Cultivating Questions

Cultivating Questions: Follow-Up On Phosphorus

by Anne & Eric Nordell

Dear Nordells,

I have enjoyed your articles thoroughly and have learned a whole lot. The recent section on phosphorus prompted me to ask about fertilizer suppliers.

We (our family) have had difficulty locating soft rock phosphate, and frankly, any other organic rock fertilizers in quantity. Locally, we are aware of none; shipping from elsewhere usually is outrageously priced. Where do you get your phosphate? How about greensand? Do you have any ideas for finding a local or otherwise reasonable distributor?

Once again, your column is most informative. Any help you can give me regarding suppliers would be greatly appreciated!


Stacy Gerry

Pleasant View, TN

The sources of rock phosphate that we use are actually closer to Tennessee than Pennsylvania. For instance, the black rock product which we applied directly to the fields for a few years is mined in North Carolina while the colloidal clay rock phosphate which we use in the composting pigpens is a byproduct of the phosphate mines in Florida.

The least expensive way we know of to purchase these minerals is to order a tractor trailer load direct from the sources. We have inquired about prices from wholesale distributors who advertize their services in Acres-USA (PO Box 91299, Austin, TX 78709 1-800-355-5313). For example, Hugh Paddock of Greenwood, IN (317-881-6143) quoted us a price in January 1998 of $72/ton for the Lonfosco brand of colloidal clay soft rock delivered to our farm from Minehead, Florida. We had intended to split a truckload with several dairy farmers in the area, but due to the low milk prices at the time we were unsuccessful at putting a group order together. Needless to say, we did not have a large enough storage area — or pocketbook — to take the whole 22 ton load!

Instead, we have relied on local distributors for rock mineral. In the 80’s, we purchased soft rock by the pickup load for $3.00/50 lbs bag from a farmer located 15 miles away. He became a distributor for Earth-Rite natural fertilizers (633 Quarry Rd., Gap, PA 17527 717-442-4171) in order to get their products delivered at discounted prices. Becoming a distributor would be one way to get these materials into your area without paying retail prices.

When this farmer retired in the early 90’s, we began purchasing rock phosphate from a Fertrell salesman who stops at the farm on his monthly route supplying feed minerals to dairy farmers in the area. This arrangement certainly is convenient, although the prices we are now paying for phosphorus have almost doubled over what we paid in the 80’s. (Fertrell is a national supplier of natural fertilizers and livestock supplements. Contact the main office at PO box 265, Bainbridge, PA 17502 #717-367-1566 for their closest distributor.

Now that the phosphorus levels in the market garden appear more than adequate, we no longer purchase rock phosphate for direct application to the vegetable fields. However, we continue to purchase rock minerals to add to the composting process and to slowly but surely upgrade our long neglected pastures. The payback for remineralizing these malnourished paddocks has been a significant increase in volunteer clover the year after applying this slow-release form of phosphorus.

Keep in mind that the rock phosphate products on the market have different analyses and physical characteristics. For example, the collodial clay products contain about 18% P and are very dusty. We think the black rock, containing over 30% P, is better suited for field application with the horse drawn fertilizer spreader pictured in the Winter 2000 photo essay. In fact, this fine, sandlike material is so free flowing we find it advantageous to mix in a coarser material, like gypsum, or some of our own screen compost. Mixing in these materials slows down the flow of the black rock and should make the phosphorus more available to the soil life and the plants.

On the other hand, we prefer the collodial clay type of soft rock for use in the composting pigpens because it does a better job of tying down the ammonia in the fresh additions of horse manure. Also, this is the only type of rock phosphate which we use as a mineral supplement for the horses and other farm animals for the important reason that it has been deflourinated. Likewise, the collodial clay products would be the safest material for continued applications to the fields in order to prevent the buildup of fluoride, cadmium, and other heavy metals.

