 |
|
|
|
|
Soil and FertilizerAcidifying soil
Ashes bad for garden
Bark mulch: does type matter?
Caliche soil
Coffee grounds a good idea?
Compost, kitchen scraps
Compost, spreading
Composting in the fall and winter
De-icing with fertilizer
Don't fertilize newly planted trees
Fertilizing trees
Fertilizing houseplants in winter
Foliar (spray) feeding, organic
Grubs in compost
Grubs in Lawn
Lawn, soil amendments
Manure, add to garden now?
Manure, spreading
Minerals missing in fertilizer
Mold in mulch
Mulch may be contaminated
Mulch near foundation wall
Mulches
Orchid, pot soil
Plastic sheet mulch: black or clear?
Plastic sheeting isn't really mulch
Potting soil: make it!
Rabbit pellet fertilizer
Rototil leaves directly into garden soil?
Soil preparation
Test before treating soil
Tree stump okay for mulch?
Trumpet plant okay for compost?
Winter composting
Wood ashes okay to use?
Acidifying soil
Q. We have lived in Alaska for the past 35 years, acidic soils that you sweeten with lime. Do we do the opposite here in NM, adding something like Miracidâ„¢ to the whole general area and possibly more to acid loving plants?
A. You have asked a good question and, although this topic has been discussed in the past, it is worthy of occasional repetition. The material you would add to your soil depends on which plants you are growing. However, you are correct - we do not recommend adding lime to soil in New Mexico. For plants adapted to calcareous soils, nothing need be added, except the addition of air by rototilling compacted soils. Plants native to more acidic soils will benefit from the addition of sulfur as elemental sulfur or sulfate (Iron sulfate). If plants exhibit symptoms of iron chlorosis (yellowing between green veins), or symptoms of micronutrient deficiency, then application of an acidifier (contains sulfur) and perhaps the deficient micronutrient in a chelated form will help alleviate the problem. Do not expect to see a response after mid-summer. After that time, the leaves are mature and will probably not absorb the missing nutrient. Apply the chelated micronutrient (trace element) in the spring. Sulfur can burn a plant in the heat of summer and is best applied in the autumn or spring.
Fertilizers that acidify may be helpful but are not essential if other acidifiers are used. There are several acidifying fertilizers sold as lawn and garden fertilizers as well as for houseplant culture. They will specify a sulfur content on the label or will specify that they are acidifying.
I suggest that you contact your local Cooperative Extension Service Office. The Extension Service Office is familiar with soil conditions in your county and with the various products sold in your county.
Back to Top
Ashes bad for NM soils 7-00
Q. Would the burnt dust from a charcoal barbecue help enrich the soil in my garden and, if so, how much of the dust would be beneficial? It seems like a good way to recycle my barbecue refuse.
A. The usual answer to questions about using wood ash to enrich soil in New Mexico is—DON’T DO IT! Now, if you are in the mountains and have a low pH soil (rare even in the mountains in New Mexico), it is possible to use the wood ashes, but very carefully). Wood ashes contain a lot of salt. The organic material has oxidized (burned off) leaving behind a lot of minerals, especially potassium. This material increases the salt content of our already salty New Mexico soils. To make the situation worse, most New Mexico soils are not deficient in potassium, so we are creating a problem without gaining any benefit. We recommend that the wood ashes be disposed in a manner that does not add them to garden soil (in a landfill or spread over a gravel driveway, etc.). Do not even add them to the compost pile. The only way to be certain that the above warning applies to your soil is to have your soil tested. Look especially at the pH results and saltiness. If your soil is acid, then you can use the wood ashes in the garden. In other parts of the country, especially east of the Mississippi River (and some areas to the west), the soils are acidic and deficient in potassium. In these areas, the addition of wood ashes is beneficial. It is from garden books written for these areas that we will find recommendations to add wood ashes to the garden.
Back to Top
Bark Mulch - Does the type of tree matter? 12-02
Q. I would like to find out what kind of trees are the best to produce wood chips/mulch for gardening purposes. Also, can mulch act as a protection layer for soil so the soil won’t be washed out when watering the plants? I really need your help for the purpose of landscaping/gardening. Eean L.
A. Almost any kind of wood chips can be used as an organic mulch. There is a difference between bark and wood chips, but bark from most trees is also useful. In the Eastern part of the U.S., there is concern that bark, especially pine bark, increases the acidity of the soil. Here in the Southwest, that is not a concern. In most cases any acidification, though mostly beneficial, will be temporary.
Another concern is the presence of allelopathic compounds (chemicals produced by one plant to inhibit the growth of other plants). Of primary concern regarding allelopathy is the bark from black walnut trees. The bark and husks of walnuts contains a chemical, juglone, that inhibits the growth of plants in the Solanaceae (tomato/potato family). Research has shown no significant effect of the wood, but the bark may be a problem if placed around plants in the tomato family.The benefit of soil protection you mentioned is only one of the benefits of mulch. On steep slopes and in rapidly flowing water, wood chips may wash away. In those locations a heavier mulch, perhaps rocks, would be best. In most other locations, wood chips are an excellent soil protector. Mulch prevents water and wind erosion and stabilizes soil temperatures (warmer in winter, cooler in summer under mulch). It also reduces evaporation of moisture from the soil, reduces weed competition with landscape plants, and prevents damage to trees and shrubs caused by lawn mowers and string trimmers. Mulch creates a better environment for many beneficial soil organisms. As it decomposes, it improves the soil. It also reduces soil compaction, increasing infiltration of water and air into the soil. This results in healthier tree and shrub roots and healthier trees and shrubs. These benefits also help herbaceous perennials.
