A. S. Claassens

Effect of direct nitrogen and potassium and residual phosphorus fertilizers on soil chemical properties, microbial components and maize yield under long-term crop rotation

Abstract. A study was conducted in a long-term field experiment initiated in 1939 at the University of Pretoria, South Africa. The aim was to assess the long-term effect of direct N and K and residual P fertilizers on chemical and microbial properties of soil and grain yield of maize in rotation with field pea. Long-term fertilization resulted in decreased total organic C (TOC) and basic cation contents, and had an acidifying effect on soil. The decrease in TOC was greater in simple fertilizer treatments (N, P, or K) whereas basic cation contents and pH declined more in balanced fertilizer treatments (NPK). Levels of total N were higher in the balanced than in simple fertilizer treatments. Soil microbial biomass and numbers of bacteria, actinomycetes and fungi were influenced by, and exhibited qualitative changes in response to, long-term fertilization. Crop rotation also exerted effects on chemical and microbial properties of the soil. Maize grain yield showed significant increases due to balanced fertilizer treatments. Responses of maize to simple fertilizer applications were not beneficial in terms of yield returns. The results suggested that judicious use of inorganic fertilizers may, in the long-term, maintain soil quality and productive capacity.

Influence of residual manure on selected nutrient elements and microbial composition of soil under long-term crop rotation

A study was conducted on a long-term field experiment at the University of Pretoria, South Africa, that was established in 1939. The aim was to investigate the effects of residual manure on the characteristics of the total and microbial biomass and their nutrient contents in the soil and on maize yield under long-term crop rotation. It was found that total C, N and available P levels were increased as a result of manure application. Seasonally, these nutrients exhibited variations that appeared to be related to influences of crop rotation. Long-term soil N content in an adjacent native site remained relatively constant while it tended to increase in the control and manured plots. Soil microbial biomass content of C, N, and P and microbial populations were affected by previous manure application, as well as by crop rotation. The biomass and numbers of microflora were generally higher in the manured plots. Manure application also resulted in higher maize yields and had substantial residual effects. Relationships between the different soil properties considered and crop yield are presented and their implications discussed.

Changes in inorganic and organic Phosphorus in soil under maize (Zea mays) cultivation

The changes in applied and residual phosphorus (P) were examined in Avalon and Clovelly soils under cultivation by using the P fractionation method of Hedley, Stewart and Chauhan (1982). Soil P was separated into six major fractions of which some fractions were subdivided into organic (Po) and inorganic (Pi) P forms: P extracted by an anion exchange resin (Resin-P); 0.5 M NaHCO3-P (NaHCO3-Pi and Po); 0.1 M NaOH (NaOH-l-Pi and Po); ultrasonication in 0.1 M NaOH at 75 watt (NaOH-ll-Pi and Po); 1.0 M HCI(HCI-Pi); and H2SO4 digestion and H2O2 oxidation, (H2SO4P). General trends in soil P were evident over 15 years of cultivation. Resin-P, NaHCO3-Pi, NaOH-l-Pi, NaOH-ll-Pi and HCl-Pi were reduced in the Avalon soil, while only the NaHCO3-Pi and NaOH-ll-Pi decreased in the Clovelly soil. It is hypothesised that NaOH-l-Pi, which showed the largest decrease, was mainly responsible for the replenishment of the labile P pool. In the Po fraction, NaHCO3-Po and NaOH-ll-Po decreased with time in the Avalon soil, in con...

Tolerance of six Namaqualand pioneer species to saline soil conditions

Saline soils pose a great problem to reclamation of mined areas in arid regions. Experiments were carried out in order to determine the tolerance of six Namaqualand pioneer species to varying levels of salinity. Both perennial and ephemeral species were used and results of all species showed that increased salinity induced a significant decrease in biomass production. With increased salinity, biomass allocation towards the leaves increased, while that towards the stems decreased in both perennial and ephemeral species, but biomass allocation towards the other plant organs varied. Of the six species tested, the perennials seemed to be better suited for rehabilitation purposes under saline soil conditions.

Soil nutrient contents, microbial properties and maize yield under long-term legume-based crop rotation and fertilization: a comparison of residual effect of manure and NPK fertilizers

A study was conducted in a field experiment that has been ongoing for over 60 years at the University of Pretoria, South Africa. The aim was to compare the residual effects of manure and NPK fertilizers on selected soil nutrients, microbial properties, C and N inputs, maize tissue nutrient concentration and yield under long-term crop rotation. Total organic C, total N and available P levels were increased owing to residuai effects of manure alone or in combination with NPK fertilizers. C input in NPK-fertilized plots was about 47% higher than in manured plots. The estimated fixed N ranged from 52 kg ha−1 in the NPK plots to 164 kg ha−1 in the control plots. Increases in nutrient contents of manured plots were not reflected in microbial properties of the soil. Microbial biomass and numbers were generally higher in the NPK-fertilized than in the manured plots. The differential responses were attributed largely to differences in decomposability of organic materials. Organic material in the NPK treatment appeared to have higher turnover rates than in other treatments, suggesting that C limitation in soils of low C but good nutrient supply may be compensated by the high turnover rate of available organic materials.

