Soil is an essential consideration when growing plants providing water, nutrients, and a physical place for roots to anchor and support the plant. Gardeners take steps each year (whether the soils are ideal or not) to build and maintain quality soil for their gardens.
What is Soil? | Tilling | Organic Matter | pH | Drainage | Fertilizer | More Information
How to Create & Maintain Good Soils
Start Before Planting
Knowing soil conditions before planting makes changes more manageable. Amendments and fertilizers are best and more easily incorporated before plant roots are present.
Conduct a Soil Test
A soil test is essential for making appropriate changes.
A soil sample can be collected anytime the soil is not frozen, overly dry, or saturated. The sample is a mixture of soil collected from 10-15 locations across the area to be tested. The soil samples should represent areas managed similarly or areas where the same crops/plants are grown. For example, conduct one soil test for your vegetable garden and a separate test for the lawn or perennial border.
Add Organic Matter and Other Amendments
The soil test report will give guidance on how to get the soil into an acceptable range for plant growth by providing information on the amount of amendments like fertilizers, minerals, and organic matter to add.
Incorporate amendments in the top 6 to 8 inches of soil (where most roots are located) and give them time to change the soil's physical and/or chemical properties.
Work Each Year to Build Good Soil
Soil is alive and complex. Plants and microorganisms use nutrients, water, and air to grow and break down organic matter. Weather, digging, and other disturbances can change its physical properties. This means the system is constantly changing. Without regular additions of organic matter and nutrients, the chemical and physical properties can change, making them less supportive of plant growth.
Building good soil is an ongoing process, requiring consistent effort to promote healthy, productive plant growth.
What is Soil?
Soil is a complex living system made up of minerals, organic materials, water, and air. It is teeming with life beyond the plant roots, including fungi, bacteria, and other microorganisms. There are thousands of different kinds of soil differing in their physical and chemical properties which were determined when the soil was formed thousands of years ago. Soil types vary in color, depth, size, texture, mineral composition, and organic matter content.
The ideal soil is comprised of approximately half solids (~45% minerals and ~5% organic matter) and half pore space (20-30% water and 20-30% air).
The properties of soil that affect plant growth include its texture, which is determined by soil particle size, with clay particles being the smallest, silt being larger, and sand being the largest. Soil structure is how these particles are held together. Texture and structure determine air and water movement in the soil, which in turn affects plant growth.
Most soils in Iowa are well-suited for vegetables, flowers, turf, and other plants in the home landscape. Occasionally, soils may need correction because of problems like poor drainage, inadequate organic matter, or high clay content, among other things.
Tilling
In the garden, soils need to be prepared for planting by removing weeds and creating good conditions for planting and sowing seeds. Often, this requires the turning or tilling of soil.
Advantages & Disadvantages
Turning or tilling the soil has benefits and drawbacks. Advantages include:
- aerates the soil
- breaks up compaction
- helps incorporate compost and other amendments
- creates a nice seed bed for sowing
- allows roots (especially large roots, bulbs, tubers) to more easily grow and expand
- helps warm the soil faster
Disadvantages to tilling include:
- increases erosion potential
- brings weed seeds to the surface for germination
- breaks apart soil aggregates, changing the water and air-holding capacity
- speeds the loss of organic material
- promotes crusting
When & How to Till
The soil should never be worked when too wet. If worked under wet conditions, the soil becomes hard and restricts root growth. If a handful of soil formed into a ball retains its shape, delay soil tillage until the water content diminishes. If a handful of soil formed into a ball crumbles when pressed with the thumb, it is ready for turning and tilling.
In the home garden, fall tilling is the most beneficial. The only exception is when the site is prone to water and wind erosion. Fall tilling removes and destroys diseased plant debris, helping reduce disease severity and eliminating over-wintering sites for pests. Fall-tilled gardens dry out and warm up sooner in spring.
A light turning of the soil is all that is typically needed in a home vegetable garden. Simply turn the soil with a fork, shovel, hoe, or rake. This only minimally disturbs the soil allowing you many of the advantages of tilling, without some of the disadvantages. Most garden plants, vegetables, perennials, and turf have most of their roots in the upper six inches of soil. Turning the soil deeper has minimal returns in increased plant growth.
No-Till
Soils in the home garden can also be managed without disturbing the soil at all. Instead of using tillers, forks, or shovels, this method of soil management utilizes the layering of organic materials to build healthy soils and create a suitable seeding/planting bed. These layers can be created using any combination of mulch, compost, composted manure, straw, dried leaves, and grass clippings, among other things.
