Soils

Basics
The information here came from Carl Rosen, Extension Soil Scientist, and books called Soil Science Simplified by Helmut Kohnke and D. P. Franzmeier (4th edition, Waveland Press, $7.50 or contact your local library) and The Soul of Soil by Grace Gershuny and Joseph Smillie (agAccess).

What is soil?
It's solid mineral particles, organic matter including microscopic organisms, water, and air. Its purposes from a plant's point of view are to provide support for roots, and water, air, and minerals for the plant.

What are the particles?
There are four types: 1) coarse gravel, larger than 1/16 inch; 2) sand, less than 1/16 inch and inert, broken down rock fragments; 3) silt, even smaller, inert, broken down rock fragments; and 4) clay, less than 1/1000 inch and becomes sticky when wet. Although clay is smaller than silt, it has a larger surface area. This is important because the larger the surface area, the greater the possibilities for chemical and physical reactions.

Clay particles contain silicon, aluminum, iron, magnesium, oxygen, hydrogen, potassium, calcium, and other elements. They are negatively charged and therefore called anions. Positively  charged particles (called cations) are attracted to them. The ability of soil to hold cations is described by its cation exchange capacity, or CEC. CEC affects the nutrient holding capacity of soil. High CEC soil has a higher reserve of nutrients than low CEC soil. If a high CEC soil is acid, more lime will have to be added to neutralize it than to a low CEC soil with the same pH. Clay particles can have CEC values ranging from 10-100 whereas sand can have CEC values below 5. Organic matter also contributes to CEC, as organic compounds have slight negative charges and attract cations.

Small, positively charged ions such as calcium and magnesium can stick to the clay particles. When this happens, the clay particles are neutralized and will stick to each other (flocculate). Sodium ions are actually smaller than calcium and magnesium, but has a larger hydrated radius than those two cations. Although sodium ions stick to clay, their large hydrated radii keep clay particles from sticking together. Clay particles containing sodium repel each other and are called dispersed.

Descriptions such as loamy sand, silt loam, clay loam, etc. refer to soil textures, or the percentages of sand, silt, and clay in the soil.

What is organic matter?
Soil organic matter includes plant roots, small animals, plant and animal residues, humus, and microorganisms. They can be dead or alive, and fresh or decomposed. Humus is dark colored, decomposed organic or microbial material containing about fifty percent carbon, five percent nitrogen, and 0.5percent phosphorus, including hard-to-degrade lignin from plant cell walls and amino acids. The kinds and amounts of humus in a soil depend on the kinds of plant residues and microbes in the soil, and climate. Humus forms better under moist and cool climates than in dry and warm ones. Humus levels decrease when plant residues are removed, soil is tilled and oxidation of humus occurs, and when erosion takes place.

Microorganisms in soil include bacteria, fungi, algae, nematodes, and protozoa. They decompose organic matter, absorb nitrate and ammonium, synthesize nitrogen-containing compounds, oxidize ammonium or ammonia to nitrate (nitrification), reduce nitrate to gas (denitrification), fix nitrogen, and sometimes cause plant diseases. They are more active when soils are warmer than 40 F, and need moisture, air, and nutrients to thrive.

Animals in soil include insects, spiders, mites, and worms. Earthworms help aerate and mix soil and form humus. Mites, spiders, and other animals also break down plant material into humus.

What is soil structure?
Structure refers to how soil particles are arranged with each other. Structure is described by the size and shapes of the particles, and on whether they are formed in distinct ways.

Groups of soil particles are called aggregates. Air spaces between aggregates can fill up with air (large pores) or water (small pores) which are taken up by plants. The amount of large pore space in soils determines the soils' aeration capacity. Rounded aggregates produce both small and large pores, which is optimum for plant growth. Air spaces are provide places for roots to grow into.

Soils can aggregate only if clay or organic matter are present (better to have both). Without organic matter, aggregates will break down when submerged in water for some time. Soils with optimum structure are said to be in good tilth. Very sandy soils, overtilled soils, and compacted soils will have no structure when wet. Rapid freezing and thawing of soil water will cause break down of aggregates.

Besides better aggregation, organic matter also provides a higher water-holding capacity and causes soil to look darker. Dark soils absorb more radiation during the day and release more heat at night. Soil temperature is important because it affects plant growth and microbial activity.

Soil color is an indication of the wetness and the amount of air in the soil. Reddish and brownish colors of soil beneath the surface indicate good aeration and little waterlogging. Grayish and olive colors are indications of waterlogging and reduction of iron. Splotchy color patterns shown changing water levels. Soil consistency is dependent on soil moisture content, and is a measure of how well soil particles hold together. Soils can have loose to cemented consistencies.

What is erosion?
Erosion occurs when soil particles are detached and moved from their original locations. Detachment is due mainly to moving water, such as rain, surface runoff, and freezing and thawing, but can also be caused by wind. Rain drops, especially large, fast-moving ones, hitting unprotected soil can cause more detachment than runoff. Detachment due to runoff increases after water collects in areas and the runoff from those areas moves at a high rate. The smaller the soil particles, the more easily they are washed away. Clay can form aggregates that do not move as easily as silt, so soils with high silt content are easily erodible.

