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Enhancing soil fertility with organic materials

Organic matter is defined as anything that contains carbon-based compounds and was or is alive. Using organic soil amendments involves some considerations. Compost, crop residue, and manure are common organic soil amendments.

What is Carbon and Why Should We Care?

Soil carbon is an important component of soil microbial communities in all farming systems. Soil aggregates are held together by organic compounds produced by microbes, fungi, and plants. In addition, soil microbes are responsiAdditionally, soil microbFurthermore, soil microbes are responsible for a wide range of nitrogen cycles, including converting ammonium into nitrogen nitrate, which is the form of nitrogen most commonly absorbed by plants. Insufficient soil moisture, soil carbon, and cold soil temperatures can all contribute to reduced microbial activity, even when there is sufficient total N in the soil.

Nitrogen (N) release is affected by the carbon to nitrogen (C:N) ratio of organic amendments. In the same way that we have dietary needs, microbes do as well. Microbes prefer a C:N ratio between 20:1 and 25:1. Soil and soil water contain nitrogen. In soil profiles with a higher C:N ratio than microbes prefer, microbes will mine N from the soil profile for their own use, disabling crop uptake (sometimes called “tying up”). As opposed to leaching or volatilization of N, it does not permanently leave the system. The microbes will die before it is available in the short term. Microbes absorb nitrogen that exceeds their dietary requirements when they consume materials whose C:N ratio is lower than their ideal diet.

Selecting organic amendments: Considerations

Efficiency of materials: Depending on the C:N ratio of the material, both the amount and the time in which N will be available to plants are affected.  N will become available during the growing season in greater proportions for materials with a lower C:N ratio. N becomes available at lower proportions in materials with a high C:N ratio. In order to meet crop N demand during peak uptake, these materials need to be applied earlier in the season and in larger quMaterial with a very low C:N ratio, such as feather meal, guano, or some liquid products, becomes available very quickly.y quickly. These products proviSimilar to synthetic fertilizers, these products can be applied in-season through fertigation or side dressing


Costs of materials:  The cost per unit of available N is more relevant when using organic amendments as an N source containing enough nitrogen. It is important to keep in mind that this is not the only factor to consider. The cost of transporting and spreading the material can quickly eat into your profits. It is expensive to move water and nutrient calculations will be affected by percent moisture.  

Consider percent moisture when calculating the true cost of organic amendments. In this case, we are estimating how much a pound of N actually costs with each product, compared to how much it costs with management. Your immediate goal may be to meet the demand of your current crop. Building soil structure and fertility is the long-term goal.

Considerations for management:  It is possible that fresh manures contain a high amount of ammonium-N, and if they are not tilled in soon after application, they can lose a substantial amount of nitrogen to the atmosphere. Weed seeds can also be found in manures.  

Compost: An Overview

Any organic material that decomposes into compost is considered compost. Composting material can range from food and yard waste to wood chips to animal manure, resulting in a variety of fertility and agronomic benefits.  Composts perform differently on farms depending on the starting material and the final product’s properties.

In addition to adding organic matter and increasing soil carbon, all composts have the benefit of providing nutrients to crops. There is no doubt that manure-based composts are more fertile than green waste composts (i.e. yard trimmings), and green waste composts would not be able to meet crop nutrient needs. Compost made from yard debris, for example, only contains 18 pounds of nitrogen, 3 pounds of phosphorus, and 8 pounds of potassium per ton, as opposed to chicken manure compost which has 44 pounds nitrogen, 26 pounds phosphorus, and 33 pounds potassium per ton. Additionally, less nitrogen will be available during the next growing season in yard debris compost.

As well as adding organic matter to soils and increasing their carbon levels, compost also provides crops with nutrients. Composts based on manure are more fertile than composts based on green waste (e.g. yard trimmings), and composts based on green waste cannot meet crop nutrition needs. The nitrogen, phosphorus, and potassium contents of yard debris compost are 18 pounds per ton, while chicken manure compost is 44 pounds nitrogen, 26 pounds phosphorus, 33 pounds potassium. Furthermore, yard debris compost will contain less nitrogen during the next growing season.

Bhumika

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