When considering organic production as a farming technique one must first come to realize and accept the reality of crop loss due to uncontrollable pest problems. There are times when non-chemical controls are just unacceptable. To help understand and accept these facts, it is advisable that a potential new producer to visit and tour an existing organic operation, attend organic grower meetings, and, thoroughly research the topic prior to planting the first seed (3). Additionally, a new producer must realize that they will be a learning curve in which 3 – 5 years maybe needed to adequately develop an understanding of the methods required to effectively produce safe foods naturally that will meet the market quality demands.
Maintenance of good soil health is the foundation of organic production. One of the most highly touted means of achieving good soil health is through increasing soil organic matter. Since most Texas soils contain 1% or less organic matter, this can be a long term, highly challenging process. High soil temperatures can cause rapid decomposition of applied organic matter, requiring large volume applications over long periods of time. Studies have shown that it is unreasonable for a grower to expect to increase soil organic matter by more than 1% (2). For instance, an acre of dry soil six inches deep weights approximately 2,000,000 pounds. To increase the soil organic matter content by 1% will require the application of 20,038 lbs/A (460 lbs/1000 2 feet). Unfortunately most organic matter sources contain 25% or more ash (inorganic matter). Consequently, to achieve a 1% increase of actual organic matter with a source comprised of 50% moisture and 25% ash, 80,000 lbs/A would need to be applied and incorporated into the soil profile (2). To further challenge a producer, this will be needed on an annual basis! It is understandable how such volume application can pose real delivery, distribution and handling problems. Fortunately, benefits from applied organic matter can be achieved at much lower rates. As little as 8712 lbs/A applied organic matter has been shown to improve soil tilth, and, 21,780 lbs/A, improved plant growth (2). Much of the response to applied organic matter is in its ability to improve; water retention and infiltration rate, soil aggregate stability, cation-exchange capacity, soil biological activity, as well as, serve as a reservoir for organic acids and plant nutrients such as nitrogen. The end result is a more favorable soil microclimate for plant root development and function. This in turn results in improved plant growth and yield (17). However, large quantities of organic matter may stimulate soil borne pathogens and insects such as wireworm, cabbage maggot, and whitefly that can cause serious damage to crops.
Nutrient management (soil fertility)
Maintenance of good soil health and optimal growth is depended upon proper soil nutrient management. Therefore, the major objective of a nutritional management program is to fertilize soils in such a manner to have a sufficient quantity of nutrients in the soil available to plants so that any one does not become yield limiting. The use of soil tests is suggested as a means of determining nutrient availability status in soils. The results from these tests will provide a producer with guidelines on how much fertilizer material will be needed for successful crop growth and yield. It should be remembered that soil tests are site specific and any given soil sample should not be used to base fertilizer needs for soils in additional fields or farms not tested.
The primary element of concern in vegetable production is nitrogen. Not only is this element normally required in large quantities to satisfy most crop needs, it is highly mobile in the soil and is easily lost due to leeching. As a result, nitrogen levels can be undergoing almost constant changes in the soil. This is especially true for light, sandy soils and in high rainfall areas. Unfortunately, soil tests generally are not reliable indicators for nitrogen availability or nitrogen fertilizer requirements(11). With organic production where nitrogen fertilization or availability is depended upon decomposition of organic matter and/or fixation by living organisms, soil test for this element may be even less reliable then with conventional production. However, through experience gained from supplemental fertilization and crop response, soil testing can be a helpful tool for nitrogen need determination. Table 4 contains information useful in interpreting soil test results.
A complement to soil analysis for monitoring plant nutrient status during the growing season is tissue analysis. Tissue for analysis can be taken any time during the growing season. If results show deficiencies of a given nutrient is present in the plant(s), a grower can then take steps to correct the situation by adjusting his fertility program.
Factors Impacting Soil Nutrient Availability and Crop Performance
There are many factors and conditions that impact plant nutrient needs. Table 5. depicts the more important factors and how they impact plant response (13). The development of a sound nutrient management program should be based on a knowledge of crop needs and soil nutrient removal ability, cation ion exchange capacity of a given soil, and the role of soil pH on nutrient availability and plant response. Table 3 of the appendix lists the general N (nitrogen)-P(phosphorous)-K(potassium) requirements for the most popular vegetables grown in Texas. Although each crop has its own specific requirement, all horticultural crops utilize N, P and K in a similar ratio. As such, supplemental nutrition should be made available in this ratio if possible.