Besides soil type and structure, other factors affecting erosion rates are slope, surface cover, and land use. The steeper a slope, the greater the runoff and erosion rates. The length of a slope and its shape also affect erosion rate. The longer a slope, the more water can collect in an area. Convex or bulging slopes have more erosion than concave ones because they are often drier, have less organic matter and vegetation, and increase in steepness downslope. The amount of cover on land and intensity of tillage also affect erosion rates. The more the soil surface is covered and the less intense tillage is, the less detachment will occur. Movement of soil particles by water or wind is also decreased by keeping the soil surface covered.

Soil particles bind water molecules tightly. Plants have to use energy to pull water away from the soil particles. It takes less energy for plants to pull water from medium-sized soil pores than from small pores. When all the soil pores are filled with water, the soil is said to be saturated. When this occurs, there is not enough air in the soil for plants. When the large pores have been drained of water, but water remains in small- and medium-sized pores, then the soils is said to be at field capacity, and water is available to plants. When plants have removed water from all the medium-sized pores and start to wilt, and water remains only in the smallest pores or as thin films on soil particle surfaces, the soil is at its permanent wilting point, and it is too dry for plants.

For optimum plant growth, soils should be between field capacity and the permanent wilting point. With or without irrigation, water must be able to enter and move through soil, and the soil must be able to store water. For this to occur, the soil must be in good tilth, and have limited surface runoff and erosion.

Excess water can be removed with drain tiles, ditches, or surface furrows. Contour plowing, terracing , or small depressions in the soil surface will slow surface runoff.

What is the chemical makeup of soil?
Ninety-five percent of soil weight, on average, is made up of oxygen (50 percent), silicon 36.5 percent), aluminum (5 percent), iron (2 percent), and potassium (1.7percent). Most of the potassium in soil is not available to plants because it is in rock fragments or in a form that plants cannot absorb. Elements available to plants make up less than two tenths of a percent of soil weight. Sulfate, nitrate, chloride, and bicarbonate are anions that are not adsorbed by clay particles and therefore move freely through soil.

Sometimes ions get washed away (leached) or build up. Too much hydrogen causes the soil to become acidic, which can affect microbial activity and nutrient availability. However, most nutrient availability or ion solubility problems occur when soil pH is below 5.0, which is uncommon in Minnesota soils. Some microbial activity is restricted when soil pH is between 5 and 6.

Too much sodium can cause soil to become alkaline and to disperse. Often the ions are bound to other ions, forming salts. Soil containing more than 0.2 percent soluble salts is considered too saline for plant growth. Most problems with alkaline soils occur when soil pH is above 8.0, which is also uncommon in Minnesota. However, iron deficiency can occur when soil pH is above 7.0  or for acid loving plants like blueberries, when soil pH is above 6.0.

Three of the elements necessary for plant growth, oxygen, carbon, hydrogen, are obtained directly from the air by plants through photosynthesis. Nitrogen, potassium, and phosphorus are the necessary elements most limiting plant growth, and often have to be supplied externally. At least, they have to be replaced after they have been taken up by plants and removed when the crops are harvested. Calcium, magnesium, sulfur, and other micronutrients (also called minor elements) often are not limiting, but sometimes need to be added to soil.

These elements, or nutrients, are added to soil through rain, nitrogen fixation, decomposition of crop residues, weathering of soil, and by the addition of external fertilizers. They are lost from soil by crop removal, erosion, leaching, and when converted to insoluble forms.

To learn more about soil nutrients, testing for nutrient levels in soil, calculating the amount of fertilizer to use, types of fertilizers, and nutrient requirements of different fruit and vegetable crops, see the University of Minnesota Extension Service bulletin (BU-5886-E) Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota by Carl Rosen.

Testing
This information was written by Carl Rosen, Extension Soil Scientist.

Soil testing to determine nutrient availability is an extremely valuable diagnostic tool for crop production and should be considered a routine practice for any operation. Results of a soil test are used primarily as a guide for making fertilizer and lime applications prior to planting. The tests can be used not only to identify low pH and nutrient deficient soils, but also to identify soils where additional amendments may not be necessary. Thus, soil testing can take much of the guesswork out of making fertilizer or lime recommendations and lead to more economically and environmentally sound nutrient management.

Effective use of a soil test is dependent on submitting a representative sample and understanding the principles upon which the results and interpretations of a soil test program are based. The following discussion will briefly address topics related to sample collection, sample analysis, and how research-based fertilizer recommendations are made.

Soil sample collection
Accurate interpretation of soil test results for making fertilizer recommendations is dependent on collecting a representative sample. Samples not properly collected will result in misleading recommendations. Even the best-equipped laboratory cannot make up for a sample that is poorly taken. The procedure for taking a meaningful soil sample is summarized below.

