The availability of potassium to plants as affected by barnyard manure

Chemical analyses were made to determine the variations in the amount and availability of potassium manure. Alfalfa, oats, and Sudan grass were grown in quartz and soil cultures which had received various fertilizer treatments. In this manner some indication of the availability of the potassium in the manure was obtained. Analyses for potassium were made on the plants from jars which had received fertilizer treatments favorable for the utilization of the largest amounts of potassium from the soil and from the soil plus manure. The amount of potassium recovered from the manure was calculated. A study was made by an indirect method to determine the effect of the decomposition of the organic matter in the manure in converting insoluble potassium minerals in the soil into forms that can be used by plants. The factors used to develop the conclusions were the differences in total and available potassium in plats which had received no treatment and manure applications, respectively, over a period of years; the variations found in the potassium content of plants which had received, respectively, no potassium and potassium in the fertilizers applied; and the total amount of potassium removed by crops from manured and no-treatment plats over a period of five or more years. A summary of the results obtained in these experiments follows: 1. The amount of potassium per ton of manure may vary from 3 to 20 pounds. That produced by the same herd may vary at different times from 5 to 12 pounds per ton. 2. All the potassium in the manure is available for plant growth before much of the organic matter in the manure is decomposed. 3. Alfalfa, oats, and Sudan grass recovered practically all of the potassium applied in manure to quartz cultures and soils deficient in available potassium. 4. From soils well supplied with available potassium, alfalfa recovered practically all the potassium applied in manure, while oats and Sudan grass recovered from half to nearly all that applied in the manure. 5. Top dressings of 100 pounds of muriate of potash on alfalfa growing in Colby silt loam gave very marked increases. Ten tons of manure and 600 pounds of 16% acid phosphate had been applied from three to five years previous to the top dressing with muriate of potash. 6. The percentage of potassium in the plant was determined by the amount available for plant growth when the supply of potassium was the factor limiting plant growth. The percentage was applied in manure. 7. The amount of dry matter produced by plant growth did not give an accurate indication of the amount of available potassium in the soil. 8. Most soils fertilized with manure had more N/5 HNO3-soluble potassium and less total potassium than adjoining soils which had never received any fertilizer treatment. 9. Crops removed considerably more potassium in addition to that added in the manure, over a period of years, from plats fertilized with manure than they did from unfertilized plats. 10. The decomposition of the organic matter in manure may convert some of the insoluble potassium minerals in the soil into forms which can be used by the plant.