Fertilizing the Vegetable Garden

Introduction

The amount of fertilizer to apply to a garden depends on the soil’s natural fertility of the soil, the amount of organic matter present, the type of fertilizer used, and the crop being grown. The best way to determine fertilizer needs is to have the soil tested. Soil testing is available through your local Extension agent, through private labs, and with soil test kits, which can be purchased from garden shops and catalogs.

Fertilizer Analysis

Fertilizers are identified by the analysis given on the package, which indicates the amount of an element present in the formulation as a percentage by weight. By law, all analyses provide three numbers, giving the percentage by weight of nitrogen (N), phosphate (P₂O₅), and potash (K₂O), respectively. Often, to simplify matters, these numbers are said to represent nitrogen, phosphorus, and potassium, or N-P-K. We should remember that it is not N-P-K, but N-P₂O₅-K₂O. For example, if we have a 100-pound bag of fertilizer labeled 10-10-10, there are 10 pounds of N, 10 pounds of P₂O₅, and 10 pounds of K₂O. To convert the P₂O₅ content to actual phosphorus content, multiply the given weight by 0.43. To convert K₂O content to potassium content, multiply the given weight by 0.83.

Fertilizers and pH

The degree of acidity or alkalinity of the soil, as measured by pH, is an important factor in the availability of soil nutrients to plants. At pH extremes, some nutrients become partially or completely locked up in the soil and cannot be used by plants, even though they are still present. For example, in a soil with a pH near 8.0, phosphates, iron, and manganese all become less available. At 4.5 or below, the availability of calcium, magnesium, and phosphorus for plant uptake is low. Other elements may become so readily available that they are toxic to plants, as happens with aluminum at very low pH. Most vegetables do best between pH 6.0 and 7.0. Lime is often added to increase the pH to a desirable level. However, the addition of lime does not eliminate the need to add fertilizer.

Elements

There are 16 elements essential to plant growth. Carbon, hydrogen, and oxygen come from air and/ or water. Nitrogen, phosphorus, and potassium are considered fertilizer macronutrients because plants require them in relatively large quantities for maximum growth and may need to be added to the soil annually. Calcium, magnesium, and sulfur are secondary macronutrients but are usually either present in sufficient quantities or are added coincidentally with other materials (e.g., lime). The other 7 nutrients, called micronutrients (or trace or minor elements), are needed in very small quantities, and most soils already contain sufficient supplies.

Healthy, well‑built soils typically supply sufficient trace elements for home vegetable gardens, so micronutrient deficiencies are uncommon when good soil‑building practices are followed. Regular additions of organic matter contribute carbon and a wide range of essential nutrients, improving soil structure, water‑holding capacity, and biological activity. Most garden and landscape plants perform best when soil organic matter levels reach at least 2%, while vegetable and flower beds benefit from levels between 5% and 10%. Many soil testing laboratories include an organic matter determination as part of their standard soil analysis or offer it as an optional add‑on, making it easy for gardeners to monitor and manage soil organic matter over time.

Synthetic fertilizers are relatively pure chemicals, and most do not carry the trace elements normally present in organic fertilizers. Therefore, the addition of trace elements through purchased or other organic materials such as manure, compost, green manures, and mulching, etc. is recommended.

Understanding Fertilizers

Types of Fertilizers

Organic Fertilizers

The word organic, applied to fertilizers, simply means that the nutrients contained in the product are derived solely from the remains or by-products of a once-living organism. Urea is a synthetic organic fertilizer, an organic substance manufactured from inorganic materials. Cottonseed meal, blood meal, bone meal, hoof and horn meal, and all manures are examples of organic fertilizers. When packaged as fertilizers, these products will have the fertilizer ratios stated on the labels. Some organic materials, particularly composted manures and sludges, are sold as soil conditioners and do not carry a nutrient guarantee, although they contain small amounts of nutrients. Most are high in one of the three major nutrients and low in the other two, although you may find some fortified with nitrogen, phosphorus, or potash for a higher analysis. Many are low in all three. In general, organic fertilizers release nutrients over a fairly long period. The potential drawback is that they may not release enough of their principal nutrient quickly enough to meet the plant’s needs for optimal growth. Because organic fertilizers depend on soil organisms to break them down and release nutrients, most are effective only when the soil is moist and warm enough for those organisms to be active. In addition to providing nutrients, organic fertilizers increase the organic content of the soil, improve the physical structure of the soil, and increase bacterial and fungal activity, particularly the mycorrhizal fungus, which makes other nutrients more available to plants.