We hope the message came through loud and clear in The Great Phosphate Debate that there are other ways to improve phosphorus availability in the soil than trucking in rock materials. Animal manures are a good source of phosphorus, particularly the droppings from poultry. Mulch materials and high carbon cover crops can promote fungal activity, in this way releasing the stores of locked up phosphorus in the soil.

Even better, from the standpoint of long-term sustainability, would be including a grass/legume sod in the rotation to increase overall biological activity and to extract the phosphorus reserves through the sod’s extensive root system. Legume cover crops and buckwheat have also earned the reputation of being natural extractors of phosphorus. In fact, recent research at Geneva, NY by Thomas Bjorkman indicates that a fall cover crop of buckwheat can provide a good bit of available phosphorus early the next spring when the soil is too cold to release this nutrient through biological activity.

In retrospect, we fear we may have done readers a disservice by focussing The Great Phosphate Debate entirely on the different ideas about building up and balancing phosphorus levels in the soil, we should also have emphasized the dangers of oversupplying phosphorus. Indeed, one of the peculiar challenges facing market gardeners is that vegetable crops use relatively small quantities of nutrients, like phosphorus, but return little in the way of organic matter to the soil. The temptation for growers of high value produce is to replenish the organic matter in the soil by applying manure or compost to the fields year after year.

Over the long run, this practice can lead to excessively high levels of nutrients, which is not the best for the crops or the environment. Although livestock producers have received the brunt of the blame for phosphate pollution here in the Northeast, several states are proposing to make vegetable growers next in line for mandatory nutrient management plans due to their role in saturating the soil with nutrients.

To provide a down-to-earth perspective on this issue, we offer the following article by Brian Caldwell on how organic growers can increase organic matter without overloading the soil with nutrients. His carefully considered arguments, and creative solutions, come from years of experience as an organic grower and Cooperative Extension agent in Tioga County, New York.

We like to think that the bio-extensive approach to market gardening minimizes the risk of overloading the soil with nutrients because the fallow lands make it possible to grow lots of cover crops to maintain soil structure and organic matter rather than relying on large quantities of manure and compost. However, we are now seeing the consequences of ignoring our own farm philosophy when we resorted to off-farm inputs to correct a phosphate deficiency.

To reverse the slow-but-steady decline in phosphorus levels showing up in our long-term, PASA sponsored soil quality trial, we initiated the all-out phosphorus building campaign described in the Winter 2000 column. This included the application of 500 lbs/acre of black rock in the market garden for three years in a row during the mid-90’s, followed by cultural methods, like adding cow manure to the compost mix, and including buckwheat and double-cut rye in the rotation of cover crops.

The impact of this “campaign for phosphorus” did not show up on the soil test reports until recently — a delayed reaction? or cumulative effect? we do not know. One thing is for sure, that when we replicated Klaas and Mary-Howell Martens’ monthly soil testing trial last year, P levels in our fields were all very high! In fact, the levels of available phosphate have risen from a low of 100 lbs of P205/acre in 1993 to over 200 lbs/acre in the last two years as measured by Brookside Lab. While these high levels are still well below the saturation point causing phosphate pollution, we clearly added more rock phosphate than necessary.

In hindsight, we wish we had waited a few years after the first application of black rock to assess the delayed reaction in P levels on the soil test reports before adding any more of this material. It might also have been more enlightening to have tried the monthly soil testing trial in the early 90’s before we applied the black rock. Then, we might have seen P levels rise during the course of the growing season following the seasonal increase in organic matter much like the results the Martens have obtained raising high yielding field crops at low nutrient levels.

To put it the other way around, now that we have boosted phosphorus levels and availability with the rock minerals, we can no longer discern the effect of the cultural methods on making the natural reserves of phosphorus more available. To the contrary, we may have inadvertently caused the fungi and bacteria in the soil to become lazy!

We hope that our misguided efforts in nutrient management might encourage other growers to rely on the soil biology first to improve phosphorus availability before resorting to lots of mined minerals or trucked-in manure. Developing ways to mobilize the low levels of native phosphate reserves may ultimately lead to the most diversified and resilient cropping systems with the least overall impact on the whole environment.

