Banding of Phosphorus Fertilizer in the Spring

In order for crops to grow and develop, soil nutrients (chemical elements) need to be absorbed by roots and distributed throughout the plant. These nutrients, coming from soil parent material or from added fertilizers, function in structural and metabolic systems, enabling the plant to carry on living processes. To obtain full yield potential and to produce high-quality crops, the soil must contain enough nutrients to support vigorous plant growth throughout the life cycle. Since the weathering of soil parent material is a slow, long-term process, assuring proper nutrition for crop plants generally requires the addition of fertilizers to the soil.

When a gardener decides to fertilize, he is faced with the choice of which fertilizer to use and the task of determining how much fertilizer to apply. No universal guidelines are available for answering these questions as soils differ in fertility and crops differ in their nutritional requirements. The gardener must apply what seems to be correct amounts of fertilizer based on soil tests, plant analysis, previous experience and advice from others. Modifications can be made during the season or in the following year based on how the crop is growing and on the amount of time remaining in the current season.

In the fertilization of crops, growers must first be concerned with supplying the primary macro-nutrients - nitrogen, phosphorus and potassium. Since organic or natural fertilizers have variable chemical composition, a balanced or adequate supply of all of the primary macronutrients from a single organic fertilizer is unlikely. Therefore, more than one kind of organic fertilizer is usually needed to provide sufficient crop nutrition in any system of organic gardening. This situation differs from that of synthetic fertilizers which are manufactured to contain one, two or all three of the macronutrients and can be purchased in practically any formulation.

The organic grower has a limited number of phosphorus fertilizers to choose from. Plant residue, farm manure and compost are, practically, too low in phosphorus to be considered for any purpose other than maintaining soil fertility following a build-up of this element in the soil. Residues from the bodies of animals are excellent sources of phosphorus, with bone meal being the most significant among available animal residues.
Rock phosphate, mined from deposits, has a high phosphorus content, but must break down into a clay form before it can be utilized by plants. This process may require from 2 to 3 years or longer depending upon the pH of the soil.

Colloidal rock phosphate, taken from a lower grade ore than regular rock phosphate, is more readily available to plants because of its infinite fineness and a process whereby nutrients in the soil are carried to the plant roots through the exchange of tiny electrical charges between the roots and the soil.

Superphosphates are manufactured by treating rock phosphate with sulfuric or phosphoric acid, thus simulating the action of acid soil and the rock.

If soil pH is too high or alkaline, essential elements such as phosphorus and iron become unavailable for plant use even though they may be present in the soil. Unavailability of mineral elements is caused by temperature, physical encapsulation by soil calcium or chemical changes associated with added nutrients.

The unavailability of phosphorus and iron can cause serious plant problems. Phosphorus is represented by the middle number on every fertilizer analysis and is responsible for flowering and root development. A phosphorus deficiency resulting in lack of flowering and poor root development clearly causes problems.

The obvious answer to rectify a mineral deficiency would seem to be to add additional quantities of the needed mineral. However, minerals must be added in such a manner that they are available for plant uptake over a period of time rather than immediately. If released too quickly, the minerals may be unavailable or inaccessible because of cold soil temperature, physical encapsulation or chemical change.

One successful approach is to concentrate the deficient or unavailable minerals in a mass or bank which cannot be acted upon as rapidly as if the minerals were evenly distributed throughout the growing area. The mineral band offers seedling plants or transplants a continuous, reliable source of needed nutrition in a concentrated zone. This approach can increase the availability of phosphorus and iron in the area of the band.

When banding superphosphate (0-20-0) to insure phosphorus availability, make a furrow 3 inches deep (for seed) or 6 inches deep (for transplants) in the planting area. Into this furrow, evenly distribute 1/2 pound (1/2 cup) of superphosphate (0-20-0) per 10 linear feet of bed. This band of phosphorus will provide a concentrated source of available phosphorus (for use as a "starter" solution) for young and growing plants. Cover the band of super phosphate with 2 inches of soil. When planting in an iron deficient, plant chlorosis-prone soil, add 1/2 pound (1/2 cup) of iron sulfate (copperas) to the 1/2 pound (1/2 cup) of superphosphate (0-20-0) per 10 linear feet of bed. If seeds or transplant roots are planted directly into this concentrated mix, growth could be stunted. The 2 inches of covering soil serves as a buffer zone. If colloidal rock phosphate (0-2-0) is used, seeds and transplants can be planted at the standard planting depth directly into colloidal rock phosphate banded into the planting furrow at the rate of 5 pounds per 10 linear feet.¶