Zai Pit Effects on Selected Soil Properties and Cowpea (Vigna unguiculata) Growth and Grain Yield in Two Selected Dryland Regions of Kenya

Erratic rainfall and temperature regimes, strongly affect agricultural productivity. To address the reduction in production, this study assessed the effect of Zai pit depths on selected soil properties and cowpea growth and grain yield. Zai pit technology was tested in two locations falling under Agroecological Zone IV (relatively dry areas) i.e. Machakos in Machakos County and Naivasha in Nakuru County, Kenya, aiming to determine the combined effect of four Zai pit depths and two levels of manure (plots with manure and plots without manure) on selected soil properties, growth and yield of cowpea. Experiment was laid out in split plot arrangement, with manure levels as the main plot factor and Zai pit depths (Flat: Z0, 30 cm: Z30, 45 cm: Z45 and 60 cm: Z60) as subplot factor, replicated four times. Cowpea (M66 variety) was used as the test crop. Inorganic nitrogen (Nin) and extractable phosphorus (Pex) were significantly (P < 0.05) higher, at 1.37 mg∙kg for Nin and 80.4 mg∙kg for Pex in Zai pits compared to flat plots which were at 0.91 mg∙kg for Nin and 47.1 mg∙kg for Pex. The values of Nin and Pex also varied depending on depths, with Z45 having highest Nin at 1.17 against the least, at 0.89 in the Z0, while Pex was highest in Z30 at 102.3 mg∙kg while Z0 having the least Pex of 89.7 mg∙kg. Generally, crops in Zai pitted plots were larger in diameter (0.46 cm) and height (34.20 cm) than crops in flat plots at (0.42 cm) and (18.11 cm) diameter and height respectively. Better performance was observed in yield, with Z45 yielding 853.33 kg∙Ha against 685.0 kg∙Ha in Flat plots in Machakos while 931.66 kg∙Ha in Z45 against 563.33 kg∙Ha from Flat plots in Naivasha. This study demonstrated great potential of Zai pit technology on crop production, as reflected on improved growth and yield of cowpeas. Combining Zai pits with manure increases soil Nin, Pex and is a guarantee of great crop performance in terms of high final yields. How to cite this paper: Oduor, S.O., Mungai, N.W. and Owido, S.F.O. (2021) Zai Pit Effects on Selected Soil Properties and Cowpea (Vigna unguiculata) Growth and Grain Yield in Two Selected Dryland Regions of Kenya. Open Journal of Soil Science, 11, 39-57. https://doi.org/10.4236/ojss.2021.111003 Received: December 27, 2020 Accepted: January 24, 2021 Published: January 27, 2021 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
Aridisols are degraded soils, mostly found in relatively dry areas experiencing unreliable rainfall regimes. The low quality of soil and low rainfall minimizes agricultural activities that can be carried out in drylands [1]. For farmers determined to cultivate these soils, they end up producing little that can only support their subsistence till next production season, making it difficult for commercialization. With the current global warming, climate change is inevitable and aridisols continue deteriorating; hence farmers produce less than previously [2]. Reduction in rainfall intensity and duration results in low soil water storage, which reduces soil moisture availability that greatly affects crop production. Water is a critical factor of crop production and before planting, farmers have to consider its source. Some farmers are able to irrigate, while those who cannot afford rely on rainfed crop production, which are proven to reduce production expectations seasonally. Alternatively, soil moisture conservation measures can be adopted, which include the use of Zai pit technology [3].