Cultivating Questions Follow Up On Phosphorous

Cultivating Questions Follow Up On Phosphorous

P.S.: We almost forgot the question about supplies of greensand! Most natural fertilizer companies carry this product. Even though greensand is mined right off the coast of New Jersey, we have not looked into wholesale distributors of this material because there are more reasonable sources of potash much closer to home.

Manures and mulch materials both contain a good bit of potassium. For that matter, alfalfa hay would probably supply as much potassium and trace elements as greensand plus a lot more of the organic matter so necessary for sustainable vegetable production. Our bias, of course, is the cover crops because they can turn the large soil reserves of potassium into plant available food right in the fields.

In terms of the overall nutrient management picture, excessively high levels of potassium may actually be even more of a problem than phosphorus under organic management. The reason we say this is that almost all organic materials contain a significant amount of this element. At the same time, these additions of organic matter stimulates biological activity which, in turn, makes available the large stores of potassium found in most soils.

For example, the lowest testing field at the start of our long-term soil quality trial showed less than 100 lbs of potassium/acre in 1993. By the year two thousand, this field had increased to over 500 lbs/acre of K. Over the same seven year period, the base saturation percentage of potassium had also doubled in this field, from a desirable 3.4% to almost 7%. The only potassium input was an annual application of 5 tons/acre of hog composted manure.

Although high levels of potassium are not considered a cause of water pollution right now, some crop consultants consider high levels a reason for concern. First of all, potassium acts like the big bully on the soil colloid, bumping off more important cations like calcium and magnesium. Secondly, high levels of K throw the nitrogen:potassium ration out of whack in the soil. Both situations, these consultants say, can make the crops — and the animals that eat them — more susceptible to insects and disease.

Whether or not this is the case, we do not see the reason to initiate a “campaign for potassium” based on greensand or other mined materials as this element seems to fend for itself quite well where compost, cover crops and crop rotation are routinely employed.

SIDE BAR: How Can Organic Vegetable Growers Increase Soil Organic Matter Without Overloading the Soil With Nutrients?

by Brian Caldwell of West Danby, New York

Part 1. The problem.

This is an issue that is just beginning to be recognized. It arises from a common practice among organic vegetable growers–that of applying compost or manure to vegetable fields nearly every year in order to fertilize crops and raise soil organic matter (OM) levels. While this is a beneficial practice in the short term, in the long run it can lead to over-fertilization and water pollution. The problem is similar to over-fertilization that occurs on livestock farms with sufficient land on which to properly spread their manure.

On most new land that is just being put into organic vegetable production, it is common and quite worthwhile to apply a big “shot” of nutrients and organic matter through heavy applications of compost and manure. After the first heavy application, amounts can be reduced in subsequent years. However, manure or compost is still usually applied at a rate that will supply at least the necessary nitrogen (N) needed for the next crop, which means that extra phosphorous (P) and potassium (K) beyond the crop requirements will be added to the soil. Over the years, soil P and K levels build to moderate, then high or excessive levels. The soil is out of balance. In fact, if manure or compost is added specifically to increase soil organic matter levels, which is a goal for many organic farmers, then usually all nutrients will be added beyond crop requirements.

Let’s look at an example from my own farm. “Field 1” is a small field of about 1/5 acre which had been the farmstead garden for many years before I moved to Hemlock Grove Farm in 1977. It had higher nutrient levels than our other fields. I have soil test data (Table 1) from this field over a period of 21 years, starting in 1978. I also have records of the nutrient-carrying materials I added to this field for 16 years, which can be extrapolated for the 21 year period, as I used similar practices over the whole time.

Though the field is small, all data have been standardized on a per-acre basis for comparison.

