Managing Soil Phosphorus

Summer 2011

Phosphorus deficient corn
Phosphorus deficient crops can be stunted; leaves may turn purple; and flowering and new shoot growth will be delayed. Often, cold soils or an improper pH for growth limit phosphorus availability, even while the soil holds plenty of phosphorus. Sideman photo.

By Eric Sideman, Ph.D.

After nitrogen (N), phosphorus (P) is the next nutrient most likely to limit crop growth on farms and in gardens. It has a much simpler cycle in farm systems, and, unlike N – the availability of which depends heavily on biological activity (see my column in the March-May 2011 MOF&G) – the availability of P usually depends simply on whether your soil has enough. Like N, soils can have too much or too little P, so monitor P rather than blindly adding more year after year.

A garden, lawn or field with excess P can become a point source of pollution, because if the soil has more soluble P than the crop needs, it can wash away with water and end up in ponds, lakes and rivers. As P promotes plant growth in soils, excess P in these waters promotes algal growth. A massive growth of algae is called an algal bloom. When a bloom dies, it decomposes, and the decomposing organisms use up the oxygen in the water, so fish and other organisms then die too.

In soils with too little P, plants will suffer. Crops will be stunted; leaves may turn purple; flowering and new shoot growth will be delayed.

A handful of key points help growers manage P in soils:

• Cold Soil. Although P availability depends on biological activity less than N availability does, if organic matter is the source of P, then biological activity is required to release it. So in cool soils, where biological activity is slowed, less P is available. Also, roots do not absorb P well from cool soils. When trying to get early tomatoes in high tunnels, you may have seen the leaves of new transplants turn purple. To avoid this purpling (and slowed tomato plant growth), wait until the soil is 60 F before setting out transplants. Adding a soluble starter fertilizer at transplant time can also help supply available P to the root zone of transplants.

• pH. Both acid and alkaline soils limit P availability. Phosphorus is most available between pH 6.5 and 6.8. In fact, if a soil test shows a pH that is way off and a slightly low P level, I recommend correcting the pH and repeating the soil test before correcting the P level. Because P is so reactive with other chemicals in the soil, it tends to get tied up easily in complex molecules and becomes unavailable. At low pH, soil aluminum levels increase and P binds with aluminum. At high pH, P binds with available calcium.

• Mobility. Phosphorus is highly reactive and binds easily with many other minerals in the soil, so P is not very soluble and does not move freely with soil water. I mentioned above that P can move into water bodies, but this is when either soil particles themselves move and carry P with them (through soil erosion), or when the soil P level is so high that even the small amount of the total P that does become soluble is still more than crops need, and it moves – both through leaching and surface water runoff – with the soil water. This is an important point for people who repeatedly add compost to their fields; you CAN have too much organic matter. With very high organic matter levels, even the small amount of soluble P released from the organic matter may be more than crops can absorb.

Since P is very immobile in soil, P amendments should be placed near plant roots. This is particularly important when side dressing (adding a nutrient source alongside a crop row during the growing season) or when banding (applying fertilizer only to the rows of crops at planting time, rather than broadcasting it). Remember that P will not move to the roots; instead, roots have to grow to the phosphorus. Furthermore, P should not be concentrated in a small band or near the soil surface, because this will cause roots to proliferate in that small, fertile area where water may eventually be limiting. Also, in dry years roots tend to grow deep, toward water, so they proliferate away from the area where P is concentrated. Either thoroughly mix P fertilizers deep in the soil, or provide an even supply of water.

• Mycorrhizae. Some of the many species of fungi living in a biologically active soil invade plant roots. These fungi are called mycorrhizae. They may seem parasitic but are actually mutualistic. While the fungi may use some carbohydrate produced by the plant, they also provide supplementary P to the plant. The fungal hyphae (threads of cells) fan out over a vast area of soil, absorbing many minerals, including P, which they pass to the plant. In nature, mycorrhizae explain the success of plants in low P soils.

