Soil Amendments
Healthy soil is the key to supporting healthy plants. Not many of us are lucky enough to have a rich loam to work with. Every soil can benefit from organic amendments to improve either the soil structure, nutrients, or pH. The difficulty is figuring out what the soil needs, how much, and how and when to incorporate it.
Amending your soil can be fairly complicated if you are interested, but most of us want to improve the soil just well enough for ordinary planting. Those of us with extreme conditions, such as compacted clay or sand, may want to amend the soil enough to make it fertile, and then if necessary, select plants that are more tolerant of our prevailing soil conditions. Healthy soil contains a mixture of air, water, minerals and organic matter. Improving your soil’s texture (or soil structure) and fertility with organic amendments will make your plants more healthy and more disease resistant. So let’s start with the basics, and see Preparing the Vegetable Garden for Winter for further information about feeding your soil so the soil can feed your plants.
Soil Texture
How does your soil feel when you squeeze a handful of it? The texture of your soil is one of the first things you need to address. How your soil feels tells you the size of the soil particles, which will indicate how your soil holds moisture and how much air is available to roots in the soil.
Loam soil has an ideal mixture of different size soil particles. When you squeeze it gently, it molds together. If you squeeze it harder it will crumble. This balance of sand, silt, clay and organic matter will hold enough water to help your plants survive short dry spells, and will drain well enough to prevent root rot when there has been excessive rain. Loamy soils do not need amendments to alter texture.
Sandy soil has large soil particles and feels gritty or grainy. It crumbles when wet but will not hold moisture well and does not store nutrients. The soil warms up quickly in spring, giving your plants a good start early in the season, but dries out easily in summer. Because sandy soil does not hold moisture and nutrients, your plants will need frequent watering and fertilizing. Sandy soils will need organic amendments that are well decomposed, such as aged manure or compost.
Clay has very small particles. When you squeeze a handful it forms a tight sticky wad. Because the particles are so small it compacts easily, leaving no room for air. Drainage is poor, leaving roots in too wet conditions. Clay soils can hold nutrients well, but are often deficient in nutrients. In spring, clay soil stays cold and wet for a long time, making it difficult for your plants to break dormancy. When it finally dries out in summer, it gets very hard. Clay soil needs improved porosity and permeability, which will improve the drainage and aeration. DO NOT ADD SAND to clay to “improve drainage”. I know, it seems to make sense, but sand + clay = cement! The fibrous amendments such as fibrous sphagnum peat, straw, or wood chips will be most effective.
Texture Amendments
There a many different amendments that can be considered for your soil. Some are best to improve the soil structure of sand, some for clay. Some are quick acting, some or long lasting. Some are good for maintenance. Initially, you may need to make rapid improvements to the soil for spring planting. But eventually you should make long lasting amendments so that maintaining good soil texture is not a huge undertaking every spring. A combination of materials is often best, and can be dug into the soil all at once. Material that is simply buried will NOT improve your soil. It must be mixed in well to a depth of at least 6 inches according to most recommendations. Personally, I think you might as well do the job right the first time. Add amendments generously and dig it in a foot or two (you might want to see the “double-digging” instructions) when you first establish a planting bed. Refreshing your soil with amendments in future years can be done to a lesser depth, but there are very few plants that only grow six inches deep into the soil. The deeper you till the soil with amendments, the more your plants will be encouraged to develop a healthy root system.
Important to know before you add uncomposted amendments: Decomposition requires nitrogen. Green materials contain nitrogen that is used in the decomposition. Brown materials do not contain nitrogen, or contains very little, and will use nitrogen in the soil as it decomposes, which means it will be competing with your plants for this resource. The decomposition process is fastest initially, so you need to either amend the soil with brown materials well ahead of planting (up to a few months of active decomposition time - when the soil is still warm) or add nitrogen by either combining your brown amendments with green amendments (still allow a few weeks of active decomposition time), or fertilize with nitrogen.
Fibrous Peat - Use the fibrous Sphagnum Peat, not the fine dusty peats that just clog the soil. The best peats come from the northern United States and Canada. This amendment has very high water retention - good for sand. Permeability is low to medium - good for sand. Decomposition is slow, perhaps years - long lasting. Peat is acidic, check the pH information below.