Zai pit technology uses holes dug in the soil for planting crops to help in soil moisture retention and nutrient/fertilizer maintenance. It is a water harvesting technology [4] that is suitable for areas with unpredictable rainfall and low soil fertility, hence subsequent crop failure. The rain water that would otherwise be lost through runoff is collected in the pits, hence stays longer and is utilized slowly by the plants for various physical, biological and chemical processes [5]. This helps in the reclamation of the drought stricken Arid and Semi-Arid Lands (ASAL) back to their originally productive status [3]. Zai pit technology also helps in reducing waste of fertilizer and manure, and increases their efficiency [1] since instead of being applied in the whole field, the manure is only applied in the pits in which the crops are to be grown. The manure, or any soil treatment, is mixed with the upper 50% of the soil, which is then returned back to the soil, while the lower 50% of the soil is piled on the lower side of the pit to trap water in case of any run-off. The technology, in combination with organic manure, especially Farmyard Manure (FYM), has great influence on selected soil properties [6], which in turn significantly influences growth and development of crops as well as the final yields. Among soil properties influenced are soil inorganic nitrogen and extractable phosphorus. P and N are two out of the three major nutrients needed in abundance by the plants throughout, for completion of their growth cycles. Once these nutrients are present in sufficient amount, the growth and development of crops (cowpea) is favoured; hence healthy crops will lead to high grain yields during harvesting. Open Journal of Soil Science Nitrogen makes up almost 80% of the earth's atmosphere. It is also found in the bodies of all living organisms including plants, taking part in their growth and development. Amount of Inorganic nitrogen (N in ) in the soil depends on the N-pools in the soil [7], as well as the amount of added organic/inorganic fertilizers, which add N in different forms. Organic fertilizers like farmyard manure undergo decomposition to release nutrients, majorly N and P that are utilized by plants [8]. Decomposition is the process by which the microorganisms in the soil attack dead biomass, primarily in search or energy and in the process, nutrients are released to the soil. If the applied manure contains higher amounts of Nitrogen than in the surrounding (i.e. C: N ratio of less than 25:1) [9], N mineralization takes place, availing it to plants in form of inorganic Nitrogen.
The various inorganic forms of N soil include ammonium ( 4 NH + ), nitrite ( 2 NO − ) and Nitrate ( 3 NO − ) [10]. 4 NH + is the most reduced form and found in anaerobic surroundings while 3 NO − is the most oxidised form of N in and exists in most aerobic environments. They are also very soluble and availing water in the Zai pits makes them readily up taken by plants.
Phosphorus is the next important plant macronutrient element in the soil and it is very important yet very limiting in plant growth and development [11], since it might be present in soil in abundance but not available for plant uptake. This is contributed by its immobility in soils, hence to make it mobile, pH must be adjusted to favorable ranges of between 6.2 -6.8 [12]. Therefore, to determine the fraction that is available for growth and development, the extractable phosphorus is always fraction measured in the laboratory [13]. It is available to plants in the form of primary orthophosphate ( 2 4 H PO − ) and Secondary orthophosphate ( 2 4 HPO − ) [14]. In plant tissues, Total Phosphorus (P T ) ranges from 0.09% to 0.6% [15]. There are limited P toxicities, while P below this level lead to deficiencies. Phosphorus deficient produces low yields, and leads to loses to farmers, hence these should be avoided. Deficiency is first observed in the lower old leaves, which display purple or reddish brown along the edges.
Nitrogen is very vital in crop growth, and a healthy plant contains 3% -4% of N in their above ground tissues [16]. This percentage is greater than any other elements' fractions in the tissues hence its significance. In the tissues, it is measured in form of Total Nitrogen (N T ) and it is crucial since it is the main component of chlorophyl [17]. Its presence is also needed since it's a major component of amino acids, which is a building block of proteins. Proteins maintain plants upright since they form structural units in plants cells, hence contributing to the plants' growth in breath (stem) and height. Nitrogen and Phosphorus are also a component of adenosine triphosphate (ATP) [18], a component that assist in energy transfer, hence allowing energy conservation and use in plant cells. Both Phosphorus and Nitrogen are part of the DNA [9], the genetic make-up that permits growth and reproduction. Therefore, N and P contribute to the general growth and development of plants and the final yields.
To determine the amount of agronomic P and N to add to the soil of optimum Open Journal of Soil Science growth and good yield, soil testing is key [12]. Such nutrients maybe abundant in soil, but inaccessible by plants, since not in their ionic form for uptake by plants. To facilitate nutrient dissolution, water needs to be availed. Zai pits deliver this water through collection from rain water. Nutrients then dissolve to ionic forms, facilitating availability to plants.
Effects on the soil properties finally lead to reciprocating effect on the growth parameters of the crops and to the final yields from the farm. In this experiment, Cowpea (Vigna unguiculata L. Walp) was used as the test crop, and there were significant observations made. Cowpea is an excellent alternative food in the world with its origin in Africa. It is a nutritive source of Vitamin B (89.0%), Iron (53.6%), Copper (51.2%) and Phosphorus (39.2%) [19]. Zai pit technology has the capability to influence growth parameters like stem diameter, height and final yield of crops.