A critical soil sulphur level for maize (Zea mays L.) grown in a glasshouse

Sulphur is an essential nutrient for the production of the amino acids cysteine and methionine, which are important for protein syntheses. Previous studies showed that sulphur deficiencies are becoming common because of the tendency to use more concentrated and liquid fertilisers. The objective of this study was determining the threshold concentration values for sulphur in different soils, above which no yield increase can be expected with increased sulphur fertilisation. Four soils, low in sulphur, were selected and treated with five sulphur levels. After four to five weeks the first deficiency symptoms appeared at the lowest sulphur levels. The plants were harvested after ten weeks. It was found that sulphur had a definite effect on yields and 10 mg kg-1 in soil can be used as a threshold value.

An inexpensive, portable rain simulator: construction and test data

Construction details of an inexpensive, portable rain simulator are given. The dismountable components are easily transported on a light truck. The flat spray nozzles used produced rain with a >90% coefficient of uniformity over a 1-m2 test plot. Drop sizes and drop size distributions were determined using a flour pellet method. The appropriate equation of motion for accelerating drops and drag coefficients were used to calculate the impact velocity of individual drops and the kinetic energy of rain produced by various nozzles. Rainfall intensity can easily be varied in the range 10–150 mm h−1 and water consumption is low.

The effects of spatial variation of certain soil properties on the winter yield of a lucerne stand

In general, agricultural fields are managed as uniform units, ignoring spatial soil heterogeneity and its effects on growth and yield of field crops. This study was conducted from June 2001—February 2002 and examines the effects of spatial variation of soil properties on the winter yield of a two-year-old lucerne stand on two soil types using geostatistical procedures. Seventy-two sampling points (nodes) were laid out on a 20 m square grid, with an additional 90 sampling points laid out on a 2.5 m square grid at six randomly selected node points to ensure that the total spatial structure would be identified. From initial soil sampling and analyses, the two experimental soils were classified as belonging to the Stella and Pyramid soil families with inter alia mean clay contents of 45% and 46%, pH(H2O) values of 7.8 and 8.8, and mean P status (Ambic) contents of 18.3 and 6.4 mg kg−1 respectively. Green biomass lucerne yield was determined on six occasions at all nodes, while soil sampling (0–300 mm layer) and analyses were done once in June 2001. Basic statistical analyses showed, for some soil properties, two distinct data populations, emphasizing the presence of two soil types. A yield prediction model (R = 0.55) contained pH(H2O), organic C, K and sand contents as variables. The geostatistical analyses of the yield model variables produced standard semi-variograms although with highly variable autocorrelation lengths. Making use of various kriging techniques, maps of soil properties and yield were compiled. These maps reveal that spatial variation of yield bears a fair resemblance to that of some soil properties and, therefore, supports the validity of the yield prediction model for this case study. This study has shown that the scale of variation of lucerne yield can be related to that of soil properties, a finding which can be useful when designing sampling schemes.

Ceramic suction samplers: A reliable method for extracting soil solutions for analysis

Ceramic samplers are used to extract soil solutions at different depths from the soil profile for chemical analysis. The advantage of this method is that samples can be taken at any time provided that the soil water potential is high enough to allow extraction of the water from the soil. An investigation was conducted to determine to what extent the CEC (cation exchange capacity) and selectivity coefficients of the samplers might influence the chemical composition of the water samples collected. This was due to criticism that the accuracy of the method may be influenced by the CEC of the samplers. The CEC of four Soil Moisture ceramic cup soil water samplers was determined. Two of these samplers had previously been installed in the field and two were new. The CEC of the sampiers did not vary much, although the used samplers had higher CEC values, which were ascribed to the contribution of clay particles contaminating the ceramic material. Selectivity coefficients for different cation combinations were calculated using the Vanselow equation. The selective adsorption of certain cations requires that negligibly small amounts of extracted soil solution be discarded. This makes the ceramic soil water samplers suitable for solution cation analysis.

The influence of varying P concentrations on the yield and abnormalities of lettuce leaves

The effect of varying P concentrations on the growth, yield and development of leaf abnormalities, nutrient content and uptake of other nutrient elements was investigated for lettuce.Three cultivars (Chaparal, Wintercrisp and Robinvale) were studied in sand cultures and soil. The frequency of changing the nutrient solution had an effect on the P level that gave the highest yields or which caused reduced yields and leaf abnormalities. The leaf P content for optimum yields varied between 0.4 and 0.9%, while higher P contents caused leaf abnormalities in Chaparal and Wintercrisp. With high P concentrations, more Ca needed to be taken up to overcome the detrimental effect of P. In soil, maximum yields were obtained with 540 mg kg−1 P applied. Soil analysis at the end of the experiment on these treatments showed that a P content of 250 mg kg−1 P (Bray II) coincided with the highest yields. Yield suppression due to high P concentrations was reduced when the Ca concentrations was increased. The leaf abnormalitie...