Ground Dry Leaves - Use shredded/ground up dry autumn leaves. Water retention and permeability is medium. Decomposition depends on how finely shredded the leaves are. In generally, finely ground leaves from a mulching vac for example, will decompose relatively quickly. Perhaps in just a few weeks. Ground leaves are generally considered an additive that will quite quickly benefit any soil type by increasing organic matter in the soil, but will require a little time to decompose, so dig them in in the fall or very early spring.
Wood Chips (cedar, redwood) - Water retention is low to medium - good for clay or silt. Permeability is high - good for clay or silt. Decomposition is slow, last years - long lasting. (Sawdust is not recommended. Although it does decompose slowly, the wood particles are so much smaller than wood chips that dramatically more surface area is exposed for decomposition that it seriously depletes nitrogen in the soil.)
Hardwood Bark - Water retention is low to medium - good for clay or silt. Permeability is high - good for clay or silt. Decomposition is slow, lasts years - long lasting.
Straw - Do not use hay!, it is full of seeds. Water retention is low to medium - good for clay or silt. Permeability is high - good for clay or silt. Decomposition is relatively slow - relatively long lasting.
Compost - Water retention is medium to high - good for sand. Permeability is low to medium - good for sand. Decomposition is moderate - about six months, good for one growing season. (Make your own compost with yard waste and kitchen scraps!) Because compost and manure permeability and water retention is roughly in a medium range, they are both excellent for annual renewal of any soil.
Aged Manure - Do not use fresh manure, the high ammonia will burn your plants. Water retention is medium - good for maintenance. Permeability is low to medium - good for sand. Decomposition is fast, days to weeks - fast acting. (Also, fresh manure has not even begun the decomposition process, which will use nitrogen. In the competition for nitrogen in your soil, the manure will win, your plants will lose. If you have fresh manure from plant eating animals available to you, put it in your compost heap. But make sure your compost heats up to at leas 130 degrees twice to kill pathogens. If it does not get that hot, do NOT use it on a vegetable garden.) Because compost and manure permeability and water retention is roughly in a medium range, they are both excellent for annual renewal of any soil.
Grass Clippings - Water retention is high - good for sand. Permeability is medium. Decomposition is fast, days to weeks - fast acting. Fresh grass clippings can be turned into the soil immediately, or used initially as a mulch and later turned in. Be aware that grass clippings may contain weed seeds and pesticides.
How much
The answer is somewhat determined by the current condition of your soil. If the soil condition is severe (high clay content and compacted, or high sand content), it would be hard to add too much. As a general guideline for moderate levels of clay or sand, you should use about three or four cubic yards of organic material for every 1,000 square feet. To make that a little easier to figure, add a two inch layer over the soil surface. I would consider that to make relatively minor improvements to seriously poor soil, but adequate for moderately poor soil. When you first dig in a bed, it would be wise to dig in much larger amounts to correct serious conditions, and dig it in very deeply. Again, refer to the instructions on “double-digging”. Use the longer lasting amendments, as well as other quick acting amendments. Be sure to mix the amendments in well. A shovel and hoe will work fine, and you well get your “work out” done at the same time. A rototiller will not get as deep as double digging with a shovel. But it will make quick work of adding amendments in subsequent years where you can use it, which is probably only for your vegetable garden or an annual bed. All the more reason to dig in any planting area well the first time. Perennial beds and shrub borders can not be tilled up every year, amendments can only be added around plantings and at a shallow depth.
Nutrient Amendments
Plants need several different nutrients that can be derived from organic sources or chemical sources. Nitrogen, phosphorus and potassium (or potash) are the nutrients in standard chemical fertilizers, N-P-K. These, and calcium, magnesium and sulfur are necessary for your plants to thrive. Minerals necessary in smaller amounts include iron, manganese, copper, zinc, and others. Organic fertilizers break down slowly and are available to plants as needed. Be sure to check recommended application rates on the packages. One note - NEVER mix lime with fertilizers. At best the lime and the fertilizer will cancel each others effect, and may have an undesirable effect on your soil.
ORGANIC NITROGEN SOURCES - Nitrogen encourages good leaf and stem growth.
Alfalfa Meal - A more balanced alternative to blood meal, it also contains phosphorous and potassium(2-1-2). Also encourages microbial activity. Excellent for vegetable gardens as an in season fertilizer, with the nitrogen moderately fast acting.
Blood Meal - Dried animal blood, is very fast acting and may contain small amounts of potassium and phosphorous (12-0-0). Follow package application rates carefully as it has a high nitrogen content - DO NOT over apply or use on seedling and very young plants. Consider alfalfa meal instead or side dress.
