In the garden, managing soils to improve tilth and maintain garden fertilization are related but not necessarily the same process. For example, compost or manure may be added as a soil amendment to improve tilth; however, they may only add nominal plant nutrients. A manufactured fertilizer may be added to supplement soil fertility levels, but it will not improve a soil’s tilth. For optimum yields and quality, gardeners need to pay attention to both soil management for improving tilth and soil fertilization.
Tilth is a term related to the suitability of a soil to support plant growth. Technically speaking, tilth is the physical condition of soil as related to its ease of tillage, fitness of seedbed, and impedance to seeding emergence and root penetration.
Soil Amendment or Fertilizer
The term soil amendment refers to any material mixed into a soil. By law, soil amendments make no legal claims about nutrient content or other helpful (or harmful) properties. Compost and manure are common soil amendments used to improve soil tilth. They may also supply nominal amounts of plant nutrients. Some of the nutrient effect seen from adding soil amendments is likely due to their effect on soil microorganisms. The organic material in soil amendments is a food source that allows microorganisms to multiply. The larger numbers increase the conversion of nutrients in the soil to plant usable forms. By contrast, mulch refers to a material placed on the soil surface.
By law, the term fertilizer refers to a material that guarantees a minimum percentage of nutrients of nitrogen, phosphate, and potassium. Organic fertilizer is derived from natural sources and guarantees the minimum percentages of nitrogen, phosphate, and potassium.
Soil Amendments
In the vegetable garden, the routine addition of organic soil amendments such as compost can optimize potential yields and quality. The goal in soil management is to increase the organic content to 3-5%, over a period of years. This level of organic material will produce nitrogen through a process called mineralization over time. However, it does need to be maintained as it will diminish over time, especially in intensive gardening such as vegetable gardening.
Common amendments may include compost, manure, compost made with manure, fall leaves, straw, coco (coconut) coir, or peat moss. Home compost has the advantage that the gardener is in control of what goes into the compost, reducing problems with salts, weed seeds, and plant diseases.
In climates with long growing seasons, another method to add organic matter is to grow green manure crops in between the vegetable growing season. In some areas, this would be a winter crop, in hot areas of the south this would be a summer heat crop. In areas like Colorado, where the entire growing season is used for vegetable production, a green manure is less practical. For additional information, refer to CMG GardenNotes #244, Cover Crops and Green Manure Crops.
How Organic Amendments Improve the Soil
On clayey soil, organic matter (over a period of years) glues the tiny soil particles together into larger aggregates, increasing pore space. This increases soil aeration and improves soil drainage, which in-turn increases plant rooting depth allowing roots to reach a larger supply of water and nutrients.
On sandy soils, organic matter holds over ten times more water and nutrients than sand.
Organic matter also encourages the beneficial activity of soil organisms and helps remediate soil compaction.
Application
General application rates for compost or other organic soil amendments are based on the salt content of the materials and soil and on the depth to which it is cultivated into the soil. Ideally, cultivate the soil amendment into the top six to eight inches of the soil. On compacted/clayey soils, anything less can lead to a shallow rooting system with reduced plant growth, lower vigor, and lower stress tolerance. Three (3) cubic yards of compost will cover 1,000 square feet approximately one (1) inch deep.
Table 1 gives the standard application rate for compost. Compost made solely from plant residues (leaves and other yard wastes) is basically free of salt problems, and higher application rates are safe when keeping the overall percentage organic material below 5%.
Compost, which includes manure or biosolids as a component, has a potential for high salt. Excessive salt levels are common in many commercially available products sold in Colorado. In compost made with manure or biosolids, the application rate is limited unless a soil test on that batch of product shows a low salt level. An amendment with up to 10 dS/m (10 mmhos/cm) total salt is acceptable if incorporated six to eight inches deep in a low-salt garden soil (less than 1 dS/m or 1 mmhos/cm). Any amendment with a salt level above 10 dS/m (10 mmhos/cm) is questionable.