Each field to be sampled should be divided into uniform areas. Each area should have the same soil texture and color, cropping history, and fertilizer, manure, and lime treatments. One sample should not represent more than twenty acres on a level, uniform field and 5 acres on hilly or rolling land. After scraping off the surface residue, samples should be collected to the 6-8 inch depth. For each sample, fifteen to twenty subsamples should be collected from randomly selected areas in the field. The soil should be mixed in a clean plastic pail and about 1/2-1 pint of the mixture placed in a sample bag. Samples can be sent to the laboratory moist; however, air-drying is recommended if they cannot be sent to a lab within a few days after sampling.

Soil samples can be collected at any time of the year, although spring and fall sampling are usually the most convenient. If soil tests from a given field are to be compared over the years, it is best that samples be collected at the same time of year. Ideally, fields used for vegetable production should be tested on a yearly basis; although every two to three years is acceptable for most nutrients except for nitrogen (nitrate), which must be tested annually in the spring. On new fields, a soil test is strongly recommended before planting. Consequences of not taking a soil test can be quite costly and in some situations has resulted in crop failure.

Sample analysis and interpretation
Fundamental goals of a soil testing program are to: 1) determine whether a particular nutrient is deficient, optimum, or excessive in a soil, 2) assess the need for fertilization, and 3) provide fertilizer recommendations to the producer based, in part, on soil test results. Processes leading up to these goals require extensive background research, both in the lab and in the field. Suitable chemical extractants must be identified that will correlate with nutrient uptake or yield. Then, crop response to fertilizer at given soil test levels must be determined. Without adequate research data, soil test results cannot be properly interpreted and may lead to improper fertilizer recommendations.

Specific fertilizer recommendations based on soil tests need to be calibrated for particular regions. Recommendations from one area, in many cases, may not be suitable for other areas due to differences in climate, soil characteristics, and cultural practices. Therefore, the best soil test recommendations are usually the result of local or regional research. In general, calibration of fertilizer recommendations to soil test values is an ongoing process and fine-tuning will occur as management practices change and more research is conducted. As an example, fertilizer recommendations need to take into account fertilizer placement. Banding of P fertilizer as opposed to broadcasting generally increases P use efficiency and therefore lower rates of P fertilizer would be required.

Soil tests are ideally suited to determine organic matter content, available P and K, as well as pH and lime requirements. Soil testing can also be used to determine secondary and micronutrient needs, which can be particularly important for some vegetable crops. The nitrate test to determine residual nitrate in the soil can also be worthwhile if manure is used or if excessive N fertilizer has been applied over the years.

A soil test valued should be considered as an index of the availability of the nutrient being extracted rather than an absolute quantity of nutrient available. Basically, the soil test value or relative level provides a probability of response to applied fertilizer. As shown in the table below, the higher the soil test value, the lower the probability of a response to applied fertilizer. Conversely, a low soil test value would have a high probability of response to applied fertilizer.

Generalized relationship between relative soil test level and probability of response to applied fertilizer.

The use of relative soil test levels is a convenient method of expressing nutrient availability because the extractant used can have a dramatic effect on the absolute value reported. When comparing soil test values from different laboratories, it is important to know the chemical methods used. If the extractants are not the same, then it is likely that the soil values will be different and may lead to confusion unless the values are categorized into relative levels as described above.

Because of the mobility of nitrate in most soils and the large fraction of nitrogen tied up in organic matter, the nitrate test in humid regions has not historically been used to determine N fertilizer requirements. Recent research, however, has shown some benefit of using the soil nitrate test especially if manure is the major source of applied nutrients. High levels of residual nitrate will lower or eliminate the need for N fertilizer. Recent studies in New Jersey on sweet corn have shown that sidedress N fertilizer was generally not required when soil nitrate N was greater than 25 ppm. The test was useful for predicting N sufficient sites, but was of limited use for making N fertilizer rate recommendations. For most vegetable crops, the use of the nitrate test in humid regions still requires further calibration research before specific recommendations can be made. 

Fertilizer recommendations based on soil tests
Once soil is submitted to a laboratory and the test results are reported, the next step is to determine fertilizer requirements. In most cases, if there is a difference in fertilizer recommendations among laboratories for a given soil sample, the reason is due to differences in soil testing interpretation philosophy. There is more than one approach to making a fertilizer recommendation. The rate of fertilizer recommended can vary depending on which approach is used. Three of the most common approaches to soil test interpretation are: 1) build-up and maintenance approach, 2) sufficiency level approach, and 3) basic cation saturation ratio approach. The build-up and maintenance approach promotes a rapid build-up to a high soil test level, plus annual replacement of the soil test level. The sufficiency level approach establishes cut-off levels above which no fertilizer is recommended. The saturation ratio approach is used for potassium, calcium, and magnesium and establishes ideal saturation ratios for these ions.

All of these approaches were developed from university research programs. Therefore, some may be more applicable to certain geographic regions than others. Many laboratories use a combination of these approaches rather than relying solely on one. In most cases, fine-tuning of the general recommendations will need to be done regardless of the approach used, but the important point to remember is that the starting point should be research-based.

