Quantitative Recovery by Alfalfa with Time of K Placed at Different Soil Depths for Two Soil Types

Knowledge of the capacity of a root system of alfalfa (Medicago sativa L.) to absorb nutrients from the soil profile is needed to evaluate fertilizer practices. Also, with the extensive perennial root system and high K requirements, alfalfa and K provide a good combination to study root activity in soil with time. This study was initiated to determine nutrient uptake patterns of the alfalfa root system over several years. Quantitative measurements were made of K uptake by two alfalfa strains from different soil depths on a Plano silt loam (Typic Agriudoll) and a Plainfield loamy sand (Typic Udipsamment). One of the alfalfa strains was predominantly taprooted, while the other was predominantly branch-rooted, especially in the surface-soil areas. Experimental treatments included a check, a surface (S) K application, and placement of K at various profile depths. The surface (S) treatment was a broadcast application of K as KC1. An injection method was used to place a solution of K as KC1 at depths below the soil surface. Treatments were placement of K at one profile position per treatment (S, 7.5, 22.5, 37.5, 52.5, 67.5, and 82.5 cm) and at two profile positions per treatment (S/22.5, S/52.5, S/82.5,7.5/22.5, 7.5/52.5,7.5/82.5,22.5/52.5,22.5/82.5,37.5/52.5, and 37.5182.5cm). Rate of K was 205 kg/ha/yr in the single-placement treatments and 410 kg/ha/yr in the dual-placement treatments. Treatment area was 1 M². Three harvests of each individual plot-area were made annually at first flower for 2 yews on the Plano silt loam and for 3 years on the Plainfield loamy sand. The oven-dried herbage was analyzed for yield and K concentration. First-year recovery of added K by alfalfa for single-placement treatments ranged from about 35% for the surface application to 15% at 82.5 cm. An increase in recovery of added K occurred in the 2nd year for the treatments down to 37.5 cm and remained relatively constant below 37.5 cm, indicating increased root activity in the soil surface and near surface layers with no further increase in activity below 37.5 cm in the 2nd year. This trend was apparent for the two alfalfa strains on both soil types. After 3 years on the loamy sand, 60% of the added K was recovered from the surface application to 25% at 82.5 cm. Quantitative similarity of K uptake from two single-placement treatments to that for the same soil depths for a dual-placement treatment indicated relatively uniform K uptake from each of the soil depths. These data show that alfalfa absorbed K most heavily from the surface soil area as compared to the Ap and deeper horizons. This K absorption pattern continued for 2 years on a silt loam soil and for 3 years on a loamy sand.