Coffee Grounds - contains about 2% nitrogen as well as small amounts of potassium, phosphorous and trace minerals. Let the grounds dry and sprinkle around plants.
Composted Manure - Use only manure from plant eating animals, and use only hot composted manure in vegetable gardens. Till it into new planting beds or work into topsoil around permanent planting. Manure adds nitrogen over a period of time and improves soil texture. Can scorch young plants.
Cottonseed Meal - ground meal from cotton seeds. Excellent source of organic nitrogen, 7.2% N. It breaks down slowly, so is available to plants over time. Is slightly acidic. Look for it in feed supply stores for best price.
Feather Meal - granulated fertilizer that delivers nitrogen in a protein form. Moderate to slow release of nitrogen. Work into the soil or sprinkle according to directions. 12-0-0 NPK.
Fish Meal or Emulsion - Fish Solubles. Dilute and apply according to directions. It is a fast acting nitrogen source - the liquid goes right to the roots without burning (9-3-0). Also contains trace elements. Odor is intense and may attract animals.
Mushroom Compost - or Mushroom Manure. This is not widely available so if you have access, by all means make good use of it. This is horse manure with brown plant materials added to be spread in mushroom beds. After a mushroom crop is harvested, it is removed from the beds and made available to local nurseries. It can be used in concentrations as high as 50-50, half topsoil, half Mushroom Manure. It has an alkaline pH, so use an acid amendment to counter around acid loving plants such as rhododendron and azalea.
Rice Hulls - high lignin content makes decomposition very slow, months. Should be composted first.
Soybean Meal - High nitrogen similar to cottonseed meal, 7.6% N. Fast release without burning. Look for both in feed supply stores for best price.
Worm Castings - quick release nitrogen, but also long lasting component. Also contains beneficial fungi. 1-0-0 NPK. Apply 10-15 pounds per square foot, or up to 1/3 in container mixes.
ORGANIC PHOSPHOROUS SOURCES - promotes root development, improves flower color and disease resistance.
Alfalfa Meal - It also contains nitrogen and potassium. Also encourages microbial activity.
Bat Guano - also sea bird guano (yes, bird and bat dung). Available as general purpose 8-4-1 NPK, high phosphorus 3-10-1, etc. Many levels of N-P-K available based on the feeding habits of the birds or bats harvested from. Excellent for container gardens and foliage growth. Availability speed is variable, and application may last anywhere from 2 to 8 months. Seabird guano is much faster acting and should be used in small amounts as a side dressing or tea until later in the season.
Bone Meal - slow release fertilizer with high phosphorus content (5-12-0). May also contain as much as 22% calcium. Phosphorous is available relatively quickly.
Rock Phosphate - long lasting, slow release, 0-3-0.. Apply according to directions.
ORGANIC POTASSIUM SOURCES - for fruit and seed production
Alfalfa Meal - It also contains nitrogen and phosphorous. Also encourages microbial activity.
Granite Dust - slow release, low potassium content.
Green Sand - Mined from mineral deposits that were once ocean floor. Also contains iron, magnesium and many other trace minerals. Also effective in improving the texture of clay soil
Kelp Meal - Spread over, and mix into the soil. Kelp is well balanced and slow-release. In all, contains 60-70 elements that the soil needs. It is, however, expensive and not easy to find.
Potassium sulfate - A very high potassium content (0-0-50)that is very quickly released and quickly leached. Add during active growing season.
Seaweed & Kelp fertilizer - Abundant in raw form that breaks down fairly quickly for composting if you live near a coast. May have a high salt content. Prepared retail fertilizers are usually a liquid concentrate, but also available in powder form to mix with water. Prepared kelp varies in nitrogen amounts, generally 1 to 5-1-3.
Wood Ashes - from yes, burned wood. Sprinkle into the compost heap or on soil in areas where the potassium is drastically low. Very fast acting. Wood ash will also increase the pH of your soil, so make sure that is acceptable by testing your pH. There is also a low phosphorous content, high calcium (which may already be too high in many soils), and can injure microorganisms. Although commonly used, I don’t suggest it.
ORGANIC CALCIUM SOURCES
Dolomite Lime - Dolomite Lime has a higher level of magnesium than Garden Lime. Also contains about 25% calcium, as does Garden Lime. A neutralized pH will allow nutrients already in the soil to become more readily available to your plants. Quick acting without the hazards of hydrated lime. Apply according to directions. Will raise the pH of your soil, test pH level first.