Weeds

Basics 
Information contained here is culled from the books Weed Ecology by Steven Radosevich, Jodie Holt and Claudio Ghersa (1997, John Wiley and Sons, 2nd ed.) and Weed Management in Horticultural Crops, edited by Milton E. McGiffen, Jr. (1998, ASHS Press). This was reviewed by Roger Becker, Extension Weed Scientist.

The Weed Science Society of America in 1994 defined a weed as "any plant that is objectionable or interferes with the activities or welfare of man". Characteristics of weeds are prolific reproduction, widespread dispersal, and rapid growth. However, generally there is more competition between crop plants than between weeds and crops. This competition is due to limitations of environmental resources such as water, CO2, light, nutrients.

Weeds can be classified several different ways:

• Taxonomy - dicots (broadleaves) vs. monocots (grasses and sedges);
• Life history - annuals (complete life cycle within a year, reproduce primarily by seed), biennials (produce leaves, stems, and roots in one year, and flowers and seeds in 2nd year) or perennials (live longer than two years and may flower several times before dying); or simple (reproduce from seed or cut root pieces), creeping (overwinters and produces new shoots from underground structures), or woody;
• Habitat - terrestrial vs. aquatic;
• Physiology -

·         a) carbon fixation during photosynthesis - C3  (carbon dioxide converted into a compound with 3 carbon atoms; examples include mustards, lambsquarters, nightshade, quackgrass, sedges) vs. C4 (carbon dioxide converted into molecule with 4 carbon atoms; more efficient process than C3; includes pigweed, purslane, crabgrass, barnyardgrass, foxtail millet);

·         b) flowering response to daylength - short-day plants, such as lambsquarters, in which flower production occurs when daylength is short or long-day plants, such as henbane. A light break during the night can inhibit flowering of short-day plants.

• Degree of undesirability - noxious or  poisonous; and
• Evolutionary strategy -

·         stress tolerators (survive inhospitable environmental conditions by slowing growth and reproduction),

·         competitors (good at obtaining resources in productive by relatively undisturbed habitats, usually produce lots of vegetative growth), or

·         ruderals (grow in highly disturbed environments, usually have short life span and produce lots of seeds).

A good reference for identifying weeds in Minnesota is: Weeds Of the North Central States, North Central Regional Publication No. 281.

Environmental factors affecting weed growth include soil type and moisture, pH, light quality and quantity, precipitation, and air, soil, or water temperatures. Biological factors affecting weed growth include insects, diseases, grazing, plant competitors, and human activities.

Once weeds become established in a field, it is difficult to eradicate them because of the seed bank, or the population of seeds in the soil. Greater than 95percent control is not enough to prevent weed populations from increasing each year! Generally, weeds produce lots of small, long-lived seeds. An example is lambsquarters seeds that were found to be 1700 yrs old and still viable! Knowing what kinds of seeds are in the seed bank allows you to predict what weeds might become future problems. Longevity of the seeds is dependent on the seeds' dormancy and condition, soil conditions (including how much manure is in it), and the location of the seed in the soil. Usually seventy to ninety percent of the seeds in the bank are from a few species, many of them produce seeds early in the season. The composition of the seed bank will change little from year to year if the cropping system in the field is the same each year. Crop rotation changes the environmental or cultural conditions in a field that favor certain weeds. Weed seeds can be lost from the bank due to 1) predation by rodents, insects, or microbes, 2) senescence or decay, and 3) germination or emergence. The natural dormancy of the seeds can be used to manage weeds - use no tillage or night tillage because many weed species require light for germination, and many weed species exhibit shallow or surface germination.

The spread of weed seeds is time and space dependent. Seeds move both vertically and horizontally, but most stay near the parent plant. The distribution of seeds is affected by tillage, herbicide use, and harvesting practices. Wind, water, animals, and humans can all spread seeds.

Weed management includes prevention, eradication, and control. Weed control can affect levels of insects and diseases, so the costs and benefits of a weed management practice need to be considered. Scout or walk fields regularly to look for weeds and other pests. Note the numbers, types, and locations of weeds. Preventative methods include using clean seed, using thoroughly composted manure, cleaning equipment and checking clothing before moving from a weedy to a non-weedy area, not moving or using weedy soil, and inspecting transplants for weeds. Cultural methods include removing weeds near ditches, fence rows, rights-of-way, etc., preventing weed reproduction, using seed screens on irrigation water, restricting livestock movement into non-weedy areas, and rotating crops. Managing crops to optimize their competitiveness also helps - plant well-adapted crop varieties at optimum times, and use optimum row spacings, proper soil amendments, and proper water management. Physical methods of weed control include uprooting, burying, cutting, smothering, or burning weeds by hand pulling, hoeing, flaming, tilling, mowing, shredding, flooding, or mulching. Biological control, the use of living organisms to decrease population or competitiveness of weeds, may act slowly so is not for emergency use. The biological control agent must be host-specific so it won't damage crops or native plants. It is useful when land values low, no closely related crops near weed, and other weed control practices are inadequate. Chemical control by herbicides is useful when cultivation difficult or expensive, and to control perennials. Using herbicides can decrease the amount of tillage and human labor needed, as well as decrease mechanical damage to crops, and allow for earlier planting and greater flexibility in choice of management systems. Herbicides can be more expensive or less effective than other methods, can injure crops, native plants, and other organisms, and can persist in water or soil.

