THE SOIL FERTILITY EFFECTS OF LEGUMINOUS LEY PASTURES IN NORTHEAST THAILAND .2. EFFECTS ON SOIL PHYSICAL AND CHEMICAL-PARAMETERS

In Part I of this series the effect of ley treatments differing in leg ume species, duration of ley and fertilization on the growth of subseq uent roselle and cassava crops was reported. This paper reports measur ements of a range of physical and chemical parameters made either duri ng or at the end of the ley phase and at the end of the test crops. Su rface soil (0-15 cm) bulk density was little affected by treatments. T he percentage of the soil mass occurring as dry aggregates > 4.8 mm di ameter at the end of the ley phase was related to grazing intensity bu t the effects were not evident at the end of the second test crop. The mean weight diameter of wet-sieved aggregates at the end of the ley p hase was higher in all the legume plots than the cassava control. Stea dy infiltration rates varied from 34-38 cm h-1 in three-year Verano pl ots to 60-62 cm h-1 in one-year Siratro plots and were inversely relat ed to the length of the grazed legume treatments. There was no evidenc e of treatment effects on soil water status at the end of the ley phas e or on soil available water characteristics. There was little evidenc e of time or treatment effects on soil pH. Cation exchange capacity va lues were very low ranging from 0.84 cmol (p+) kg-1 in the 0-5 cm laye r at the end of the ley phase for the cassava control to a high of 1.4 0 cmol (p+) kg-1 for the fertilized three-year Siratro plots. Soil min eral N levels during the cropping phase were very low (usually 1-2 mg kg-1) with the higher values in the plots previously under legume. The re were marked treatment effects on soil total N. Gains during the ley phase ranged from 49 to 191 kg N ha-1 in the surface 15 cm of soil wi th some additional input at 15-30 cm. Maximum gains occurred after two years of legume treatment and were usually lower in unfertilized plot s. Rate changes during cropping were relatively low (e.g. a rate of lo ss (k) of 0.047 yr-1 for fertilized Siratro by first-order kinetics). Generally values for soil carbon paralleled those for nitrogen content . Yield of the first test crop (roselle) was not significantly correla ted with any of the measured soil physical parameters but it was, stro ngly, with soil carbon, total nitrogen and CEC (R2 values ranging from 0.54 to 0.78). Mineral nitrogen was also correlated but not as strong ly (R2 values ranged from 0.51 to 0.67).