All parameters are affected proportionately depending on the size of the Zai pits being used. The pits in this experiment had varying depths and were being compared to those plots which the technology was not applied to. Effect of the two different sites (but falling under the same agro-ecological zone) was also investigated. This experiment was carried out in two sites, Machakos and Naivasha, with the aim of determining the combined effect of Zai pit technology and manure levels on selected soil properties, growth and yield of Cowpeas.

Site Description
The study was carried out in two locations, which were Katumani in Machakos county and Naivasha in Nakuru county, Kenya. These two locations, Figure 1,   [20]. It lies in the altitude of 1000 to 2000 m above the sea level [21] and is lo-

Experimental Design and Treatment
The experiment was laid out in a Split-Plot Design, where fields "with manure" and "without manure" represented the main plot factors while the sub-plot factor was comprised of Zai pits of different sizes: Without Zai (Z 0 ) i.e. Flat, Zai pits measuring 60 × 60 × 30 cm (Z 30 ), Zai pits measuring 60 × 60 × 45 cm (Z 45 ) and Zai pits measuring 60 × 60 × 60 cm (Z 60 ). These treatments were replicated four times in both the two study sites.
During the digging of the Zai pits, soil from the pits was piled on the side of the pit for the first 50%. This represented the top soil, which was then mixed with 1.2 kg (for Machakos) and 0.6 kg (Naivasha) of Farmyard Manure for each pit, representing application rates of 9.8 t/ha and 4.9 t/ha for the two locations respectively, as recommended after soil fertility analysis of the sample from the respective study locations, followed by cattle manure analysis, which determined their nutrients content. Analysis method was similar to the method described in the "Soil characterization" (below), after which the mixture was returned back into the pits. This was then followed by planting of the Cowpea seeds, where 15 seeds were planted per pit in the pitted plots (containing 12 zai pits) while 3 seeds per hole (in flat plots containing 60 holes). These gave seed density of 120,000 seeds per hectare. The lower 50% of the soil was piled on the lower sloping side of the pit and was never returned back to the pit. It remained on the side of the pit to reduce the rate of the run-off during precipitation and hence increase the amount of the rainwater collected in the pits.

Soil Characterization
Before Prior to land preparation, soil samples were taken to the lab for initial characterization. Samples for chemical analysis were collected using Soil auger at Open Journal of Soil Science equidistant points forming a Zigzag at a depth of 15 cm below the ground surface. These were then mixed in a bucket to form composite samples, after which the representative samples were put in a brown khaki bags for analysis. The samples were airdried in an open-air space for 7 days, after which it was sieved using <2 mm sieve. Sieving was then preceded by testing different soil parameters. Soil texture was investigated, through carrying out particle size analysis (PSA) using hydrometer method, as outlined by [22]. Infiltration rate was measured in situ [23] using the Double ring infiltrometer method [24] [25] [26].
Bulk density was determined by the core ring method, where undisturbed samples were collected by driving core rings into the soil up to a depth of 15 cm using compaction hummer. Then, the core ring was removed with its content (soil). The extending soils on the core sampler were then trimmed carefully with a knife. This was to ensure that the volume of the soil sample fits or is the same to the volume of the core ring.
At the lab, the core rings were placed in the moisture can and weighed together with its content. The core ring plus its content was then place in an oven and dried at 105˚C for 24 hours, to ensure a constant weight was achieved. After drying, the core ring was then weighed, and the dried weighed recorded. Bulk density was then determined by finding the quotient of the mass of solids and the total volume of soil in the core ring (which has a diameter of 5 cm and a height of 5 cm): While using the same sample(s), mass of water lost was determined by finding the difference between the initial weight of wet soil and the (final) oven-dried weight of soil. This mass was then converted to volume by using the density of water; 1 g•ml −1 [27] which was then used to determine the Volumetric water content (Ө) of the soil by dividing volume of lost water by the total volume of soil in the core ring.