Tillage disturbs soil, and breaks, cuts, or tears weeds from soil so they dry up and are smothered. It works best when the weeds are small, the soil is dry, and the weather is hot. It needs to be done every 10-14 days after seedling emergence. Plowing, disking, and harrowing are used before planting to prepare seedbeds, kill emerged annuals, suppress perennials, and burying seeds. Rotary hoeing is done after seeding and before crop emergence. It is most useful to remove small-seeded weeds among large-seeded crops. Cultivators are used to uproot or cover weed seedlings. Cultivation works best before weeds reach the 2-3 leaf stage. Mowing is used to cut weed flowers before they can set seed. Mulching covers weeds. Natural mulches can also add organic matter and nutrients to soil and are generally inexpensive, but are bulky to transport and can be difficult to apply over large areas. They should be applied to a depth of 4 inches. Synthetic mulches, generally made of plastic, can decrease soil water loss and increase soil temperatures, but can be difficult to remove. Also, holes must be made in the plastic for the crop plant to grow through. Weeds can grow in these holes and at the edges of the plastic. Weeds will grow under clear plastic. Black plastic keeps light out so weeds won't germinate under it, but will not warm the soil as well as clear plastic. Infrared transparent mulches warm soil but prevent weed growth. Fast growing smother crops, such as buckwheat, or cover crops, such as rye, can be used to suppress weeds, but can delay planting.

Herbicides work by inhibiting plant growth, cell division, photosynthesis, or metabolism. They are classified based on how they work, when they are applied (preplant, preemergence, or postemergence), and whether they are selective against weeds (such as metolachlor and trifluralin) or nonselective (such as paraquat and glyphosate), so must not come in contact with crop plants. Symptoms of herbicide injury include formation of multiple shoots, twisting, whitening or yellowing of leaves, leaf death, and abnormal seedling development. Herbicide injury to crop plants can occur with drift, as a spray or vapor. Most injury due to drift occurs from herbicides affecting plant growth or amino acid synthesis. Injury to crop plants can also be due to carryover, when an herbicide stays longer than it's needed. Persistence of an herbicide in a field depends on its movement into air or in soil of the herbicide, its adsorption to soil, and how fast it is broken down. Because most herbicides are adsorbed by clay or humus, and water interferes with this adsorption, risk of carryover is less in soils that are coarse-textured, contain low organic matter, or that are irrigated frequently. The rate an herbicide is broken down is dependent on temperature and moisture. The higher the temperature and amount of moisture, the faster an herbicide will break down.

Weeds can develop resistances to herbicides, although there is less resistance to herbicides than to insecticides or pesticides. This is because of the relatively low persistence of many herbicides, and the lower fitness of some resistant (versus susceptible) weeds. Also, susceptible weeds are usually still able to produce seed and there is often a large reservoir of susceptible weed seeds in the seed bank.
 

Tillage and cultivation equipment
Information in this section came from Greg Bowman's book, Steel in the Field (1997, The Sustainable Agriculture Network, Beltsville, MD, $18) and an article titled "Mechanical weed control offers alternative for growers" (March 1998 issue of The Great Lakes Vegetable Growers News). Also, Jim Bender provides some advice on choosing equipment in his book Future Harvest (1994, University of Nebraska Press, Lincoln, NE).

Rotary Hoes 
Rotary hoes have ground-driven wheels with curved teeth. They till soil to a depth of one to two inches, and can be used pre- or postemergence, if the crop is more deeply rooted than the weeds. Ideally, they should be used when the weather is dry and hot. They can also be used to aerate crusted soils.

Harrows 
Flex-tine harrows have spring wire tines and are used for broadcast weeding (light tillage) over and between rows. They work best when weeds are in the white root stage or newly germinated, if the crop is well-rooted. They can be used for a variety of crops and row spacings without much equipment modification. Tines can be lifted above the crop, and they move over obstructions. They can be used at high speeds preemergence, but are not useful when weeds have 4 or more leaves, and are 1.5 or more inches tall.

Spike-tooth harrows have pointed spikes the stir soil to a depth of one and a half inches.

Cultivators
Cultivators have arrow-shaped sweeps that uproot and bury weeds. A variety of shanks, stabilizers, and hitches provide flexibility for the grower. Models with S-tines can be used for loose soils to shake weeds loose. Single shank cultivators can be used in firmer soil with varying amounts of residue. Rolling cultivators have shovels and gangs of wheels with curved teeth (spiders) or notched disks that slice through soil. Those with spiders can cut through low amounts of crop residue. Disks can be used with moderate residue.