Table 4. Guide to Interpreting Soil Test Results
|ELEMENT||CROP||ELEMENT RATINGS (ppm)|
|V. LOW||LOW||MODERATE||HIGH||V. HIGH|
* Indicates the soil test level at which addition of a nutrient is suggested or a comment statement is generated.
Reference: Soil, Plant and Water Testing Laboratory-Texas AgriLife Extension Service.
Predicted crop response to fertilizer where soil test indicates element to be:
Very high = no crop response; High = no crop response; Medium = 75-100% of maximum expected yield predicted without fertilization; Low = 50-75% of maximum expected yield predicted without fertilization; Very low = 25-50% of maximum expected yield predicted without fertilization;
Table 5. Factors impacting nutrient needs of plants.
Pest and Disease
|Roots require oxygen for respiration and nutrient uptake
Nutrient uptake is an indirect result of photosynthesis, cannot occur with light
Needed to dissolve nutrients in order to be absorbed by roots
Influences growth, which in turn influences rate of uptake, more growth greater the need.
Older the plant the less the absorption efficiency
The more extensive the root system the greater the potential for nutrient absorption
The reservoir for nutrients, buffer for leeching
Breakdown organic matter to release nutrients
The greater the inherent content the less the supplemental needs
The heavier the soil the greater the ability to store nutrients
Regulates soil supply availability
Can disrupt organs such as roots and lessen the absorption potential
Table 6 lists the ratio of these elements found in tissue samples across plant types (fruit, vegetables, and ornamentals). Based on this data, N, P, K is found in all horticultural plant species in a 5-1-2 ratio. Where possible, soil nutrient availability should be in this ratio as well. The data for soil removal by a given yield of vegetable crops follows this ratio to a certain extent, Table 4 of the appendix. Unfortunately, most organic fertilizer sources, Table 5 of the appendix, do not allow for the precision of supplying in the normal removal rates for vegetable crop nutrients or in the ratio shown in Table 6. In addition, the actual percentage of N, P, and K contained in the usual organic sources is low. Consequently, huge volumes of these materials are needed to meet the crop demands.
Table 6. Average N – P – K content of horticultural crops.
|Crop Type||% N||% P||% K||N:P||N:K|
|2.0 – 6.0
2.0 – 7.2
2.4 – 5.6
|0.2 – 0.7
0.15 – 0.3
0.3 – 0.7
|1.5 – 3.5
1.0 – 2.5
1.5 – 4.0
Fertilizer ratios needed to equal leaf analysis: 15 – 3.5 – 11.25
Cation exchange capacity (CEC) determines availability of nutrients in soils. Cations are positively charged particles(ions) of elements, the most important of which are Ca, Mg, Fe, NH4, Na and H2. Clay soil particles and organic matter have negatively charged ions. As a result, the cations of the various plant nutrients can be attached to and held on the surface of these soil particles. CEC is a measure of the quantity of cations that can be absorbed or held by a soil. Soil organic matter has a very high CEC. Thus, soils with high organic matter typically have a higher CEC than soils with low organic matter. These soils also typically can store and supply nutrients to plants more readily than soils with a low organic matter content. Table 7 lists the typical CEC of some soil texture classes (27).
Table 7. Typical CEC of some soil texture classes.
|Soil Texture||Typical CEC range
Clay and Clay loam
|2 – 6
3 – 8
7 – 15
10 – 18
15 – 30
Soil nutrition availability is also influenced by soil pH. Simply stated, pH is a measure of soil acidity or alkalinity. Technically, pH refers to the hydrogen ion concentration within a soil. The pH scale ranges from 0-14. A value of 7 represents a nutireal soil, values above 7 represents an alkaline or basic soil, and, values below 7, an acid soil. Contrary to popular belief, the pH scale is not linear but logarithmic in scope (7). Consequently, a soil with a pH of 6 is 10 times more acid than on with a pH of 7 and one with a pH of 5 is 100 times as acid as one with a pH of 7. In itself pH is almost non-consequential because it has no direct effect on plant growth. However, pH can impact plant growth based on its influence on the availability of essential plant nutrients and on the concentration of elements toxic to plants. Optimal pH for most vegetables range between 6 and 7. Table 6 of the Appendix contains the optimal pH ranges for selected vegetables and their tolerance of acid or alkaline conditions. Table 8 presents nutrient deficiency symptoms induced by pH problems(7).