Note: dS/m or mmhos/cm is the unit used to measure salt content. It measures the electrical conductivity of the soil.
Table 1. Routine Application Rates for Compost
| Site | Incorporation Depth1 | Depth of plant-based or other low-salt2 compost before incorporation | Depth of manure- or biosolid-based compost or compost for which the salt content is unknown3 before incorporation |
|---|---|---|---|
| One-Time application such as a new lawn or perennial garden | 6-8 inches | 2-3 inches | 1 inch |
| Annual Application for new vegetable and flower gardens (first three years) | 6-8 inches | 2-3 inches | 1 inch |
| Annual Application for established vegetable and flower gardens (older than three years) | 6-8 inches | 1-2 inches | ½ inch |
| 1 Incorporation Depth: Cultivate compost into the top 6-8 inches of the soil. On compacted/clayey soils, anything less may result in a shallow rooting depth predisposing plant to reduced growth, low vigor, and low stress tolerance. Application rates need to reduce when deep incorporation is not practiced. 2 Plant Based Compost: Plant based composts are derived solely from plant materials (leaves, grass clippings, wood chips, and other yard wastes). Also use this application rate for other compost known, by soil test, to be low in salts. 3 Compost Made with Manure or Biosolids: Use this application rate for any compost made with manure or biosolids unless the salt content is known, by soil test, to be low. Excessive salts are common in many commercially available products sold in Colorado. Based on soil tests of commercially available compost, this application rate may be too high for products extremely high in salts. |
Compost needs to be thoroughly mixed into the upper six to eight inches of the soil profile. Do not leave compost in chunks, as this will interfere with root growth and soil water movement.
As the soil’s organic content builds in a garden, the application rate should be reduced to prevent ground water contamination issues. A soil test is suggested every four to six years to establish a base line on soil organic matter content.
If using a green manure cover crop, till the cover crop in before it reaches four inches in height depending on the goals of the cover crop. Some cover crops can be left in place to function as a green mulch.
In the vegetable garden, do not plow in woody materials such as bark or wood chips. They may interfere with seedbed preparation and may result in soil nitrogen depletion.
Precautions When Using Compost and Manure
Manure, compost made from manure, and bio-solids may be high in salts that will interfere with crop growth. Do not add more than one inch per season without conducting a soil test to evaluate potential salt build-up.
Due to a health issue (E. coli contamination), fresh manure additions should be made at least four months prior to the harvest of any edible crops. In other words, apply fresh manure only in the fall after crops are harvested.
Fresh manure or unfinished compost products may be high in ammonia. Avoid application of products with an ammonia smell; they could burn roots and leaves. Manure and compost may be a source of weed seeds.
Nutrient Release Rates from Compost and Manure
Gardeners need to understand that the nutrient release from compost and manure is slow, taking years. Adding compost or manure to improve soil tilth is not the same as fertilizing.
The typical nitrogen release rates from manure are only 30% to 50% the first year (fresh manure), 15 to 25% the second year, 7% to 12% the third year, 3% to 6% the fourth year, and so on. With compost and composted manure, the release rate is even slower, 5% to 25% the first year, 3% to 12% the second year, and 1% to 6% the third year.
Because the nitrogen percentage of compost and manure products is typically only 2% to 4%, the amount of actual nitrogen released to support crop growth is very small.
For soil with 4% to 5% organic matter, the mineralization (release) of nitrogen from soil organic matter will likely be sufficient for crop growth.
For soils with 2% to 3% organic matter, the mineralization of nitrogen from soil organic matter will not likely be sufficient for heavy feeding vegetable crops. Supplement with 0.1 pound nitrogen fertilizer per one hundred square feet.
For typical garden soil with 1% organic matter or less, the mineralization of nitrogen for soil organic matter will be minimal. Add 0.2 pounds of nitrogen fertilizer per one hundred square feet.