Brush Hoes
Brush hoes have plastic bristles that uproot weeds by rotating through soil. Shields hang above the soil surface and protect crops, but this is an aggressive and precise tool, so an operator to steer the shields is necessary. It can be used for weeds up to ten inches tall, and in slightly moist soils.

Finger Weeders 
Finger weeders have steel cones with rotating fingers that push soil and uproot weeds. They are used for in-row weeding of small weeds up to one inch tall. They work best in loose soils and when the fingers pass very close to the crop row. Therefore, slow driving speeds and precise cultivation are necessary.

Torsion Weeders
Torsion weeders are also used for in-row weeding. They have spring-loaded steel rods that undercut weeds on either side of a crop row. They work best for weeds in the white root stage.  

Basket Weeders
Basket weeders have rolling pairs of wheels connected with spring wire. They can be used for weeds up to two inches tall, in soil that is not crusted. They weed through one inch of soil without moving soil into the crop row.

Flame Weeders
Flame weeders use propane-fueled burners to rupture the cells and kill the growing points of weeds. They can also kill the crop plants, and cannot be used with all crops. They work best when weeds are small and dry, and when the weather is dry and winds are calm. Because the growing points of grasses are often close to or below ground, they are not as effective with grass weeds as with broadleaved weeds. Soil next to weeds should be smooth, and weeds should not be protected by clods of soil, so it is best to cultivate after flame weeding. Flame weeders are available as portable backpack or tractor-mounted models.

Herbicides
Information in this section came from Weed Management in Horticultural Crops, edited by Milton E. McGiffen, jr. (1998, ASHS Press), and Weed Ecology by Steven Radosevich, Jodie Holt and Claudio Ghersa (1997, John Wiley and Sons, 2nd ed.).

Herbicides are synthetic chemicals used to kill or suppress weeds. They can be applied preplant, before seeding or transplanting; preemergence, after seeding but before the crop or weeds emerge from the soil; postemergence, after the weeds emerge; or lay-by, the last operation in the field before harvest. Herbicides applied to soil must be mixed into soil by water or mechanical action, such as tillage preplant. They are then absorbed by plant roots, tubers, corms, bulbs, or seeds. Their effectiveness depends on how far they are from the plant in the soil, and how well the weeds tolerate them. Herbicides applied to foliage injure weed leaves and stems. They can injure plants upon contact or move through the plant and suppress root or shoot growth.

Herbicides are usually formulated in water, but other carriers can be used, including diesel oil, which enhances the penetration of the herbicide into waxy leaves or bark, fertilizer solutions, or dry materials for granular application. Liquid sprays include water-soluble or wettable powders and liquids, emulsifiable concentrates, and water-dispersible liquids and granules. Wettable powders are not truly dissolved in water, and require agitation to keep the particles suspended in water. Different companies formulate the same active ingredient in different ways to enhance the effectiveness, handling, or application of the herbicide, decrease toxicity to animals, including humans, or improve weed control or crop selectivity. Some formulations include crop protectants.

Herbicides are categorized based on their chemical structure, use, effects on plants, and toxicity. Dinitroaniline herbicides prevent cell division at growing points, so must be applied before weeds have emerged. They are not water-soluble, so must be mechanically incorporated into soil, and will not leach. They are effective against most annual grasses and broadleaved weeds, but not those in the legume, mustard, or nightshade families. They can last four to twelve months in soil, and can damage sweet corn, spinach, and beets.

Acid amide herbicides work the same way as dinitroaniline herbicides, but can also have an affect on plant metabolism. They also must be applied preemergence through mechanical incorporation. However, they are more water-soluble than the dinitroaniline herbicides, so can leach through soil when water is applied. They are effective against most annual grasses and specific broadleaved weeds.

Urea, uracil, and triazine herbicides cause the loss of chlorophyll and interrupt photosynthesis. The timing of application depends upon the crop. They can be applied to soil or foliage, but they only move from the roots upward, and not from leaves to roots. Most of these herbicides can leach through soil with irrigation or rain, and they persist for months to a year in soil. They are effective against most weeds.

Thiocarbamate herbicides suppress root or shoot development, and cause deformities. They are are water-soluble so can be applied with water preemergence. They can move laterally through soil. If incorporated into soil in a dry form, they persist for a long time, until the soil is wetted. Then they last four to eight weeks. They are easily absorbed through all parts of the plant, and move readily through the plant. They are effective against many different grasses and annual broadleaved weeds.

Phenoxy-type herbicides include 2,4-D, dicamba, and picloram. They affect plant growth and development, are only applied postemergence. They are effective against most broadleaves, and can cause crop injury.

Clomazone and norflurazon bleach leaves, are readily taken up by plant roots, and are applied preemergence. They do not move through soil. Clomazone lasts four to eight weeks, while norflurazon can persist for up to a year. They are effective against many grasses and broadleaved weeds.

Selective grass herbicides, such as sethoxydim and clethodim, affect the growing points of grasses. They are only applied postemergence. Nonselective herbicides including glyphosate and paraquat can be used pre- or postplant but preemergence. They will kill crop plants as well as weeds, although paraquat is ineffective against large grasses. DCPA and bensulide are herbicides used preemergence. They are effective against most annual grasses and some broadleaved weeds.

