Building Nutrients in Organic Systems
Monday, February 3, 2014
Understanding the way the various nutrient sources behave will help you to manage them better
by KEITH REID
A prime tenet of organic farming is that crops can get their nutrients from the soil, as long as active soil biology is cycling these nutrients effectively. This is also critical on farms that use mineral fertilizers – and every farmer would benefit from optimizing soil biology – but organic farming is completely dependent on it.
The unstated assumption is that there is already a supply of nutrients in the soil to be cycled. While this is true in most soils, sometimes soil fertility has been depleted so missing nutrients need to be replaced. What options are available, and what are the pros and cons of each?
Legumes for nitrogen. Nitrogen (N) is perhaps the easiest nutrient to replace in an organic system because legumes are able to fix nitrogen out of the air. Nitrogen fixed by grain legumes, like soybeans, is taken off the field when the crop is harvested. However, forage legumes (including grain legumes disked into the soil at an immature stage) leave a significant amount behind. Complex rotations that include forages are necessary to provide nitrogen to the non-legume crops in the rotation.
The challenge is to match the release of the nitrogen from the forage plow-down to the time when the following crop needs it. This is pretty straightforward for a crop like corn, which takes up most of its nitrogen in midsummer after the soil warms up. The organic nitrogen compounds have lots of time to be broken down into available forms by soil organisms.
Cool season crops that need most of their nitrogen earlier in the season are more difficult. The legumes would need to be plowed down in late summer or early fall, so there is time for mineralization to take place before winter. However, this opens up the risk of losses before the crops take up nutrients next spring.
Composts and other organic materials. Composts provide phosphorus, potassium and micronutrients as well as nitrogen, but there is a huge variation among different composts in the amount of each of these nutrients. Analyzing these materials is just as important for the organic farmer as manure analysis is for a conventional farmer.
Most of the nitrogen in compost will be in the organic form, with a small amount present as nitrate. The nitrate is immediately available, but like the nitrogen from legumes, the organic N will need time to break down to release available forms of nitrogen. Similar to legume N, compost application will need to be timed so that organic N can break down in time for the crops to use it. Be cautious about trying to overcome this lag by applying more compost, as the release of mineral N could continue after the crop no longer needs it, leading to nitrate leaching.
The concentrations of phosphorus (P) and potassium (K) in compost will depend on the materials that went into the process, and are generally highest in manure-based composts. In a livestock operation, this is an effective way of recycling back to the land the minerals that were removed with the crop harvest.
Unless there is supplemental feed coming onto the farm, however, the recycling is incomplete because of the nutrients leaving the farm as milk and meat. The other loss from the system is during the compost maturation process, when potassium can leach out of the pile if it is not shaped to shed most rainwater.
Mineral fertilizers. While not normal practice on an organic farm, there are times when some form of mineral supplementation is the only practical way to maintain crop yields. As a general rule, the materials that are allowed in organic production are simply mined and applied, with little or no treatment to increase the nutrient availability. Application rates will need to be quite high if there are a lot of non-nutrient compounds mixed in with the material, or if the solubility is so low that only tiny amounts are released at one time. A complicating factor is that some materials dissolve slowly in acidic soils but not at all in alkaline soils. This is particularly true for the soft phosphate rocks, and also for some of the rotted granite materials sold as potassium supplements.
There has been some experimentation with mixing these materials into a compost pile, so the acids released during the composting process will help to dissolve them, but it is unclear how effective this is. If you have a soil with a pH above 7, buying composted manure as a phosphorus source is likely a better option than rock phosphate. Otherwise, you should be looking for an acidulated rock phosphate (a.k.a. single superphosphate) as a P source and potassium sulfate as a K source.
In summary, the management of nutrients on organic farms is not so different from conventional farms. Organically managed fields may have a lower level of dissolved nutrients in the soil at any one time than those receiving mineral fertilizers, but both should provide an adequate supply of nutrients to the crop when it needs them. In each case, understanding the way the various nutrient sources behave will help you to manage them better. BF
Keith Reid is Soil Scientist, Agriculture and Agri-Food Canada, Guelph.