Eggshells - Also will raise pH, having the mild effect of liming. Let them dry out and crush them. A blender works great. Sprinkle on the soil, around vegetables, fruit tree, containers, roses...
Epsom Salts - Contains calcium and magnesium, will also have some pH effect, raising it to neutralize acidity. Often used instead of lime.
Gypsum - 23% calcium.
Limestone - Garden Lime is pulverized limestone. Note the pH effects of lime before applying.
Oyster-Shell Lime - Similar to dolomite lime, so it will raise the pH of your soil. Also contains high content of calcium as well as other nutrients and minerals.
ORGANIC MAGNESIUM SOURCES
Dolomite Lime - Made from seashells rather than limestone. Dolomite Lime has a higher level of magnesium than Garden Lime, about 10%. Also contains calcium, as does Garden Lime. A neutralized pH will allow nutrients already in the soil to become more readily available to your plants. Quick acting without the hazards of hydrated lime. Apply according to directions. Will raise the pH of your soil, test pH level first.
Epsom Salts - Contains calcium and magnesium, will also have some pH effect, raising it to neutralize acidity. Often used instead of lime.
Limestone - Garden Lime is pulverized limestone. Note the pH effects of lime before applying.
OTHER ORGANIC MINERAL SOURCES
Iron Sulphate - Used to supply readily useable Iron when deficiency is indicated in Azaleas, Camelias, Citrus and Gardenias, as well as in the lawn. Soils with a high pH already contain high levels of unuseable iron, so this product will not be effective without lowering the pH first.
PH CONSIDERATIONS
Different plants need different pH levels or ranges to thrive. 5.5 to 7.5 is considered an optimal, neutral range for most plants, an environment that allows nutrients to be available to plants most readily. Micro-environments, specific planting areas for acid or alkaline loving plants, may need pH adjustments to provide the best environment. Likewise, soil in geographic areas may tend to be acidic or alkaline, in which case general adjustments may need to be made. Soil test kits are widely available for as little as a few dollars. Kits that test pH, nitrogen, potassium and phosphorus as many as 40 times per kit, are available for as little as $20. Before making dramatic amendments affecting these levels, or if plants are not thriving, it would certainly be cost effective to invest in a kit. You can save time and money by adding only appropriate nutrient and pH amendments. Many of the amendments described above include a notation regarding pH, but there are also specific amendments available to alter pH in your soil. Be sure to check application rates on the packages. Most will indicate how much to use to raise or lower your pH to desired levels, assuming you know the current pH of your soil.
pH scale: 0-3 strong acid
4-6 weak acid
7 Nuetral
8-10 weak alkaline
11-14 strong alkaline
ORGANIC PH AMENDMENTS
Dolomite Lime - Made from seashells rather than limestone. Dolomite Lime has a higher level of magnesium than Garden Lime. Also contains calcium, as does Garden Lime. A neutralized pH will allow nutrients already in the soil to become more readily available to your plants. Quick acting without the hazards of hydrated lime. Apply according to directions. Will raise the pH of your soil, test pH level first.
Epsom Salts - Contains calcium and magnesium, will also have some pH effect, raising it to neutralize acidity. Often used instead of lime.
Garden Lime - (pulverized limestone) Mix into the soil as directed to neutralize acid soil, or raise the pH to an alkaline level.
Garden Sulpher - This mineral and nutrient is commonly used to raise acid in the soil for plants that require it. Turning Hydrangea blooms from pink to blue is the most common use.
Hydrated Lime - This powdery lime can be caustic, use garden or dolomite lime instead. It is quick acting but effect is short lived. Often used to neutralize “dog spots” in the lawn.
Iron Sulphate - Used to supply readily useable Iron when deficiency is indicated in Azaleas, Camelias, Citrus and Gardenias, as well as in the lawn. Soils with a high pH already contain high levels of unuseable iron, so this product will not be effective withough lowering the pH first.
Wood Ashes - from yes, burned wood. Sprinkle into the compost heap or on soil in areas where the potassium is drastically low. Wood ash will also increase the pH of your soil, so make sure that is acceptable by testing your pH. There is also a low phosphorous content, high calcium (which may already be too high in many soils), and can injure microorganisms. Although commonly used, I don’t suggest it.