Why Organic Matter is Important for Organic Farms
Organic matter is an amazing material in so many ways. But it’s also a very humble material that is composed of the most basic building blocks of life on this planet. Organic matter is critical on farms of all types and not just organic farms and having large amounts of organic matter present in your soil can help you grow nutritious and bountiful crops.
Before I can go into why organic matter is important I first want you to fully understand what organic matter is. The term organic matter is one of the many terms used to describe it, some terms you might be more familiar with would be compost or perhaps humus. They all essential described the same thing which would be the decayed remnants of plants and animals. Compost, humus and organic matter are what is left over after the decay process is completed and all that remains of the plant or animal is the basic carbon and nutrients its body contained. This leftover has a very critical role to play in soil and plant health as it provides many things that new plants need to grow successfully.
The ability of soil organic matter to release nutrients in a slow steady fashion is arguably one of the most important reasons why you need it. For example for every percentage point of organic matter you have in your soil you will get,
Now if we compared that to what nutrients are needed for a high yield corn crop you will find that you need around 120 lbs of nitrogen if you have less than 2% organic matter in your soil. But if you can increase the amount of organic matter in your soil the amount of nitrogen you need to apply goes down. This saves you money as a farmer and if you can provides nutrients essentially for free you can cut down on your need to purchase and apply nutrients. In many cases you can greatly increase how much nitrogen is left in the soil by planting and cultivating in a legume cover crop. For example a good stand of alfalfa can leave 100 lbs of plant available nitrogen in your soil after it has been cultivated and decomposed. The amount of organic matter and nutrients that a legume cover crop can put into your soil makes them a very good addition to any farming operation.
Next to nutrients the ability of organic matter to retain water in the soil and soak it up like a sponge is one of the most important things about organic matter.
The water retention of any soil is different from all others depending on its soil texture. The soil texture is defined by the percentages of the three different soil particles sand, silt and clay. In a general sense the smaller the particle the more water holding capacity that soil will have. So for example a soil that has more clay will be able to retain more moisture than a soil that is composed mostly of sand. To help clear this up take a look at the diagram to the left. You will see a range of water holding capacities going from coarse sand to clay with the the general trend being more water held in the more clayey soils.
So how organic matter comes into this is that it can dramatically improve the ability of all types of soils to absorb water and keep it available for plants. It can be most effective with the sandier soils and can give you the best of both worlds by keeping your soil moist but not over saturated with water. Nailing down exactly how much water organic matter can hold is hard because it varies from location to location. But to throw some numbers at you, it has been reported by soil scientists that 1% organic matter can hold 16,500 gallons of plant available water per acre. So if you can maintain two or even three percent organic matter in your soil you can have up to 50,000 gallons of water available for plants.
To put this in perspective lets look at the water requirements for corn one of the most common crops grown on the planet. One inch of an average rainfall will drop around 27,000 gallons per acre. To get a 200 bushel of corn harvest you will need 22 inches of rain to grow that, which multiplies out to 594,000 gallons of water/acre/year. Now if you have a field that has 2% organic matter it can hold 33,000 gallons plus whatever the natural water holding capacity of the soil is. When you multiply the 33,000 by the required 22 inches of rain you get 726,000 gallons/acre/year of potential water capacity. So assuming you get the rain you need your soil will have more than enough moisture to grow that crop. Keep in mind the real world it is never this accommodating even with high percentages of organic matter if you get all your moisture in the early spring your crop could still suffer come late summer.
The term soil tilth is a way of describing what condition the soil is in for growing crops. A soil with a good tilth will have a crumbly loose texture, have a good rate of water infiltration, be well aerated, and can hold high amounts of water to be available for plant growth. To see an example of good soil tilth you can usually look in your nearest road ditch. The conditions under which most road ditches grow are great at producing excellent soil tilth. They mimic closely the conditions or the Midwest before agriculture came in that they are,
Good tilth is made possible by making sure to have lots of organic matter in your soil. The organic matter promotes stable aggregates which gives you that loose crumbly texture. It also improves both water infiltration and water retention since organic matter acts like a sponge and soaks up as much water is possible. Good soil tilth is critical at being able to grow all sorts of crops easily since it creates conditions that are ideal for root growth and general plant success.
To learn more about this check out some the sources at the bottom of this article and other information available on the Green Living Library.
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