Synthetic Fertilizers

In general, synthetic fertilizers act more quickly than organic types, though some organic materials release their nutrients quite rapidly. It isn’t possible, therefore, to make a blanket statement about the long-term effects of fertilizers, except that organic materials such as manure and plant waste do usually help improve the soil structure while adding nutrients. In contrast, chemical fertilizers do not affect soil structure. General-purpose synthetic fertilizers have the advantage of being readily available to the gardener and relatively inexpensive. When over‑applied, some synthetic fertilizers can be detrimental to earthworms. Excess nutrient salts, soil acidification, and ammonia release from nitrogen fertilizers (such as urea) may reduce earthworm survival and reproduction. Following recommended application rates helps protect soil‑dwelling organisms.

Complete vs. Incomplete Fertilizers

A fertilizer is said to be complete when it contains nitrogen, phosphorus, and potassium (10-10-10, 5-10-10, 5-30- 5, etc.). An incomplete fertilizer contains one or two of the major components. Examples would include triple super phosphate (0-46-0) and potassium nitrate (13-0-44), ammonium nitrate (33.5-0-0). An incomplete fertilizer might be used in situations where the soil tests very high for phosphorus and potassium. In this example, a nitrogen-only fertilizer such as ammonium nitrate (33.5-0-0) would be an appropriate choice to use.

Fertilizer Formulation

Fertilizers come in many shapes and sizes. Different formulations are made to facilitate types of situations in which fertilizer is needed. Packaging for all formulations must show the amount of nutrients contained, or the analysis, and sometimes it tells how quickly a nutrient is available. Some of the formulations available to homeowners include granular solids, water-soluble powders, and slow-release spikes, liquids, and tablets.

In most fertilizers, a “filler” is added to bulk up the fertilizer. This is done to lower the analysis, allowing a more even spreading pattern as compared to spreading a small amount of a high-analysis material over an area. A filler is generally an inert material such as sand, lime, ground corn cobs, etc. Some fillers are used to enhance the fertilizer’s handling qualities

Not all types of formulations are commonly used by homeowners for vegetable gardens. The most common and readily available formulations are:

Dry/Granulated Fertilizers

This is the most common type of fertilizer applied to the garden. Examples are 10-10-10, 5-10-5, and 5-10-10. The manufacturer treats the material so that it has large more evenly sized grains. Granules spread more evenly and easily. Sometimes granules are coated to prevent moisture absorption.

Liquid Fertilizers

Liquid fertilizers come in a variety of different formulations, including complete and incomplete. All are made to be diluted with water; some are concentrated liquids themselves, while others are powders or pellets. Fertilizer solutions are often used to water-in transplants, providing an immediately-available supply of nutrients for fast root growth and plant establishment. Liquid fertilizers may also be applied to plant foliage, where the nutrients are absorbed directly through the leaf surface. This foliar feeding provides nutrients to the plant very quickly.

There are several fertilizer options for liquid application. Commercial soluble fertilizers—often sold as transplant starters—are widely available and easy to use. Granular fertilizers are generally not appropriate for this purpose because most of their phosphorus is not readily soluble. Organic liquid products, such as fish emulsion and liquid kelp (seaweed extract), also provide good nutrient sources. Always follow the label directions when using any of these materials.

Application Methods

There are different methods of applying fertilizer depending on its formulation and the crop needs.

Broadcasting

A recommended fertilizer rate is spread over the growing area and incorporated into the soil with a rototiller or spade. Broadcasting is used over large garden areas or when time or labor is limited.

Banding

Banding is one way to meet the needs of many plants (especially tomatoes) for phosphorus as their first roots develop. When fertilizers are broadcast and worked into the soil, much of the phosphorus is locked up by the soil and is not immediately available to the plant. By concentrating the phosphorus in the band, the plant is given what it needs even though much of the phosphorus remains locked up.

Narrow bands of fertilizer are applied in furrows 2 to 3 inches from the garden seeds and 1 to 2 inches deeper than the seeds or plants. Improper placement of the fertilizer band—especially when it is too near the seed row—can expose young roots to excessive salts and cause root burn. The best technique is to stretch a string along the row where the seed is to be planted. With a corner of a hoe, dig a furrow 3 inches deep, 3 inches to one side, and parallel with the string. Spread 1/2 of the suggested fertilizer rate in the furrow, then cover it with soil. Repeat the banding operation on the other side of the string, then sow seeds underneath the string.

For widely spaced plants, such as tomatoes, fertilizers can be placed in bands 6-inch bands long for each plant or in a circle around the plant. Place the bands 4 inches from the plant base. If used in the hole itself, place the fertilizer at the bottom, work it into the soil, and cover with about 2 inches of soil before putting the plant in the hole.

