Farming Magazine - August, 2009


Organics: Improve Soil Humus Levels—Just Add Manure

By Diane Wells

A soil’s health and fertility must be maintained by encouraging the presence of humus. Humus is the relatively stable organic component that wears many hats in the soil community. It acts as a sponge and stores nutrients and water. It improves the soil’s ability to form a crumbly structure, or soil aggregates. It feeds soil microbes, which then slowly release nutrients to plants over time. It harbors macro-organisms that consume crop pests and contribute to soil health. It binds toxins in a way that prevents roots from coming into contact with them. It improves the soil’s cation exchange capacity. In essence, it is the soil’s life support system, so how do you go about supporting it? By incorporating organic matter, and livestock manure is an excellent source.

The National Organic Program, which oversees the production, labeling and handling standards for organic agricultural products, has a few regulations when it comes to using manure as a soil amendment. If you want to be federally certified organic, you must adhere to the following rules: The use of livestock manure should be used to maintain or improve soil organic matter content in a way that does not contaminate crops, soil or water with pathogenic organisms, heavy metals, prohibited substance residue (this is in reference to the program’s list of synthetic and nonsynthetic materials not permitted for organic use) and plant nutrients. Manure that contains a prohibited synthetic substance may not be used, although the use of manure from conventional, nonorganic farms is allowed. Many organic farmers have objected to this allow-ance, stating that manure from nonorganic sources could contain residues from prohibited substances, such as antibiotics and heavy metals. The NOP has stood by its rule; the importance of recycling manure seems to outweigh the risk of spreading nonorganic substances on organic land. Certifying agents can require the screening of nonorganic manure, though, if the presence of prohibited substances is suspected to be sufficient enough to violate the organic farm’s integrity.

Raw manure may be used to produce a crop. If that crop is for human consumption, it must be applied no less than 120 days prior to harvest if the portion to be eaten comes into direct contact with soil, or no less than 90 days prior to harvest if the portion to be eaten does not come into direct contact with soil. Composted manure may also be used, as long as the pile had an initial carbon-to-nitrogen ratio of between 25:1 and 40:1, and adequate temperatures were maintained. Adequate temperatures range between 131 and 170 degrees Fahrenheit for three days in an in-vessel or static aerated pile system, or for 15 days in a windrow composting system during which the materials are turned at least five times. Unlike raw manure, there are no restrictions as far as when composted manure can be applied during the growing season.

The organic, or solid, component of manure is comprised of undigested feed, bacteria and bedding. Liquid manure contains up to 4 percent solids, while slurry has between 4 and 10 percent, semi-solid between 10 and 20 percent and solid has a 20-percent solids content or more. Roughly 20 to 35 percent of the carbon contained in these solids becomes humus, and many years can pass before it is stabilized, exhibiting those characteristics we value. Humus levels have been reduced by 50 percent on some farms due to historical tillage practices and the use of chemical fertilizers that do not contain organic matter. How do you know where your soils stand? Compacted soils are the primary indicator that humus levels are wanting. If you want to see numbers, there are test kits and labs that can rank your soil’s humus composition. The Luebke test ranks on a scale of 0 (no content) to 100 (high content); the LaMotte humus screening test ranks on a scale of 1 (low) to 5 (high).

We all know that applying manure to the soil also boosts nutrient (nitrogen, phosphorus and potassium) and micronutrient (boron, calcium, copper, manganese, sulfur, iron and zinc) levels. Up to 50 percent of the nitrogen, 90 percent of the phosphorus and 100 percent of the potassium are available to plants during the year of application. Over time, soil microbes will consume the insoluble nutrients and minerals contained in the organic material and excrete them in soluble forms that plants can then readily uptake. This can make it a little tricky when it comes to figuring application rates, but plants actually prefer a slow nutrient release, and a slow release reduces nutrient loss to surface and groundwater.

Not all manure is created equal with respect to nutrient content. The extent to which it contributes to soil fertility varies according to livestock species, age, diet, the amount and type of bedding material used, and how the manure is collected and stored. Generalized estimates on nitrogen, phosphorus and potassium levels for a particular livestock species’ manure do exist, from veal calves to tom turkeys and everything in between, but the numbers you should concern yourself with are the numbers produced by the lab you send your manure samples to. If you are applying livestock manure to your soil, you should be sending a representative sample of that manure to an agricultural testing lab at least once a year, and more frequently if you adjust one of the variables involved in its production. Once you know your “manure numbers,” you can adjust manure application rates and amend your soil accordingly.

Recycling manure has many benefits, not just for the soil and you, the farmer, but also for the community you live in. It can be misused, though, and its overuse or improper applications can lead to nutrient leaching and pollution. Stay informed, and consider it one of many tools in your soil management toolbox.

The author, a brand new contributor to Farming, is a biologist who lives and farms in Vermont’s Northeast Kingdom.