as compost or manure, that would contribute to the soil organic matter content but require relatively large amounts to satisfy plant nutrient needs. The lowest concentration of nutrients are found in compost teas – extracts made from soaking compost in water. These are extremely variable in their nutrient content, dependent on temperature and the length of extraction time. The value of compost teas in managing soil-borne and foliar plant diseases has not been been conclusively proven in rigorous scientific tests. Their value as a source of nutrients would only be as a secondary source in a regular nutrition program. Manures can vary greatly in nutrient content. It is difficult to guarantee the analysis because it varies from season to season and according to the length of time the manures have been exposed to decomposition. Manures are typically more complete than most inorganic fertilizers, because they contain many of the essential nutrients required for plant growth and development. Dry chicken manure is the most concentrated animal manure with respect to primary nutrients, and it can serve as the only garden fertilizer applied. Dairy manure is much less concentrated and usually has less nitrogen readily available to plants. Dairy manure may require additional commercial nitrogen fertilizer. Steer manure from animals fattened on concentrated feeds is richer in nutrients than dairy manure. If it has been handled to prevent nitrogen losses, it can usually be the sole source of nitrogen.
With synthetic or inorganic fertilizers, there is the choice of whether to use a “complete” fertilizer (one that provides nitrogen, phosphorus, and potassium) or a single source one (one that provides only nitrogen, phosphorus, or potassium). When looking at single source fertilizers, there is also the question of what impact will it have on soil pH. Ammonium sulfate based fertilizers lower pH and nitrate ones tend to raise it. There are also synthetic fertilizers that contain more plant-essential nutrients than nitrogen, phosphorus, and potassium. These products are usually needed only when deficiencies of the additional elements are known or when fertilizing plants grown in containers.
Choosing whether to use a granular, water-soluble powder, or liquid formulation of fertilizer is one of convenience. The powder and liquid fertilizers are easy to mix with water for application to containers and small garden areas with irrigation water or through a hose end sprayer.
Q: Is foliar feeding a good way to fertilize plants?
A: Attempting to supply nutrients need in large quantities by plants, such as nitrogen, in sufficient quantities with this method is very expensive and inefficient because only very small amounts of nutrient elements can enter a plant through leaf tissue. However, it is feasible to supply adequate amounts of certain formulations of micronutrients by this method to correct micronutrient deficiencies. Spray the foliage using equipment like that used for pesticide application. Because plant response can be affected by droplet size and other technicalities, this method is not recommended for general fertilizer application. To fertilize vines and fruit and nut trees, certain nutrients can be applied to the foliage in combination with pesticide sprays during the latter part of the dormant season. Check labels for specific instructions regarding legal requirements and usage.
Q: When and how much should I fertilize my lawn?
A: Lawns respond primarily to nitrogen and should be fertilized in the seasons they are most actively growing. Lawns primarily composed of tall fescue and other cool-season grasses should receive most of their fertilizer in the spring and fall, while those of Bermuda, St. Augustine, and other warm-season grasses should be fertilized first in the late spring/early summer when they have fully greened up and last about a month before the first frost is expected. Tall fescue and other cool-season grasses perform well when provided 4 to 6 pounds of actual nitrogen per 1,000 square feet per year, and Bermuda and other warm-season grasses perform well with 0.5 to 1.0 pounds of nitrogen per month they are growing. It is best if at least a portion of the nitrogen applied is in a slow-release or water-insoluble form.
Q: How should I apply fertilizer to a large area like my lawn or large beds?
A: The broadcast method of applying fertilizer is very effective for large lawn areas, bed, or garden plots. It consists of uniformly distributing dry or liquid fertilizer materials onto the soil surface. Many home gardeners use a drop spreader or spinning-type spreader to facilitate uniform application of dry fertilizer. Before preparing the seedbeds for planting vegetables, annuals, or turfgrass, fertilizer may be broadcast and promptly worked into the soil to prevent nitrogen losses through ammonia volatilization. Broadcasting is an effective means of applying nitrogen and potassium to existing large gardens and turf areas but is not effective for applying phosphorus, which is immobile and must be incorporated into the soil to ensure uptake by roots.
Q: How much fertilizer does a vegetable garden need?
