Observations that are frequencies rather than measurements often call for special types of statistical analysis. This paper comments on circumstances in which methods for one type of data can sensibly be used for the other. A section on two-way contingency tables emphasizes the proper role of χ2 a test statistic but not a measure of association; it mentions the distinction between one-tail and two-tail significance tests and reminds the reader of dangers. Multiway tables bring new complications, and the problems of interactions when additional classificatory factors are explicit or hidden are discussed at some length. A brief outline attempts to show how probit, logit, and similar techniques are related to the analysis of contingency tables. Finally, three unusual examples are described as illustrations of the care that is needed to avoid jumping to conclusions on how frequency data should be analysed.

Much of the watermelon (Citullus lanatus) cultivation in Brazil is conducted in sandy soil and topographic conditions that favour the erosion process. Therefore, conservation tillage methods are critical for the sustainability of the production chain of this vegetable crop. We studied different tillage methods and cover crops in watermelon cultivation in the area of the reform of degraded pasture. For this purpose, two tillage methods were established as experimental treatments: minimum tillage preparation with subsoiling only, and no tillage. As cover crops white lupine (Lupinus albus) and bristle oat (Avena strigosa) were seeded. As control, watermelon was cultivated with conventional tillage, without prior cultivation of cover crops. For the experimental design, randomised blocks in a factorial arrangement with four replications were used. After liming and phosphate fertilisation of the soil, cover crops were cultivated in soil with minimum tillage and no tillage to produce straw to be used for soil cover, where subsequently the watermelon was grown. The productivity of dry mass and nutrient accumulation in the shoot of cover crops, the soil properties and the watermelon agronomic performance were evaluated. White lupine had better performance in the production of dry mass and nutrient accumulation in shoot than bristle oat. There were differences among treatments for soil penetration resistance, where in conventional tillage the values were lower in the first 30 cm of depth in relation to no-tillage cultivation. The tillage method also affected the fertility of the soil at a depth of 0 to 20 cm. The no tillage provided increased nitrogen leaf content in watermelon regardless of cover crops but restricted root growth in relation to minimum tillage and conventional tillage. Watermelon had similar commercial production by different treatments, with reduction only in no tillage on bristle oat straw.

The rate of regrowth in realtion to total dry matter yield is important for smallholders fodder crop production. A simple regression technique is used to compare the rates of regrowth of eight perennial fodder crops cut at the same time. A brief interpretation and discussion of results from data illustrative of the model' use in given.

Little is known about the adaptability of mamey sapote (Pouteria sapota) to acidic soils high in aluminum (Al). A 2 year field study was conducted to determine the effects of various levels of soil Al on dry matter production, stem diameter and nutrient concentration in tissues of four cultivars of mamey sapote. Soil Al treatments were statistically different for all variables measured in the study. Cultivars and the year × cultivar interaction were not significant; therefore, results were averaged over cultivars and years. Increasing soil Al concentration from 3.5 to 7.8 cmol kg−1 resulted in an increase in total dry weight, but higher soil Al concentrations resulted in dry weight and stem diameter reductions. Increments in soil Al resulted in a significant reduction in the concentration of leaf, stem and root calcium. The Al concentration in leaf and stem tissues was not significantly affected with increments in soil Al, but there was a significant decline in the concentration of Al in root tissue suggesting that mamey sapote may exclude Al from roots.

Internal browning disorders, including brown fleck (BF), in potato (Solanum tuberosum) tubers greatly reduce tuber quality, but the causes are not well understood. This is due, in part, to the highly variable data provided by visual value-based rating systems. A digital imaging technique was developed to quantify accurately the incidence of internal browning in potato tubers. Images of tuber sections were scanned using a flatbed scanner and digitally enhanced to highlight tuber BF lesions, and the area of affected tissue calculated using pixel quantification software. Digital imaging allowed for the determination of previously unused indices of the incidence and severity of internal browning in potato tubers. Statistical analysis of the comparison between digitally derived and visual-rating BF data from a glasshouse experiment showed that digital data greatly improved the delineation of treatment effects. The F-test probability was further improved through square root or logarithmic data transformations of the digital data, but not of the visual-rating data. Data from a field experiment showed that the area of tuber affected by BF and the number of small BF lesions increased with time and with increase in tuber size. The results from this study indicate that digital imaging of internal browning disorders of potato tubers holds much promise in determining their causes that heretofore have proved elusive.

The total N uptake, crude protein content, and recovery of N in rice grains varied with varieties. Increased levels of N application gave increases in total N uptake and crude protein content in the grains, though recovery of N in the grains decreased with increased levels of nitrogen application. Irrespective of varieties and levels of application, three split dressings of nitrogenous fertilizer gave greater total N uptake, crude protein content, and recovery of N in the rice grains than where all the N was applied at the time of transplanting of seedlings.

Field and plant tissue tests under Sudan Kenana conditions revealed that rice does not respond to K but response to P was very clear, both from visual symptoms, tissue analyses and in yields. As early as the seedling stage, a concentration of leaf blade-P of about 1300 ppm is considered optimum. Yield, and level of soil application, corresponding to this value were 5000 kg/ha paddy rice and 42·8 kg/ha P2O5 respectively. A low yield (2400 kg/ha) was obtained from plants receiving no P2O5. Phosphate application reduced the grain protein.

