The question of whether to use manure on tomato plants has a nuanced answer: yes, this traditional soil amendment can be highly beneficial, but only when specific precautions are followed. Tomatoes are heavy feeders, requiring a steady supply of nutrients throughout their long growing season for a substantial harvest. Manure serves as an excellent, slow-release source of these elements, enriching the soil more comprehensively than synthetic fertilizers alone. However, using manure incorrectly, especially when fresh or from an unknown source, can easily damage or destroy a tomato crop. Successful use depends entirely on proper preparation and application techniques.
Essential Nutrients Manure Provides
Manure provides a balanced array of macro and micronutrients that support the growth cycle of a tomato plant. It is a natural source of nitrogen (N), phosphorus (P), and potassium (K), the three primary macronutrients. Unlike the rapid delivery of synthetic fertilizers, manure delivers these nutrients in an organic, slow-release form as soil microbes break down the material.
Phosphorus is important for fruiting plants, promoting strong root development and stimulating abundant flowering, which translates into a higher fruit yield. Potassium contributes to the overall vigor of the plant, helping to regulate water uptake and improving the plant’s resistance to various diseases. In addition to NPK, manure also introduces micronutrients like calcium, magnesium, and iron, which are necessary for plant health.
Manure’s most significant contribution is the addition of organic matter to the soil. This organic material improves soil structure by fostering aeration and increasing its capacity to hold both water and nutrients. A soil with good structure supports a thriving community of beneficial microorganisms, which enhances the availability of nutrients to the tomato roots.
Mandatory Preparation: Curing and Composting
Using manure in its fresh state is highly discouraged for tomato plants and can cause severe damage. Fresh manure contains excessively high levels of soluble salts and nitrogen, which can lead to a condition known as “root burn” or “fertilizer burn.” This damage occurs when the high salt concentration draws water out of the plant roots, effectively dehydrating them.
To make manure safe and beneficial, it must undergo a process of aging, curing, or hot composting. Aging involves allowing the manure to sit for a minimum of six months to a year, which permits the volatile nitrogen compounds to dissipate and the salt levels to stabilize. Hot composting is a faster, more active process that requires maintaining the pile temperature between 131°F and 170°F for a specific period, often 15 days, with multiple turns.
This heat treatment is crucial for two reasons: it kills pathogens and eliminates weed seeds. Raw manure can harbor human pathogens like E. coli and Salmonella, and the sustained high temperatures of hot composting sterilize the material, making it safe for use on food crops. Composting also destroys viable weed seeds that pass through the animal’s digestive tract, preventing the introduction of new weed problems into the garden.
Application Timing and Methods
The successful use of prepared manure depends on timing the application to meet the tomato plant’s nutritional needs. The best method is to incorporate the aged or composted manure into the soil before the tomato plants are transplanted. This pre-planting application ensures the nutrients are evenly distributed and available to the young roots immediately after planting.
A common guideline for incorporation is to spread a two- to three-inch layer of prepared manure over the garden bed and then till or turn it into the top six to eight inches of soil. This deep incorporation helps to improve the overall soil structure and fertility where the tomato root system will be developing. After the initial pre-planting amendment, additional nutrients can be supplied as the plants mature.
As the tomato plants begin to flower and set fruit, they require a supplemental boost, and a technique called side-dressing is used. Side-dressing involves applying a small amount of composted manure in a band around the base of the plant, keeping it at least six inches away from the main stem to prevent stem burn. This application is usually timed when the first fruits begin to form and can be repeated every four to six weeks during the peak growing season.
Specific Hazards for Tomato Plants
Tomatoes are sensitive to contaminants and nutrient imbalances that can result from improper manure use. The most significant risk is contamination by persistent herbicides, particularly aminopyralid and clopyralid. These broadleaf weed killers are used on hayfields and pastures, and their residues can pass through an animal’s digestive system and remain active in the resulting manure, even after composting.
When this contaminated manure is applied, it can cause severe damage to susceptible plants like tomatoes, potatoes, and beans, even at extremely low concentrations. Symptoms include cupped, stunted, or twisted foliage, often described as “fern-like” growth, which ruins the plant’s ability to fruit. Gardeners must ask their manure source about the feed or hay given to the animals to ensure it was not treated with these persistent chemicals.
Another common hazard is nitrogen toxicity, which occurs when too much nitrogen is supplied, especially early in the season. Over-fertilization with nitrogen promotes excessive, lush vegetative growth—thick stems and abundant leaves—at the expense of fruit production, leading to an outcome of “all vine, no fruit.” Finally, using manure that has not been sufficiently aged or composted carries the risk of salt burn, where the remaining high salt content in the material dehydrates the roots and impairs water uptake.