Phosphorus Fertilizers

Phosphorus levels reported on a University of Maine Soil test should be in the 20 to 40 pound per acre range. Recommendations for P amendments are meant to build the soil reservoir to that range and to compensate for crop removal that season. If the test reports more than 40 pounds of P per acre, consider using soil amendments with no or little phosphorus.

Organic matter and the activity of soil microbes should be the basis of an organic fertility program for phosphorus. Crop residue, livestock manures and compost recycle P around the farm and should meet most P needs. Maintaining a good P level should be possible with typical yearly applications of materials used for N, e.g., seed meals, mulches, cow manure. To build P from a low level, you will probably need a material with more phosphorus.

Cage layer manure is a cheap material that is high in P and N, but it is unpleasant to work with; it can easily be misused and inhibit seed germination because it has so much free salt; it can pollute ground and surface water because the nutrients in it are so soluble; it does nothing to improve the soil because it contains so little organic matter; and in soils with a pH that is already in the correct range, it can make the soil too alkaline.

Rock phosphate, a natural mined material, is loaded with P, but only about 1 to 3 percent of that P is available; the rest is tightly bound in complex compounds that are slow to break down. Colloidal rock phosphate is a fine material left after processing rock phosphate. It is lower in total P but a bit higher in available phosphorus.

Bone meal is high in P but is very expensive for the amount of P available. Bone char (burned bone meal) has more available P and is a much better value than bone meal.

Value is something we will all be watching over the coming decades as we approach a limiting P supply (“peak phosphorus”) globally, so manage P carefully and don’t blindly add more each year. For more information on peak phosphorus, see “New threat to global food security as phosphate supplies become increasingly scarce,” a Nov. 2010 Soil Association report; and “Recycling animal and human dung is the key to sustainable farming,” by Kris De Decker, Energy Bulletin, Sept. 16, 2010;

Eric Sideman is MOFGA’s organic crops specialist. You can address your questions to him at 568-4142 or [email protected].

Too Much P, Too Many Weeds

At MOFGA’s 2009 Spring Growth Conference, New York grower Klaas Martens said that velvetleaf, lambsquarters, pigweed and galinsoga, all nonmycorrhizal weeds, seem to grow better in soils that are high in P and salt – soils that do not support mycorrhizae. Fields frequently amended with manure, such as those closest to the barn, are high in P and salts, so haul manure to farther fields. Likewise, bringing in a lot of compost can increase fertility too much. To reduce excess soil P, grow a few crops of grain, which help remove P and put carbon in the soil. “I don’t think it’s the absolute amount of P that’s the problem, but how much there is in relation to carbon,” said Martens.

Martens said that a researcher at the University of Wisconsin noticed that in some trials, mycorrhizal fungi on crop roots slowed the growth of nonmycorrhizal weeds up to 90 percent. So mycorrhizae not only help feed crops but also help crops compete with weeds.

The Martens grew grains to lower soil P concentrations in order to control velvetleaf. By the third year, the velvetleaf was getting shorter, with smaller-diameter stems, and late in the season its leaves yellowed and fell off. The weakened plants were attacked by a fungus, whiteflies and a virus and were gone by mid-August. Most of the seed produced was not viable. “No magic organic spray killed the weed.” Instead, the Martens created conditions that no longer favored nonmycorrhizal weeds.

From “The Martens Farm: We All Do Better Together,” by Jean English, The Maine Organic Farmer & Gardener, June-Aug. 2009.


An editorial error occurred in Eric Sideman’s March-May 2011 column, “Managing Nitrogen Fertility.” The first two sentences read: “Nitrogen (N) is the nutrient most commonly limiting crop growth and yield on organic farms. This is especially true when creating a farm from an old, abandoned field and when transitioning from organic to conventional fertilizing practices, because N, unless managed, is easily lost from soil.” The second sentence should have read, “…when transitioning from conventional to organic…,” which we hope is the direction growers are taking. The editor regrets her error.

– JE

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