Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 30, 2002
Publication Date: N/A
Interpretive Summary: Green manure crops may increase soil phosphorus (P) fertility for succeeding crops by releasing the P contained in their tissues during decomposition. Phosphorus released during green manure decomposition may not be measured by common soil P tests but needs to be accounted for in P fertility recommendations. We conducted a greenhouse study to evaluate the effect of green manures on soil P fertility and on a succeeding sorghum crop's P uptake. We found that P uptake of sorghum increased as the P content of three preceding perennial green manure crops increased. These results suggest that green manures can increase P uptake by a succeeding crop. We also used a common soil P test to measure changes in soil P fertility during this experiment. Differences in P uptake among different green manure crops and by sorghum following different green manures were not reflected in different soil test values. These results support the idea that a different soil test may be necessary to measure soil P availability to green manure and succeeding crops. These results will be useful to agricultural scientists, extension personnel, and consultants for making P recommendations to growers.
We conducted a greenhouse study to evaluate the effect of green manure growth and subsequent soil incorporation on P and biomass yields of a succeeding sorghum [Sorghum bicolor (L.) Moench] crop. We conducted two experiments, one using four perennial forage green manure crops and one using four winter cover crop green manures. Among perennial forages, we found that the P yield of the succeeding sorghum crop increased as the P yield of three green manure crops increased. This relationship between P yields was not a N effect. In the winter cover crop experiment, we found evidence that lupine (Lupinus alba L.), even though it produced two to three times more biomass and therefore contained two to three times more N and P than any of the other cover crops, had a detrimental effect on the succeeding sorghum crop. Among the other three winter cover crops there were small differences in P yield but these differences did not result in different P uptake by the succeeding sorghum crop. Sorghum biomass, however, was greater when sorghum followed the three cover crops than when sorghum was planted in pots with no previous cover crop. We also evaluated the ability of the Bray-1 soil P test to measure changes in soil P bioavailability due to green manure P uptake, green manure decomposition, and sorghum P uptake. In general, it seems that Bray-1 P has limited potential to predict differences in P uptake and release among different types of green manures and by sorghum following green manures.