Wood chips and bark are not the only beneficial organic mulches. Old straw or hay, dried grass clippings, pecan shells, and many other organic materials are useful as organic mulches. Inorganic materials such as rock, paving and flagstone, and other inorganic materials are also useful. Each material has its limitations and its strengths. For example, pecan hulls are a common New Mexico waste material useful in the landscape, so it is inexpensive. However, the pecan shells usually contain some residual pecan nutmeats which attract rodents and insects (roaches, etc.), a potentially negative characteristic that must be considered. Rocks don’t wash or blow away as easily as organic mulch, but they become hotter and may injure some plants. Choose mulch materials with their strengths and weakness in mind, and choose appropriate sites in which to use them.
Back to Top
Burned leaves from spray-on foliar fertilizers 9-00
Q. Last year I burned a lot of plants using a spray-on fertilizer in the summer. Is there something organic that I can use without hurting my plants?
A. In defense of the spray-on fertilizers, a lot of people use them effectively. It is important to be careful not to make the concentration too strong and to apply them in the early morning while the temperatures are cool and evaporation minimized. The plants should be well-watered when these materials are applied. To specifically answer your question: Yes, there are organic spray-on treatments. You can purchase fish emulsion fertilizer and, after diluting it according to the directions, apply it to your plants. You can also make compost tea or manure tea by soaking compost or manure in water until the water takes on the brown color of the compost or manure. This may also be sprayed over the plants. Don't make them too concentrated and, as with the commercial spray-on fertilizers, apply them early in the morning to well-watered plants.
Back to Top
Caliche soil – what is it? 9-01
Q. I moved here from the Midwest. My neighbor explained that the reason I had trouble with my garden this year is that my soil is "caliche" soil. What is caliche soil? Artesia
A. Caliche is a term used often in New Mexico when speaking of soil. I think some people just mean the soil is very calcareous; that is, the soil contains a high level of calcium and has a high pH (is alkaline). Others think of caliche as a layer of rock (limestone) at some level below the surface of the soil. I spoke to Dr. Robert Flynn, NMSU Extension Agronomist, about this, and he gave me some specific definitions which are important for gardeners to understand their soil. Caliche is defined as an amorphous (non-crystalline) mass of calcium carbonate (limestone) mixed with clay. The cement-like layer below the soil surface that is often called caliche is termed a "petrocalcic horizon" by soil scientists. Many New Mexico gardeners will find that their usable soil depth is affected by an impermeable petrocalcic horizon as some depth below the soil surface. This affects drainage and can provide a source of calcium to thwart efforts to alter pH. This will also influence shrubs, trees, and other deeply rooted plants. In order to learn the specific soil conditions influencing your garden, it is wise to send a soil sample to a soil-testing laboratory. This will provide information about soil physical and chemical characteristics allowing proper management of the soil.
Back to Top
Can I add manure to my garden in fall?
Q. Is fall a good time to add manure to the garden?
A. Now is the proper time. Once the crops have been harvested and you are ready to clean your garden for the winter, you can apply manure to the garden. Winter precipitation will help leach the various salts present in manure below the root zone of next year's crops, if there is winter precipitation. It is possible to apply manure in the spring, but waiting until spring to apply manure to the garden increases the chances that your seedlings and transplants will be injured. If you want to apply manure in the spring, the wisest thing to do is to compost it with chipped tree trimmings and lawn waste over the winter, then apply the compost to the garden in the spring.
Back to Top
Can I rototil leaves directly into garden soil?
Q. Is it OK to rototil fallen leaves and old vegetable plants into the garden rather than building a compost pile? I've done it before, but some years the leaves and old plant stems are still there in the spring.
A. In general, yes. However, there are some warnings. In particular, if any of your garden plants had fungal or bacterial diseases, remove them from the garden. Turning them into the soil almost guarantees disease problems next year. Most gardeners will remove and dispose of diseased garden plants. That is the safest way to deal with them. Tree diseases are not usually a problem for the garden plants, so the leaves from the trees should not be a problem.
Even if you remove diseased plants from the garden, be sure to rotate your crops the next year. This is good advice even if you didn't have diseased plants. Crop rotation helps prevent the build-up of disease organisms in the soil. Crop rotation means that you move the crop you grew in one spot to another place the next year. The reason that the material turned into the garden doesn't decompose over the winter is probably due to the fact that the soil was too dry that winter, or the plant debris was too woody. In the spring when you rototil the garden, rake out any uncomposted material and finish composting in a compost pile, or use it as mulch between the rows in the garden. Adding manure as you turn the debris under in the fall and making sure the soil is moistened in dry winters will maximize the in-situ composting.
Back to Top
Coffee grounds a good idea? 5-02
Q. What are the reasons I would want to put coffee grounds in my plants? I read an explanation but I was told it was just a way to add acid to the soil so that some plants will grow better. I am taking an earth science class right now in college. We are getting ready to make a terrarium. Is it a good idea to put a small amount of coffee grounds in there?
A. The coffee grounds will indeed acidify the soil, as will other organic matter. If the plants you will be using require acid soil, coffee grounds or a compost/peat-based soil will be beneficial. The addition of organic matter to mineral soils is helpful because the decomposing organic matter slowly releases nutrients into the soil, increases the water-holding capacity of sandy soils, and improves the structure of clay soils.
