Fertilizing Dry Beans

Soil Sampling

A valid soil analysis can be used to evaluate nutrient application needs to maintain a healthy dry bean crop. The validity of a soil test is directly related to how well the sample was collected. The soil sample must represent the area sampled; consequently, a good sample is a composite of 15 to 20 soil cores taken from an area uniform in soil type. If the soil is not uniform in the field, separate samples should be taken from each area with major differences in soil properties or management practices. Take samples down to a minimum of 1-foot deep. 

Air-dry all soil samples thoroughly within 12 hours after sampling by spreading the soil on a clean surface where the soil will not be contaminated. Keep the sample cool until it can be dried. Do not oven-dry the soil because soil test results can be changed. Place the air-dried soil in a clean sample container for shipment to the soil test laboratory. 

Submit a carefully completed information form with the soil sample. This form provides information so fertilizer application suggestions can be tailored to your specific situation. Take soil samples for NO₃-N analyses every year for optimum fertilization of crops. Analyze soil for availability of the other nutrients, pH, and organic matter content every three to four years. 

Other information can be obtained from the Colorado State University Soil, Water, and Plant Testing Laboratory on the Colorado State University SPUR Campus in Denver (970-491-5061).

Nitrogen Suggestions

Nitrogen fertilizer may not be needed if dry beans follow crops that have been fertilized with high amounts of N. For crops that leave large amounts of residue in the soil, higher levels of fertilizer N may be required to aid in straw decomposition. The general rule is to apply about 15 pounds N per ton of residue up to 3 tons or 45-50 pounds of additional N. 

Dry beans are legumes that biologically fix N through a symbiotic N fixation process. Inoculate bean seed with the specific host bacteria if dry beans have not been grown recently in a field and in fields where the presence of N-fixing bacteria in the soil is questioned due to historically poor nodulation. Where needed, an appropriate Rhizobium inoculant can be applied to the seed or in the seed furrow at planting. 

Because legumes can fix N if nodules function properly, some of the N requirements of the plants are met through this process. However, N fixation is limited in heavy clay soils or compacted soils due to poor soil aeration. If residual NO₃-N levels in the soil are low, apply N fertilizer (Table 1). Avoid excessive N levels because they inhibit nodule formation, stimulate heavy vine growth, delay maturity, provide conditions favorable to insect activity, and enhance white mold and bacterial diseases. When residual soil NO₃-N levels are high (greater than 30 ppm), consider planting varieties that are resistant to white mold and early maturing to compensate for potentially delayed maturity. Additionally, any N application should take place before the crop starts to bloom to avoid a delay in maturity. 

Nitrogen fertilizers may be surface broadcast then incorporated, or band applied. Band application can be accomplished with the use of planter attachments that place the fertilizer band 2 inches below and 2 inches beside the seed row. If band application is utilized, the N rate should be less than 20 pounds of N per acre to avoid burning the seedlings. Additional N can be applied mid-season through fertigation. 

Table 1. Suggested nitrogen rates for irrigated dry beans (expected yield*: 2,500 pounds per acre). 

Phosphorus Suggestions

Dry beans respond to applied phosphorus (P) on soils with low or medium levels of extractable P. Phosphorus deficiencies are most likely on soils that are acidic (pH<6.0) or alkaline (pH>7.5) and low in organic matter (such as eroded knolls or land-leveled areas). Suggested fertilizer P rates (Table 2) are for band applications related to soil test levels. The main soil tests for extractable P in Colorado soils are the Olsen sodium bicarbonate (NaHCO₃) and Mehlich-3 tests. Values for both tests are shown in Table 2. 

Place P fertilizers in the root zone because P is not mobile in soil. Band application at planting is the most efficient placement method for P, and suggested rates for band application (Table 2) are about half those for broadcast application. Phosphate fertilizers may also be surface broadcast and tilled into the soil, but if applied using this method, twice as much P fertilizer will be needed. Fertilizers applied directly to the seed, or “popup” fertilizer placement is not suggested for beans because they may injure the seedlings in dry soil, especially at high fertilizer rates. Monoammonium phosphate (MAP, 11-52-0), diammonium phosphate (DAP, 18-46-0), and ammonium polyphosphate (10-34-0) are equally effective per unit of P if properly applied. Base your choice of fertilizer on availability, equipment, and cost per unit of P. 

Table 2. Suggested phosphorus rates as banded applications for irrigated dry beans. 

Potassium Suggestions

Most Colorado soils are relatively high in extractable K, and few crop responses to K fertilizers have been reported. However, some sandy soils or highly eroded soils with exposed subsoils may be low in extractable K. Suggested K rates related to soil test values (NH₄OAc) are given in Table 3.

The main K fertilizer is KCl (muriate of potash, 0-0-60). Broadcast application followed by incorporation into the soil prior to planting is the usual application method. Potash applications can also help in mitigating heat stress. 

Table 3. Suggested potassium rates for irrigated dry beans. 

Zinc Suggestions 

The availability of soil zinc (Zn) decreases as soil pH increases; consequently, most Zn deficiencies are reported on soils with pH levels higher than 7.0. Zinc deficiencies also are found on soils that have been leveled for irrigation where the subsoil is exposed, on soils with high levels of free lime, sandy soils, or soils low in organic matter. Maturity may be delayed in dry beans grown on marginally Zn-deficient soils, so Zn applications may hasten maturity without increasing yields. Suggested fertilizer rates provided in Table 4 for banded and broadcast applications of Zn are based on use of ZnSO₄. Effective Zn chelates, such as ZnEDTA, may be applied at about one-third of the Zn rates shown in Table 4. Band application of all Zn fertilizers with starter fertilizers is more effective than broadcast application.  

Zinc deficiencies also may be corrected by foliar sprays of a 0.5 percent ZnSO₄ solution applied at a rate of 20 to 30 gallons per acre. However, it is difficult to prepare this solution in the field due to its low solubility, so ZnEDTA or other soluble Zn sources can be used. A nonionic surfactant (wetting agent) increases plant absorption of the applied Zn. Foliar Zn applications may correct deficiency symptoms or overcome delayed maturity without necessarily influencing dry bean yield. 

Table 4. Suggested zinc rates for irrigated dry beans.