Why organic resources and current fertilizer formulations in Southern Africa cannot sustain maize productivity: Evidence from a long-term experiment in Zimbabwe

Abstract

Sustainability of maize-based cropping systems is a major challenge for southern Africa, yet the demand for maize as staple food and animal feed in the region continues to increase. A study was conducted on a sandy clay loam (220 g clay kg-1 soil) at Domboshawa in Zimbabwe to investigate the long-term effects of organic resource quality and application rate, and nitrogen (N) fertilization on soil chemical properties and maize (Zea mays L.) productivity. Crotalaria juncea (high quality), Calliandra calothyrsus (medium quality), cattle manure (variable quality), maize stover and Pinus patula sawdust (both low quality) were incorporated into soil at 4.0 t C ha-1 (high rate) and 1.2 t C ha-1 (low rate) at the start of each cropping season for nine consecutive years. At both high and low application rates, each of the five organic resources was applied in combination with or without mineral nitrogen (N) fertilizer at 120 kg N ha-1. The nine-year period saw maize grain yields declining by 22% to 84% across treatments despite increases in soil organic carbon, total N and available P from 6% to 80%. Crotalaria, Calliandra and manure led to a less steep yield decline. Exchangeable calcium (Ca), magnesium (Mg) and potassium (K), and soil pH explained much of the variation in yield patterns observed under the different organic resource applications. Maize grain yield was positively correlated with exchangeable Ca (r = 0.51), Mg (r = 0.62) and K (r = 0.53), and soil pH (r = 0.49), but negatively correlated with other soil properties over the 9-year period. We concluded that declining soil exchangeable basic cations were the underlying causes of decreasing maize productivity, and was aggravated by use of low rates of organic resource inputs, particularly with N fertilization. Current nutrient management and fertilizer recommendations that emphasize inorganic N, P and K significantly undervalue the role played by organic resources in sustainability of maize cropping systems in southern Africa.

Fertilizer management and soil type influence grain zinc and iron concentration under contrasting smallholder cropping systems in Zimbabwe

“…Thus, if smallholder farmers apply organic nutrient resources to their fields, there is a good chance of increasing the grain Zn nutritive value of their crops. Apart from the crucial role of organic nutrient resources in sustaining maize productivity in southern Africa 16 , organic nutrient resources proved to contribute significantly to DTPA-extractable soil Zn availability as well as grain Zn concentration but not DTPA-extractable soil Fe and grain Fe concentration.…”

“…Integrated soil fertility management can include: combined applications of mineral NPK fertilizers and locally available organic nutrient resources, legume-cereal rotations/intercrops, the use of appropriate germplasm and good agronomic practices for increased soil and crop productivity. Whilst evidence of increased crop yields with ISFM is well established in SSA 14–16 , there has not been much work on establishing the effects of the environment and farmer management options including soil type, organic matter management and crop choices on grain Zn and Fe concentration.…”

Nitrogen effect on zinc biofortification of maize and cowpea in Zimbabwean smallholder farms

“…When soil and foliar Zn fertilizers were applied, the largest grain Zn concentration in maize was achieved when N was applied as mineral N fertilizer at a rate of 45 kg N ha −1 compared to when N was applied as sole organic or combinations of organic and mineral N fertilizer. In smallholder farming systems, cattle manure, which has additional benefits of supplying micronutrients and increasing soil pH (Manzeke et al., 2012; Mtangadura, Mtambanengwe, Nezomba, Rurinda, & Mapfumo, 2017), is unfortunately only within reach of resource‐endowed households (Masvaya et al., 2010; Swift, Frost, Campbell, Hatton, & Wilson, 1989; Zingore, Murwira, Delve, & Giller, 2007). Our findings suggest that intermediate‐resourced and resource‐constrained farmers who often do not own cattle could still harvest more grain Zn even with smaller additional mineral N fertilizer applications, which would also likely improve cereal and legume grain yields.…”

Nitrogen rate impacts on tropical maize nitrogen use efficiency and soil nitrogen depletion in eastern and southern Africa

“…While organic fertilizer and crop residue have been proposed as being more affordable sources of nutrients, they do not currently provide enough nutrients to meet the needs of maize grown on small-holder subsistence farms, much less the needs of maize in an intensified production scenario (Bationo et al 1998 ; Franzluebbers 2002 ; Mtangadura et al 2017 ). In addition, crop residues are also used as feed and, therefore, are often not available for soil fertility improvement.…”