Plants grown in terraria, if native to tropical rain forests, will be best adapted to soils containing much organic matter. If the terrarium plants are native to arid soils, the organic matter may not be beneficial. An answer to a similar question may be found in the Yard and Garden archives at the NMSU College of Agriculture and Home Economics web site at http://cahe.nmsu.edu/ces/yard/2000/111800.html and http://cahe.nmsu.edu/ces/yard/2001/120801.html. In the first of these web pages, the answer provides some cautions regarding use of coffee grounds in flower pots. The second discusses coffee and other beverages used to irrigate plants.
Back to Top
Coffee grounds in pots 11-00
Q. Is it safe to put old coffee grounds on my houseplants?
A. It is okay as long as you don't overdo it. In fact, there may be some benefits. However, if you put too many into the pot, there may also be some detriment. There will be some nutrients released as the coffee grounds decompose. This is good, but the decomposition process can cause some problems. As the coffee decomposes, various fungi (which are necessary to decompose the coffee grounds) may increase resulting in disease in the houseplants. The process of decomposition produces heat which may be sufficient to damage or kill the houseplant if there is a thick layer of coffee grounds. In a thick layer, the coffee grounds may become soggy. This sogginess may create an environment conducive to the growth of damaging fungi and bacteria, which may then kill your houseplant. Various organic acids may also be released into the soil creating an unfavorable environment for some plants. Use the coffee grounds sparingly, and be careful with water after application of coffee grounds to the soil. Concerns about increasing acidity will also be eliminated by sparing use of coffee grounds around the plants.
Back to Top
Composting in the fall and winter
Q. What will happen to leaves I put into my compost this fall? Will they wait until spring when it gets warmer to decompose?
A. If you add green grass clippings, manure, or some other source of nitrogen to the leaves, they will decompose through the winter. The heat in the compost is generated by the composting organisms; it does not come from the air. Of course, the air may cool the compost and slow the decomposition process around the edges of the compost, but the center should generate enough heat to continue composting through the winter. This depends on the balance of materials in the compost. A source of nitrogen, such as green grass clippings or manure, should be mixed with a source of carbon such as brown leaves or shredding branch trimmings. The nitrogen and carbon are the foods needed by the composting organisms such as fungi and bacteria. They will generate heat and decompose the landscape debris so that in the spring you will have a supply of compost to add to your garden as a soil conditioner. Just remember that you will need to supply moisture to the compost and to turn the compost materials through the winter.
Back to Top
De-icing with fertilizer?
Q. A neighbor used fertilizer to melt ice on his sidewalk. He said that this is better than de-icing salt because it won't hurt the plants. Is this true?
A. Fertilizer isn't labeled for use as a de-icing salt, and though some will function in this manner, they will not be as effective as products made for de-icing. Regarding the reduced damage to plants, your neighbor is partially correct. Some de-icing salts can be harmful to the landscape. There are others, however, that are less harmful to the landscape yet manufactured for the purpose of de-icing. Fertilizer will be less damaging than the first of the two types of de-icing salt, but fertilizer can cause some damage if the fertilizer salts accumulate to an excessively high concentration in the landscape near the sidewalk. These salts can be diluted by irrigation, but if there is poor drainage, some damage may appear in the landscape.
Back to Top
Don't fertilize newly planted trees
Q. I live in Ruidoso. In late December 1996, I planted ten dwarf Alberta spruce; in April 1997, I planted ten green mound junipers, and then seven mugo pines. All the plants are very small, but I used no fertilizer when I planted them. They are looking good now, but I wonder when I should begin fertilizing them. Also, what type of fertilizer should I use? I have them surrounded with pine bark to prevent water loss. I want to xeriscape my new home at this 7000 ft. elevation.
A. Regarding fertilization of newly planted trees -- don't! For at least the first year, their nutrient needs will be minimal. During this time they are establishing their root systems, and fertilizer (especially nitrogen which stimulates stems and leaves) will not be appropriate. You may begin a light fertilization a year from now as the leaves are forming, but for this year, be patient. The type of fertilizer is not especially critical. You can purchase fertilizers which are specially formulated for trees, or you can use a general purpose fertilizer. Just be certain that the fertilizer is not a "weed-and-feed" product containing a postemergence herbicide which could harm the trees.
When applying the fertilizer around the tree, don't put it too close to the trunk and be certain not to put it all in one spot - it can burn the tree if too much salt is absorbed by the roots. General purpose fertilizers contain nitrogen, phosphorus, potassium, and in some cases trace nutrients. Nitrogen is needed for growth of stems and leaves. Phosphorus is needed for good plant health and energy in the plant. It stimulates flowering and fruit development, not your main purpose in growing pines, juniper, and spruce. The potassium is needed for good root development and plant health. Trace nutrients are used in very low concentrations but are sometimes a limiting factor in plant growth in New Mexico. The nutrient the trees will need most is nitrogen; however, they also need the others.