Table 8. Nutrient deficiency symptoms induced by pH problems.
|Phosphorous||Stunting, slow growth, delayed maturity, short internodes, purple or dark green foliage; old leaves die back. More severe in cold weather|
|Potassium||Young plants have dark green leaves with small stems and short internodes. Older leaves scorched on margins; weak stem; fruit shriveled, uneven ripening. Young leaves can crinkle and curl|
|Boron||Tip of growing plant turns inward and dies; bud becomes light green; roots are brown in center; flowers do not form. Leaves are small, crinkled, deformed with areas of irregular areas of discoloration|
|Calcium||Young leaves turn yellow then brown; growing tip bends; weak stem; short dark roots.|
|Iron||Young leaves are yellow between veins on older leaves. Initiates first from top to bottom; veins, margins, and tips stay green.|
|Magnesium||Leaves are thin, lose green color from between veins on older leaves. Start out from bottom of plant up; tend to curve upward.|
|Manganese||Tissue between veins turns white; leaves have dead spots; plant is dwarfed. Starts out on very young leaves|
|Zinc||Terminal leaves are small; bud formation is poor; leaves have dead areas. Yellow intervein mottled regions.|
In some situations soil pH may require adjustment. To lower soil pH, sulfur applications are recommended, whereas, to increase pH, applications of lime or limestone are suggested. Quantities of sulfur and limestone needed to adjust pH can be found in Tables 9 and 10. In many areas of Texas, high pH is the problem. Unfortunately, the water sources in most of these areas are also prone to be high in free calcium carbonate. Therefore, the addition of sulfur to lower pH has little or no effect on reducing pH because large quantities of Ca are often applied with subsequent irrigation. As a result, the sulfur is neutralized. Producing crops with a high tolerance of high pH is the only solution to the problem.
Table 9. Correcting Soil pH -Amount of Lime Required to Increase pH.
|(ECC) Effective Calcium Carbonate
Equivalent by soil test
*Neutralizing equivalent based on calcium being 100 percent.
Source: Adopted from KCES. Horticulture Tips by Charles Marr and Dave Whitney.
Table 10. Amount of Sulphur (95% S) Needed to Lower the Soil pH
(as Measured to Approximately pH 6.5 – Weights are expressed in Pounds per Acre)
|Broadcast application to whole soil mass (6-inch depth)|
|Soil pH found by measurement||Sandy Soils||Loamy Soils||Clayey Soils|
Source: The Fertilizer Handbook.
Fertilizing crops organically
Once a crop’s nutrient needs are determined, the next decision to be made is designing an effective fertility program is what source for nutrients best fit the overall production system. The nutrient sources most often used in organic production are: green manure and cover crops; manure; compost; and sludge. Availability and cost usually determines which of the above is used on a given crop. However, a sound fertility program will include green manure and cover crops in combination with one or more of the other sources. Green manure and cover crops containing a N-fixing legume are the most economical and beneficial means of supplying nutrient (11). These crops are usually planted in rotation with economic or cash crops and serve to improve soil tilth and water holding capacity as well as to replenish soil nitrogen and other nutrients (17). A green manure crop is so called because it is one, which is planted for the purpose of plowing into the soil while still green and prior to harvest maturity. Depending upon species, some green manure crops may require as many as 120 – 160 days of growth in order to fix 100 – 200 lbs N/A. Research suggest that available N from a green manure crop will increase over a 4-6 week period following soil incorporation and then return to pre-incorporation levels. Therefore, crops following a green manure crop may need to receive supplemental applications of N from another organic source such as compost or manure tea (11). Legumes are the green manure crops of choice due to their efficiency in fixing N. These crops can add as much as 30 –125 lbs N/A if properly grown and managed (2). Suggestions for green manure crops, seeding rates and N contributions can be found in the following Table 11.
Table 11. Green manure crops for use in enhancing soil health.
|Crop||Season||Seeding rate (lbs/A)||Type||Nitrogen value (lbs/ton dry matter)|
Source: Growing Vegetables Organically. George Boyhanon, Darbie Grandberry, W. Terry Kelly and Wayne McLaurin. Univ. Georgia Cooperative Extension Service B1 011.