Fertilization
Soil fertilization is the addition of soil nutrients to support crop growth. While some soil amendments do add nutrients, amending the soil to improve soil tilth is not the same as amending the soil to provide nutrients.
Manufactured fertilizers are popular with gardeners because they are readily available, inexpensive, easy to apply, and generally provide a quick release of nutrients for plant growth. Application rates for any fertilizer depend on the content and the amount of nutrient to be applied. In products containing multiple nutrients, the application rate is always based on the nitrogen content.
Starter Fertilizers
In setting out transplants, starter solutions often promote early growth. Because transplants have been hardened-off (growth slowed to prepare the plant for movement to the exposed, windy, outdoor environment), the nitrogen in the starter solution gives the signal to resume active growth. Because phosphorus is less available in cold soils, phosphate may also be helpful in spring and before soils have thoroughly warmed.
A starter fertilizer is any water-soluble fertilizer added to the irrigation water. Common examples include MiracleGro, Peters, Schultz Plant Food, Fertilome Root Stimulator, and Plant Starter Solution, etc. They generally contain ammonium nitrate since it is readily usable by the plant. Some products claim that vitamins or hormones promote plant growth. These claims are not supported by research findings.
Nitrogen Applications
Nitrogen is the nutrient needed in largest quantities by plants and the one most frequently applied as fertilizer. It is annually applied in the form of manufactured fertilizer, organic fertilizers, and/or organic soil amendments. Application rates are critical, because too much or too little directly affects crop growth.
The standard annual application rate for home vegetable gardens is two pounds of actual nitrogen per one thousand square feet (0.2 pound actual nitrogen per one hundred square feet). When organic matter is present, adjust the rate accordingly to account for nitrogen released by the organic matter. [Table 2]
Manufactured solid nitrogen fertilizer can be broadcast and watered in or broadcast and tilled into the top few inches of soil. It can be banded three to four inches to the side of the seed or plant row. Do not place the fertilizer in the seed row or root injury will occur. Some soluble types are applied in the irrigation water through a process called “fertigation.” “Organic” nitrogen fertilizers are typically tilled in or can be applied in irrigation water.
Table 2. Standard Nitrogen Fertilizer Application Rates for Gardens
| Low Soil Organic Matter (below 2%) | Moderate Soil Organic Matter (2-3%) | High Soil Organic Matter (4% or higher) | |
|---|---|---|---|
| Nitrogen Needed | 0.2lb actual N per 100 sq. ft. | 0.1lb actual N per 100 sq. ft. | 0 |
| Application rate of 21-0-0 Ammonium Sulfate | 1lb fertilizer per 100 sq. ft. (about 2 cups) | 0.5lb fertilizer per 100 sq. ft. (about 1 cup) | 0 |
| Application Rate of 34-0-0 Ammonium Nitrate | 0.6lb fertilizer per 100 sq. ft. (about 1 1/3 cups) | 0.3lb fertilizer per 100 sq. ft. (about 2/3 cup) | 0 |
| Application Rate of 45-0-0 Urea | 0.4lb fertilizer per 100 sq. ft. (about 1 cup) | 0.2lb fertilizer per 100 sq. ft. (about 1/2 cup) | 0 |
Nitrogen “Side Dressing”
Plant need for nitrogen varies. Beans, peas, tomatoes, and vine crops (cucumbers, squash, pumpkins, and melons) are examples of vegetables with a lower need for nitrogen as they produce their own through a symbiotic relationship with bacteria called rhizobia. High nitrogen levels promote excessive growth of the plant at the expense of fruiting.