We are very sad to say goodbye to Dr Lindsay Innes, OBE, FRSE who is retiring from the post of Book Review Editor that he has held for 15 years. During that time, Lindsay has worked tirelessly to ensure that regular readers of Experimental Agriculture have always been able to find out what books they should be reading, and why, by consulting the Journal's concise, incisive reviews by renowned experts from a bewildering range of fields. Lindsay, whose career description reads like a summary of all the top jobs in International Agricultural Research, always knew the best person to ask to review a book and the result was that we got the best reviews. We will miss him but we thank him sincerely for his work for the journal and we wish Lindsay the best of luck for the future.

Soyabean (Glycine max) is a relatively new crop for small-scale farmers in Zambia which has been adopted following the introduction of new cultivars, greater opportunity to obtain credit, easier marketing and an attractive guaranteed price. However, low yields limit production partly due to the lack of a planting method that establishes optimal populations. The present method is to plough and plant in the same operation, dribbling the seed behind the ox-plough. This often leads to uneven depth of planting, and hence to poor seedling emergence and erratic stands. Alternative planting techniques evaluated on farmers' fields for three seasons (1985/86–1987/88) suggest that farmers should replace their practice of planting behind the plough with either hand seeding following a plough–harrow operation or the use of a modified ox-drawn planter (Taparia).

Ideally each block of an experiment should be made up in the same way with respect to treatments, that is, the design should be ‘orthogonal’. In practice that can be difficult to achieve, especially if the blocks have been chosen to fit the fertility pattern of the field. Sometimes it is impossible, in which case each block will have to contain its own selection of treatments. A number of simple and useful possibilities exist.

Whatever non-orthogonal design is chosen some of the contrasts of interest (perhaps all of them) will be evaluated less efficiently, but that can be compensated by the smaller error mean-square given by a better blocking system. Also, where blocks do differ in their content, comparing their means will provide additional information about treatment effects. Sometimes the information may be worth the trouble of recovery.

Special attention is given in this paper to total balance (including balanced incomplete block designs), supplemented balance, square and rectangular lattices and alpha-designs. The reinforcement of a design is explained and the advantages considered.

Problemas de diseño en la experimentación con cultivos. III

A field experiment was conducted to assess the effect of competition between a leucaena hybrid and maize (Zea mays L.) when planted simultaneously in an alley cropping system. The leucaena hybrid (a cross between L. diversifolia and L. leucocephala) was planted at hedgerow spacings of 3 and 5.25 m, while maize was planted in rows 75 cm apart between the hedgerows. The spacing between the leucaena hedgerow and maize was varied by removal of 0, l or 2 rows of maize to give three spacing treatments of 37.5, 75 or 112.5 cm between leucaena and maize. A control plot of leucaena alone was also included in the treatments. The growth and yield of individual maize rows were virtually unaffected by the presence of leucaena, but maize had a significant influence on the growth and yield of leucaena. At full maize canopy development, photosynthetically active radiation reaching the leucaena was reduced in all treatments, resulting in a 75% yield reduction in leucaena at the closest spacing. Overall, maize grain yield reached 10.3 t ha−1 in the continuous maize plots (37.5 cm treatment). This was reduced by up to 40% after removal of two maize rows in the closest leucaena row spacing treatment. The implications of these results for the practical establishment of leucaena hedgerows with a maize crop are discussed.

Shifting cultivation of dry rice on steep hill-sides continues to be widely practised in Sarawak, Malaysia. Fertilizer experiments have shown that economic increases in yield can be obtained by mixing 0·9 kg. (2 lb) of mono-ammonium phosphate pellets (11 per cent N: 21 per cent P) with 2·4 kg. (5·3 lb) of seed just before dibbling. In certain areas additional increases might be gained by further increasing the rate of application of phosphorus, from the use of potassium, and from top-dressing with nitrogen. Closer spacing (30·5 cm. or 12 in. square) than that traditionally used should also increase yields per unit area.

In three field experiments, peat pH increased in parabolic relation to liming at the rate of 0.0544 pH unit/t/ha between 0–40 t/ha lime, but only 0.0224/t/ha between 40–72 t/ha. Groundnut and sorghum seed yield and crop residue also showed parabolic responses to peat pH and liming, but groundnut seed yield correlated poorly with soil pH (r2 = 0.38, P = 0.05), compared with its crop residue as well as with sorghum seed and residue (r2 = 0.75, 0.76 and 0.79, respectively, P = 0.01). Groundnut was generally more tolerant of peat acidity (pH 3.35–3.65) than sorghum, giving higher plant survival in unlimed plots, optimum seed and residue at pH 4.60 compared with 5.70 for sorghum, and reduced responsiveness to pH increase. Seed and residue of both crops decreased with continuous cropping, probably because of increasing pest and disease incidence and depletion of available soil nutrients.