Vol Vol ume umetri of wa c water cont ter Total Vo ent lume = Soil pH (water) was determined by a 1:2.5 soil to water ratio, as described by Okalebo et al. [22], Nitrogen was determined according to Kjeldahl method [13], Phosphorus, Potassium, Magnesium, Sodium, Manganese and Calcium were determined using Mehlich I (Double acid) method, according to Mylavarapu et al. [28], while available Iron, Copper and Zinc were extracted in a 1:10 ratio (w/v) with 0.1 M HCl, which were later determined using AAS (Atomic Absorption Spectrophotometer). Soil carbon was determined calorimetrically according to Okalebo et al. [22].
Soil moisture was monitored weekly using soil neutron probe, Troxler 4300 At the end of the experiment, soil was sampled for lab analysis, mainly for extractable phosphorus and inorganic nitrogen. Composite samples were obtained from each of the subplots. They were obtained from each of the Zai pits using an auger up to a depth of 20 cm from the surface of the soil (surface vary from one pit to the other depending on the depth of specific pits), and taken through the drying process as described above. Inorganic N was then determined using 2 M KCl as extracting solution, using the ratio 1:5 (soil: solution) according to Estefan et al. [13] while extractable P was determined by using 5 M sodium bicarbonate (NaHCO 3 ) as the extracting solution [13].

Data Analysis
The data was first tested for normality using "Proc univariate Normal plot", after which it was subjected to the analysis of variance (ANOVA) using proc GLM in Statistical Analysis Software (SAS) version 9.2 [30], followed by mean separation using Tukey's Honestly Significant Difference (HSD) at α = 0.05.

Changes in Selected Soil Properties as Influenced by Zai Pits Technology and Manure Application
Difference in soil characteristics were observed at the end of the experiment, with respect to the initial soil characteristics (Table 1), in terms of soil inorganic nitrogen and soil extractable phosphorus. Similar differences were also observed among plots with different manure levels (main plot factor) and different Zai pit sizes (sub-plot factor). Data analysis revealed a significant difference (P ≤ 0.001) due to location (Table 2) in inorganic nitrogen (N in ) and extractable phosphorus (P ex ) in soil samples analysed at the end of the experiment.
Further, mean separation at P ≤ 0.05 shows that manure application had a significant effect in the level of N in at the end of the experiment. This was also the case when means of P ex were separated (P ≤ 0.05) ( Table 2). Zai pit depth, the sub-plot factor was also significant at P ≤ 0.05 for the soil N in . Interaction between Manure and Zai pit technology significantly (P ≤ 0.05) affected soil N in as well as soil P ex at P ≤ 0.001, with higher N in from 45 cm Zai Depth in both two sites (Table 3).
In the experiment, there was a significant improvement in the soil chemical parameters. Inorganic Nitrogen (N in ) level in the soil was increased from 0.23% at onset of the experiment to an overall mean of 0.69% at the end of the experi-Open Journal of Soil Science ment while total phosphorus (P ex ) increased by 29.9% in Naivasha soils. In Machakos, an upward trend was observed, with an increase of 62.5% in N in and 19.4% in P ex . Soil analysis also revealed a significant difference in manure treatments. Plots with manure application revealed a tremendous alteration in soil chemical parameters, e.g. in Table 3, N in was constantly higher and significantly different between plots that were applied with manure and those plots that had "No Manure" application, this trend was observed in both Machakos and Naivasha study sites.

Cowpea Growth Responses to Zai Pits and Manure Application
Cowpea height and stem diameter were measured 14, 35, 56 and 70 Days after planting (DAP), as shown in Figure 3. Running ANOVA revealed that stem diameter and plant height differed significantly (P ≤ 0.001) across sites ( Figure   2), with Naivasha producing both the highest mean pant height of 32.9 cm as well as the largest stem diameter of 0.51 cm. It is also worth noting that plant heights from Naivasha were all higher than those from Machakos, as observed in Figure 2, where all the heights from Naivasha are above the horizontal line. It was also noted that manure application on both sites influenced cowpeas stem diameter, since the mean diameter ( Figure 2) in Machakos was high, at 51.51% compared to Naivasha, with mean separation using Tukey's HSD revealing a significant difference at P ≤ 0.05. ANOVA also revealed a significant difference of mean plant height at P ≤ 0.01 due to manure application.