Cover crops are also an important and beneficial component of no-till systems. Select crops that can be terminated without tilling. Those that winter kill are the best option. This includes crops like buckwheat, field peas, cowpea, oats, and oilseed radish, among others.
Organic Matter
Organic matter (also sometimes referred to as hummus) is the dark brown to black organic compound that results from the decomposition of vegetative and animal matter.
As plants grow, the amount of organic matter in the soil becomes progressively lower unless organic materials are added. Commercial fertilizers cannot be a substitute for organic matter. While it can be a source of nutrients, the primary benefit of organic matter is more than that. Organic matter builds good soils by:
- improving the physical condition or structure of the soil
- increasing water absorption
- increasing aeration, allowing more oxygen to roots
- increasing nitrogen and other nutrient retention
- supporting the soil microorganism population
Sources of organic matter include compost, well-rotted animal manure, cover crops, and yard waste.
Compost
Compost is decomposed plant material. When fully composted, the final product resembles soil in color and texture.
Composting is a good way to recycle plant refuse from the landscape and can be done at home. However, it must be done well to avoid problems with weeds and disease organisms.
Compost can be spread and incorporated into the top few inches of soil with a shovel, garden fork, or tiller. This can be done any time of the year, but in spring before planting is easier.
Learn more in this article: Composting FAQs.
Composted Manures
Manure is a cost-effective soil amendment but must be used carefully to avoid contamination by microorganisms that cause food-borne illnesses like salmonella, E. coli, roundworms, and tapeworms.
Use well-composted manure, which reduces pathogens, odor, and risks of fertilizer burn.
Apply composted manures in the fall. Edible crops should not be harvested within 90-120 days of the manure applications (120 days for any vegetables that come into contact with soil, such as carrots and lettuce, and 90 days for vegetables that are not in contact with the soil, like corn and peppers). Applying manure in the fall allows adequate time between the application and the harvest. As a general rule, do not apply manure after the garden is planted.
Incorporate the composted manure into the top 6 to 8 inches of soil. This helps prevent a loss of soluble nitrogen to the atmosphere. If applying manure in the fall, consider also applying mulch or planting a cover crop to avoid nutrient runoff over the winter and early spring.
Learn more in this article: Using Manure in the Home Garden.
Yard Waste & Other Undecayed Organic Materials
Tree leaves, lawn clippings, peat, sawdust, straw, chipped limbs, and other plant materials are good sources of organic matter. Coarse materials like limbs and tree leaves are best shredded with a chipper-shredder or lawnmower before incorporating. This makes it easier to uniformly mix the materials into the top few inches of soil.
Mix undecayed plant materials thoroughly with the top 6 inches of soil, incorporating no more than 2 inches per year.
Whenever a sizable quantity of sawdust or other coarse fibrous material is added to soil, extra nitrogen must be applied with it. Sawdust and most coarse materials are high in carbohydrates (cellulose) and low in nitrogen. Soil microorganisms cannot get enough nitrogen from these materials to adequately break them down so they absorb additional nitrogen from soil reserves. This means that nitrogen in the soil that growing plants could have used is instead used by microorganisms. Plants affected by nitrogen deficiency will show yellowing and stunted growth. To avoid a nitrogen shortage, apply 1 pound of nitrogen per 1,000 square feet by adding composted mature or a high-nitrogen commercial fertilizer like urea (46-0-0) or similar high nitrogen analysis fertilizer (i.e., 27-7-3).
Fall is the preferred time to add yard waste or sawdust. This allows partial decomposition of the material, releasing essential plant nutrients before the following growing season. It is always wise to watch plant growth closely when large quantities of organic matter have been used. A side dressing applied mid-growing season may be necessary.
Cover Crops
Various cover crops can be planted and tilled into the soil to increase organic matter. Some cover crop species can be sown in late August and will grow sufficiently to be tilled under before winter. Most can be sown in the fall and turned under the following spring. Select crops that can be easily terminated so they don't become troublesome weeds. Those that winter kill are the best option. This includes crops like buckwheat, field peas, cowpea, oats, and oilseed radish, among others.
Soil pH
Soil pH is an important factor as it affects the availability of nutrients for plant uptake. In Iowa, pH typically ranges from 5.5 to 7.5, with many areas of the state trending toward the higher end of that range.
