Concentrating Manure near Crop Plants Boosts Soil Fertility

An important principle of conservation agriculture is that of targeted, precision applications of organic or inorganic fertilizer near crop plants, as opposed to evenly dispersing inputs over an entire field. In a row-crop setting, this can be done by banding fertilizer next to the crop plants. In systems such as zai or Foundations for Farming, fertility inputs can be placed within small planting basins in which the crop seeds are sown.

During the first growing season of a zai study that ECHO is conducting in South Africa, soil nutrients were higher in manured than in non-treated zai pits. In season two, we wanted to further document the effect of targeted (near sorghum seedlings) versus dispersed cow manure. To do this, we added flat-ground treatments including 1) flat ground with broadcast manure and 2) flat ground with the manure concentrated near the sorghum plants. Each treatment was replicated four times in a randomized complete block design.

Cow manure was applied about three weeks prior to sowing sorghum seeds on 15 October 2012, with each plot receiving the equivalent of 400 g of manure per zai pit (or flat-ground planting station with the same 75 cm x 75 cm spacing as the zai pits). In the flat ground plots, the manure was lightly raked into the top 5 cm of soil. In the zai pits, the manure was mixed with 5 cm of soil at the bottom of the hole. Soil was sampled the same week that sorghum seeds were sown. A soil probe was used to collect soil near the sorghum at a depth of 0 to 10 cm.

We found that, within the flat ground treatments, the soil concentration of nearly every nutrient was significantly higher with concentrated than with broadcast manure. Broadcast manure did not even raise the measured soil nutrient concentration above that with no fertilizer. Figure 7 illustrates what we found with soil nitrate; the trend also held true for phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese and zinc. Nutrient concentrations were even higher with the manure placed in zai pits (30 cm wide X 20 cm deep), although it should be noted that the zai pits had already received a previous application of manure at the beginning of season one of the study. By the 15th week after seeding in season two, when the plants were producing grain heads, the sorghum plants had grown 14 cm taller with preplant-concentrated than preplantbroadcast manure.

These results help validate the concept of targeting fertility inputs near crop plants to get the maximum benefit, a practice that ECHO has encouraged over the years. By placing manure or other organic or inorganic fertilizer near the plants, fertilizer rates can be reduced. Using less fertilizer (than you would to treat a whole field) is important to avoid injury to crops from “salt burn” that results from over-application of fertilizers. We did not see salt burn on the sorghum plants in this study. See EDN 84-3 for related information on “micro dosing” fertilizers.