RainCity Soil & NPK 101

With spring comes prepping the beds and amending the soil for preparation of planting. Since this is only the second year we've maintained a significant garden, we had some learning to do.... Initially I didn't even know what 10-10-10 meant!

RainCity Soil

According to "Growing Vegetables West of the Cascades" (by Steve Solomon) our soil here isn't really very good (Or more accurately, it has very poor balance). This due to our heavy and persistent rains, which have leeched a great deal of nutrients out of the soil. "Unfortunately, the nutrients that provoke plants to become highly nutritious- calcium, magnesium, and phosphorus- are the ones most readily lost." (p27) The second problem is that this leeching effect has not been done equally with all nutrients. On the other side of the equation, soil here has "an overly high level of an otherwise useful mineral, potassium. Our soils usually have lots of potassium- maybe too much". (p27) Presumably, this means that when we are amending the soil in our garden, we need to minimize potassium that goes in.

Mr. Solomon recommends that all garden soils in our region need to address the issue of calcium and magnesium deficiency, as this is what gets leeched out the most and has the biggest impact the nutrition of what we're growing. One recommendation for this is using equal parts of two different kinds of lime - calcium carbonate, and dolomite lime, and using 50 lbs per 1,000 square feet on a garden that is new, or has never been limed. Thereafter, he recommends regular, smaller soil amendments of calcium and magnesium to maintain the availability of those nutrients in particular (p32).

So the soil in our little corner of Earth has some unique characteristics to consider....

I'm a data-oriented sort of person... so I need to understand *why* we need to amend the soils. Generally speaking it's to 'feed the plants what they need'- I got that, but I prefer a bit more detail.

NPK (Nitrogen-Phosphorus-Potassium)

N (Nitrogen) I found Nitrogen: The Essential Element published by some folks over at the Center for Environmental Research at Cornell University. It's not long, but supplies some history, the role of nitrogen, The Nitrogen Cycle, and some pointers on use. It's a great write up.

Plants use nitrogen to produce protein (I can see how that's important), but according to the Nitrogen Cycle, the quantity of nitrogen in the soil is perpetually in flux (being used by the plant and flowing away into the environment). Beyond getting my head around the Nitrogen Cycle in general, my main take away is that by using organic matter to amend the soil and provide nitrogen, the Nitrogen Cycle is slowed way down so that a continuous supply is available as the organic material decomposes. This slow and steady application of nitrogen (via decomposition) is preferable to chemical alternatives, which blasts the plant with a one-time pulse of nitrogen, and then quickly leeches away in water or converts to gas and escapes into the atmosphere.

Nitrogen at the time of planting (particularly the pure chemical kind) pretty much goes to waste; The time the plant really needs it is when it's really producing. It's my sense that it's pretty hard to provide too much nitrogen -though not impossible. The worst thing I could find about excessive nitrogen was that it could over stimulate leafy production in the plant, skew it's nutrition content in a negative manner, and make it taste not so good. Clearly this isn't what anyone goes for, but essentially the downside is "grows too fast".

P (Phosphorus) Phosphorus is required for all forms of life. One of its key functions is energy transfer at the cellular level. It is also needed for manufacturing nucleic acids (DNA & RNA). Neat huh? Maybe I should have taken bio chem...

Given it's biological role for life in general, it is no surprise that phosphorus plays a key part in the the process of photosynthesis. It is pivotal in stimulating early growth and root formation in plants... ultimately speeding up maturity, flowering, and seed production

The availability of phosphorus for the plant gets complicated- just because it is present, doesn't mean the plant can absorb it. Unlike nitrogen, which flows away in the water and air constantly, phosphorus likes to bind with other things rendering it into a 'solid state'. As plants absorb phosphorus along their roots and the concentration in the soil surrounding them decreases, more phosphorus diffuses towards the area to correct the imbalance. This is called a "concentration gradient". The problem here is that phosphorus has a very low solubility and it likes to link to other minerals. The concentration gradient lends itself towards making the phosphorus more immobile and difficult for the roots of a plant to absorb, because it is constantly passing by minerals that it can easily bind to, rendering it inert. Further complicating this is that the solubility of phosphorous is linked to soil Ph. The more acidic a soil is, the more likely phosphorus will bind to iron or aluminum to form compounds that are insoluble and unavailable to the plant. The more alkaline, the more likely phosphorus will bind to calcium... and (wait for it) render it insoluble and unavailable to the plant. This means for phosphorus, the ideal soil Ph is 6.5 to 7.5 (as neutral as possible). Oh yea... soil temperature also impacts this whole equation as well. I've decided not to delve to deeply into that for now.

Given the issues with binding, absorption, and diffusion, the more incorporated phosphorus is with the soil overall, the better it is for the plants (to minimize the negative effects of the concentration gradient). The time to amend soil with phosphors is prior to planting. Furthermore, given the impact that Ph has on the solubility of phosphorus and its propensity to easily get trapped in a solid-state form (when Ph is anything but neutral), it seems quite worthwhile to focus on the Ph of the soil. If the soil is too acidic or too alkaline, the phosphorus is likely already there, but entirely immobile.

It appears that the most limiting factors that can be actively controlled in relation to phosphorus are the Ph balance of the soil, and the level of nitrogen in the soil, which plays a key role in the plants ability to absorb phosphorus. (A plant deficient in nitrogen will quickly also be deficient in phosphorus.)

K (Potassium) Potassium plays a variety of functions in plants. It's involved in protein synthesis, cell development, regulation of water absorption, energy, carbon dioxide, and electrical balance. In my layman's understanding of things, it's a key element that controls the plants metabolic state. When potassium is absorbed from the soil, it produces a concentration gradient (like Phosphorus) so that potassium from further out flows in to restore an equilibrium. (Thus, it is good to have potassium evenly spread throughout the soil.)

I'm still trying to get my head around the Potassium Cycle- but here's a shot at it: Potassium has a distinct relationship with clay. (And here in RainCity, the glaciers scraped everything down to the clay bed during the last ice age, so we've got plenty of that. I know. I've had to shape it with a spade.) There is 'inorganic structural K' caught inside the clay, 'exchangeable K' that binds to the surface of clay, and 'solution K' that plants can readily absorb. The key here is that as plants absorb potassium (remember the principle of the concentration gradient), potassium from the 'exchange K' on the surface of clay moves into the solution in the soil, creating a ripple so that 'inorganic structural K' then moves into place to equalize the 'exchangeable K'... repeat infinitely. In the Potassium Cycle, 'solution K' does leech out of the soil easily via weathering, but if the soil has a high clay content, the impact of weathering is effectively negligible (it's always being replaced); One article went so far as to refer to it as "musical chairs".

I think that the reason that Steve Solomon said that when working with soil west of the Cascades we have "maybe too much" potassium is because we are all pretty much gardening on the clay bed that the glaciers left us... so potassium is uniquely captured (and diffusing) into our garden in a nearly infinite loop. Joined with this fact is that plants *love* potassium, and will happily absorb more than they require with no toxic effect. The problem here for those of us growing well balanced, nutritious vegetable gardens is that excessive potassium intake can result in calcium and magnesium deficiency due to competition between the nutrients. I believe the conflict in nutrient absorption, coupled with our abundance of clay, explains his "maybe too much" assertion.

My favorite sites that helped me with my rudimentary bio chem education:

Information on Soil Fertility A 'basic science' site hosted by NASA.

Fertilizers and Plants A education page hosted by Ballance, a New Zealand fertilizer company.

The Real Story of N-P-K (Plus) A really solid little write up that helped me allot.