Here yellow shoulder developed well into harvest as tissue K levels decreased. Photo: Steve Bogash
There are a number of cultural practices that growers can implement to dramatically decrease these problems. Proper irrigation management, careful attention to balancing specific nutrients, and the use of either plastic or organic mulches have all been proven in field trials to significantly increase fruit quality.
While weather is a factor that remains beyond growers control, shelters such as high tunnels have been shown in trials to be "unusually effective" at increasing fruit quality through reducing rain splash on fruit and even improving light quality when using 'diffusion-type' plastic films. Episodes of fruit cracking often follow rain especially in larger fruit.
In addition, keeping rain off the foliage all but eliminates a number of fungal and bacterial diseases by keeping the leaves dry and preventing rain drop splashing of disease causing spores. Spider mites, whiteflies, and powdery mildew remain pest challenges for shelter-grown tomato plants.
In order to produce the greatest quantity of the highest quality tomatoes (peppers too.) growers must:
- Pay careful attention to soil preparation prior to planting. In the case of soilless media, the selection of the appropriate media has long term consequences for the management of plant nutrition.
- Select only those varieties that perform well as well as meet individual grower market requirements.
- Understand their water resources thoroughly as pH and alkalinity have direct implications in water treatment and the selection of nutrients.
- Use moisture testing soil appliances such as tensiometers in order to meet plant demands as conditions change.
- Be prepared to test both the soil and plant tissue as part of a concerted program to meet changing plant demands.
- Have a well designed, easy to maintain, nutrient injection system.
- Be prepared to apply nutrients on a regular basis to meet plant demands.
- Be prepared to apply foliar nutrients in addition to 'plow-down' and injected nutrients based on plant demand.
Radial cracking can be caused by insufficient or imbalanced Ca and Mg. Photo: Steve Bogash
Preplant soil preparation
The first step in creating a high yielding, high packout tomato crop is preplant soil testing. Incorporate at least 30-50% of nutrient requirements at soil preparation. Organic growers will want to incorporate 70-80% as organic fertilizers for injection post planting are substantially lower in N,P, & K versus powdered concentrates. Slower release fertilizers such as greensand as a potassium source and aragonite as a calcium source have demonstrated high potential to further reduce Blossom End Rot and Yellow Shoulder.
Perhaps no other issue is as open to argument as is 'what varieties are best to grow'. Most important in this selection is, what do your customers expect from you? Then comes, What can you produce profitably to meet their expectations? Penn State Extension has been trialing tomato varieties in both field and high tunnel since 2000. Reports are available by contacting the author.
Currently the varieties that meet most growers' needs for red, slicing tomatoes are: BHN 589, Scarlet Red, Conestoga, BrandyBoy, Primo Red, Fabulous, Charger, and Mountain Fresh Plus. There are many hundreds of tomato varieties with new ones coming to the market every year. Consider yield, flavor, fruit appearance, disease resistance, and training / pruning needs when selecting.
Water resources, pH and alkalinity
Tomatoes and peppers have the best nutrient uptake at a soil solution pH of 6.2-6.5. This will maximize potassium uptake as well as create the situation where it is possible to keep all of the other nutrients in the sufficient zone. Regular testing of your irrigation source for pH and alkalinity will provide you with the information to adjust your pH through the addition of acids. Note: low pH is seldom a problem, but where the irrigation pH is below 5.8, the use of alkaline fertilizers is indicated.
The injection of alkaline materials such as lime is only recommended where the pH is below 5.5. Sulfuric acid is the most common material used to reduce pH and alkalinity. Organic growers have had good success with citric acid pH reduction. Nine ounces of powdered citric acid will reduce the pH of most water by about 1 full point on the scale or 10X. Conventional grower can use the NC State / Purdue alkalinity calculator to get extremely close to a proper dose. The use of a calibrated pH meter is highly recommended to test the irrigation stream regularly as the online calculator and citric acid recommendation are simply tools to get you close and water changes based on rainfall and source.