Crops such as potatoes, corn, and cole crops (broccoli, cauliflower, cabbage, and kale) use large amounts of nitrogen and need supplemental applications during the growing season (referred to as side dressing). For example, home garden potatoes often show nitrogen deficiency from August into fall. Symptoms start as a yellowing of lower leaves and progress into a general browning and dieback of the vine. When nitrogen stress hits, potatoes become more susceptible to diseases, including early blight and Verticillium wilt. [Table 3]
Fertilizers commonly used in the home garden for side dressing include ammonium sulfate, ammonium nitrate, and water-soluble fertilizers such as MiracleGro, Peters, etc. Ammonium sulfate and ammonium nitrate should be sprinkled over the soil and watered in or applied to a furrow to the side of the plant. Over-application will burn roots, stunting or killing plants. Water-soluble fertilizers have a low burn potential but are significantly more expensive.
Table 3. Timing of Nitrogen Side Dressing of Vegetable Crops
| Vegetable | Timing |
|---|---|
| Asparagus | 1. Early Spring 2. At end of harvest season |
| Sweet Corn | 1. 12 inches tall 2. One month later |
| Leafy greens | 3-4 weeks after emergence |
| Onions | 3-4 weeks after emergence |
| Potatoes | Late July to Early August |
| Tomatoes, peppers, and eggplant | When the first fruits reach 1” diameter |
| Cole crops (broccoli, cabbage, cauliflower) | 1. 2-3 weeks after transplanting 2. 4-5 weeks after transplanting |
Phosphorus and Potassium Applications
Phosphate and potash fertilizers are best added in the spring or fall when they can be cultivated into the soil. Many Colorado soils have sufficient levels of phosphorous and potassium and are rarely deficient.
A soil test is the best method to determine the need for phosphate and potash. With a fertilizer containing nitrogen and phosphate and/or potash, the application rate is always based on the nitrogen percentage because nitrogen is most critical to plant growth.
Phosphate and potash fertilizers are best applied in the spring or fall when they can be tilled into the soil.
Phosphorus
Phosphorus levels are adequate in the majority of established Colorado soils. Deficiencies are most likely to occur in new gardens where the organic matter content is low and in soils with a high pH (7.8 to 8.3). Excessive phosphorus fertilizer can aggravate iron, zinc deficiencies, and increase soil salt content.
Routine application of compost or manure will supply the phosphorus needs in most Western soils.
Where phosphorus levels are believed to be low, the standard application rate without a soil test is ¼ to one pound triple super phosphate (0-46-0) or ammonium phosphate (18-46-0) per one hundred square feet.
Potassium
Potassium levels are naturally adequate to high in most Colorado soils. Deficiencies occasionally occur in new gardens low in organic matter and in sandy soils low in organic matter. Excessive potash fertilizer can increase soil salt content.
Routine applications of compost or manure will supply the potassium needs for most Western soils.
Where potash levels are believed to be low, the standard application rate without a soil test is ¼ to ½ pound potassium chloride (0-0-60) or potassium sulfate (0-0-50) per one hundred square feet.
Managing Soil Compaction
On clayey soils, soil compaction is a common problem, limiting crop growth potential. Soils are typically compacted in the construction process. Walking on wet soils, cultivating wet soils, and the impact of rain are other common forces compacting soils.
The following are suggested to help minimize soil compaction in the garden:
- Add organic matter to clayey soils. Incorporate to the average rooting depth eight to eighteen inches deep.
- Avoid cultivating or working a clayey soil when wet. To evaluate, squeeze a handful of soil. Then try to crumble it. If it crumbles, it can be worked. If it will not crumble but stays in mud balls, it is too wet to be worked.
- Avoid cultivating other than to prepare a seed bed or till in organic matter and fertilizers. For weed control, use mulch, hand removal, or shallow cultivation only.
- Use a raised bed with established walkways and avoid walking on the growing bed.
- Mulch the soil, year round, to minimize the compaction forces of rain and sprinkler irrigation. Winter rains on bare soil are a major compaction force. This also helps manage weeds and reduces irrigation need.
This publication, reference GardenNotes #711, is developed as part of the Colorado State University Extension Master Gardener Program.