It would be wise to take a soil sample to determine what your soil needs to support good tree growth. Your local County Extension Service can advise you as to the proper method to collect the sample and tell you where you can get the soil tested. They can also help you understand the results of the soil test if you need help. As to the xeriscape, you are off to a good start. Some people who live at lower elevations in New Mexico may question the mugo pines and dwarf Alberta spruce, but at your elevation they are appropriate if irrigated properly. The spruce will need more irrigation than the pines, but the needs of each species can be supplied with a properly designed irrigation system. The use of mulch is also a good idea, especially when starting trees as it not only reduces evaporation of water from the soil, it keeps grass from competing with the developing root system of the trees, and it keeps lawn mowers and weed whackers away from the tender bark of the young trees.
Back to Top
Fertilizing Plants in Winter
A. Some houseplants will be dormant or their growth will slow in the winter. You can cease fertilizing during this time or at least reduce the amount of fertilizer that they receive.
Houseplants that are actively growing should continue to receive fertilizer with a relatively high level of nitrogen. Nitrogen in fertilizer (the first of three numbers on the label) stimulates growth of leaves and stems. Flowering plants that may be dormant now but preparing to bloom in the spring will benefit from a fertilizer with a relatively greater amount of phosphate (the second of the three numbers). Phosphate benefits development of flowers and fruit. Finally, potash (potassium, the third number) benefits plant health. All are important, but the relative concentrations may determine whether you stimulate vegetative or flowering growth.
These fertilizers may be purchased as liquids that are diluted with water or soluble solids that must be dissolved in water. They may be from organic sources or mineral sources. These diluted or dissolved fertilizers should be used according to the directions on the label. When it is cloudy and during the shorter days, fertilize less. You can make the lower recommended concentrations on the label, or apply the fertilizer less often. When the days lengthen and growth begins, you can increase application of fertilizer. Do this by increasing the concentration of fertilizer (never exceeding the recommendations on the label) or by applying fertilizer more frequently.
It is always wise to apply the fertilizer to a soil that is already moist. If the soil in the container is dry, water to moisten the soil and apply the fertilizer solution a few days later.
Back to Top
Grubs in compost 11-00
Q. I have a composter and it is working well. However, there are many, many grubs in the pile. I am afraid to put the compost in my garden or around the yard. Do I need to kill them or will they be okay? I am assuming they are grubs. They are small white/brown worms.
A. Grubs are common in composting material. They are fat, white, larvae of beetles. They have a brown head capsule and legs and a dark gray area at the end opposite their head. There are other "worms" that inhabit compost, so be certain yours are really grubs. Grubs may actually participate in the composting process. There are many types of grubs, so yours may not be a problem. However, it is possible that they are a problem. You should consult with your local Cooperative Extension Service agent for information about the grubs common in your area. If you have a problem, you may use various insecticides labeled for use in the location (vegetable garden, flower garden, etc.) where you will apply the compost. It is also possible to kill the grubs by heating the compost in a plastic bag placed in the sunlight for several hours or days prior to applying the compost to the garden. However, treatment will probably not be necessary.
Back to Top
Grubs in Lawn
Q. I hope you can give us some advice on dealing with grubs in our lawns. We are the Renaissance Homeowners Association, and our groundskeeper has pointed out a number of brown spots on the neighborhood lawns that he claims are caused by grubs. Is there a way we can be certain that this is our problem and know how to deal with it? We are averse to using highly toxic chemicals if it can be avoided. I hope you can advise us about this. We will appreciate your guidance.
A. To determine if grubs are indeed present, cut across the boundary between the brown and the green grass with a shovel, making an angle between two cuts. This allows you to lift the sod and look at the roots and crown (where leaves and roots meet) of the grass plant. If grubs are present, you will find some dingy-white, fat grubs in the crown or root zone of the grass. If the grubs are causing major problems, the grass may lift like a rug from a floor - its roots have been eaten by the grubs and there is nothing holding it to the soil. If you do not see any grubs present, check in a few other places. If there are still no grubs found, take a sample of grass from the boundary area of the damage (some brown, some still healthy) to your local County Extension Office where the Extension Agent can look at the sample and, if necessary, send it to the Extension Plant Pathologist in Las Cruces to determine if fungal disease is causing the problem.
Before taking in a sample, however, be sure that the brown areas are getting the same amount of water as other areas of the lawn. Do this by placing empty jars or cans on the lawn while watering. Place some in healthy areas and some in brown areas. If they catch equal amounts of water in both areas, irrigation is uniform and probably not the problem. If there is less water collected in the brown areas, adjusting the irrigation system to provide uniform coverage may be necessary (look for clogged sprinklers, turned sprinkler heads, or incorrect replacement sprinklers).
If grubs are the problem, there are insecticides labeled for controlling grubs. If used properly, these should not be a problem, but keep children and pets away from direct contact with the treated grass for a couple of days. There is an organic/biological treatment using parasitic nematodes to reduce the grub infestation. Nematodes are very small (almost microscopic) round worms that feed on insects. You can purchase the nematodes from insectaries that raise these biological control agents. The nematodes will be shipped to you in insulated, cool containers. They must be applied as soon as possible after receiving them. Early morning application to a recently watered lawn is best. After application (by watering can or sprayer) irrigate briefly to wash the nematodes from the grass blades to the crown and root zone of the grass. If you must delay a few days to wait for cooler weather or other reasons, the nematodes must be refrigerated (not frozen) until use. Do not store under refrigeration more than a few days.