Entomology

Plant Pathology

Postharvest Handling 
Harvested horticultural products are still "alive". That is, metabolic processes continue even after fruits, vegetables, and flowers are cut from a grown plant. Like humans, such products breathe, using up carbohydrates or proteins and oxygen and producing carbon dioxide, water, and heat. This breathing process is called "respiration". Knowing the respiration rate of a cut product can allow you to predict how long it can be stored. This knowledge can help you plan a harvest schedule. For example, market gardeners should harvest long storing produce (generally those with low respiration rates) the night before market, but harvest short storing produce (generally those with high respiration rates) as late as possible.

Because respiration rates are temperature dependent, harvest produce when temperatures are cool for the longest possible shelf life. On sunny days, shade harvested produce if there is a delay in taking them to the packing shed.

The right time to harvest depends on the product characteristics, end use, and climatic conditions. For example, No. 1 fresh market carrots should have a diameter of one inch at the crown. However, processing carrots should be larger because they will later be cut up into smaller pieces. Harvesting and quality standards have been developed for many horticultural commodities. Starch indices have been developed for apples, and the maturity of an apple can be determined based on the amount of starch in the flesh, as determined using iodine staining. However, indices may be variety specific, generally are used for commonly grown varieties and may not be applicable to locally developed varieties. Other produce characteristics used to determine maturity and quality include color, size, specific gravity, shape, firmness, sugar content, acidity, and juiciness.

Although mechanical harvesters have been developed for iceberg lettuce, celery, nuts, processing tomatoes, and some other crops, horticultural products for the fresh market are generally hand harvested. This is because mechanical harvesters can cause a lot of damage to the product, and buyers of fresh produce will not buy product damaged in any way. Also, nicks and bruises on produce can shorten the shelf life of the product. Respiration rates generally increase when wounding occurs. Also, open wounds provide entryways for pathogens. When hand harvesting produce, trim fingernails and wear gloves to avoid nicking product. Provide cushioning in harvest containers. If possible, pack produce in the field. The less the product is handled, the lower the possibility that damage will occur.

Packaging
Packaging protects the product from damage, including that occurring when the product is dropped, compressed, or shaken. It can also protect the product from water loss, if the packaging contains moisture barriers, such as wax or plastic liners. Special packaging can modify the atmosphere around the product, lengthening its shelf life. With modified atmosphere packaging, carbon dioxide in the atmosphere surrounding the product increases and the oxygen decreases, as the product continues to respire. In some cases, modified atmosphere packaging can include an ethylene absorbent. Ethylene is a gas that is normally produced by many ripening fruit. It can hasten fruit ripening and softening, so can shorten product shelf life. On the other hand, ethylene gas is used by the fruit industry to uniformly ripen mature, green tomatoes and bananas.

Curing
Potatoes and onions should be cured before storage. During the curing process, wounds are healed over and skins are dried. To cure potatoes, keep them at fifty to sixty degrees Fahrenheit and ninety-five percent relative humidity for ten to fourteen days. Then put them into cool storage. Sweet potatoes should also be cured, but at eighty-five degrees Fahrenheit and ninety-five percent relative humidity. To cure onions, below eighty degrees Fahrenheit, eighty percent relative humidity air through the pile until the outer scales are dry and the neck is light. Other products should be precooled before storage. 

Precooling
Once product is taken to the packing or storage shed, it should be cooled to the lowest possible temperature the product can withstand to remove field heat. For products originating from temperate climates, the precooling temperature is often forty degrees Fahrenheit or lower. For tropical produce, this temperature is fifty-five degrees Fahrenheit. This cooling should occur before the product is placed into final storage. Delays in lowering product temperature result in shortening product shelf life, so should occur as soon after harvest as possible. Precooling methods include room cooling, forced air-cooling, hydrocooling, icing, and vacuum cooling.

Room cooling is the most inefficient method. Product is simply placed in a cooler, and over time, the product temperature decreases nearly to the cooler temperature. This process takes a long time, but requires no other equipment than a cooler.

In forced air-cooling, product, often packed, is placed so that fans can push or pull cool air through the product. Cooling time is faster than with room cooling. The only equipment necessary is a forced air cooler, which can be made relatively inexpensively. However, because the air being pulled or pushed through the product is of lower relative humidity than the inside of the product, some loss of water can occur from the product. Forced air-cooling is applicable to a wide range of product.

With hydrocooling, cool water is sprayed over the product, or the product is immersed in cool water. To avoid introducing microorganisms into the product, clean debris off the product before hydrocooling it. Also, hydrocooler water should be chlorinated with fifty to one hundred ppm chlorine. If the product will be immersed in the hydrocooling water, the water should be slightly warmer than final storage temperatures. Because the amount of moisture inside the product will be less than one hundred percent, hydrocooler water will tend to move into the product. If there are microorganisms present in the hydrocooler water, they will move into the product along with the water. Hydrocooling can only be used with product and packaging that can withstand wetting. It should not be used for product that needs to be cured before storage, such as onions, potatoes, and winter squash.

