Tomatoes and Marigolds
Growing up we always planted marigolds in with our tomatoes. This practice was folklore wisdom that got handed down from generation to generation. In this case, folklore wisdom is proven correct with scientific research. Researchers at Newcastle University have published a study in PLOS ONE, showing that limonene is the component of marigolds that repels tomato whiteflies. The research team says it could be possible to develop a limonene product that could be hung in greenhouses to deter whiteflies. Study leader Dr. Colin Tosh commented, “There is great potential to use limonene indoors and outdoors, either by planting marigolds near tomatoes, or by using pods of pure limonene. Another important benefit of using limonene is that it’s not only safe to bees, but the marigolds provide nectar for the bees which are vital for pollination.” The team is conducting further studies on developing a three-companion-plant mixture to repel three major insect pests of tomato: whiteflies, spider mites, and thrips.
A wide variety of soil textures can be used for tomato production. Ideal soil textures range from sandy to fine-textured clays as long as the soil is well drained (tomato roots will not tolerate being waterlogged), has good structure, and is well aerated. Planting dates can be determined by the type of soil a grower has. loam and clay loam soils are the most productive soils for production. A mixture of one-quarter to one-half sand, one-quarter to one-half silt, and no more than 27% clay gives loam soils the best possible properties for plant growth. These mixtures supply adequate drainage and good soil structure. Conversely, heavier clays can be a serious problem due to poor soil drainage and excess soil moisture for extended periods, making tomatoes more prone to disease.
One of the foundations good gardening is maintaining and building a microbial-active soil enriched with organic matter and a balanced mineral diet. Humus-building practices and addition of minerals not only supply plant nutrients but also increase tolerance to insects and diseases, help control weeds, retain soil moisture, and ensure produce quality. Depending on the soil type, soils with no history of organic management will probably need additional fertilization to be incorporated during bed preparation or banded to the side of the row at planting. Soil tests provide a baseline for your soil so it’s always a good idea to conduct a soil test to determine the proper amount of nutrients to apply.
In general, tomato plants have a high requirement for the macro elements potassium (K) and calcium (Ca) and the micronutrients iron (Fe), manganese (Mn), and zinc (Zn). Without a good supply of both K and Ca for plant uptake and utilization, the fruits will lack the recommended soluble solids content (sugars) and will be more susceptible to physiological disorders such as blossom end rot. Smaller requirements of the elements nitrogen (N), magnesium (Mg), phosphorus (P), boron (B), and copper (Cu) also are needed for proper plant development.
Tomato plants grow best when the soil pH is between 6.0 and 6.5. Liming to this range improves plant growth and optimizes fertilizer efficiency. If the pH is less than 5.5, Mg availability decreases significantly. Consequently, this increases the level of available Mn and aluminum (Al). Unless a deficiency of magnesium is noted, application of high-calcium (nondolomitic without magnesium) lime is advised. In addition, when a low pH is coupled with low Ca, blossom end rot is likely to occur. However, when soil tests are high in both P and pH levels, Zn will become insoluble, resulting in deficiency symptoms such as cupped leaves and splotchy chlorosis.
Minda Daughtry is the Horticulture Agent for North Carolina Cooperative Extension in Lee County.