Manure is a naturally derived organic material, primarily consisting of animal waste often mixed with bedding like straw or sawdust, used to improve garden soil. Its function is two-fold: it acts as a slow-release source of plant nutrients (nitrogen, phosphorus, and potassium) and significantly enhances the soil’s physical structure. The organic matter helps increase the soil’s capacity to hold water and air, benefiting root development and microbial activity. Using this material correctly builds long-term soil health, but requires careful preparation and application to avoid risks to plants and human health.
Selecting and Preparing Manure
The source of the manure dictates its nutrient concentration, often referred to as its “hotness” or likelihood of burning plants if used fresh. Manures from different animals offer various nutrient profiles. For example, poultry, particularly chicken manure, is typically the most nitrogen-rich and therefore the hottest. Rabbit manure is also highly concentrated, often containing four times more nutrients than cow or horse waste.
Cow manure is generally milder, providing a balanced nutrient mix and greater bulk for organic matter. Horse manure is moderate in nutrients but is notorious for containing viable weed seeds. Sheep and goat manures are also moderately high in nitrogen and potassium.
Preparation is mandatory before incorporating most manures, as fresh material carries significant risks. Fresh manure is high in soluble nitrogen (ammonia), which can severely burn plant roots and leaves. Raw animal waste can also harbor human pathogens, such as E. coli and Salmonella.
The process of aging or composting stabilizes nutrients and destroys these harmful elements. True composting requires the material to reach a sustained temperature of at least 131°F to 140°F for several weeks, effectively killing most weed seeds and pathogens. If high-heat composting is not feasible, manure should be aged for six months to a year until it is dark, crumbly, and has an earthy smell. Never use waste from carnivores, such as cats or dogs, due to the high risk of transmitting parasites.
Determining Application Timing
The timing of manure application is critical for food safety, allowing pathogens to die off before harvest. For edible crops where the consumable part touches the soil (e.g., carrots or leafy greens), non-composted manure must be applied a minimum of 120 days before harvest. For crops that do not contact the soil, a minimum waiting period of 90 days is required.
Applying manure in the fall is often the preferred schedule for vegetable gardens. This timing allows the material to break down over winter, ensuring the necessary waiting period passes before spring planting and summer harvest. Fall application also protects the soil from nutrient loss due to heavy winter rains.
If applying in the spring, it should be done as early as possible and fully incorporated into the soil, ideally four weeks before planting seeds. Never apply fresh or poorly aged manure directly to actively growing plants, as the high salt and nitrogen content will cause root burn. For perennial plants like rhubarb or asparagus, fertilize after the harvest season is complete.
Application Methods and Rates
The most common method for preparing a new garden bed or amending soil before planting is to till or dig the manure into the soil. A layer of aged or composted manure, approximately one to two inches thick, should be spread evenly over the soil surface. This material is then incorporated into the top six to eight inches of soil to ensure a thorough mix that maximizes the benefits of the organic matter.
Top Dressing and Side Dressing
For established gardens, top dressing is a gentler technique where a thin layer of well-composted manure is spread over the surface around plants. This acts as a slow-release amendment and mulch, helping to suppress weeds and conserve soil moisture. Side dressing is a variation where a small amount of material is placed in a ring a few inches away from the base of established plants to avoid scorching the stem and roots.
Manure Tea
A third technique is creating a liquid fertilizer known as manure tea, used for watering plants or as a foliar feed. This is made by soaking aged manure in water, which extracts the soluble nutrients into a liquid form immediately available to plants. This liquid is typically diluted to a light amber color before use to ensure the concentration is not too high.
Determining Application Rates
Application rates depend heavily on the manure type and the existing soil condition. A general guideline for moderately rich, composted manure is to apply 20 to 50 pounds per 100 square feet, or about one pound per square foot. Manures with a higher nitrogen content, such as chicken manure, require smaller amounts because of their potency. Excess application is detrimental, as it can cause nutrient runoff, introduce an imbalance of phosphorus, and lead to salt buildup that damages plant health.