Treatment is most effective in August and early September while the grubs are small and more easily managed. Later treatment is less effective since the grubs will cease feeding and migrate more deeply into the soil for the winter. In the spring, they may feed briefly but will quickly form pupae and begin their metamorphosis from grubs to june beetles. Treatment is not effective once they form the pupal stage. If this problem persists, an insecticide analog of nicotine (chemical name Imidacloprid) may be applied in late June to be present as soon as the eggs hatch. This is a chemical insecticide, but it has low toxicity to mammals (people/pets). It has a three-month residual in the soil so, if applied properly, it is waiting for the grubs when they hatch.
Whether you use biological, organic, or chemical control measures be sure to read, understand, and follow all label directions to maximize both effectiveness and safety of the treatment.
Back to top
Kitchen scraps in compost
Q. Is it ok to put kitchen scraps into the compost? Can I get diseases using them?
A. It is good to put some types of kitchen scraps into your compost. If there is sufficient heat generated in the compost, plant disease organisms will be killed. Human disease organisms, if that was your concern, should also be killed by composting process. However, human diseases are not carried by plants. You should avoid adding to the compost kitchen scraps which contain fats and oils such as meat, foods cooked in large amounts of butter or oil, and such. Most people avoid breads and bread products because they also contain oil and because they are very attractive to mice and ants.
Just think, what you didn’t eat from the kitchen may be composted and used to grow food in the garden which is then eaten. This gives a new meaning to your mother’s admonition to eat everything on your plate! At the time she said that, didn’t you wish broccoli could be turned into strawberries?
Back to Top
Lawn, soil amendments 12-01
Q. I have a small front yard (30’x 50’). It faces south. For the last year it has been used as a parking lot. I would like to rototill in some leaves from my mulberry tree and some pecan shells. Would this be a good type of mulch? I plan on planting grass next spring.
A. When planting grass, it is a good idea to work organic matter into the soil. These materials, leaves and pecan shells, will help to loosen the soil to help the lawn. The "parking lot" has compacted the soil creating a need for the soil to be de-compacted. The pecan shells will be slow to decompose and will maintain a porous soil allowing water to permeate the soil. The leaves will decompose rapidly if kept moist (even in the winter). The humus formed by the leaves will serve as sponges and hold the water in the soil as well as helping keep the soil loose and aerated. Other organic materials may also be added to improve the characteristics of the soil and enhance grass development.
However, I do have a concern regarding your plans to rototill. Is the tree in the vicinity of this lawn area? Tree roots will extend beyond the ends of the branches and may be damaged by the work you do in preparing the soil for the lawn. There is a term "critical root radius" which defines the zone in which there should be no rototilling or trenching (any activity that injures the roots). This is calculated by measuring the diameter of the tree trunk in inches at chest height (about 4 feet above the ground), multiplying by 1.5, and reporting the results (critical root radius) in feet. So, a 4-inch diameter trunk has a critical root radius of 6 feet. Within six feet of the trunk, do not damage the tree’s roots. It is important that you do reduce the compaction of the soil by rototilling or digging, but avoid the critical root radius.
Back to Top
Minerals missing in fertilizer 7-02
Q. As I read the label on my fertilizer bag, I remembered something that I learned in school many years ago - that plants need more than just nitrogen, phosphorus, and potassium. Those three minerals are all that this bag of fertilizer contains. Where can I get the others?
A. There are many other nutrients needed by plant besides nitrogen (N), phosphorus (P), and potassium (K). It is just that N, P, and K are the nutrients most often deficient in soils, so these are the nutrients most commonly available in bags of fertilizers. Plants also need large amounts of hydrogen and oxygen that are readily available in water, so we apply these when we water. Plants also need a lot of carbon that is available in carbon dioxide. We provide this nutrient each time we exhale. Except for carbon, hydrogen and oxygen, all other plant nutrients must be extracted by plants from the soil.
In New Mexico, we often see nutrient deficiency problems in some of the nutrients other than N, P, and K. Iron deficiency appears as chlorosis, or yellowing of leaves while the veins in the leaf remain green. Iron chelate compounds are applied to alleviate this problem. Zinc deficiencies are also observed and zinc chelates are also available. A chelate is an organic molecule that loosely binds to the nutrient needed. This nutrient is easily given up to plants by the chelating compound. Unfortunately, it is also given up easily to the soil. Some nutrients are present in our soil in excess. One of these is calcium that can react to form insoluble compounds with other nutrient elements. Thus, the element may be present in the soil but since it is not soluble, it is not available to the plants.
Nutrient availability is often the cause of nutrient problems in New Mexico. In addition to chemically binding with other nutrients, calcium and other minerals common in our soil cause our soils to be alkaline. In alkaline soils, many minerals do not readily dissolve. Since plants must absorb minerals in dissolved form, they are not available to the plants even though they are in the soil. Some plants are better able to extract minerals from alkaline soils; others are less able. Plants native to alkaline soil regions do better in our soils. Common vegetables and flowers, however, do better in slightly acid soils. For these plants, we must supply the unavailable nutrients in the form of fertilizer, or modify the soil to release those nutrients.
We can modify the soil by adding organic matter. Organic matter consists of material being decomposed to humus and humic acids. The decomposition or organic matter releases carbon dioxide into the soil. When dissolved in water, carbon dioxide forms a weak acid that acidifies the soil. The carbon dioxide with the humic acids release nutrients bound in the soil. We can also apply acidifying agents, usually compounds of sulfur. By acidifying soil, we temporarily make minerals more available to the plants. In time, the soil again becomes alkaline, so we must continue to modify the soil to grow plants needing more acidic soils.