Year      Soil P    Soil K       Soil pH        Soil OM

1              25         400           6.1               3.2

2              37         400          6.0               3.4

12            43          515          6.7               3.3

21            82          685          7.0               3.7

Table 1. Soil Test Data, Field 1

This data shows the problem. Soil nutrient levels are all in the high range after 21 years, which seems good, but if I continue the same practices, they will get too high.

Phosphorus levels are already very high, and going up faster than anything else. (Cornell test values are on a scale that reads lower than typical values, so my current P level would probably be measured at over 500# by most labs). High soil P does not hurt crop plants, but can contribute to water pollution. Note that soil organic matter levels have increased only slightly, from 3.2 to 3.7%. Soil nitrogen levels are so variable because of weather conditions that they are not routinely measured, but OM level gives an indication of how much is stored in the soil.

Using guestimates as to the nutrient composition of the applied compost, hay mulch, manures, etc (but not including N from cover crops) and of the amounts that typical mixed crop vegetable harvests may have removed over the period, I’ve made a rough nutrient budget for this field over the 21 years.

The field did not seemingly get heavy applications of organic fertilizers, averaging only 6 tons per acre per year of beef or sheep manure, with occasional additional applications of hay mulches, commercial and homemade compost, and wood ashes. (In retrospect, the 500#/A of rock phosphate we put on one year looks like a mistake.) Adding all this up, though, gives an estimated total N-P-K addition of 3500-2200-3650 to this field over 21 years. I further estimate crop removal at 1500-200-2000 over that time (note how little P is actually removed by vegetable crops). So, net additions to this field were around 2000-2000-1650 pounds per acre of N-P-K. No wonder test values went up!

Where do excess nutrients go? Extra added P and K are mostly held in the soil in unavailable forms, but most nitrogen is not. Some of the nitrogen is held in the increased amount of soil organic matter after 21 years- a .5% increase holds about 400# of N. But most (over 1500# /A or about 70#/A/year in this case) of the excess nitrogen will not be held in the soil, but will leach into groundwater or volatilize into the air. In many situations, such as typical home gardens, this is not a problem, since only a relatively small amount of nitrogen is in question. But if this practice is done on a widespread basis or on large farms, there is potential for significant groundwater pollution. The same situation occurs when excessive chemical fertilizer is applied.

I believe that there is no good reason to continue to increase these soil nutrient levels. The field produces good yields and quality. It has clearly reached a “mature” state in which heavy applications of brought-in organic materials are unsound. A field like this needs an approach that produces crops and maintains soil OM levels without the “booster” type approach.

Part 2. How do we raise soil OM levels without causing this problem?

High levels of soil organic matter are desirable in many ways. Higher OM improves soil water holding capacity, aeration, infiltration, nutrient holding and release, and more. But how do we achieve high soil OM sustainability over the long term? And how high should it be?

Virgin soil had much higher organic matter levels than current cultivated soils. How did high soil organic matter levels arise naturally (presumably, without groundwater pollution)? The answer is: very slowly, and in the absence of tillage and crop removal. Intensive tillage is the primary culprit in “burning up” soil organic matter at a very high rate, requiring that we add outside sources of OM to the soil. Under natural, untilled forest or prairie conditions, highly carbonaceous organic litter (leaves, etc.) Is added to the soil surface each year, and roots die within the soil. No additional P or K is added to the soil system, except what weathers slowly from the rocks. Nitrogen is added in small amounts from precipitation and bacterial fixation, but held tightly in the vegetation and decomposing surface litter. Small amounts of nutrients are sequestered away each year in humus and “locked up” OM that is not available to decomposers or oxygen. Nutrients cycle around and around, with relatively slow breakdown of soil OM, and accumulation of high-carbon OM on the soil surface. In this way, soil organic matter can build up very gradually over thousands of years, to levels around 10% in many virgin mineral soils.