A cover crop is one which is usually planted during the cropping season in which a field is usually left fallow. Such crops may or may not be harvested as a cash crop. Although non legume cover crops do not contribute much N, they can trap it and mine other nutrients from deep with in the soil profile and bring them to the surface where they can then become available to the following cash crop once they are plowed into the soil (11). Some research also suggests that cover crops maybe key to the development of humic acid fractions of the soil. Increasing humus in soils is one of the primary precepts of organic production. Decomposing organic matter can also contribute P, K, S, Ca, and Mg to the soil nutrient pool. An additional benefit of green manure and/or cover crops is that they tend to reduce N leeching (17).
The use of cover crops, however, can have some adverse effects in cropping systems. They can serve to deplete soil moisture supplies, limit options for cropping sequences, and temporarily immobilize plant nutrients, increase pest problems and increase production cost. Therefore, the key to effective and profitable use of such crops lie in creative management designs to enable a producer to take advantage of their benefits within a rotation without missing income opportunities as a result of a missed cash cropping season (17). Animal waste or manure is the oldest fertilizer source used by man to produce a crop. These waste products are still widely used today and are the backbone of organic fertilization programs. The more commonly used manure and their relative nutrient content can be found in Table 5 in the appendix. Although manure is a good organic fertilizer it should be remembered that the use of fresh manure is to be avoided. Ammonia is released during the decomposition of fresh manure, which can be injurious to plants (25). In addition, fresh manure contains a large quantity of moisture which cause problems in handling and uniformity of distribution in a field. It also tends to increase the cost of transportation and handling. Aged manure is a better choice but problems with uniformity of distribution still occur. Therefore, properly composted manure is the most desirable choice for use in crop production. The composting process, if properly employed, reduces the moisture content and kills most of the harmful bacteria. Pulverization of the composted organic fraction reduces transportation costs and improves uniformity of distribution. Although compost is a relatively economical source of plant nutrients, composts can be quite variable in nutrient content depending upon the source and materials used in the process. This situation can prove to be a challenge to a producer in determining its composition and to determine how to use it effectively (11). Properly composted animal waste is essential to prevent human health problems. Research has confirmed that the human pathogen, Esehercichia coli 0157:W7 contaminated soil from contaminated manure can be transmitted to produce grown on this soil (28).
There are food safety issues arising from the use of manure in that animal feces contain high levels of human pathogenic organisms which can be transferred to crops on which it is used. Aged or properly composed manure tends to reduce the risk from the use of animal waste materials. However, it is important that all farms using manure follow good agricultural practices to reduce any microbial risk that may exist (22). These include:
- Consider the source, storage, and type of manure
- Store manure as far away as practical from areas where fresh produce is being grown and handled. If manure is not composted, age the manure at least six months prior to field application.
- Where possible, erect physical barriers or wind barriers to prevent runoff and wind drift of manure particles.
- Store manure slurry for at least 60 days in the summer and 90 days in the winter before applying to fields.
- Compost manure using proper temperature and turning techniques
- Plan manure application in a timely and careful manner
- Apply manure in the fall or at the end of the season to all planned vegetable fields preferably when soils are warm, non-saturated, and cover cropped
- Use only properly decomposed manure on crops like lettuce and leafy greens
- Avoid planting root or leafy crops in the year that manure is applied to a field
- Incorporate manure into the soil
- Do not harvest vegetables until 120 days after manure if possible.
- Document rates, dates and locations of manure applications
In addition to animal waste, commercial composting operations often utilize other waste products such as plant debris, tree and shrub trimmings, discarded food, and food processing waste. The actual elemental content of these products will determine the nutritive value of the resulting compost. As a result, consistency in nutritive value often becomes a problem. Poor quality or immature compost can tie up N in the soil and decrease N availability to plants (11). C:N ratio of a compost is an important consideration. During composting microorganisms require carbon for growth and energy for protein synthesis. Decomposition of organic matter wastes depends on proper balance of C and N. Rapid decomposition occurs when C:N ratio is between 15 and 35:1. Favorable ratio results in the loss of NH4(ammonia) while higher ratios can slow the process (19). Other factors affecting proper composting are; temperature, pH, and oxygen supply, Optimum composting conditions are: moisture content between 40 – 60 %; temperatures between 55 – 600 C; pH 5.0 – 9.0, and, 30 % free air content. Compost quality factors include age, moisture content, particle size, pH, salt concentration and purity (volume of sand, soil and other non-organic materials).