Back to Top
Mulch could be contaminated 5-01
Q. Last year (April 2000) I had my China Berry trees cut down. The tree service sprayed a root killer on the exposed stumps. Now (in March) I had the stumps ground down. Can I use the mulch left over from the stumps or is it contaminated?
A. The answer to this question depends on which material was applied as a root killer. Some materials that may be used in this manner cannot be absorbed by plants from the soil (or wood chips), others breakdown in sunlight or by microbial activity, and others are persistent and remain active in the soil for an extended period of time. If the product used was one of the first two mentioned (not absorbed from the wood chips or broken down by light or microbes), then after one year there should be no cause for concern.
If it is the last class of materials that persist and remain active, then there will be a problem. In this last case, it would not be wise to use the wood chips for mulching or for composting. Contact the tree service to determine what product they used and if it will be persistent and active. Ask also for its mode of decomposition in the environment. This should give you some idea regarding the probability of problems if you use the wood chips for mulching. You can also conduct a bio-assay by taking some of the wood chips, mixing them with soil or potting soil, and placing the mixture in flower pots. Plant some bean or radish (broadleaf plants) and some corn or grass seeds in the pots. Use five replications (different pots) for this bioassay. Plant an equal number in five pots of soil or potting soil without the wood chips as a control for comparison purposes.
If the chemical remains in the wood chips and causes one or both of the plant types (broadleaf or grass) to develop symptoms or die, then don’t use the wood chips in the landscape. The control pots without wood chips in the soil should show no symptoms. If the control pots also show symptoms, then the problem is due to the conditions under which you are growing the plants (or the particular potting soil you used). Over-watering or under-watering is the most likely problem in the growing conditions. If both the control and the wood chip pots show symptoms of problems, rerun the bioassay with different potting soil and with more care to watering.
Back to Top
Mulch near foundation wall 3-02
Q. How close to a house foundation should mulch be spread?
A. This will depend on the type of mulch and potential pests. An organic mulch, such as bark mulch, close to a home can create problems with roaches if they are a problem in your area. Under plastic mulch (with rocks) we have seen carpenter ants that wandered into homes (probably not causing structural damage to the home). Other ants may also benefit from this, increasing their access to your home.
If mulch is used in this manner, the flower bed and other plantings should be at least two feet from the foundation. If you have termite problems, plantings and organic mulch next to the foundation can exacerbate the problem. If termites are a potential problem in your area, application of a chemical termite barrier in the soil around your home before installing the mulch may be a good idea. Contact some pest control companies for estimates. This will depend on the type of mulch and potential pests.
On the beneficial side, I think that a two-foot band of plastic mulch, covered by rock mulch to shed water from around a foundation can reduce potential termite problems by keeping the soil around a foundation dry (at least here in New Mexico where there is little soil moisture).
Back to Top
Mulches—part I, what's best 11-99
Q. What is the best type of mulch to use as a top layer over soil?
A. There is really no "best" mulch to use. The answer depends on the purpose for the mulch and the plants you are growing. Mulches may be used for several purposes. They may be used to stabilize soil to prevent wind and water erosion. They may be used to reduce the evaporation of water from the soil by creating a protective boundary between the soil and the air as well as shading the soil from the sunlight. Because the mulching material usually has a greater porosity than the soil, moisture in the soil does not easily move to the air boundary by capillary action, thus evaporation is reduced. By isolating the soil from the air which changes temperature rapidly, mulches maintain more constant soil temperatures, preventing rapid temperature changes from day to night and as the weather suddenly changes.
Mulches are often used to reduce weed growth. Annual weeds must germinate from seed each year, but they do not germinate if covered so that they are not in the light. Perennial weeds will grow through most mulches, but may be weakened by the effort to reach the light and by the expenditure of additional food reserves to do so. Finally, mulches can help reduce or delay soil compaction, increasing water and air permeation into the soil. It is also easier to manually remove weeds from non-compacted soils. So, with so many purposes, it is difficult to choose a best mulch without knowing the purpose. There are many different mulching materials. Inorganic mulches, which consist of rock, sand, gravel, cinders, and other natural materials derived from non-living sources are excellent mulches for some purposes. Inorganic mulches serve most purposes for mulches quite well. They reduce erosion effectively, and on slopes larger sizes of organic mulches are superior to most organic mulches because they remain in place and do not float away. Inorganic mulches isolate the soil from drying winds and sunlight, effectively reduce erosion, and help maintain constant soil temperatures. However, inorganic mulches tend to heat the air around them more than organic mulches. They also hold that heat longer into the night to the detriment of plants that respond well to distinct diurnal (day to night) temperature variations. If the inorganic mulch is thick enough, weed problems may be reduced.
These mulches can also help prevent soil compaction. Organic mulches are those derived from once living things, grass clipping, bark, compost, and such. They may reduce erosion but are themselves sometimes subject to erosion by wind because they are light or by water because they float. They can reduce water loss from the soil but, because they are porous and absorb water themselves, are not as effective in this as the inorganic mulches. They can help maintain stable soil temperatures, but in the case of finer particle and decomposed organic mulch, plant roots may grow in the mulch material where they are again exposed to drying and temperature changes. This does not make them inferior to inorganic mulches but does reveal the differences. Organic mulches do not usually become as hot as the inorganic mulches and are not as likely to cause heat injury to plants and may not hold heat as long into the night. Organic mulches decompose releasing nutrients into the soil. This enriches the soil but requires that the organic mulch be replaced periodically. Organic mulches create a good environment for earthworms and, because of the action of the earthworms, soil permeability is enhanced. As organic mulches decompose, they release carbon dioxide into the soil. This can injure some of our arid lands plants which are adapted to well-aerified soils containing high levels of oxygen in the spaces between soil particles. Such plants do poorly when mulched with organic mulches.
