Date of this Version
FIELD CROPS G-22, Cropping Practices Issued November 1990, 7,500
The Soil Sample
The starting point for any fertilizer program, regardless of the tillage system, is obtaining a representative soil sample. Without knowing which nutrients are needed, the questions of what fertilizer and how much to apply become irrelevant. Two types of soil samples should be obtained: the traditional, zero to 8-inch sample for general fertility (pH, phosphorus, potassium, organic matter, etc.) and deep, subsoil samples for nitrate. Subsoil samples for nitrate should be taken to a depth of at least 3 feet.
The problem of how to obtain a representative soil sample is particularly relevant in ridge-tilled fields. On conventionally tilled fields, mixing occurs and fertility levels may be fairly uniform throughout the top 6-8 inches of soil. In a ridge-till system, much less soil mixing occurs. This is a concern where phosphorus fertilizer has been banded in the past, since phosphorus is relatively immobile in the soil. This is in contrast to nitrogen which is relatively mobile and will dissipate away from a band with time.
If ridges are maintained for several years and phosphorus is banded each year in the same relative location, concentrated phosphorus zones can develop. These can cause wide variations in soil test results and fertilizer recommendations. In order to know how to sample a ridge-tilled field, it helps to know the fertilization history of the field, including how fertilizer was applied in the past, and how long the practice was maintained.
This situation presents somewhat of a dilemma: should the band be avoided or included in the soil sample? This question is still being researched. For now, the best suggestion is to sample in the shoulder of the ridge (Figure 1), where some soil mixing will have occurred. Any bands placed in the ridge, or fertilizer broadcast and located in the furrow, will be mixed with bulk soil during planting, cultivation and ridging. Consequently, the shoulder of the ridge may be representative of the field in general. Also, the shoulder of the ridge will normally be drier than the furrow and easier to sample.
Band placement of phosphorus, with or without other nutrients, can result in concentrated zones of phosphorus building up in the soil (Figure 1). After several years of repeatedly banding phosphorus in the same location, it may be advisable to separately sample the banded zones and areas between bands. If soil phosphorus levels are quite high where bands have been repeatedly applied, it may not be necessary to apply phosphorus until these levels are reduced.