Thanks for posting. I have a similar problem, although not quite as severe, but the P levels in my veg garden got into the high range, mostly from adding compost year after year, since I hardly use fertilizers.

I use some leaf compost and horse manure...both rather low in P.
Crops typically remove nutrients in about a 3-1-2 ratio of NPK.

You done it now TX. I think this discusion will go about like trying to convince McDs to put kosher pork sandwichs on the the menu.

I do wonder if P is now the pc whipping boy target. Yes, there are lakes and streams that have too much N and P, but I think that not all gardens are so guilty.

wayne - there are at least two distinct issues. One is the impact of excess fertilization, either organic or synthetic mineral, and other organic soil amendments to natural waters. The other is the impact of applying excesses ton soil quality.

I would generally agree that the home veggie garden probably is not much of a contributor to the former, although residential lawn and landscape care has been and commercial agriculture still is. There the issue is both nitrate nitrogen and phosphorus.

In the latter, excessive use of manures and compost can quickly and rather easily lead to nutrient excesses that have adverse impact on the results enjoyed (or not) in the home veggie garden. In that situation excess P levels tend to be the most troublesome because they tend to be lasting in most soils. I am but one example - because I added some composted cow manure where it wasn't needed, I am now struggling with trying to recover from a soil P level of >300 ppm by Melich III extraction, and the plants sure showed it.

Most of the publications focus on the impact of excessive manure and fertilizer applications to natural waters. That's because of the scope of the issue and the attendant politics (farmers vs. environment). As a result, we tend to loose sight of the issue in our soils.

This post was edited by TXEB on Thu, Sep 5, 13 at 20:34

I live in the corn belt and have miles of field crops around me...I am aware of farm issues...not so much concerning lawn issues..I put nothing on it for years at a time.

I have noticed no adverse issues with P or N in my gardens..about 8,000 sq. ft. I likely have a very high organic matter content. In fact it is i\ a vintage year for everything but melons which got herbicide damage [rare] in a remote location.

wayne - the ultimate question is how much of any nutrient you're adding via any amendment or fertilizer, and where it ends up. If you add more than the plants take out, then the excess either accumulates or it is lost somewhere to the environment. The relative amounts lost vs. accumulating will certainly depend upon things like soil texture, soil pH, rainfall/irrigation amount and water quality.

In my case our soils (alkaline calcerous clay) tend to accumulate P and hold onto to it. Any apparent losses over time are primarily to complexation with the calcium that is largely abundant (>5,000 ppm), and that is a very slow process -- 5-10 years to sequester ~ 100-150 ppm P.

Just to put this into perspective, a typical finished compost spec is an NPK of 1.0 - 0.6 -0.3 with a bulk density of 1000 lbs per cubic yard. If that were to be applied at a coverage of 1", that would amount to 3 yards per 1,000 sq ft, or 3,000 lbs of compost per 1,000 sq ft. Assuming that 80% of the P and 90% of the K were available in the first year after application, then if I did the calc's on this right, that 3000 lbs of compost over 1,000 sq feet would contribute 312 ppm P as P2O5 and 178 ppm K as K2O to the soil. Is that too much? For me it is way, way too much. For others given their conditions and what they are growing it may be acceptable, but that's a lot of P to be adding. It's the equivalent of adding 140 lbs of an X-10-6 fertilizer over that 1,000 sq. ft in a year.

tox - did I get the calc right, or did I loop it somewhere?

pt03 - are you following this? Is that result reasonable or just plain looney?

This post was edited by TXEB on Fri, Sep 6, 13 at 0:36

Tex, I figure 180 lbs of 18-10-6 That would be a lot of fertilizer for most applications. However, I doubt that most gardeners have that much compost to apply for that size patch. Some of their compost is just returning the spent residues. I recently applied about 1600 lbs of moist leaf compost over a very wide area. Also some horse manure with high hay content. I return crop residues in place.

I have natural but amended clay loam with perhaps some silty clay loam. If there is any runoff on half the gardens, it goes to a pasture that is lower. The other part would go to a lower area in a field. that uses a lot of nutrients.

wayne - sounds about right to me. The area use was just for a quick, easy reference to do the calculation. If we reduced it to 100 sq feet the rate would be 18 lbs of your fert calculation. The impact on the soil, calculated at "plow depth" (45,500 lbs/1,000 sq ft) would be the same for nutrient additions stated in ppm.

I believe the salient point is how easily nutrient levels can get out of whack by chasing soil organic matter through additions of OM without consideration of the nutrient amounts being added. I regularly hear folks talk about and recommend applying compost at inches to soils. The calc above was done for 1-inch.

What this illustrates is that raising soil OM levels needs to be a long-term pursuit, accomplished by making additions with nutrient levels in mind. The side benefit is if we get that part right the soil will be steadily improved over time, and the runoff issues will take care of themselves.

Chalk one up for wood chips. Even my soil, started at 5ppm P (very very low), only needed two large compost applications to get to 80ppm. Wood chips will release fewer nutrients, more slowly, and fall leaves help feed the earthworms, which you have to have, specially in thick clay. For the rest of fertilizer needs there is urea, man-made or factory-made.

What kind and how much of soil organic amendments to add to any soil depends on what that soil type is. Nutrient loss through clay soils is less then through sandy soils simply because the soil particles are smaller and more tightly packed, and in clay soils organic matter opens up the spaces between the mineral soil particles so moisture, nutrients, and plant roots can move around more freely. Organic matter in sandy soils fills in the pore spaces and helps hold moisture and nutrients in place so the plant roots can access them.
So, by adding organic matter to the soil here the soils pH moved from 5.7 to 7.2 and the P and K levels went from low to high optimum and the Calcium and Magnesium became balanced without the use of any synthetic fertilizers and no lime. That did not happen overnight, however, but did happen in less than 5 years and has stayed consistent since.

"...the Calcium and Magnesium became balanced... "

What does that mean, "balanced"?