Synthetic mulches, those which are manufactured from petroleum products (plastic mulches), are also useful; however, if used improperly, they may cause severe landscape problems. Rolls of plastic mulch are used in field agriculture to reduce herbicide and irrigation requirements. In these cases there are spaces between the plastic rolls. In landscape applications, the plastic mulch is often used over the entire landscape with no gaps through which weeds may grow. The plastic interferes with the infiltration of water and air into the soil. Plant roots require both water and air, so even if there is an irrigation system under the plastic, landscape plants often do poorly under a solid (impermeable) plastic mulch. To remedy this, porous plastic mulches have been developed. These remedy the problem of water and air infiltration into the soil but often develop problems over time as wind deposits dust and sand above the mulch material. Weeds then germinate in this layer of dust; their roots then penetrate into the soil below through the holes in the plastic. These weeds are difficult to remove manually because their roots are interwoven with the mulch material.
Back to Top
Mulches—part II
Q. What is the best type of mulch to use as a top layer over soil?
A. This week the answer continues from last week when we discussed the characteristics of various mulches. This week we will focus on the characteristics of plants as they relate to selecting the appropriate mulch. This is somewhat less complicated, but also the characteristics of specific plants are sometimes less well known.
Many plants grown in New Mexico landscapes are not native to New Mexico. The majority of these are from regions which are cooler, moister, and have less alkaline soils. The appropriate mulch for these plants should help alter the environment and soil to simulate that from which the plants are found naturally. Some plants will adapt to a wide range of conditions - these are the easiest to grow under our conditions. Other plants have a very narrow range of environmental adaptability - these plants are much more difficult to grow under New Mexico environmental conditions. Proper choice of mulch material is especially important with those plants with less environmental adaptability.
Organic mulches maintain a lower temperature around plants and are best for plants from cooler locations, whether from cool locations outside New Mexico or high elevations within our state. These organic mulches will decompose, and in the process neutralize some of the alkalinity of the soil. The carbon dioxide from the decomposition of these mulches dissolves in the soil water, lowering the pH of this soil water.
If it is desirable to use inorganic mulches around plants native to cooler climates, it is important to provide shade. Inorganic mulch may be used around plants requiring cooler conditions if the plants are in the shade of buildings or other structures. It may also be possible to plant other, larger plants to provide the necessary shade and cooling. Plants native to New Mexico soils, especially those from sandy and gravelly soils such as are found in arroyos, are often dependent on the porosity of the soil to provide high levels of aeration and high oxygen levels around the roots. The carbon dioxide from decomposing organic mulch is often harmful to these plants. Such plants benefit from an inorganic mulch of cobbles, gravel, or crusher fines, a byproduct of the crushing of rocks, to make gravel. Plants native to arroyo environments are often adapted to the higher temperatures present above the inorganic mulch.
Back to Top
Orchids and potting soil 2-01
Q. I have a Dendrobium orchid which is potted with bark. When the time comes, should I replace with bark or potting soil, or does it matter? Is Miracle Grow okay to use to fertilize?
A. The Dendrobium and many other orchids grow naturally on the bark of trees. These "epiphytic orchids" need very good drainage. That is why they are grown in the bark instead of soil. When repotting, use fresh bark (sold in garden stores for growing orchids) or a similar potting medium. Some people add lava rock to help increase drainage. Potting soil is not recommended for this type of orchid, but I have seen people succeed with potting soil. It is much easier for root rot to develop in potting soils which do not drain as readily. You might want to find an orchid society in your area and benefit from advice from orchid growers familiar with your local conditions. A members of an orchid society should be able to look at your plant to determine which Dendrobium it is, and then give advice based on that. Regarding fertilizers, Miracle Growâ„¢ is okay, as are many other houseplant fertilizers. Don’t overfertilize. Too much nitrogen fertilizer will speed the decomposition of the bark and development of toxic compounds which can injure your orchid.
Back to Top
Plastic sheet mulch: black vs. clear
Q. I have always heard that black plastic mulch in the garden warms the soil faster, but now someone told me that clear plastic causes the garden to warm more rapidly. That doesn’t really make sense to me, but that person is very knowledgeable so I wanted to check.
A. Clear plastic does indeed speed the warming of soil, if the garden is in a sunny location, because it allows the sunlight to pass through to the soil, warming the soil directly. It then holds the warmth next to the soil. Black plastic absorbs the light directly and is itself heated. It then must warm the air under it and the air warms the soil. So why do people use black plastic rather than clear plastic? Because weeds can grow very well under the clear plastic in the early spring. The black plastic, but excluding light from the soil, inhibits weed growth.