The single most important technique in growing great fruit is careful water management in the root zone. This is especially true during fruit fill. Depending on the weather, tomatoes can use up to 2" of water per week during fruit fill. Medium flow drip tape (.5 gallon per minute per 100') needs to run for approximately 9 hours per week in order to apply 1" of water. More important than the amount of water is getting it applied evenly over time. Plants that experience erratic waterings causing severe wet to dry soil conditions will produce greater numbers of cracked and split fruit. This is especially true of larger fruit. Research has shown that the larger the fruit, the greater the likelihood of skin cracking. Some of the smaller "cherry and grape" types of tomatoes also have a greater sensitivity to cracking as they have been bred to have thinner skins.
Properly installed tensiometers are an excellent tool in determining how long and often to irrigate. Both our field and high tunnel plantings at SEAREC get tensiometers installed at 6, 12 and 18" shortly after planting.
Cracking can be reduced or managed by paying careful attention to irrigating and through managing Ca and Mg" tissue levels. Photo: Steve Bogash
Soil and Tissue Analysis
The key to good plant tissue analysis results is in taking a good sample and getting that sample to a testing lab while still in good condition. Knowing the correct parts of a plant, how much plant and the right time to collect plant tissue are important to this process.
For example, useful tomato tissue testing requires 10-15 entire leaves, including the petiole taken from the most recently fully expanded leaves. These are usually the 4th or 5th leaf down from the growing tip. These leaves are now at their full size and have changed to a mature coloration. Older leaves than these store excess nutrients and can indicate plenty of some nutrients, while younger leaves are still taking up nutrients, so may indicate a deficiency that does not exist. Collect leaves only from plants that appear average in your planting. Stunted plants or extra large plants will be poor indicators of the overall nutrient needs of a planting.
Each sample should be all the same cultivar as mixed varieties will degrade the usefulness of the results as well. If necessary send in multiple samples. One method to manage the number of tissue samples is to select one or more 'indicator' varieties, then base your nutrient applications on those results versus testing every variety separately every two weeks. The downside to this is the potential to under or feed varieties that have special needs. In our high tunnel plantings at the Penn State Southeast Research and Extension Center, we've experienced certain heirloom varieties that yielded little but a heavy canopy when sharing a nutrient program with high yielders such as BHN 589.
Like yellow shoulder, Blossom End Rot is determined by tissue levels at the time the fruit is pollinated and fertilized. Photo: Steve Bogash
Tomatoes go through 3 relatively different stages of growth during their lives. First the plant increases rapidly in size leading up to flowering. Sampling the plants at the first flowers will give you a snapshot of their early fruiting needs and go a long way to providing nutrient information needs to prevent Yellow shoulders, blossom end rot and other related, non-pathogen caused diseases.
Two to three weeks later, as the first fruit is sizing up rapidly, is another window to tissue test. The quickly growing fruits are using quite a lot of calcium, magnesium and potassium along with other nutrients. This test will help you to assess your fertilizer program and answer the question: Are you keeping up with your plants needs?
Unless you are only planning a single harvest, testing again at the beginning of the harvest will enable you to again assess your fertilizer program for continued fruiting(s). Testing every two weeks is an efficient program that will keep you current with your plants nutrients needs over a changing season.
The tissue sampling program described above is very simple and easy to apply to determinate tomato varieties, indeterminate types are somewhat more complicated in later stages, but can be successfully sampled as well. Peppers benefit from this same program. Be sure to indicate the stage of the plants that you are testing on your sample submittal form. If in doubt, talk to your laboratory prior to collecting samples, most labs are very helpful in making suggestions so they get the best samples and you get the best information.
Definitions of terms on tissue analysis results
There is not enough of the nutrient present to meet even the most minimal requirements of the plants. In tomatoes, deficiencies are commonly found in Ca, Mg and K. Shortages of these nutrients cause many of the common packing house losses as tomatoes are downgraded due to cracking, blossom-end rot, and yellow shoulders. Depending on the specific nutrient and the degree of deficiency, you may be able to make up the difference with foliar applications, injected fertilizers, or a combination of injected and foliar applied nutrients.
