Phosphite: A Natural Fungicide
What is the difference between phosphite and phosphate. Phosphate is an atom of phosphorus with four oxygens attached. Phosphates are what plants take up and use for the majority of their plant nutrition. Very stable, it is used in the plant cell wall, in DNA (genetic code) and RNA (messengers), and for energy transfer. The bad part about phosphate is that it may cause water quality concerns when it leaves the soil.
Phosphite is an atom of phosphorus plus only three oxygens. It is much more mobile, both in the soil and in the plant, but highly reactive so it gets tied up very quickly. Phosphite acts like a natural fungicide, suppressing many soil fungus but it also acts like a bio-stimulant, increasing early root growth, leading to healthier stems and more early leaf growth. Phosphite is the reduced form of phosphate and its highly reactive nature allows it to be beneficial to plant growth, both as fungicide and abio-stimulant. It is also natural and not harmful to humans, animals, or p
lants.
Some reports say phosphite can also be a fertilizer, however; most reports say it should not replace phosphate fertilizer. I think both statements can be true depending upon the soil conditions. First, do not rely on phosphite for your total P fertilizer. Under just right conditions, phosphite could be oxidized to form phosphate in warm well aerated soils but in cold wet soils (reducing conditions); it is not a plant nutrient.
As a natural fungicide, phosphite allows the plant roots to control soil-borne pathogens like phytophthora, pythium, fusarium, rhizoctonia, anthracnose, sclerotinia (white mold in soybeans) and powdery mildew. On alfalfa, phosphite applications help with crown rot. It does not kill pathogens, it just suppresses them. In order to work effectively, phosphite can be applied in-furrow at planting to suppress these fungus and also later as a foliar feed on the leaf. The phosphite has to enter the plants xylem (water conducting vascular system) in order to work effectively. There it enhances early root growth leading to healthier and thicker stalks, and more leaves. It is a preventative product. It can not heal or cure fungal disease that already occurs. It also has no activity on the foliar leaf diseases that occur later in the growing season, only soil pathogens.
Usually, phosphite is applied as potassium phosphite but there are other forms including calcium and magnesium phosphite. Phosphite have been used on lawn and turf for over 50 years but now farmers are using it in corn, soybeans, hay crops, and vegetables as a natural fungicide and bio-stimulant. One other factor, phosphite stimulates and activates the plants natural defense system called Systematic Acquired Resistance (SAR) which helps the plant prepare defenses against future fungal pathogenic attacks. Usually, the phosphite is active in the plant for 60 days, so for best results, two applications (either in-furrow as a first application plus a later foliar application) are recommended.
One form of phosphite sold on the market is Xylem Plus. It is a potassium based phosphite with 7 phosphite molecules in a chain. It suppresses disease in the soil, also in the plant, and also actives the SAR or natural plant defense network. It promotes early growth in both corn and soybeans.
Two problem occur for farmers with phosphite application, especially on soybeans. First, not many drills are set up for soil application of a natural fungicide at planting. This can partially be compensated by applying the phosphite early with your herbicide and applying right when the soybean plant forms its first (1st) trifoliate.
The second problem is that phosphite works and should be applied per foot of row. That means 15 and 30-inch soybeans require less product than drilled soybeans at 7.5 inch spacing. The product has to protect the newly planted seed and germinating root right at the source. Again, it can also be applied with a herbicide as a foliar spray with similar results. Results vary depending upon the year, the weather, soil conditions, pathogen population, soil structure, but yield increases of 10-30 bushel on corn and 5-10 or more (18-25) bushels on soybeans are possible. Applications rates vary from 24-32 ounces/acre/application with generally two applications needed per year.