Managing Available Phosphorus

canstockphoto26719966by Jon Frank

Have you ever baked a cake? If you want the cake to turn out well you need to have the right amounts and ratios of ingredients. What would happen if you decided to modify the cake recipe and double the liquids, while cutting the flour and dry ingredients in half? It would mix just fine in a bowl, but when you take it out of the oven you would have some glop that nobody wants to eat, and you wouldn’t dare call it a cake. You must understand the right proportions to make modifications, or else you need to follow a recipe.

In this same way, you need to maintain the right levels and ratios of available nutrients in soil if you want to produce nutrient-dense foods. It is especially important to keep your eye on the big three: calcium, phosphorus and potassium. If you get these three right in your soil, everything else is a piece of cake.

The easiest problem to fix is low potassium. Just add it, and your problem is over. Raising a low calcium level is a bit more challenging because it takes microbial digestion. Harder still is to build up low phosphorus in soil. Once phosphorus is built up it becomes much easier — all you have to do is maintain a consistent level.

When considering the overall influence of growth versus fruiting energy in soil, the primary reproductive energy comes from phosphorus. Manganese gets honorable mention, but phosphorus is the big one.

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When looking at phosphorus in soil, it is important to view this nutrient in relation to other elements. For example, the phosphorus to potassium ratio should be around 1:1 with a level of around 175 pounds per acre of each. At this level these elements are well supplied, but are not interfering with calcium.

Another ratio to keep an eye on is calcium to phosphorus. Look for an 18:1 ratio as ideal. When the ratio gets narrower than 12:1, phosphorus begins to interfere with the function of calcium. What does that mean? Produce tastes like garbage. I have seen many soils showing 2-3,000 pounds of phosphorus and potassium with only 1,800 pounds or less of calcium. Instead of 18:1, the ratio becomes 0.7:1. This is just like that gloppy cake recipe. There is no way a soil like this can grow nutrient-dense foods until the levels and ratios are corrected.

As hard as it is to build phosphorus in soil, it is much harder to reduce an extreme level of phosphorus. It only comes down at a snail’s pace. Many zealously amended soils will need 20 years to reduce all that phosphorus. Here is my best advice in such a situation; move your garden or crops. Another option is to excavate the top 6 inches of soil and replace with unamended topsoil. A third option is to dilute existing soil with 6 inches of unamended topsoil. All three of these approaches are much quicker than trying to bring down phosphorus to the right level by cropping it out.

On the other hand, many new gardens are critically deficient in phosphorus. They are so low that the plants are practically on life support. Phosphorus is the P in ATP. ATP is the prime mover in the energy cycle in plants. Plants need phosphorus to make sugars, transport minerals and transport sugars.


When phosphorus is undersupplied in a plant, everything suffers — especially quality.

• Use soft rock phosphate as the base. About 50 pounds per 1,000 square feet should do it. Repeat one more time if needed. Do not waste your time with hard rock phosphate.
• Use 11-52-0, an acid phosphate, in conjunction with soft rock phosphate. This will supply the plants’ need for phosphorus right away, and the acid assists in making soft rock phosphate more available.
• Use other acids in the fertility program. This again helps the soft rock phosphate become available.
• Use a liquid fish that has been stabilized with phosphoric acid as part of the nutrient drenches.

I have used this exact program on hundreds of gardens all across the pH spectrum and it works very well. Rarely does it take more than 2-3 years. What really slows this process down is trying to do it 100 percent organic. The other acids and the commercial phosphate are the key to getting soft rock phosphate available. Do not use 18-46-0 or DAP. It is a useless, high-pH fertilizer that just ties up with calcium.

The strategy I use to raise phosphorus is to get a big hammer and swing it as hard as I can. In many instances, a low phosphorus reading of 20 pounds per acre might move up to 70 or 80 pounds in one year. I normally repeat the program from the first year and see phosphorus further rise to 150-180 by the beginning of the third year on the program. Then it goes into maintenance mode where small doses of soft rock phosphate are added whenever the soil test shows phosphorous between 150 and 250 pounds per acre. The maintenance dose is 12 pounds per 1,000 square feet or 500 pounds per acre.


As phosphorus rises to optimum levels, commercial phosphate can be taken out of the soil management program.

As phosphorus rises to the optimum level, commercial phosphate is taken out of the program. Here is what I don’t suggest; apply low doses of phosphorus, use mycorrhizea and hope for the best. This approach keeps the soil depleted for a long time and rarely yields nutrient-dense produce. If you want nutrient-dense foods you must get available phosphorus to around 175 pounds as quickly as possible. At this level of available phosphorus, mycorrhizea go dormant and aren’t much use to roots. The best use of mycorrhizea is to use it on low fertility soils where remineralizing with phosphorus is not economical.

Phosphorus in soil is very similar to a dry sponge. If water is dribbling on a sponge, no runoff will occur until the sponge is saturated. First you have to saturate the sponge. The same principle applies to phosphorus. You have to apply quite a bit at first to saturate the sponge; and then available phosphorus will show up all of a sudden.

On the Morgan soil test here is what I do for specific levels of available phosphorus:

• 1-120 pounds per acre: Swing the big hammer, don’t be timid, move past this insufficiency as fast as you can by using soft rock phosphate, 11-52-0, other acids, and phos acid liquid fish.
• 120-150 pounds per acre: Swing a smaller hammer but make sure to take action by using all the tools listed above.
• 150-250 pounds per acre: Maintenance mode, use soft rock phosphate in maintenance doses and 1-2 pounds of 11-52-0 if lower than 200 pounds per acre.
• Above 250 pounds per acre: Leave it alone, this soil does not need any more phosphorus. Check it next year.

Jon Frank is the owner of International Ag Labs, based in the solidly Ag community of southern Minnesota. He is a soil consultant with over 14 years of experience in his field. He is the founder of High Brix Gardens, the market garden/backyard garden division of IAL. He is fascinated with the correlation between minerally rich soil and nutrient-dense food and its subsequent impact on human health. International Ag Labs, Inc. 800 W. Lake Ave., Fairmont, Minnesota 56031 United States;; 507-235-6909.

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