Most vegetables and other plants grow best in slightly acidic soil with a pH of 6.0 to 6.8. Within this range, root growth, plant vigor, nutrient availability, and microbial activity are optimal. Beyond this range, plant growth can be limited because nutrients become unavailable, and soil microbes work less effectively.
Conduct a soil test, and if it reveals a less-than-ideal pH, add amendments to change the soil pH.
Learn more in this article: The Importance of Soil pH.
Changing Soil pH
The most commonly used amendments to increase or decrease soil pH in the home garden are lime and sulfur. These amendments need to be mechanically incorporated into the soil with a tiller, shovel, or garden fork to a depth of 6 to 8 inches, ideally before planting.
Amendments used to change soil pH can be applied in spring or fall when the soil temperature is above 55°F. Changing soil pH may take weeks or months. For this reason it is preferred to add amendments in the fall and retest in the spring to be sure the pH has changed to an adequate value.
More details on how to apply amendments and determine how much to add can be found in this article: How To Change Your Soil's pH.
Drainage
Most plants thrive in well-drained soils. These soils have adequate pore space that allows excess water to drain away, preventing it from displacing air. Oxygen-deprived soils negatively affect the root health of plants.
Causes of Poor Drainage
Pore space in the soil is influenced by particle size and compaction. Soil is composed of particles of sand, silt, and clay that vary in size. Sand has the largest particle size and clay has the smallest. When soils have a large percentage of sand, they tend to have more pore space and are more sharply drained. Soils with a high percentage of clay tend to have much less pore space, which impedes water drainage.
When soils are not managed well, they can become compacted. Frequent walking or driving over soil compresses the soil pores and limits the air and water that can move into the soil. Over-tiling can break down soil aggregates, reducing pore space. Improper soil management can also lead to the formation of hard pans or impermeable layers that are impervious to water, allowing water to collect higher in the soil profile, displacing air, and negatively affecting root health.
Testing & Improving Soil Drainage
If soil drainage is a concern conduct a percolation test. If testing confirms poor drainage, steps can be taken to improve the soil conditions. Drainage can be improved by incorporating compost or other soil amendments, planting cover crops, or building a raised bed, among other things.
Learn more in this article: Testing and Improving Soil Drainage.
Fertilizers
Many garden plants, especially those in vegetable gardens, lawns, and annual beds, benefit from fertilizer applications.
Rate
The rate and frequency of application depend on the nutrient analysis of the fertilizer, plant species, soil type, and other environmental factors. Always start with a soil test to determine the amount of fertilizer needed. Adding too much fertilizer can be as undesirable as using too little.
Type
There are several types of fertilizers, from synthetic to organic, granular to liquid, and water-soluble to slow-release. Which you choose depends on what you are growing, when you are applying, how much is needed, soil type, and gardener preference, among other things.
All plants require 17 essential mineral nutrients to grow and complete their life cycle. These nutrients are made available to the plant from the soil. Some nutrients, like nitrogen, phosphorous, and potassium, are used in large quantities (macronutrients). Others, such as iron, boron, and manganese, are needed in very small quantities (micronutrients). Because macronutrients are used in large quantities, they are the most important nutrients to consider when applying fertilizers, as they are the most likely to be deficient in the soil. Most Iowa soils contain sufficient micronutrients; additional supplementation is rarely needed.
All commercial fertilizers, including specialty fertilizers (such as organics), state the guaranteed analysis on the bag. This series of three numbers indicates the percentages of nitrogen, phosphate, and potash in the fertilizer. The first number refers to the percentage of the fertilizer's weight is nitrogen. The second (middle) number gives the percentage of phosphate (which contains phosphorous), and the third refers to the amount of potash (which contains potassium). For example, a fertilizer analysis of 10-6-4 contains approximately 10% nitrogen, 6% phosphate, and 4% potash.
Application
Once the rate and type of fertilizer are known, the amount of fertilizer needed can be determined and applied. It is often best to apply fertilizers before or during planting. It is more practical to incorporate the material into the top few inches of soil when plant root systems are not in the way and can possibly be damaged.
More information on fertilizers, including application methods and recommended rates, can be found in this article: Fertilizing in the Home Garden.
More Information
- Garden Soil Management (PDF)
- Soil Testing Resources for Home Gardeners
- Creating Raised Bed Planters
- Composting Yard Waste (PDF)
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