Abstract

Alfalfa (Medicago sativa L.) fertilization with P and K becomes increasingly important in low desert southwest to sustain high yields and maintain stand persistence as management intensifies. A field trial was conducted in 2018 and 2019 on a sandy clay loam soil at Maricopa Agricultural Center. Combinations effect of three P rates (0, 100, and 125 lb acre^-1 P₂O₅) and three K rates (0, 100, and 300 lb acre^-1 K₂O) on yield were investigated. Phosphorus fertilized plots had significantly (P<0.05) higher total alfalfa hay yield in each year. The results demonstrated that the 125 lb acre^-1 P₂O₅ rate combined with 100 lb acre^-1 K₂O appeared adequate to maximize yield as compared to higher P or K fertilizers alone.

Introduction

Alfalfa (Medicago sativa L.) has an annual economic value of $9 billion in the United States (USDA NASS, 2018). In Arizona, there are currently 260,000 acres of alfalfa producing 2.16 M tons of hay with a cash value of $451 million. Increasing the productivity and improving the profitability of alfalfa in the low desert southwest United States is of great importance for dairies and livestock. Growers intensively manage and frequently harvest alfalfa to achieve and sustain high yields with high nutritive value that dairies depend upon. Among the potentials for enhancing production, profitability, and nutrition efficiencies is through effective use of fertilizers. For many soils in the low desert of Arizona, phosphorus (P) as a phosphate fertilizer is very commonly applied prior to planting alfalfa. Potassium (K) is assumed to be abundantly available in desert soils; therefore, not typically applied to crops. Nitrogen (N) fertilizer is generally not applied for alfalfa production since alfalfa can obtain its own N from N-fixing nodules. Specific information about the interactions and effects of P and K on alfalfa yield and quality for Arizona has not been developed. The University of Arizona Cooperative Extension Field Crop Program investigated and demonstrated the importance of balanced fertilizer applications to maximize alfalfa yield.

Rational for Research

Frequent alfalfa harvesting may lead to reduction in yield and stand persistence. To sustain yields and maintain stand persistence, alfalfa fertilization with P and K becomes increasingly important as management intensifies. Proper P nutrition is essential to maximize alfalfa stand development, productivity, and persistence. Currently, Arizona alfalfa production systems are not fertilized with K due to the assumed high levels of total K in the low desert soils. Potassium is usually at high concentrations in the desert soils of California and Arizona; although K deficiency in alfalfa can occur on sandy soils and on soils with a history of crops that remove a large amount of K such as alfalfa and cotton. The response of alfalfa to potassium (K) fertilizer often varies with soil type, the initial soil test P and K levels, irrigation and harvest management, and yield level. Alfalfa can remove large amounts of K (60 lb/ton) under intensive production systems (Robert, 2004) such as those in Arizona.

For decades, in many intensive agricultural systems, producers increased single, high nutrient P fertilization inputs in order to achieve higher yields, often leading to soil nutrient accumulation. There has been research suggesting that a balanced application of P and K fertilizers is needed to achieve increased yield and extend stand longevity rather than single high nutrient applications. Another study revealed that the imbalance of soil P and K may result in reduced crop yields and that balanced levels of P and K have a positive effect on yield in soils where K is not lacking.

The objective of this study was to determine the yield response of irrigated alfalfa to various combination of P and K fertilizers.

Results

• P has significant, while K has slight effect on yield individually;

• The main effect of P fertilizer had positive and significant effect on hay yield (Figure 1);

• The summation of cuttings were significantly higher at both 125 and 100 lb acre^-1 P₂O₅ rates;

• The combination of 125 lb a^-1 P₂O₅ with 100 lb a^-1 K₂O rates gave highest yields (Figure 1);

• Unfertilized plots (0/0, P2O5/K2O) gave the least hay yield;

• P & K interaction has synergetic effects on yield (Figure 2);

• The annual P application increased Olsen-P and tissue level P (PO4-P)

Figure 2. The annual P application (left) increased Olsen-P, tissue level P (PO4-P) and alfalfa yield significantly (P<0.05). The Olsen-P and plant-P (PO4-P) significantly and positively correlated to yield (Middle). Balanced fertility synergetic P and K fertilizers effect on alfalfa yield at Maricopa Ag Center (right). Sources of fertilizers are MAP-monoammonium phosphate (11-52-0) and KCL-potassium chloride (0-0-60).

Conclusions

• P & K interaction has synergetic effects on yield;

• Highest fertilizer application did not result in significantly increased yield;

• Balanced PK at 125 lb. acre^-1 P₂O₅ and 100 lb. acre^-1 K₂O rates produced the highest productivity;

• The annual P application (left) increased Olsen-P and tissue level P (PO4-P) increased by annual P application;

• A conservative approach to identifying fertilizer application rates may be more profitable;

• Additional long-term research and detail economic analysis required.