607 Oh Phosphorus, Where Art Though?

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#RealisticRegenAg | Following up on the episode earlier this season borrowing a movie title (Dude, Where’s my Nitrogen?), I’m going to focus on the elusive nutrient, phosphorus. This is an offshoot of a talk I gave this winter on using cover crops for break years between forages. It was delivered in person and to participants online. It was recorded so I’ll put a link to the talk in the description. The high-level summary – you need to replace what you export. No biostimulant or cover crop is going to make new phosphorus for you. Stay tuned as I dive into this topic.

Welcome to the sixth season of Plants Dig Soil, a podcast about #RealisticRegenAg. I’m your host, Scott Gillespie, and I’m an agronomist from the western Canadian prairies specializing in climate-smart agriculture for farmers and agribusiness. I discuss scientifically proven practices that benefit the planet and, just as importantly, farmers' economic sustainability. Be sure to visit my website, www.plantsdigsoil.com, for information about my consulting services and my book “Practical Regeneration: Realistic Strategies for Climate Smart Agriculture”.

Resources mentioned:
Cover Crops for Break Years – my talk at the Alberta Forage Industry Network AGM
https://youtu.be/_3pHwncZI74?si=HI1Y80KIR62koFtK

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https://www.plantsdigsoil.com/podcast/o-phosphorus-where-art-though

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Phosphorus is the limiting nutrient in most agricultural soils. It definitely is in my area. I work in the southwestern prairies of western Canada, in Alberta, just north of the Montana border. The soils are called the brown soils because they don’t naturally accumulate much organic matter. Not surprisingly they are brown. Moving north and east through the prairies there is more precipitation which drives more organic matter accumulation, and the soils darken to eventually be called the black soils.

The common feature of all the soils is that phosphorus is naturally low. In the early times of farming on the prairies fallow was used to accumulate moisture and build up nutrients. While they may not have understood exactly what was going on, the year away from growing plants was allowing the microbes to break down organic matter and release nutrients. For every 1% organic matter there is approximately 1000 lbs. of nitrogen and 100 lbs. each of phosphorus, potassium, and sulfur plus smaller quantities of micronutrients.

Every crop has different amounts of nutrient requirements, and every crop will export different amounts depending on variety and yield, but a ballpark rule is that for every 2 parts of nitrogen export there is 1 part phosphorus. As an example, a wheat crop at 40 bu/ac will export 60lbs of nitrogen and 25lbs of phosphorus. Canola at 35 bu/ac will export 65lbs of nitrogen and 35lbs of phosphorus. Recall that organic matter releases 10 parts of nitrogen to every 1 part of phosphorus. You can see the problem we are facing. While fallow can give us enough nitrogen, phosphorus soon becomes limiting.

The fallow system worked very well in the early days of agriculture. They were able to keep growing crops without adding anything to the system. They thought they had a system that was going to keep working indefinitely. The system crashed eventually. The dust bowl of the 1930’s was mostly due to a prolonged drought cycle, but it was also because the organic matter in the soil had been burned up by tillage and nutrient export.

When phosphorus fertilizers first started showing up on the market they gave tremendous results, more so than nitrogen. Crops responded extremely well to any amount of phosphorus. Once phosphorus was taken care of nitrogen became the limiter, and so new fertilizers that contained nitrogen could be added for even greater effect.

So where am I going on this? Back to the talk I gave recently to the Alberta Forage Industry Network. I was asked to speak about break years between forages. These are the years where a stand is terminated, and the ground is prepared for another stand of forages. The reasons for this are varied but after 3-4 years production declines due to plant aging, weeds, lost fertility, and compaction. My angle was looking at the use of cover crops to address those issues.

Cover crops can be very effective in suppressing weeds. We can use fall, spring, or summer seeded ones in combination with tillage and/or herbicides to get them under control. However, we can’t use cover crops to make more nutrients, other than nitrogen, which can be put back into the system with legumes.

Why do I say this when many are saying that cover crops can mine soil phosphorus and eliminate the need for fertilizer? The answer to this lies in the history of the land.