This is probably the greatest area of confusion for growers in determining how much fertilizer to apply as sufficiency is often expressed as a percentage. At the low end of the scale (less than 50% sufficiency), there may enough nutrient present to carry a small crop or light fruit load, but not enough to prevent all nutrient related maladies or grow a profitable crop. As the percentage moves closer to 100% sufficiency in all nutrients and they are kept in balance, the likelihood of a great crop that moves easily through packing increases. Nutrient balance is especially important as nutrients can compete for binding sites. The balance between K, Ca and Mg is one that directly impacts fruit quality. Once flowering has begun in earnest, look for plant tissue levels of 3% plus on potassium, 3% on calcium and. 8-1% for magnesium by dry matter.
Once a nutrient is above 100% sufficiency, some other nutrient is probably lacking as all nutrients are carried in plants in a balance. Toxicities can be expressed as phytotoxicity (leaf scorch or burn), or more often as deficiencies as other nutrients are no longer as available. In extreme cases, nutrient toxicities can kill plants.
Once you have your tissue test in hand, the next step is in determining what to apply and when.
Nutrient Injection System
Proper and timely application of injected nutrients requires equipment that is up to the task. Since most growers will inject more than one concentrate into the irrigation stream at a time, multiple proportional injectors make the job much easier. These are typically plumbed in series, so that each ingredient is added to the stream. The first injector should be reserved for pH and alkalinity modification.
If injecting acid on a regular basis, consider an injector that can handle the levels of acid you will be adding. Most growers that the author has worked with require 2-7 ounces of 93% sulfuric acid per 100 gallons of irrigation water in order to modify the irrigation stream to a pH of 6.2-6.5. This level of acid in a 1/100 ratio injector in within the capability of most equipment designed for fertilizers. Be prepared to rebuild your injectors every two years or as the manufacturer recommends for reliability and accuracy. Other injectors should be added in series as needed. Higher concentrations of acid (generally above 2%) in the irrigation stream may require 'acid-proof' parts.
Application of injected nutrients
The potassium / nitrogen (K/N) ratio deserves careful consideration. It is advisable to have plenty of nitrogen available to rapidly grow a strong plant. At flowering and during subsequent fruit fill, the ratio between K and N becomes critical. Experiments in Southwest Michigan indicate that a 2K to 1N ratio is necessary to produce quality fruit during fruit filling. This ratio will tend to enhance fruit firmness as well as reduce yellow shoulder. Potassium plays a key role in water relations and epidermis (skin) elasticity. Sites that are very poor in fertility may benefit from a 3K to 1N ratio. Tissue N levels of 4-6% are advisable as plants are growing out from a transplant. However, tissue N levels need to be brought down to 3.5-4.5% once fruit is maturing or suffer the likelihood of soft fruit.
The Michigan research also indicated that a single foliar application of boron at. 25 lb/A reduced shoulder checking. In some, but not all cases foliar calcium applied both with and without boron was also beneficial. Growers need to be cautious in applying micronutrients such as boron as excessive amounts can result in fruit defects and phytotoxic damage to skin and leaves.
Once tomato plants are fruiting look for the following tissue nutrient levels (by dry matter):
|Nutrient||Element||Tissue nutrient level|
A good program to begin injecting nutrients is to base your initial applications on general plant population requirements. An acre of tomatoes needs approximately #.5 of N per day. From this starting point, you can adjust your program as necessary based on soil and tissue analysis.
A tomato acre has 4,840 plants (plants 18" apart with rows on 6' centers). Start with a balanced fertilizer such as 20-20-20 until the first flowers appear, Switch to a high K, low N fertilizer at this point such as 9-15-30, 5-10-27, or 8-16-42 and adjust other nutrients based on tissue results.
Calcium is a little special as it does not move from older plant leaves, so plants need it regularly for reduced cracking, firmness and preventing blossom end rot. Tissue testing will reveal whether you need to adjust this nutrient. It is not uncommon to have to regularly tweak levels of Mg and Ca.
Foliarly Applied Nutrients
Keeping up with rapidly ripening fruit demands when a single tomato plant may be ripening #8-15 of fruit at a time requires careful attention. High yielding tomato varieties often benefit from the application of very small amounts of Ca, Mg, B, and K to foliage. Research has demonstrated a synergy between the application of potassium to the roots coupled with a foliar application of potassium. Regular applications of Ca, Mg and K help in maintaining sufficient levels of these nutrients during highest demand.