Cowpea stem diameter varied with Zai depth significantly (P ≤ 0.001) ( Figure  2) while an interaction between Manure application and Zai pit technology had significant effect in plant height at P ≤ 0.05. Mean square comparison using Tukey at P ≤ 0.05 also revealed that there was significant difference between Zai pit depths on mean stem diameter, with 45 cm and 60 cm depths producing the largest diameter while 0 cm (flat) and 30 cm depths producing smallest diameters. Difference was also observed with respect to location, where plants from Naivasha site were significantly taller ( Figure 2) than those from Machakos. ANOVA at the same time revealed that time (DAP) of measurement had an effect on the stem diameters (Figure 3   Plant tissue analysis revealed influence of nutrients in the tissues by the applied treatments. Close observation of Table 4 reveals that location significantly (P ≤ 0.001) influenced both total nitrogen (N T ) and total phosphorus (P T ).
Manure application also had a significant effect on both N T and P T (Table 4) from the analysis of plant tissues. Further, ANOVA shows that an interaction between location and manure significantly influenced total nitrogen in the plant tissues, based on comparison of means between crops that received manure and crops that had "No Manure" application.  Furthermore, ANOVA revealed a significant difference in the number of pods and pod weight of cowpea at P ≤ 0.001 and 100 seed weight and stover weight at P ≤ 0.05 (Table 5), which were greatly influenced by manure application. Zai depth, the sub -plot factor influenced the final yields (Figure 4), since it revealed significance at P ≤ 0.001 while an interaction between Manure (Main plot factor) and Zai depth (Sub-plot factor) influenced 100 seed weight (P ≤ 0.001) as well as the stover weight at P ≤ 0.05. Flat plots produced low yields from both the two sites, revealing the great roles played by the Zai pits in growth, development and final yield of Cowpea.

Discussion
Improvement in soil nutrient status can be attributed to the effect brought about by addition of manure, which after decomposition, released nutrients in available forms like ammonium ion ( 4 NH + ), Nitrite ion ( 2 NO − ) and Nitrate ions ( 3 NO − ) that are available for plant uptake [31]. Secondly, presence of enough soil moisture in the soil (after collection in Zai pits) that dissolve plant nutrients from complex solid substances which existed in the soil to ionic inorganic forms that can be detected and up-taken by plants [16].
Phosphorus is contained in organic forms in bodies of microorganisms found in the manure, as well as in complex metal compounds in the manure [15]. This makes phosphorus not readily available for plant utilization, hence plants might suffer deficiencies even when P is present in the soil. On favorable conditions i.e. a pH range of 6.0 -7.7, phosphorus is converted to orthophosphate forms ( 2 4 H PO − and/or 2 4 HPO − ) that are available to plants, hence ready for uptake by plants [31]. It should be noted that in very acidic soils, P combines with Iron oxide and/or aluminum oxides while in very alkaline soils (high pH), P combines with Magnesium or Calcium (Ca). These reactions form complex substances that reduce P availability to plants due to impaired mobility.
In the present study, manure application to the soil may have improved soil particle aggregation that might improve water infiltration in the soil. Zai pits acted as a trough, which trapped the infiltrating water, thereby reducing percolation hence making the water remain within the rooting depth of the crops (Cowpea). This can be affirmed by the results of Adeyemo et al. [32] and reinstated by Kausar et al. [33] that, combining Zai pits and manure application improved the soil water retention, providing enough moisture for plant growth.   Means with the same letter within a column are not significantly different at P ≤ 0.05.
Significant difference in inorganic nitrogen and extractable phosphorus in the two locations can be attributed to the difference in the rainfall occurrence in the two locations. Given that the two locations are situated within similar agroecological zones (AEZ IV), climate change over time has resulted in slight difference in weather conditions hence varying rainfall received [3]. This then explains the difference in soil moisture content that resulted into difference in decomposition rates of the applied manure hence the difference in nutrient contents. The higher extractable phosphorus in Naivasha can be attributed to the fact that humid conditions (locations) facilitate rate of decomposition and release of nutrients.
Therefore, the rate of release of P into soil solution for plant utilization was higher in Naivasha site compared to Katumani in Machakos county.
