Open full view…

Happy Birthday, Phosphorus!

David Blanchard
Thu, 05 Sep 2019 09:58:20 GMT

Phosphorus was first isolated as an element in 1669, so 2019 marks the 350th anniversary of this discovery. In addition to being a key component of cell membranes, DNA, and bones, phosphorus in the form of phosphate ions moving between ADP and ATP molecules is the basic energy currency that powers the metabolic processes of life on Earth. But besides being essential to life, phosphorus has a dark side: excessive concentrations in the environment can lead to serious water quality problems. For an excellent overview of global environmental problems and the role of excess phosphorus, read "A Safe Operating Space for Humanity", which you can download free here: To acknowledge the importance of phosphorus the Soil Science Society of America and the American Society of Agronomy are celebrating Sept. 15 - 21 as "Phosphorus Week." The societies jointly publish the Journal of Environmental Quality, which has an editorial focus on environmental issues from an agricultural perspective, and the Sep tember 2019 issue has a special section on agriculture, phosphorus, and the environment, which can be accessed here (scroll down to get to the phosphorus section): Most of the papers are open access and can be read without a subscription to the journal. Those interested in wise stewardship and environmentally responsible use of this essential, nonrenewable resource will find a lot of interesting information here. Because reading time can be hard to come by at this time of year, here are a few key points: 1. Loss of P from farm fields to water bodies is very site-specific and depends on management practices, soil type, weather events, and local hydrology. So it isn't easy to assess the risk of water pollution from individual fields. But on a large scale, there's a strong correlation between soil test P (STP) levels and P pollution of water. Pollution can be mitigated by maintaining STP at the so-called agronomic optimum, which varies regionally and with crop and soil type, but is typically around 20-30 ppm for a Melich-3 test. STP above this gives no crop yield benefit but increases the risk of water pollution. Organic farms that use a lot of compost or other organic fertilizers like pelleted poultry manure often have STP of 200+ ppm. 2. No-till can actually increase the risk of P pollution because applied nutrients aren't being mixed into the soil profile by tillage and thus accumulate to high levels in the surface layer. P at the soil surface is vulnerable to transport when dissolved in water moving across the soil surface, especially during heavy rainfall or snow melt events. 3. I especially recommend the paper "Options for Improved Phosphorus Cycling and Use in Agriculture at the Field and Regional Scales", which is focused on biological aspects of P management and includes discussion of intercropping, cover cropping, and mycorrhizal fungi as management tools. Spoiler alert: all of these work better at lower rather than higher P levels.

Fri, 06 Sep 2019 08:48:51 GMT

Great info here, David. Will check these out. Question for you, how do you avoid excess P in your system?

David Blanchard
Sun, 15 Sep 2019 08:57:27 GMT

Sorry to be slow responding to this question. We got hit pretty hard by Hurricane Dorian last weekend, and only got power and Internet back a couple of days ago. To avoid building our soil P any further (it already tests in the high to excessive range) we have stopped buying in off-farm compost, and are using only farm-generated feedstocks to make on-farm compost. These feedstocks are manure from our cattle and poultry, hay made on-farm (used as bedding for the stock in winter), and all the waste products of the vegetable operation. The idea is to cycle the nutrients we already have, with small additions in the form of purchased poultry feed and some purchased hay to make up for the nutrients leaving the farm through produce sales. The main nutrient we need to top up each year is nitrogen, which we're trying to get as much as possible from leguminous cover crops. These will cycle existing mineral nutrients like P, K, and Ca, which are already at adequate to high levels, and pull N in from the air. In cases where the legumes haven't fixed enough N, due to poor stands, win ter kill, etc., we're using alfalfa pellets as a source of N that doesn't carry an excessive load of P as well. Why do we care about avoiding excessively high nutrient levels? The potential for water pollution from agricultural sources is highly site-specific. We are farming moderately steep 3 to 6% slopes, with a well drained, highly erodible sandy loam soil and a lake at the bottom of the hill. We get lots of heavy rainfall events and lots of snow in the winter, which can melt fast in the spring with the ground still frozen, generating a lot of surface flow into the lake. Pretty much a textbook case of high hydrological connectivity between farm fields and a water body. Other farms will have less, or even more, of such connectivity, but I think all environmentally responsible farmers should be aware of how much nutrients they're applying to their land in relation to the needs of their crops, and to be aware of where excessive nutrients may end up. Also, see the article about phosphorus and soil biology recommended above... a lot of the soil biology we're interested in, like mycorrhizal fungi and N-fixing bacteria, function better at lower rather than higher nutrient levels. I've attached a photo of some of our vegetable beds, 4' wide with 2' permanent sod paths between, laid out across the slope and slightly terraced as an erosion-fighting measure, and cover cropped for the off-season with oats and winter peas. [IMG_5772](//

Dawson Mehalko
Sun, 15 Sep 2019 12:03:17 GMT

Jesse, you need to get this fella on the pod!