Horse manure is a common organic soil amendment used by home gardeners. The simple answer is yes, it is an excellent resource for boosting tomato health and production, but only when processed and applied correctly. As a rich source of organic matter, it offers significant long-term benefits to soil structure and fertility, making it a popular choice for heavy feeders like tomatoes. Using fresh or improperly handled manure can lead to plant damage or crop failure, so preparation is crucial.
The Nutritional Advantage for Tomato Growth
Horse manure is relatively mild compared to other animal fertilizers, such as chicken manure, which has a higher nitrogen concentration that can burn tender roots. Well-rotted horse manure contains essential macronutrients like Nitrogen, Phosphorus, and Potassium, typically in moderate amounts. For example, a ton of well-rotted manure may contain approximately 11 pounds of nitrogen, 2 pounds of phosphorus, and 8 pounds of potassium.
The primary benefit of horse manure is its high percentage of organic matter. This material acts as a soil conditioner, improving the physical qualities of the garden bed. Incorporating it helps break up dense clay, improving drainage, and increases the water-holding capacity of sandy soils. This enhanced soil structure allows tomato plants, which require consistent moisture and well-aerated roots, to thrive.
Essential Preparation: Converting Manure into Fertilizer
Fresh, or “hot,” horse manure should never be mixed directly into a vegetable garden because it poses several risks. The high concentration of soluble salts and ammonia in fresh manure can cause severe chemical burn to roots and foliage. Fresh material may also contain viable weed seeds, pathogens, and parasites.
The essential process for making horse manure safe and beneficial is proper composting or aging. This transforms the material into a stable, nutrient-rich soil amendment. For active composting, the pile must reach and maintain temperatures between 130 and 160 degrees Fahrenheit. This heat is necessary to kill weed seeds and harmful pathogens.
To ensure uniform decomposition, the pile requires regular turning. While simply stockpiling manure can take six months to a year, a well-managed compost pile can be ready in three to six months. The finished product, known as cured or well-rotted manure, should be dark brown, crumbly, and possess an earthy smell, indicating stabilization.
Safe Application and Timing in the Tomato Patch
Once the horse manure is fully composted and cured, timing and application methods optimize tomato growth. The ideal time to incorporate finished compost is in the fall, allowing winter rain and microbial activity to integrate the material. If fall application is not possible, mix the manure into the soil in early spring, at least three to four weeks before transplanting seedlings.
The goal is to evenly incorporate the material across the planting area. Spread a layer of composted manure one to three inches thick over the soil surface and work it into the top six to eight inches of the garden bed. This distributes nutrients where tomato roots can easily access them, enriching the soil structure uniformly.
For established tomato plants, cured manure can be used as a side-dressing or mulch during the growing season for slow nutrient release. When using this technique, keep the manure a few inches away from the plant stem. Direct contact can trap excessive moisture, which may encourage rot or disease development around the plant crown.
Warning: Persistent Herbicide Residues
A serious risk associated with using horse manure is the potential for contamination with persistent broadleaf herbicides. Chemicals such as aminopyralid, clopyralid, and picloram are commonly used to control weeds in hay fields and pastures. These chemicals pass through the horse’s digestive system and remain active in the manure, as they are not broken down by digestion or composting.
Tomatoes, peppers, and potatoes are extremely sensitive to these chemicals. Contamination can cause severe stunting, leaf cupping, and deformed growth, even at low concentrations. Damage from contaminated manure can take years to remedy, making prevention the best course of action. Gardeners should always ask the manure source about hay and pasture management practices to ensure no persistent herbicides were used.
Testing for Contamination
To check for contamination, a simple bioassay test can be performed before widespread application. This involves planting sensitive indicator plants, such as peas or beans, in a small sample of the manure mixed with potting soil. If the seedlings show signs of curling, stunted growth, or deformation within two to four weeks, the manure should not be used on tomatoes or other susceptible plants.