A: Vegetable crops primarily need nitrogen but many crops also benefit from some complete fertilizer containing phosphorus and potassium. A combination of organic and inorganic fertilizer often works well because a broader range of plant-essential nutrients are provided, those in the organic form are usually slower release, and many organic fertilizer materials (such as manures) also add organic material to the soil which improves its structure. When using a combination of manure and inorganic fertilizer, apply the manure about a month before planting at a rate of no more than one pound per square foot of garden area. Then follow up with an application of inorganic fertilizer at or just after planting at a rate of ½ to 1 pound of fertilizer material per 100 square feet of garden. If no organic material is added, double the rate of inorganic fertilizer applied. Apply additional organic or inorganic fertilizer at the pre-planting rate once seedlings are 3 to 4 inches tall or 2 to 4 weeks after transplants are set out, except for tomatoes, melons, squash, and similar crops. These should not receive additional fertilizer until they have begun to set fruit. Continue fertilizing crops with nitrogen at 3 to 4 week intervals until they are harvested.
Q: How can I most efficiently fertilize a row of newly planted seeds?
A: Band placement is a subsurface application method in which narrow bands of fertilizer are placed several inches to the side and/or below seeds. The soil type, and fertilizer determine whether bands are placed to the side and below the seed or only directly below the seed. Fertilizer placed too close to seeds may damage roots or inhibit seed germination.
In vegetable gardens, band application of a phosphate-containing fertilizer at planting time may be more effective than broadcasting because phosphorus has limited mobility in the soil. With a band application, the phosphorus will be closer to the plant roots for uptake. Ammonium phosphate (16-20-0 or 11-48-0) or several of the complete fertilizers, such as 5-10-5, 8-16-16, or 12-12-12 are acceptable materials. Dig a shallow trench 2 to 4 inches (5 to 10 cm) to one side of the row and 2 to 4 inches (5 to 10 cm) below where the seed is to be placed. Place 1 to 2 pounds (0.45 to 0.9 kg) of fertilizer per 100 feet (30 m) of row in the bottom of the trench and cover it with soil. When using furrow irrigation, place the fertilizer band between the seed or plant row and the irrigation furrow. If sprinkler irrigation is used, band the fertilizer on either side of the row.
Q: What is the most efficient way to apply fertilizer to vegetable plants as they are growing?
A: Side dressing is a method appropriate at critical plant growth stages. Side dressing is the application of dry fertilizer beside actively growing plants to replace nutrients that have been leached or used up in the growth process. For example, after vegetable plants are well established and at least 3 to 4 inches (7.5 to 10 cm) tall, side dressing with nitrogen is very effective at rates similar to those for banding fertilizer. The trench should be at least 4 inches (10 cm) from the side and below the plant row to prevent burning the roots. Subsequent irrigations will move the nitrogen into the root zone. In tomatoes, melons, squash and similar crops, do not side dress with large amounts of nitrogen until after the vegetable plants have begun to set fruit because the fertilizer will encourage vegetative growth at the expense of fruiting and of storage organ production.
Q: How much should I fertilize landscape trees and shrubs?
A: Mature trees and shrubs do not need fertilizer if they are reasonably healthy. With few exceptions, trees do not respond to supplemental fertilization in California. In fact, fertilizing healthy woody plants can be harmful by encouraging excessively vigorous growth. As long as plants have good leaf color, some annual shoot growth, and acceptable canopy density, there is no reason to fertilize them. Usually when fertilizer is needed, only nitrogen is necessary. It is easiest, quickest, and most effective to apply fertilizer to the soil surface for these plants. Broadcast about 1 to 3 pounds of actual nitrogen per 1,000 square feet of planted area for shrubs or per 6 inches of trunk diameter for trees.
Trees and shrubs less than 2 years old often benefit from fertilization. Apply 0.2 pounds of nitrogen with an area no more than 3 feet by 3 feet around the base of each plant. Apply immediately after planting, about 2 months later, then the following spring.
Q: How and where can I obtain a soil test?
A: Look at this guide, Selected Plant and Soil Laboratories in
Northern and Central California. Analyses performed include: plant and soil nutrients, plant pathology,nematology, irrigation water testing, hazardous wastes and pesticide residue. Laboratories were identified by performing internet searches for diagnostic testing related to crop production. Laboratories provided in this listing are for informational purposes only and do not constitute endorsement or recommendation by University of California Cooperative Extension.
Our thanks to Dennis Pittenger, Area Environmental Horticulturist University of California Cooperative Extension, Central Coast & South Region/Los Angeles County/U.C. Riverside for providing content for the soil & fertilizer section of California Gardening.