Recommended application rate: Apply one pound of 10-10-10 or two pounds of 5-10-5 (or 5-10-10) per 100 feet of row.

Starter Solutions

Another way to meet the need for phosphorus when setting out transplants is to use a liquid fertilizer high in phosphorus as a starter solution. Follow directions on the label.

Side-Dressing

Dry fertilizer is applied as a side dressing after plants are up and growing. Scatter fertilizer on both sides of the row 6 to 8 inches from the plants. Lightly cultivate it into the soil, avoiding damaging the roots, and water thoroughly.

Recommended application rate: Apply one pound of 10-10-10 or two pounds of 5-10-5 (or 5-10-10) per 100 feet of row.

Foliar Feeding

Foliar feeding is used when insufficient fertilizer was used before planting, when a quick growth response is wanted, when micronutrients (such as iron or zinc) are locked into the soil, or when the soil is too cold for the plants to use the fertilizer applied to the soil. Foliar-applied nutrients are absorbed and used by the plant quite rapidly. Absorption begins within minutes of application and, for most nutrients, it is completed within 1 to 2 days. Foliar nutrition can be a supplement to soil nutrition at a critical time for the plant, but not a substitute, since greater amounts of plant material are needed than what can be absorbed through the plant leaf at any given time. At transplanting time, a phosphorus spray will help establish young plants in cold soils. For perennial plants, early spring growth is usually limited by cold soil, even when the air is warm. Under such conditions, soil microorganisms are not active enough to convert nutrients into forms available for root absorption; yet, if the nutrients were available, the plants could utilize them. A foliar spray will provide the necessary nutrients immediately, allowing the plants to begin growing. 

Liquid feeding is appropriate for container plants to provide the nutrients they need throughout the growing season. The commercial forms are generally cost-prohibitive for continued use in the garden where a greater soil volume is available to the plant roots.

Instead, liquid feeding is more of a one-time procedure, either as a transplant starter or as a foliar feeding to correct a deficiency in a major or trace element.

If a foliar feeding is desired, follow directions carefully. Using too much fertilizer, especially the synthetic forms, can quickly burn the foliage. It is also best to apply foliar nutrients early in the morning before the sun heats the air temperature. As the ambient air temperature increases, the likelihood of a phytotoxic reaction increases.

It is also best to apply foliar nutrients early in the morning before the sun heats the air temperature. As the ambient air temperature increases, the likelihood of a phytotoxic reaction increases.

Timing of Fertilizer Application

Soil type dictates the frequency of fertilizer application. Sandy soils require more frequent applications of nitrogen and other nutrients than do clay-type soils. Other factors affecting the frequency of application include the type of crop, the required level of crop productivity, the frequency and amount of water applied, and the type of fertilizer applied and its release rate.

The type of crop influences the timing and frequency of application since some crops are heavier feeders of nutrients than others. Root crops require less nitrogen fertilization than do leafy crops. Corn is a heavy nitrogen feeder of nitrogen, and may require nitrogen fertilization every 4 weeks. A general rule of thumb is that nitrogen is for leafy top growth; phosphorus is for root and fruit production; and potassium is for cold hardiness, disease resistance, and general durability. Proper use of nutrients can control plant growth rate and character. Nitrogen is the most critical nutrient in this regard. If tomatoes are fertilized too heavily with a nitrogen fertilizer or sidedressed before fruit set, the plants may be all vine and no fruit. This is also the case with potatoes, which will show excessive vining and poor tuber formation. If slow-release fertilizers or heavy amounts of manure are used on crops that form fruit or vegetables, leaf and vine growth will continue into late summer, and fruit and vegetable development will occur very late in the season.

Remember that a nitrogen application will have its greatest effect for three to four weeks after application. If tomatoes are fertilized heavily on June 1, there may be no flower production until July 1, which will, in turn, delay fruit ripening until late August. For this reason, it is important to plant crops with similar fertilizer needs close together to avoid improper fertilizer application rates.

Additional Resources

VCE Publication 452-129 Soil Sampling for Residential Areas (https://www.pubs.ext.vt.edu/452/452-129/452-129.html)

VCE Publication 452-701 Explanation of Soil Tests (https://www.pubs.ext.vt.edu/452/452-701/452-701.html)

VCE Publication SPES-384NP Your Soil Test Report Simplified; A Guide for Homeowners (https://www.pubs.ext.vt.edu/SPES/spes-384/spes-384.html)