Carefully follow the label directions and double check your math as there is always the potential to burn leaves and fruit through mis-applied foliar materials. Avoiding applications during the hottest part of the day once temperatures reach 85F will greatly reduce the potential for phytotoxic reactions from spray materials. Materials that blended specifically for foliar application often have adjuvants that increase the uptake of nutrients through leaf tissue.
More on fruit quality
Both organic and plastic mulches have been associated with reduced fruit cracking. This is due to increased control of moisture in the root zone as well as a tighter variation in temperatures between daytime and nighttime. Increased organic matter also plays a role in moisture management as well as nutrition buffering.
Yellow shoulder is a physiological disorder of tomatoes that is characterized by discolored regions under the skin that show through and reduce the quality of the fruit. The disorder can range from very mild with some internal spotting to quite severe with large areas that are hardened and yellow to white. This wide variation in symptoms has spawned a number of names for the same primary disorder: yellow shoulder, yellow eye, green shoulder, yellow tag, gray wall, and internal white tissue. It is very important for growers to understand that yellow shoulder is not a delayed ripening, but an actual disorder of the affected tissue.
The cells in the affected sectors of the fruit are generally smaller in size and have a more random arrangement than that of normal cells. This happens very early in fruit development, which makes early leaf analysis extremely important in prevention, as uniform color requires more K+ than the amount required to sustain yield.
Note: The cause of the yellow shoulder disorder in tomato fruit has baffled scientists for the last 30 years. Many scientists believe there are several causal factors for yellow shoulder including: environment (specifically, high temperature >90°F), nutrition, genotype (cultivar) and virus. The interaction of these factors under field conditions is very difficult to evaluate.
This disorder can be triggered by insufficient exchangeable K+, excess magnesium in relation to calcium, and pH above 6.7. Management options to reduce yellow shoulder include increasing K+ to above 3% by dry matter at fruit set, adjusting the soil solution pH to 6.4-6.7 and increasing the Mg/Ca ratio to 1/6 or better (with 1/2-1/3 being ideal). Leaf analysis testing at the first flower initiation is extremely important in preventing yellow shoulder as once fruit is hanging and damaged it will not improve.
Also, certain cultivars are less susceptible and others at higher risk, so cultivar selection is integral to a program to manage this color disorder. Certain processing varieties have been identified as less susceptible, but much research remains to be done on fresh market varieties in order to identify those that are more or less susceptible. Some growers have anecdotal reports of cultivars that are more or less susceptible, but no research has been done to conclusively identify those cultivars.
The practice of letting the fruit hang longer in order to "color up" does not work and has the potential to increase the danger from fruit rots. Increasing K+ through the drip lines once there is abundant fruit hanging is also unlikely to lessen symptoms as this disorder shows up very shortly after fruit set.
Quite a bit of work has been done by UC Davis and Ohio State on prevention of this disorder in processing varieties. They have developed the Hartz formula for anticipating the risk of yellow shoulder. This formula can be readily accessed online. You will need recent soil and leaf analysis results in order to use the formula.
For most growers the best practices to prevent yellow shoulder will be to intensively leaf test tomatoes from first flower cluster for Ca, K+ and Mg levels. From this information, a grower can adjust their nutrition program to reduce the potential for this disorder. Typically, eliminating yellow shoulders also greatly reduces Blossom End Rot.
Here the entire cluster of grape tomatoes has yellow shoulder. Having sufficient K while in balance with Ca and Mg levels at fruit set could have prevented these losses. Photo: Steve Bogash
High Tunnel Production
Repeated use of the same field location for high tunnel production has been shown to increase the soluble salt content of the soil. A good practice for growers is to periodically thoroughly water the soil in a shelter between crops in order to leach the leftover fertilizer salts out of the soil. This will require several inches of water over 1-2 weeks. If you've been growing in a high tunnel for more than 3 years and using fertigation to apply nutrients, get your soil tested for salts. Sometimes salt levels are so high that a white crust can be seen on the soil surface.