Icing is used for product that can withstand direct contact with ice, such as beets and broccoli.The ice can be finely crushed, flaked, or in a slurry with water. Packaging used with icing must also be able to withstand wetting.

Vacuum cooling is used for product with high surface-to-mass ratios, such as leafy vegetables, cauliflower, and sweet corn. The product is placed in a vacuum, so that the atmospheric pressure around the product is reduced. This reduces the water vapor pressure around the product, and when it is lowered below that inside the product, water evaporates from the product. This causes a one percent weight loss for about every forty degrees Fahrenheit decrease in temperature. Vacuum cooling equipment can be very expensive, so is used where there is a large quantity of product.

Storage
The most important components of proper storage are temperature and relative humidity. For recommended storage conditions for various horticultural commodities, consult USDA Agriculture Handbook No. 66, The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks, by Robert E. Hardenburg, Alley E. Watada, and Chien Yi Wang (1986). This reference also provides information on how long different commodities can be stored.

Storage temperatures should be low as possible to slow product metabolic processes and discourage pathogen growth, but high enough to avoid freezing or injuring the product. Tropical products, as well as some more temperate products such as basil, cucumbers, peppers, and tomatoes, are chilling sensitive, and turn black or develop sunken spots if held at too low temperatures. Water makes up ninety-five percent or more of horticultural commodities. For this reason, storage relative humidity should be ninety-five percent or higher to slow the rate of product water loss and shrinkage. High storage relative humidity can be accomplished by installing misters or containers of water in storage rooms. Be careful not to allow standing water on the product, to discourage pathogen growth.

Unless ethylene is being used deliberately, keep product away from ethylene sources, such as internal combustion engines, ripening fruit, decomposing produce, cigarette smoke, deteriorating rubber products, and propane-powered forklifts. Keeping storage rooms well-ventilated (one air exchange per hour) helps keep ethylene levels low.

Building plans for storage facilities are available from the Cornell University (James A. Bartsch, Agricultural Engineering Bulletin 453, call 607-255-2280), University of California (James F. Thompson and Robert F. Kasmire, leaflet 21449), and North Carolina State University (M.D. Boyette, L.G. Wilson, and E.A. Estes, document AG-414-2). Storage buildings should be well insulated, painted white to reflect sunlight, and be equipped with moisture barriers.

Transportation
For proper care, product temperatures during transit should be the same as storage temperatures. Precooled product in unrefrigerated transportation systems can heat up during long trips if outside temperatures are high. When transporting loads containing product that must be kept cold mixed with product that should be kept at a warmer temperature, insulation, ice, or cold packs around the cold product should be used. If possible, ship chilling sensitive product separately from those that need to be kept cold. Also, separate ethylene-producing product (apples, pears, muskmelons) from ethylene sensitive product (leafy greens, broccoli), and odor producing product (garlic, onions) from those that can absorb odors.

References:  

General

• Knott's Handbook for Vegetable Growers

Entomology

Organic production

• Bender, Jim. 1994. Future Harvest: pesticide-free farming. University of Nebraska Press, Lincoln, NE, 159 p. ISBN 0-8032-1233.

Plant pathology

Postharvest handling and technology

• Hardenburg, Robert E., Alley E. Watada, Chien Yi Wang. 1986. The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. USDA Agriculture Handbook #66, 136 p.
• Kader, Adel A. et al. 1992.  Postharvest Technology of Horticultural Crops, University of California, 304 p., ISBN 0-931876-72-9.
• Kitinoja, Linda. 1995. Small-Scale Postharvest Practices: A Manual for Horticultural Crops.
• Wills, Ron, Barry McGlasson, Doug Graham, Daryl Joyce. 1998 (4th ed). Postharvest. An Introduction to the Physiology and Handling of Fruit, Vegetables and Ornamentals. CAB International, 262 p., ISBN 0-86840-560-4.

Soils

• Kohnke, Helmut and D. P. Franzmeier. 1995. Soil Science Simplified (4th ed), Waveland Press, Prospect Heights, IL,162 p.
• Gershuny, Grace and Joseph Smillie. 1995. The Soul of Soil: A Guide to Ecological Soil Management (3rd ed), agAccess, Davis, CA, 158 p.

Weeds

• Bowman, Greg. 1997. Steel in the Field. Sustainable Agriculture Network, Beltsville, MD, 128 p. ISBN 1-888626-02.
• Radosevich,  Steven,  Jodie Holt and Claudio Ghersa. 1997. Weed Ecology: implications for management (2nd ed). John Wiley and Sons, NY, 589 p. ISBN 0-471-11606-8.
• McGiffen, jr., Milton E (ed). 1998. Weed Management in Horticultural Crops. ASHS Press, Alexandria, VA. ISBN 0-9615027-8-9.
• Weeds Of the North Central States, North Central Regional Publication No. 281.