Manuring is the practice of adding organic matter, primarily animal waste or decomposed plant material, to soil. This process enriches the soil, providing a foundation for healthier and more vigorous plant growth. Manure serves as a natural fertilizer, contributing to the overall productivity and sustainability of agricultural and gardening systems.
How Manure Enhances Soil
Manure improves soil quality by providing essential plant nutrients. It contains macronutrients like nitrogen (N), phosphorus (P), and potassium (K), along with various micronutrients such as copper, manganese, and zinc. These nutrients are released gradually as the manure decomposes, providing a sustained supply for plant uptake throughout the growing season. This slow release mechanism helps to prevent nutrient leaching, making them more available to plants over time.
Beyond nutrient provision, manure increases the soil’s organic matter content, a component of healthy soil. Increased organic matter leads to improved soil structure, promoting the formation of stable aggregates. These aggregates create pore spaces, enhancing aeration for root respiration and improving water infiltration and retention, allowing the soil to hold more moisture during dry periods.
The addition of manure also stimulates the activity of soil microorganisms. These microbes break down organic matter, accelerating nutrient cycling and converting organic forms of nitrogen and phosphorus into inorganic, plant-available forms. This heightened microbial activity fosters a more diverse and resilient soil ecosystem, contributing to soil fertility and plant health.
Different Types and Application Methods
Manure characteristics vary depending on the animal species, their diet, age, and how the manure is managed. For example, swine manure has a higher proportion of immediately available ammonium nitrogen, while dairy manure often contains more organic nitrogen. Cow and horse manure are commonly available and are considered “cold” manures, decomposing slowly and releasing nutrients over a longer period. Chicken manure is a “hot” manure, rich in nitrogen, and can scorch plants if applied fresh due to its high nutrient concentration.
For garden beds, digging in well-rotted manure is a common method. This integrates the organic matter and nutrients throughout the root zone. A general guideline is to apply a layer of manure 1 to 2 inches thick, then mix it into the top 6 to 8 inches of soil.
Top-dressing involves spreading a thin layer of manure around existing plants or on the soil surface, allowing nutrients to slowly leach into the soil with rain or irrigation. This method is useful for established plants or when preparing beds in the autumn, providing a protective layer against winter rains and allowing incorporation by soil organisms. Liquid manure, often diluted, can be applied through irrigation systems or directly poured around plants, providing a quick nutrient boost. For large areas, broadcasting solid manure with specialized spreaders is common, often followed by tillage to incorporate the material and reduce nitrogen loss.
Important Considerations for Manuring
Using fresh manure directly on plants can cause “burning” due to its high salt and nutrient content, particularly nitrogen. Fresh manure may also contain weed seeds from the animals’ diet. Furthermore, raw manure can harbor human pathogens like E. coli and Salmonella, posing a food safety risk if it comes into contact with edible crops.
Composting or aging manure is a recommended practice to mitigate these risks. Proper composting involves maintaining specific temperatures, between 131°F and 140°F (55°C and 60°C), for several weeks, with regular turning to ensure all material is exposed to the heat. This process effectively kills most weed seeds and pathogens. Aged manure, while not reaching the same pathogen-killing temperatures as hot compost, still offers reduced risks compared to fresh manure.
Over-application of manure can lead to environmental concerns, particularly nutrient runoff into waterways. Excess nitrogen and phosphorus can contribute to water pollution, causing issues like algal blooms. To prevent this, apply manure at rates consistent with crop nutrient needs and consider soil test results. Avoiding application on frozen ground or immediately before heavy rainfall can also reduce runoff.
When handling manure, wearing gloves and washing hands thoroughly after contact helps prevent the spread of any remaining pathogens. For edible crops, a waiting period of at least 120 days between raw manure application and harvest is recommended for crops that contact the soil, and 90 days for those that do not.