Learn how organic amendments improve the soil of your vegetable garden and how to apply such soil soil amendments. Our next post will covers precautions to take when applying soil amendments, so visit our site or page tomorrow to learn more.
In the vegetable garden, the routine addition of organic soil amendments such as compost will optimize potential yields and quality. The goal in soil management is to increase the organic content to 4-5%, over a period of years.
Common amendments include compost, manure, compost made with manure, fall leaves, straw, and peat moss. Home compost has the advantage that the gardener controls what goes into the compost, reducing problems with salts, weed seeds, and plant diseases.
Another method to add organic matter is to replant the fall garden with a green manure crop such as winter rye or Austrian peas. Some of these cover crops fix small amounts of nitrogen in their roots that is tilled into the soil for plant use.
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 oxygen levels and improves soil drainage, which in-turn increases the rooting depth allowing roots to readily 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.
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.
Table 1 gives 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.
Compost, which includes manure or biosolids as a component, has a potential for high salts. Excessive salt levels are common in many commercially available products sold in Colorado. On compost made with manure or biosolids, 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. 711-3
Note: dS/m or mmhos/cm is the unit used to measure salt content. It measures the electrical conductivity of the soil.
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 organic content builds in a garden soil, 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.
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.
Table 1. Routine Application Rate for Compost
Depth of compost before incorporation1
Plant Base Compost and other compost known to be low in salts3
Compost made with manure or biosolids for which the salt content is unknown4
One-time application—such as lawn area
Annual application to vegetable and flower gardens – first three years
Annual application to vegetable and flower gardens – fourth year and beyond
- 3 cubic yards (67 bushels) covers 1,000 square feet approximately 1 inch deep.
- 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 plants to reduced growth, low vigor and low stress tolerance. The 3-4” inch depth is shown as an illustration of how application rates need to adjust when the deep cultivate is not practiced.
- Plant based composted are derived solely from plant materials (leaves, grass clippings, wood chips and other wards wastes). Use this application rate also for other compost known, by soil test, to be low in salts.
- 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.