Mean separation revealing difference in soil from plots WITH manure application having higher mean inorganic nitrogen and extractible phosphorus is due to the fact that addition of manure is equivalent to addition of more nutrients into the soil hence this enriches the soil with plant nutrients. These can be supported by Chen et al. [34], as well as Deng et al. [35] who observed that animal manure is a renewable source of plant nutrient elements such as N, P, K and other macronutrients.
Zai pits also influenced soil N in and P ex that were analyzed at the end of the study, with different Zai pit depths recording different levels of inorganic N. Zai pits at 45 cm depth recorded highest mean N in while flat plots gave the least N in . This is attributed to the fact that flat plots lacked enough soil water, therefore inadequate soil moisture that reduced the decomposition rate. Incomplete de- to ammonia gas that escapes into the atmosphere [39], therefore only few amounts remain in the soil for plant uptake that relatively lowered the tissue N T .
Total N and P are significantly affected by manure application into the soils, in that soils where manure is added reflect a great amount in plant tissues [40].
This is attributed to the fact that after decomposition, the added manure release plant nutrients into the soil, which plus on top of the nutrients that were already in soil initially, hence during uptake, a significant amount are utilized by the crop [36], compared to crops from plots that were not supplied with manure. In some plots where N T was high, it was noticed that there was a corresponding higher level of P T . This is better explained by the findings by Hefner et al. [40] who stated that synergies exists on the uptake process of some plant nutrient, hence N facilitated uptake and utilization of P (an immobile nutrient) up to a certain extent.  [41] who stated that stem diameter has a strong correlation with available plant moisture, up to a certain point, just before water-logging that affects "normal" crops' growth and development.
Tentatively, Wan et al. [42], states that xylem and phloem of growing crops increases in size continuously with time, provided that all the growth (both soil and weather conditions) factors are optimal for growth, up to the maximum de- Results from plots treated with manure application showed a continuously higher mean values in number of pods, 100 seeds weight and stover weight at harvest. This shows that manure resulted in a significant alteration in plants nutrients, and coupled by its effect on soil moisture, crop performance can be enhanced a great deal [43]. This can also be explained by the findings of Astiko et al., [44] that supplying crops with adequate water as well as plant nutrients favors absorption and utilization of the water and plant nutrients which accelerates development of generative organs like branches and higher number of pods which in turn boosts the 100 seeds weigh and the final stover weight at harvest.
The variation in yield data indicates that any change in moisture levels in soil is proportionally reciprocated by plant performance. Manure contains plant nutrients, hence on decomposition, macronutrients like nitrogen and phosphorus are made available for plant uptake. Nitrogen is present in different forms. The inorganic form of N, i.e., ammonium-N ( 4 NH + ) is immediately available to plants after a short while of application. Therefore, manure influenced plant performance in terms of yield. Same conclusion was also drawn by Goldberg et al. [45] who achieved positive results on short term effects of livestock manure on soil properties. Conditions for better performance were also created by enough soil moisture content which was a result of collection by Zai pits. An interaction of the two led to higher 100 seeds weight as well as stover weight at harvest.

Conclusion
To increase productivity on the current scarce and low agriculturally productive lands especially in dryland areas, investing in better agricultural inputs and technologies is key. Use of Zai pit technology is one of the interventions that

Recommendation
Since ASAL areas experience erratic rainfall regimes, when using Zai pits in the farms, the size of the pits matters a lot. Findings from this research have proven that a pit with 45 cm depth performs better in terms of moisture retention as well as on plant nutrient delivery, which has seen an increase in stem diameter, height and higher final yields. It is also important to point out that soil moisture boosts yield and general productivity up to a certain level, beyond which no difference is observed. This is evident in Naivasha where pits with 60 cm depth produced least yields. They could store water for longer, till they become waterlogged for a period of time. Combining Zai pit technology with manure application guarantees highest yields since they provide nutrients, which is evident by improved inorganic nitrogen and extractable phosphorus in soils, as well as improving soil structure that aids in boosting soil moisture availability at the root zone. These in turn brought about higher N T and P T in cowpeas tissues, thereby contributing to significance in yields from plots that contained Zai pit technology. We also recommend that this research should be carried out over more than two seasons to assess how often manure should be added based on duration of residual effect of the applied manure.