When this land is cleared and repeatedly tilled, OM levels drop rapidly down to less than 2% in the absence of manure or compost applications. A sick soil. But remember, our real goal for a farm field is to preserve or increase soil quality, not just its OM content. We tend to be in a frame of mind that says, “the more OM, the better.” While there is some truth to this, under any given tillage and cropping regime there is an “equilibrium” level of soil OM. Generally, the less tillage, the higher this equilibrium level. OM levels can be maintained above equilibrium only by continuous heavy applications of compost or manure that carry far more nutrients than the crops can use. This is wasteful and leads to pollution. (The Biodynamic goal of the farm as a self-contained organism helps to avoid this problem, because it discourages importation of large amounts of nutrients.) Research at the Rodale Research Center has shown that soil biological activity, quality, and fertility can be very high, even at modest (2.5-3.0%) soil OM levels, if large portion of the OM is in the “active” form, i.e. in the process of being broken down. So, the key soil quality strategy in farming is not merely accumulating a high soil OM level, but cycling it rapidly and effectively. It is counterproductive to shoot for virgin soil OM levels on tilled farm fields.

It is important to realize that the constant production of tilled crops, especially vegetables which return few residues to the soil, is the harshest way to treat your soil. Sod crops in rotation are the only tried and true way to increase long term soil OM levels without negative “side effects.” Sod accomplishes this because the soil is not tilled, and extensive root systems are formed. Traditional field crops rotations often involved applying manure or compost to a field only once in every 4- or 5- year cycle. Organic matter levels and soil nitrogen were greatly enhanced by at least 2 years of a sod hay crop. Phosphorus and potassium did not build up in such systems, but were instead mostly cycled around the farm through feed and manure.

An ideal rotation for vegetable growers, from a soil and nutrient standpoint, would be to substitute vegetable crops for the heavy feeding (field corn) and light feeding (small grains) crops in this traditional rotation. Heavy feeding vegetable crops would include intensive greens, brassicas, sweet corn, leeks, cucurbits, etc., while light feeders would be root crops, beans and peas, etc. A sod crop of legumes and grasses will provide a maximum OM contribution, while supplying its own nitrogen. If hay is harvested, there may be a net removal of P and K. These nutrients can then either be sold off the farm, or fed or otherwise recycled within the farm.

An experimental method of increasing soil OM without heavy nutrient loading is to use high-lignin, relatively low nutrient OM sources such as wood chips. These interact in a limited way with the soil, because of their high lignin content and low surface to volume ratio, but do provide an excellent OM source over the long term. Cornell did a 15-year study in the 1950’s and 1960’s in which 10 T/A/year of hardwood chips was added to experimental plots of Honeoye silt loam, a rich soil type. Soil OM and other soil quality levels were dramatically raised, with some positive (and some limited negative) effects on vegetable crop yields. Little soil nitrogen was “tied up”, contrary to expectations.

Recently, Quebec research on positive results from the use of chipped hardwood branch wood (“remial”) was reported in the Maine Organic Farmer and Gardener magazine (12/98-2/99 issue). The authors stressed the importance of fungus organisms in the soil. There is growing opinion from some soil scientists, notably Dr. Elaine Ingham of Oregon State University, that many of our agricultural soils are overbalanced toward bacterial, rather than fungal, populations because of the highly available nutrient sources we use. There may be other benefits to favoring soil fungi–perhaps establishing large and varied fungal populations in our soils could also help reduce fungal pathogen populations.

So, what are the take home lessons here? Mine are–

1. Significantly increase the sod and light-feeding crops in your rotation on “mature” fields.

2. Reduce tillage and keep soil covered with cover crops and mulches.

3. Don’t waste nutrients by excessive manure or compost applications. This is particularly important if your P levels are in the “high” range. Rely more heavily on getting nitrogen from legume cover crops and sod than from manure or compost.

4. If you want to increase soil OM levels further, try experimenting with spreading wood chips on your fields in moderate (10T/A or less) amounts. They can be spread just before spring tillage, or even better, left as mulch on the surface until next spring.