As the season advances and the weather warms, it is important to remember that the plastic is still functioning. Clear plastic continues to warm the soil into the summer, perhaps making it too hot for the plants you are growing. In fact, a process called solarization uses clear plastic in the summer to heat the soil to a temperature which will kill disease organisms and weed seeds in the upper layers of the soil. Such heat is not desirable at the roots of our garden plants. Clear plastic may be used early in the spring to warm the soil for early planting of warm season vegetables but, especially here in New Mexico, it is important to remove the plastic or cover it with another mulch or reflective material as the weather warms. Black plastic has the potential to burn whatever touches it. The shading provided by the plants and the rapid heat loss from the thin film, the low heat holding capacity of the plastic film, allow us to use the black plastic as a mulching material without the excess soil heating that clear plastic provides.
Mulch is an important tool for limiting weed problems in the garden and in conserving moisture as well as cooling the soil in mid-summer. However, it can be used to speed the heating of soil in the spring to allow earlier planting of plants which need warm soil. Proper choice of mulch material and then its proper use allows us to maximize the benefits.
Back to Top
Plastic sheeting: is it really mulch?
Q. You recently wrote about the benefits of black plastic mulch versus clear plastic mulch. My question is about whether the mulch is tilled into the soil to capture the sun's infrared rays below the surface...or whether we simply lay the black plastic over the surface of the soil. I always thought of mulch as something that is mixed into the soil?
A. The English language can sometimes be fun, or sometimes a real pain, because it changes so rapidly. I was using the term "mulch" in a traditional sense, as do most professional horticulturists. I have noticed that in recent times many people, especially home gardeners and the media, use the word "mulch" when traditionalists would say "compost" or "soil amendment." I believe the confusion has developed because when we create a mulch from garden trimmings and debris, we encourage people to shred or grind it. This shredding or grinding has begun to be called "mulching" by many people, including some landscape professionals. We also encourage the shredding or grinding (mulching) of garden debris before adding it to a compost pile. Here is where the problem begins to develop. Then, once the compost has been formed by decomposition of the garden wastes, it is applied to the garden. It is usually tilled into the soil but may sometimes be applied to the surface as a "mulch".
This is a further opportunity for "word confusion." The application of the word "mulching" to the grinding process and the use of compost as a mulch has allowed enough confusion to cause many people to call compost "mulch". To a horticulturist, a mulch is a material applied to the surface of the soil to reduce the evaporation of water, to moderate soil temperatures, to reduce wind and water erosion, sometimes even to create an aesthetically pleasing background for the plant, and to limit weed growth. This term, mulch, is the same whether the mulching material is plastic, rocks, sand, or if it is shredded landscape debris. We use the term compost to describe the decomposed organic materials which are added to the soil as a soil amendment. So, the plastic mulches I discussed are to be placed over the surface of the soil and not incorporated into the soil. I apologize for the confusion and appreciate your letter. I am sure you are not the only one who was confused by this terminology. The English language is one in which the correct definition of the word is determined by the majority usage. Someday we horticulturists may need to coin a new word for what we call mulch, but for now, I will explain how I use the word rather than invent new words.
Back to Top
Potting soil: make your own 12-99
Q. Can I make my own potting soil, or must I buy that from the store?
A. Yes, you may make your own potting soil, just as horticulturists have done for centuries. It is important to prepare your potting soil carefully. With today's use of herbicides, we must be certain that any materials used in potting soil are not contaminated with a chemical which will damage the plants that you intend to grow. This is not a complaint against herbicides but rather a warning that they must be considered if certain soils from the garden are used or if soil and plant material for composting are obtained from unknown sources.
Make your potting soil by combining the basic ingredients in proportions appropriate for the plants you grow. The basic ingredients include some or all of the following: native soil, peatmoss, compost, vermiculite, sand, perlite, shredded coconut husk, and other similar materials. Some of these (peatmoss, compost, coconut husk, and similar materials) hold water and nutrients. Some of the other materials are added to facilitate drainage and allow air into the soil. These are very important elements of the soil because waterlogged soils are not good for most plants. Some of the ingredients (native soil, vermiculite, and others) may provide several of these benefits to the soil but usually cannot be used alone as a potting soil. Potting soils may be made from native soil to which amendments have been added.
Some potting soils may actually contain no true soil. It is important that the soil be prepared with the plants to be grown in it in mind. It is not possible to give a generic recipe for home-made potting soil because different plants have different soil requirements. Many cacti and succulents do well in a potting soil containing a high percentage of mineral soil, especially preferring sandy or gravely soils. Other plants prefer a very organic soil which may be composed of a high percentage of compost or peat moss to which a little vermiculite or sand has been added. Other plants may require other variations.
Native soil may also contain plant disease organisms and should be pasteurized by heating moist soil to about 160 degrees. This may be done in the kitchen oven, but causes the kitchen to have a very unpleasant odor. Portable ovens used in the garage or other means of uniformly heating the soil to this temperature may be used to avoid the unpleasant odors. Home-made compost and other non-sterile materials should also be pasteurized before using them in potting soils. New Mexico native soils, if used in potting soils, may often provide problems as our soils have high concentrations of mineral salts. These minerals are necessary plant nutrients but are often present in excess and may cause injury to the plants.
Addition of compost or sand helps to dilute these substances and also allows improved drainage to allow leaching of salts from the soil and water from the pot. This characteristic of our soils and the fact that soil varies from location to location makes specific recommendations regarding your soil difficult. You will need to experiment and determine mixtures of your soils that work best for each of the plants that you grow.
Store-bought potting soil is actually a rather new convenience for gardeners. The ingredients have been around in the garden stores for many years, but only in the last few decades have gardeners had the option of purchasing ready-to-use potting soil. This is the result of increased development of potting soil for use in |