Early success with cover crops mining nutrients from the soil is related to the additional phosphorus added in the form of chemical fertilizers. Something that perplexed scientists was that prior to 1970 they could run rate trials with different crops and different products and get responses. After 1970 they were finding less effect. They could now grow crops with no fertilizer or just add some and get a small boost. What was happening was that the soil was tying up phosphorus. They would add fertilizer, the crop would take some, and the rest was tied up. Expand this over the decades and you get a lot of phosphorus locked up in the soil.

As with all things, an equilibrium was finally reached. Some of this phosphorus would show up each year for the crop through chemical and microbial processes. It didn’t mean we didn’t need to add fertilizer anymore. It just meant that adding enough to replace what was exported was all that was needed. The nutrients that ended up in the grain were a combination of what was added in the year of production, plus some from the previous year, the years prior, and some from decades prior.

Cover crops are finding this legacy phosphorus and making it available to the next year’s cash crop. As long as the breakdown of the tissue is close to the next year’s crops uptake, this newly found phosphorus grows a crop without the need of additional fertilizer. Just like the fallow system, it appears a system has been found that works without inputs. The proponents of this system think the microbes are mining soil particles for phosphorus. While there might be some of this going on, it would be a very small portion of the total crop needs.

It's hard to argue with anyone that thinks the system is working so I don’t bother anymore. I know that eventually they will hit a breaking point. The best example of this is the Glenlea organic rotation at the University of Manitoba. For the first decade they were able to increase phosphorus in the system with cover crops alone. However, in the years after this first decade things were showing signs of stress. At the 15 year mark the system crashed. There is a stark photo that I included in the presentation from this. In the foreground is lush alfalfa where manure had been added. In the background is yellow stunted alfalfa where no manure was added.

So, here’s the bottom line: You need to replace the nutrients that are exported. The early settlers of the Prairies eventually needed to replace nitrogen and phosphorus once the organic matter was depleted. Current growers will need to replace what is taken out each year. Phosphorus isn’t as dramatic as nitrogen. It has buffers in the soil. But over time it does deplete, and it does need replacing. For organic and conventional growers’ manure or compost is a great source of phosphorus. It also comes along with many of the other nutrients that are required. Conventional producers have the added tool of chemical fertilizers such as mono-ammonium phosphate (MAP). There is nothing wrong with using this to replace your exported phosphorus. The soil microbes and plants don’t care about the source.

In fact, even organic producers use it. It’s just that it gets incorporated into a plant that is then digested by an animal first. Nearly every organic farm that has animals will keep its manure. It’s too valuable to sell or it’s not possible to sell if the animals are on pasture.

Organic crop producers are permitted to use conventional manure if no organic is available. Most will use it because phosphorus is one the greatest limiters on the Prairies, as illustrated by the Glenlea organic trial. The manure from conventional producers came from animals that ate crops that were grown using MAP or other chemical fertilizers. They usually import feed for their animals, and this feed usually comes from cash crop farms. If it were possible to put the manure back on those fields they would. However, beyond a few kilometers of trucking manure is expensive to haul. Crops on the other hand can be transported hundreds or thousands of kilometers by truck or rail.

Without conventional production, organic production would not be possible. The only way for organic to become more closed loop is to add fertilizers such as struvite, a product derived from city water purification systems, into their permitted substances. This would be one step in closing the loop on a circular economy. Struvite is available in a product called Crystal Green for conventional producers, but the cost prohibits most from using it. If this were happening the phosphorus that us humans pee out would go back to the land. Until that happens, we must rely on mined phosphorus.

If you want to dive deeper into this concept, and other regenerative practices that need a little more critical thought, why not check out my new book? As announced previously it is out now and available wherever you buy books, print or for e-readers. Particularly relevant will be chapters 3-5 that cover what regenerative agriculture is, the long view (10,000 years in my case) on regeneration, and the three pillars propping up regenerative agriculture.

Thanks for listening. If you’re not already subscribed, please be sure to do so. There are also links in the description for my monthly newsletter delivered through email and LinkedIn. I also have a community on WhatsApp where I post interesting articles nearly daily throughout the work week. Talk to you again next time.