A misnomer surrounding the use of organic verses inorganic fertilizer is that the organic sources are better for plant growth and the environment. Claims are often made that plant responses are more dramatic with the use of organic sources. In actuality, the source of N etc is irrelevant to a plant. For instance, N has to be in a certain form, nitrate (NO3) or ammonium (NH4), before it can be taken up by plants. Consequently, if an organic source is used the N has to be converted to the nitrate form just as it has to be when inorganic forms are used. Table 12 lists the essential elements for plant growth and their forms available to green plants (24). Noticed improvement in plant response with inorganic forms is really due to the effect of organic matter on soil tilth, aeration and water holding capacity of the soil (25) and not to N. Once in the plant the source of N has no baring on the use of this element in growth. In some instances, natural or organic sources can actually contain dangerous levels of contaminants such as salts, boron and heavy metals. Such contaminants can have a disastrous effect on plant growth and yield. With regard to environmental friendliness of organic or inorganic sources, organic sources have just as high a potential to cause N leaching into the ground water as does inorganic sources. What determines the rate of leeching is the quantity and use patterns of the fertilizers (17).
Table 12. Elements and chemical forms available to green plants
Form available to green plant
|N03- (nitrate ion), NH4 + (ammonium ion)
HPO4 +and H2PO4- (mono and dihydrogen phosphate ions)
K+ (potassium ion)
Ca++ (calcium ion)
Mg ++ (magnesium ion)
S04 +and S03 +(sulfate and sulfite ions)
Fe ++, Fe +++ (ferrous and ferric ions)
Mn ++, Mn +++ (manganese ions)
Cu +, Cu ++ (cuprous ions)
Zn ++ (zinc ion)
B03 C (borate ion)
Mo04 +(molybdenum ion)
Cl – (choride ion)
Co ++ (cobalt ion)
Often times during the production of a crop, additional N is required during the growing season. Supplemental N can be applied in a side or top dressing with a good quality compost. Another method of supplying supplemental N during the growing season is through the application of manure tea. The use of this product may or may not have merit in large scale farming operations. Adding an organic source such as chicken manure or steamed bone meal to water, stirring the mixture for several days, and then draining off the liquid makes manure tea. The tea is then applied to crop either as a soil drench , band or injected through a drip irrigation system. The basic formula for the above is three pounds of manure/25 gallons of water (2).
Sewage sludge has also been used as a organic fertilizer. Sludge is the solid material removed from sewage treatment plants. It generally is available in three forms; raw, digested, and, activated (previously treated sludge that has been aerated so that aerobic decomposition can occur) (32). Of these only the activated sludge should be considered for use in limited situations in agricultural crop production. The risk of contaminating food with human pathogen is a major concern with these products. Table 13 contains a listing of organic sources for elements other than N.
Table 13. Organic nutrient sources for essential elements other than nitrogen.
Ca, Ng, S
|Poultry liter, colloidal, soft and hard rock phosphate
Cover crops, mined granite, greensand, basalt, feldspar, langbeinite and Potassium sulfate
Kelp and sea weed extracts and powders, dolomite, gypsum, keiserite, langbeinite, limestone, rock phosphate, and, oyster, clam and crab shells
Liquid or powdered seaweed extract, kelp meal, rock powders
Certain meat processing industry by-products such as blood and bone meal have also been touted as organic fertilizers. However, these material also have serious food safety issues regarding their use due to the potential transmission of certain diseases to humans.
New organic farming operations and/or transition fields (those previously used for conventional crop production) may initially experience nutrient deficiencies when using organic fertilization systems until the nutritional benefits from these systems begin to become available. In these situations there are other materials approved for use. Table 7 of the appendix lists some suggested products approved for use in Texas by TDA. There are other growth enhancing products being used that are not fertilizers but are auxin or hormonal in nature. Kelp and other see weed extracts fall into this category. Test results, however, with many of these products have shown marginal benefits from their use.
Most conventional farming operations follow very intensive soil preparation activities such as mold board plowing, disking and bedding. Unfortunately, intensive tillage can be counter productive to an organic farming system due to its negative impact on soil organic matter content. Long-term tillage practices can reduce soil carbon 30 – 50 % (17). Consequently, in organic production where soil organic matter is the key component , conservation tillage may be a better alternative. Conservation tillage is a practice in which at least 30 % of the soil surface is covered by residue from a previous crop. The biggest draw back to this practice is a concern of potential weed buildup and some allopathic responses. However, if properly managed, these problems can be minimized.